[John L Stewart]

Go to this link-

http://greygum.net/sbench/sbench101/

& click on 'Inverted Triodes'.

Steve Bench is a real live whiz & has come up with some neat stuff. In this case he uses ten 5687s direct driving a loudspeaker from their grids. The plates are run negative, plate & grid functions interchanged.

The plate voltage does have an influence on grid current. You can easily see it in any of the plate family curve sets showing grid one +Ve as it would be in a Class AB2 or Class B amplifier part of the time. Same goes for G2 current plots, take a look.

This effect has been known since DeForest stumbled on the grid.

With the Plate -ve with respect to the Cathode & the Grid +ve it is possible to build an amplifer with low OP Impedance & very high IP Impedance. Terman mentions the use of this device to form a convenient interface for the measurent of high voltages.

Would I build Steve's amp? Probably not, but interesting to see.

Cheers, John

[Big Bad Bob]

On 03/26/11 12:10, John L Stewart so witilly quipped:
Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). Those
tubes typically fail emissions tests. I've never once seen a heater
burn out. Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

[Engineer[_2_]]

On Mar 27, 2:45*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. *Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). *Those
tubes typically fail emissions tests. *I've never once seen a heater
burn out. *Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. *And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

John, I'm just completing my "Scratch 20" mono amplifier (2 x 6L6,
6SN7 driver, 6SL7 amp and phase splitter, 5U4 rectifier) and am
contemplating adding in-rush protection. Would you put a thermistor
in the 6.3 VAC heater line (the 5U4 can fend for itself!) or in the
120 AC input line?
Cheers,
Roger

[John L Stewart]

Originally Posted by Engineer[_2_]

On Mar 27, 2:45*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. *Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). *Those
tubes typically fail emissions tests. *I've never once seen a heater
burn out. *Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. *And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

John, I'm just completing my "Scratch 20" mono amplifier (2 x 6L6,
6SN7 driver, 6SL7 amp and phase splitter, 5U4 rectifier) and am
contemplating adding in-rush protection. Would you put a thermistor
in the 6.3 VAC heater line (the 5U4 can fend for itself!) or in the
120 AC input line?
Cheers,
Roger

If used the thermistor should be connected in the primary circuit of the power transformer. I recently had a look at the drop across the thermistor in one of the twin coupled amps I did a few years ago. Drop after warmup when running was about one volt, so no need for concern of low voltage operation. And very cheap insurance.

Cheers, John

[[email protected]]

On Mar 27, 2:45*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. *Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). *Those
tubes typically fail emissions tests. *I've never once seen a heater
burn out. *Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. *And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

The filaments always burned out even in the most durable tubes.

[[email protected]]

On Apr 2, 10:14*pm, flipper wrote:
On Sat, 2 Apr 2011 19:04:58 -0700 (PDT), wrote:
On Mar 27, 2:45 pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2..4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). Those
tubes typically fail emissions tests. I've never once seen a heater
burn out. Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

The filaments always burned out even in the most durable tubes.

Always burned out in what?

I'm with BigBadBob on this one. Heater burnout is unusual and it's the
cathode coating that usually goes first.- Hide quoted text -

- Show quoted text -

Heaters/filaments can burn out in any vacuum tube , even in a CRT .
Cathode stripping is more common , though . Loss of emission .

[NX211]

On Apr 4, 2:11*am, wrote:
On Apr 2, 10:14*pm, flipper wrote:



On Sat, 2 Apr 2011 19:04:58 -0700 (PDT), wrote:
On Mar 27, 2:45 pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). Those
tubes typically fail emissions tests. I've never once seen a heater
burn out. Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

The filaments always burned out even in the most durable tubes.

Always burned out in what?

I'm with BigBadBob on this one. Heater burnout is unusual and it's the
cathode coating that usually goes first.- Hide quoted text -

- Show quoted text -

Heaters/filaments can burn out in any vacuum tube , even in a CRT .
Cathode stripping is more common , though . *Loss of emission .

Actual filament burn out is very rare but it does occasionally
happen. Usually after rough physical handling or excessive voltage.
Most times though they glow happily then slowly fade away.

[Nordic Breeds WA4VZQ[_2_]]

Filament inrush current _is_ a big deal with high power transmitting
tubes. I have known of cases where the inrush current was so high that
the intense magnetic field created by the inrush current shook the
filament so much that it was broken. This was with a tube whose filament
requirements were 7.5 volts at 104 amps (a ruggedized version of the
3CX10000A3). The tube was used in industrial dielectric heating.

Burle Industries, the successor to RCA's transmitting tube business
published a number of guidlines and application notes to maximize the
life of their tubes. The following are direct quotes from these
publications. These notes are rather long and not directly applicable to
the _miniscule_ tubes used by the readers of this newsgroup, but I
thought they might be interesting.

If anyone here wants to build audio gear using the tubes I mentioned
earlier, I have 5 NOS ones with a matching filament transformer and
sockets! A pair of these will loaf at 40 kilowatts output. :-)

73, Dr. Barry L. Ornitz WA4VZQ {my ham call} at
Hotmail.com

From the "Application Guide for BURLE Power tubes."

A filament starter should be used to raise the filament voltage
and current gradually in order to limit the high initial surge of
current through the filament when the circuit is first closed. The
starter may be a system of time-delay relays which cut resistance
or reactance out of the circuit, a high reactance filament trans-
former, or an adjustable auto-transformer. Regardless of the
method of control, it is important that the filament current never
exceed the value specified in the published data. In equipment
which utilizes automatic "run-up" of filament voltage with volt-
age regulators, provision should be made for a limit-switch cut-
off of the filament-voltage supply in the event of malfunction or
"runaway" of the regulator to prevent damage to the filament
due to overvoltage. The limit switches should be set at or below
the maximum permissible filament voltage for the tube involved
and should be checked periodically to assure proper operation.

From "Techniques to Extend the Service Life of High Power
Vacuum Tubes."

On/Off Filament Cycling

When a cold filament is turned on, damage is
caused by two effects: (1) The current inrush into a
cold filament can be up to 10 times the operating
current if the filament supply is of very low impedance;
(2) Grain reorientation occurs at about 600 to 700
degrees centigrade; this is called the Miller-Larson
Effect. The grain reorientation will result in a
momentary plastic state that can cause the wire to
grow in length and therefore become thinner.
To illustrate the first effect, one can use an
ordinary 60 watt light bulb. The light bulb should
operate at 0.5 amps at 120 volts. This translates to a
hot resistance of 240 Ohms. An Ohmmeter reading of
a room temperature 60 watt light bulb will be less than
20 Ohms or, a hot to cold ratio of greater than 12.
Power tube filaments will have about a ten to one hot
to cold ratio because they operate at a lower
temperature than light bulbs.

The second effect is aggravated by variations
in the cross sectional area of the filament wires along
their length. This will cause “hot spots” in the thinner
cross sections due to the greater current densities.
The higher temperatures at the hot spots cause
increased growth during the warm-up through the 600
degree Centigrade temperature when they have a
higher current density than the rest of the wire, and a
much higher power dissipation per unit length due to
also having a higher resistance. Each time the
filament is turned on, the wire becomes thinner until
the hot spot temperature gets into a runaway
condition. During the runaway, the filament
temperature reaches the melting point during the turn
on surge, and the light bulb (or filament) produces the
familiar flash bulb effect that signals its demise.

Light bulbs nearly always burn out during the
turn on inrush surge. Each Off/On cycle removes
several hours of life from the light bulb (or filament)
and this trauma is approximately proportional to the
cube of the inrush current. By limiting this inrush
current or, by preferably eliminating it, the life of the
power tube filament can be lengthened several times.

The Egyptian Ministry of Information
operates a two megawatt transmitter in the medium
wave band. This Doherty transmitter uses four tubes,
two carrier tubes, and two peak tubes working through
a combiner. Even though the transmitter was
operating more than 22 hours per day, the filaments
were turned off and back on every night. The very
expensive tubes (about $100,000 each) were lasting
only 5,500 hours, and a stock of 10 spares was
expected to last 18 months for the four sockets. After
leaving the filaments on continuously and rotating the
spare tubes annually, the station has yet to have a
tube failure after seven years.

In Greenville, North Carolina, VOA has four
500 kW transmitters with vapor cooled tubes. The
transmitters are used for 12 to 15 hours per day.
Initially, the filaments were turned off and on two or
three times per day during gaps in the schedules.
Three years ago, VOA began leaving the filaments on
at all times except for major maintenance. As a result,
the expenditures for tubes for these transmitters has
dropped from about $420,000 per year to under
$100,000. This was done at a cost of $15,000 per
year in added electrical power cost. For every extra
dollar spent on added filament power, we had a return
of more than 20 dollars in reduced tube costs — eat your
heart out Wall Street!

Calculations for both of the above situations
indicate that each Off/On cycle of the filament was
reducing the life expectancies by over 75 hours. An
unexpected result was that it appeared to be
independent of the filament construction - straight
wire, hairpin, and mesh filaments all benefited nearly
equally by leaving the filaments on continuously. The
mesh filament was expected to suffer more from
Off/On cycling because of flexing of the many welds.
Also, the mesh filament structure will grow in overall
diameter as the wires lengthen. It is constructed in a
manner similar to a Chinese finger trap which
changes overall diameter as the axial length is
changed. This mesh filament construction causes an
additional failure mode because the overall mesh
diameter can grow enough to cause a grid-to-filament
short before a wire breaks. In many large power
tetrodes, the grid to filament spacing is less than two
percent of the overall filament diameter and the
diameter tends to grow more at the bottom of the
filament due to gravity.

From "Handling and Operating Considerations When
Using BURLE Broadcast-Type Tetrodes."

Filament Warm-Up

Most filamentary tubes used in transmitter service are
rated for 15 seconds minimum heating time. This warmup
is necessary to allow the grids and the filament to
reach an equilibrium temperature and avoid arcing due to
momentary shorting between these elements after the
application of high voltage. Shortened warm-up time
cycles can be used by step-starting the filament. It is
possible to start tubes with as little as 3 seconds heating
time, but this procedure can cause subtle internal changes
which may result in shortened life times. Such short
cycles may be used as emergency starts, but never as
normal procedure. In most cases, it is advantageous to
use the full recommended minimum heating time, then
apply the other voltages before drawing full plate current.

This technique results in less thermal stress to all the
elements concerned and assists in prolonging tube life. It
is good practice to allow filaments to run continuously
and minimize start-up stresses. If this method is impractical,
the filament should be preheated for 10 to 15 minutes
before the application of any other voltages. This
procedure can substantially increase tube life expectancy.
During starting, it is also important to limit initial
filament current surges to the value listed in the particular
tube bulletin. The ratio of hot to cold resistance of thoriated
tungsten wire is about 10:1. If the applied voltage
were supplied by a low impedance filament supply, starting
currents could reach as high as 1500 A for the 8807,
as an example, where the maximum permissible current
is 300 A. In a practical application, surge currents of 600
to 700 amperes can be realized and these high surge
currents will damage the filament structure.

Methods for holding surge currents below the limiting
value include high reactance transformers, a resistive
starting network or manual control of filament voltage by
a Variac. A combination of the first two can also be used.
Many new transmitters employ the “Pulsistor”, a type of
variable resistor. This device is placed in the filament
primary circuit and it limits the surge current by changing
resistance, having a high resistance when cold and a low
resistance when hot.

~~~~~~~~~~

[[email protected]]

On Apr 11, 7:30*pm, "Nordic Breeds WA4VZQ"
wrote:
Filament inrush current _is_ a big deal with high power transmitting
tubes. *I have known of cases where the inrush current was so high that
the intense magnetic field created by the inrush current shook the
filament so much that it was broken. *This was with a tube whose filament
requirements were 7.5 volts at 104 amps (a ruggedized version of the
3CX10000A3). *The tube was used in industrial dielectric heating.

Burle Industries, the successor to RCA's transmitting tube business
published a number of guidlines and application notes to maximize the
life of their tubes. *The following are direct quotes from these
publications. *These notes are rather long and not directly applicable to
the _miniscule_ tubes used by the readers of this newsgroup, but I
thought they might be interesting.

If anyone here wants to build audio gear using the tubes I mentioned
earlier, I have 5 NOS ones with a matching filament transformer and
sockets! *A pair of these will loaf at 40 kilowatts output. *:-)

* * * * 73, Dr. Barry L. Ornitz *WA4VZQ * * * *{my ham call} at
Hotmail.com

From the "Application Guide for BURLE Power tubes."

A filament starter should be used to raise the filament voltage
and current gradually in order to limit the high initial surge of
current through the filament when the circuit is first closed. The
starter may be a system of time-delay relays which cut resistance
or reactance out of the circuit, a high reactance filament trans-
former, or an adjustable auto-transformer. Regardless of the
method of control, it is important that the filament current never
exceed the value specified in the published data. In equipment
which utilizes automatic "run-up" of filament voltage with volt-
age regulators, provision should be made for a limit-switch cut-
off of the filament-voltage supply in the event of malfunction or
"runaway" of the regulator to prevent damage to the filament
due to overvoltage. The limit switches should be set at or below
the maximum permissible filament voltage for the tube involved
and should be checked periodically to assure proper operation.

From "Techniques to Extend the Service Life of High Power
Vacuum Tubes."

* * On/Off Filament Cycling

When a cold filament is turned on, damage is
caused by two effects: (1) The current inrush into a
cold filament can be up to 10 times the operating
current if the filament supply is of very low impedance;
(2) Grain reorientation occurs at about 600 to 700
degrees centigrade; this is called the Miller-Larson
Effect. The grain reorientation will result in a
momentary plastic state that can cause the wire to
grow in length and therefore become thinner.
To illustrate the first effect, one can use an
ordinary 60 watt light bulb. The light bulb should
operate at 0.5 amps at 120 volts. This translates to a
hot resistance of 240 Ohms. An Ohmmeter reading of
a room temperature 60 watt light bulb will be less than
20 Ohms or, a hot to cold ratio of greater than 12.
Power tube filaments will have about a ten to one hot
to cold ratio because they operate at a lower
temperature than light bulbs.

The second effect is aggravated by variations
in the cross sectional area of the filament wires along
their length. This will cause �hot spots� in the thinner
cross sections due to the greater current densities.
The higher temperatures at the hot spots cause
increased growth during the warm-up through the 600
degree Centigrade temperature when they have a
higher current density than the rest of the wire, and a
much higher power dissipation per unit length due to
also having a higher resistance. Each time the
filament is turned on, the wire becomes thinner until
the hot spot temperature gets into a runaway
condition. During the runaway, the filament
temperature reaches the melting point during the turn
on surge, and the light bulb (or filament) produces the
familiar flash bulb effect that signals its demise.

Light bulbs nearly always burn out during the
turn on inrush surge. Each Off/On cycle removes
several hours of life from the light bulb (or filament)
and this trauma is approximately proportional to the
cube of the inrush current. By limiting this inrush
current or, by preferably eliminating it, the life of the
power tube filament can be lengthened several times.

The Egyptian Ministry of Information
operates a two megawatt transmitter in the medium
wave band. This Doherty transmitter uses four tubes,
two carrier tubes, and two peak tubes working through
a combiner. Even though the transmitter was
operating more than 22 hours per day, the filaments
were turned off and back on every night. The very
expensive tubes (about $100,000 each) were lasting
only 5,500 hours, and a stock of 10 spares was
expected to last 18 months for the four sockets. After
leaving the filaments on continuously and rotating the
spare tubes annually, the station has yet to have a
tube failure after seven years.

In Greenville, North Carolina, VOA has four
500 kW transmitters with vapor cooled tubes. The
transmitters are used for 12 to 15 hours per day.
Initially, the filaments were turned off and on two or
three times per day during gaps in the schedules.
Three years ago, VOA began leaving the filaments on
at all times except for major maintenance. As a result,
the expenditures for tubes for these transmitters has
dropped from about $420,000 per year to under
$100,000. This was done at a cost of $15,000 per
year in added electrical power cost. For every extra
dollar spent on added filament power, we had a return
of more than 20 dollars in reduced tube costs � eat your
heart out Wall Street!

Calculations for both of the above situations
indicate that each Off/On cycle of the filament was
reducing the life expectancies by over 75 hours. An
unexpected result was that it appeared to be
independent of the filament construction - straight
wire, hairpin, and mesh filaments all benefited nearly
equally by leaving the filaments on continuously. The
mesh filament was expected to suffer more from
Off/On cycling because of flexing of the many welds.
Also, the mesh filament structure will grow in overall
diameter as the wires lengthen. It is constructed in a
manner similar to a Chinese finger trap which
changes overall diameter as the axial length is
changed. This mesh filament construction causes an
additional failure mode because the overall mesh
diameter can grow enough to cause a grid-to-filament
short before a wire breaks. In many large power
tetrodes, the grid to filament spacing is less than two
percent of the overall filament diameter and the
diameter tends to grow more at the bottom of the
filament due to gravity.

From "Handling and Operating Considerations When
Using BURLE Broadcast-Type Tetrodes."

* * * * * Filament Warm-Up

Most filamentary tubes used in transmitter service are
rated for 15 seconds minimum heating time. This warmup
is necessary to allow the grids and the filament to
reach an equilibrium temperature and avoid arcing due to
momentary shorting between these elements after the
application of high voltage. Shortened warm-up time
cycles can be used by step-starting the filament. It is
possible to start tubes with as little as 3 seconds heating
time, but this procedure can cause subtle internal changes
which may result in shortened life times. Such short
cycles may be used as emergency starts, but never as
normal procedure. In most cases, it is advantageous to
use the full recommended minimum heating time, then
apply the other voltages before drawing full plate current.

This technique results in less thermal stress to all the
elements concerned and assists in prolonging tube life. It
is good practice to allow filaments to run continuously
and minimize start-up stresses. If this method is impractical,
the filament should be preheated for 10 to 15 minutes
before the application of any other voltages. This
procedure can substantially increase tube life expectancy.
During starting, it is also important to limit initial
filament current surges to the value listed in the particular
tube bulletin. The ratio of hot to cold resistance of thoriated
tungsten wire is about 10:1. If the applied voltage
were supplied by a low impedance filament supply, starting
currents could reach as high as 1500 A for the 8807,
as an example, where the maximum permissible current
is 300 A. In a practical application, surge currents of 600
to 700 amperes can be realized and these high surge
currents will damage the filament structure.

Methods for holding surge currents below the limiting
value include high reactance transformers, a resistive
starting network or manual control of filament voltage by
a Variac. A combination of the first two can also be used.
Many new transmitters employ the �Pulsistor�, a type of
variable resistor. This device is placed in the filament
primary circuit and it limits the surge current by changing
resistance, having a high resistance when cold and a low
resistance when hot.

~~~~~~~~~~

Correct. Small tubes (true now , just like before) have similar
problems , but of course they are nowhere near handling such extremes
in power levels as described above . Filaments DO go bad .

[Big Bad Bob]

On 04/11/11 16:30, Nordic Breeds WA4VZQ so wittily quipped:
Filament inrush current _is_ a big deal with high power transmitting
tubes. I have known of cases where the inrush current was so high that
the intense magnetic field created by the inrush current shook the
filament so much that it was broken. This was with a tube whose filament
requirements were 7.5 volts at 104 amps (a ruggedized version of the
3CX10000A3). The tube was used in industrial dielectric heating.

hmm... that's interesting. Looks like you need a special warmup
sequencer on it. DC motors use 'starting resistors', where consecutive
series resistors use a timer to sequence them, and the final position is
"no resistor". Saw something like that in the quote from the manual, so
they thought of it too.

If anyone here wants to build audio gear using the tubes I mentioned
earlier, I have 5 NOS ones with a matching filament transformer and
sockets! A pair of these will loaf at 40 kilowatts output. :-)

40 kw - that's like the amp at the beginning of the original 'Back to
the Future'. "Heavy"

[Big Bad Bob]

On 04/11/11 20:49, so wittily quipped:
On Apr 11, 7:30 pm, "Nordic Breeds
wrote:


Methods for holding surge currents below the limiting
value include high reactance transformers, a resistive
starting network or manual control of filament voltage by
a Variac. A combination of the first two can also be used.
Many new transmitters employ the �Pulsistor�, a type of
variable resistor. This device is placed in the filament
primary circuit and it limits the surge current by changing
resistance, having a high resistance when cold and a low
resistance when hot.

~~~~~~~~~~

Correct. Small tubes (true now , just like before) have similar
problems , but of course they are nowhere near handling such extremes
in power levels as described above . Filaments DO go bad .

perhaps a switching power supply for tubes _could_ have a 'warmup cycle'
on the filament supply, ramping it up from 2VDC to 6.3VDC over a period
of 10 seconds. It's a 'feature' worth considering. A similar feature
might turn the bias supply on FIRST, then ramp up the HV side.

Anyway, this kind of thing gives me a reason to consider that sort of
stuff in any future designs.

[Nordic Breeds WA4VZQ[_2_]]

"Big Bad Bob" wrote in message
m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC over a
period of 10 seconds. It's a 'feature' worth considering. A similar
feature might turn the bias supply on FIRST, then ramp up the HV side.

Anyway, this kind of thing gives me a reason to consider that sort of
stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a regulated
DC supply to power it using the ancient Fairchild uA723 regulator with a
pass transistor. I set the current limit to 50% more than the normal
filament draw. When first turned on, the supply regulates the current
for about 10 seconds with the voltage rising slowly. Once the current
starts dropping, the supply switches to regulate the output voltage.
This supply not only protects the tube during startup but it also
increased the stability of the oscillator about ten fold.

73, Barry WA4VZQ {ham_call} at Hotmail.com

[Big Bad Bob]

On 04/12/11 15:01, flipper so wittily quipped:
On Tue, 12 Apr 2011 12:08:55 -0700 (PDT), mike s
wrote:

On Tuesday, April 12, 2011 4:49:49 AM UTC+1, wrote:


Correct. Small tubes (true now , just like before) have similar
problems , but of course they are nowhere near handling such extremes
in power levels as described above . Filaments DO go bad .

Perhaps, but consider why 6.3V was adopted as the standard heater in the USA in the mid 1930s and by the mid 1940s in the rest of the world. It's because this is the on-charge voltage of 3 lead acid cells, i.e. a 6V car (automobile) battery.

So the fact that from the outset the heaters of standard octal 6??, 6??GT etc. were designed for use with either AC or DC from a high capacity, low impedance, supply would suggest to this engineer that messing about with slow start heater supplies is probably not a smart use of resources. Better to spend the money on some other "magic" that's less likely to go wrong - fancy cables, or hand made capacitors, or something equally daft.


You have a good point. On the other hand, tube operating
characteristics are based on, as the data sheets say, "acceptable
serviceability" so while unprotected 6.3V heaters may provide
"acceptable serviceability" it could be the case that employing some
simple inrush limiting would make for 'sterling', or fantastic,
serviceability.

On the other other hand, a heater filament that lasts till the cows
come home is of little practical use if the cathode has long since
been depleted.

Theorizing can't really answer the question. You need case studies and
they were undoubtedly done, once upon a time, but are now probably
lost in some musty old store room somewhere.

legacy tech doesn't get much R&D does it?

a bit of googling led me here

http://frank.pocnet.net/other/docs/T...um_Tubes.pd f

sorry for the line wrap (no breaks in the link)

pg 11-14 discusses cases of heater burnout (most were caused by
overvoltage) and the use of surge suppression resistors (thermistor,
whatever), and also the 'instant on' feature where a small amount of
current flows through the heaters when the unit is off [helps prevent
startup surge]. A lot of TVs in the 70's use this, and a lot of tube
computer monitors do this for the picture tubes. Unfortunately this
practice has unintended consequences.

there was also a section regarding 'drawing too much plate current' (pg
18) before the cathode warms up completely, causing damage to the tube.
As I recall someone (earlier posts in this newsgroup) suggested
operating tubes with under-heated cathodes to change their
characteristics, supposedly improving THD or linearity or whatever.
Apparently this sort of thing can have unintended negative consequences
on tube life as well. In any case these aren't things I ever looked
into [making tubes last longer] and there are a lot of ideas covered in
the PDF.

chapter 10 is about improving tube life. first item up is COOLING,
which is no surprise. Put a fan in your gear. Second item is voltage
and current regulation (discussed here a few times) and not allowing
heater voltage to exceed ratings. Other items mentioned include shock
mounting and maintenance practices.

FYI for anyone interested in looking at it.

[John L Stewart]

legacy tech doesn't get much R&D does it?

a bit of googling led me here

http://frank.pocnet.net/other/docs/T...um_Tubes.pd f

sorry for the line wrap (no breaks in the link)

pg 11-14 discusses cases of heater burnout (most were caused by
overvoltage) and the use of surge suppression resistors (thermistor,
whatever), and also the 'instant on' feature where a small amount of
current flows through the heaters when the unit is off [helps prevent
startup surge]. A lot of TVs in the 70's use this, and a lot of tube
computer monitors do this for the picture tubes. Unfortunately this
practice has unintended consequences.

there was also a section regarding 'drawing too much plate current' (pg
18) before the cathode warms up completely, causing damage to the tube.
As I recall someone (earlier posts in this newsgroup) suggested
operating tubes with under-heated cathodes to change their
characteristics, supposedly improving THD or linearity or whatever.
Apparently this sort of thing can have unintended negative consequences
on tube life as well. In any case these aren't things I ever looked
into [making tubes last longer] and there are a lot of ideas covered in
the PDF.

chapter 10 is about improving tube life. first item up is COOLING,
which is no surprise. Put a fan in your gear. Second item is voltage
and current regulation (discussed here a few times) and not allowing
heater voltage to exceed ratings. Other items mentioned include shock
mounting and maintenance practices.

FYI for anyone interested in looking at it.[/quote]

-----------------------------------------------------

That is a very good document. I see Tomer has even covered the problem of cathode interface (p33) which showed up in a lot of equipment during & after WW2.

The TEK 580 Series Scopes in their distributed vertical amp used about a dozen of the 6DK6 which were fixed for that problem. Seems to me that several of the dual triodes meant for switching circuits also had cathodes fixed for the interface problem. They would include the likes of 5814, 7119 & 5687 & many others.

I used lots of those back in the day. But not for audio!

Cheers, John

[Big Bad Bob]

On 04/13/11 08:54, John L Stewart so wittily quipped:

http://tinyurl.com/3ehf65m

OK - tiny links are good to stop line wrap, but also used by trolls to
re-direct to ugly sites - I suppose I should've provided both. Too late
in AM.

That is a very good document. I see Tomer has even covered the problem of
cathode interface (p33) which showed up in a lot of equipment during&
after WW2.

had to re-look at it - yeah, duty cycle averaging is great for plate
dissipation calculations, not so great for the effects of high current
short duration pulses on the rest of the components.

radar systems would have similar problems. this is probably where it
was discovered.

[Big Bad Bob]

On 04/13/11 12:04, flipper so wittily quipped:
there was also a section regarding 'drawing too much plate current' (pg
18) before the cathode warms up completely, causing damage to the tube.
As I recall someone (earlier posts in this newsgroup) suggested
operating tubes with under-heated cathodes to change their
characteristics, supposedly improving THD or linearity or whatever.
Apparently this sort of thing can have unintended negative consequences
on tube life as well. In any case these aren't things I ever looked
into [making tubes last longer] and there are a lot of ideas covered in
the PDF.

The "starved filament" theory is for DHTs, not heater types.

http://greygum.net/sbench/sbench102/dht.html


hmmm... I didn't get that from earlier (this guy appears to be
heater-starving his triode-configured power tubes operating as class A
while simultaneously increasing current to 'murder' the Miller effect
http://groups.google.com/group/rec.a...002936ec633e90 ).
I found it interesting to even USE heater starvation when I first read
it but now [from what you say] the theory doesn't even apply to
indirectly heated cathodes, and according to Tomer the reduced heater
voltage may actually damage the tube. What I get out of it is that
there can be unintended consequences of operating outside of the normal
range, things others have already discovered. So yeah, I'll just stick
with normal operating ranges for these kinds of things. I think that
was my original reaction anyway.

[Big Bad Bob]

On 04/13/11 12:26, flipper so wittily quipped:
That is a very good document. I see Tomer has even covered the problem of
cathode interface (p33) which showed up in a lot of equipment during&
after WW2.

had to re-look at it - yeah, duty cycle averaging is great for plate
dissipation calculations, not so great for the effects of high current
short duration pulses on the rest of the components.

That section deals with "sleeping sickness" and that is not caused by
the 'high current pulses' but the period of cutoff.

um, ok. high current pulses separated by long periods of (dull, boring)
extremely low current and/or cutoff [will that do ya?]. Damn I hate
having to get wordy when I'm trying to be brief and save the eyeballs of
anyone looking (and my precious time as well), just to preempt this sort
of reply.

[Big Bad Bob]

On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:
"Big Bad wrote in message
m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC over a
period of 10 seconds. It's a 'feature' worth considering. A similar
feature might turn the bias supply on FIRST, then ramp up the HV side.

Anyway, this kind of thing gives me a reason to consider that sort of
stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a regulated
DC supply to power it using the ancient Fairchild uA723 regulator with a
pass transistor. I set the current limit to 50% more than the normal
filament draw. When first turned on, the supply regulates the current
for about 10 seconds with the voltage rising slowly. Once the current
starts dropping, the supply switches to regulate the output voltage.
This supply not only protects the tube during startup but it also
increased the stability of the oscillator about ten fold.

every time someone confirms the idea of proper voltage regulation in
tube amps I raise the project's priority level. If I put such a beast
into hardware I suppose I oughta 'open source' the board layout at least
(after getting ideas from this newsgroup). Start filaments before B+,
ramp up filaments, bias on first, and sbort-circuit protection like a
computer P.S.. Yeah.

/me pondering how to do all this with a single toroid.

[Big Bad Bob]

On 04/13/11 13:32, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:05:54 -0700, Big Bad Bob
wrote:

On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:
"Big Bad wrote in message
m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC over a
period of 10 seconds. It's a 'feature' worth considering. A similar
feature might turn the bias supply on FIRST, then ramp up the HV side.

Anyway, this kind of thing gives me a reason to consider that sort of
stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a regulated
DC supply to power it using the ancient Fairchild uA723 regulator with a
pass transistor. I set the current limit to 50% more than the normal
filament draw. When first turned on, the supply regulates the current
for about 10 seconds with the voltage rising slowly. Once the current
starts dropping, the supply switches to regulate the output voltage.
This supply not only protects the tube during startup but it also
increased the stability of the oscillator about ten fold.

every time someone confirms the idea of proper voltage regulation in
tube amps I raise the project's priority level. If I put such a beast
into hardware I suppose I oughta 'open source' the board layout at least
(after getting ideas from this newsgroup). Start filaments before B+,
ramp up filaments, bias on first, and sbort-circuit protection like a
computer P.S.. Yeah.

/me pondering how to do all this with a single toroid.

The time I went 'whole hog' with DC heaters I put a CCS under each
tube so there isn't any 'surge' at all.

I'd guess about an extra $1 per tube? how was your warmup time?

[mick]

On Wed, 13 Apr 2011 16:23:18 -0500, flipper wrote:

On Wed, 13 Apr 2011 13:42:13 -0700, Big Bad Bob
wrote:

On 04/13/11 13:32, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:05:54 -0700, Big Bad Bob
wrote:

On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:
"Big Bad wrote in
message m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC
over a period of 10 seconds. It's a 'feature' worth considering.
A similar feature might turn the bias supply on FIRST, then ramp up
the HV side.

Anyway, this kind of thing gives me a reason to consider that sort
of stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a
regulated DC supply to power it using the ancient Fairchild uA723
regulator with a pass transistor. I set the current limit to 50%
more than the normal filament draw. When first turned on, the
supply regulates the current for about 10 seconds with the voltage
rising slowly. Once the current starts dropping, the supply
switches to regulate the output voltage. This supply not only
protects the tube during startup but it also increased the stability
of the oscillator about ten fold.

every time someone confirms the idea of proper voltage regulation in
tube amps I raise the project's priority level. If I put such a
beast into hardware I suppose I oughta 'open source' the board layout
at least (after getting ideas from this newsgroup). Start filaments
before B+, ramp up filaments, bias on first, and sbort-circuit
protection like a computer P.S.. Yeah.

/me pondering how to do all this with a single toroid.

The time I went 'whole hog' with DC heaters I put a CCS under each
tube so there isn't any 'surge' at all.

I'd guess about an extra $1 per tube?

All the parts, including heatsinks, were junk bin and, so, 'free'.

CCS was a pass transistor and current sense resistor under each heater
with a 324 opamp closing the loop. The one 324 holds 4 for 4 tubes.

how was your warmup time?

That was a long time ago and I don't recall. But I don't remember it
being 'much' longer.


I suppose you could just use a LM317 & 1 resistor for each heater now.
Those things are stupidly cheap and no op-amp supply needed. Allow 1.25v
across the resistor and another 2-3v across the 317 to let it regulate
properly. Although where you get a 4.16666666666666667 ohm resistor for a
300mA heater from I've no idea!

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

[Big Bad Bob]

On 04/13/11 14:23, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:42:13 -0700, Big Bad Bob
wrote:

On 04/13/11 13:32, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:05:54 -0700, Big Bad Bob
wrote:

On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:
"Big Bad wrote in message
m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC over a
period of 10 seconds. It's a 'feature' worth considering. A similar
feature might turn the bias supply on FIRST, then ramp up the HV side.

Anyway, this kind of thing gives me a reason to consider that sort of
stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a regulated
DC supply to power it using the ancient Fairchild uA723 regulator with a
pass transistor. I set the current limit to 50% more than the normal
filament draw. When first turned on, the supply regulates the current
for about 10 seconds with the voltage rising slowly. Once the current
starts dropping, the supply switches to regulate the output voltage.
This supply not only protects the tube during startup but it also
increased the stability of the oscillator about ten fold.

every time someone confirms the idea of proper voltage regulation in
tube amps I raise the project's priority level. If I put such a beast
into hardware I suppose I oughta 'open source' the board layout at least
(after getting ideas from this newsgroup). Start filaments before B+,
ramp up filaments, bias on first, and sbort-circuit protection like a
computer P.S.. Yeah.

/me pondering how to do all this with a single toroid.

The time I went 'whole hog' with DC heaters I put a CCS under each
tube so there isn't any 'surge' at all.

I'd guess about an extra $1 per tube?

All the parts, including heatsinks, were junk bin and, so, 'free'.

CCS was a pass transistor and current sense resistor under each heater
with a 324 opamp closing the loop. The one 324 holds 4 for 4 tubes.

opamp - 'high tech' solution, heh.

how was your warmup time?

That was a long time ago and I don't recall. But I don't remember it
being 'much' longer.

good enough assessment - that suggests that a rampup wouldn't take 'much
longer' either by limiting current during the initial phase through
limiting voltage.

/me ponders microcontroller running the power supply. At ~$17 per
controller board (unassembled), it would make things VERY interesting.

[Big Bad Bob]

On 04/14/11 02:31, mick so wittily quipped:
On Wed, 13 Apr 2011 16:23:18 -0500, flipper wrote:

On Wed, 13 Apr 2011 13:42:13 -0700, Big Bad Bob
wrote:

On 04/13/11 13:32, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:05:54 -0700, Big Bad Bob
wrote:

On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:
"Big Bad wrote in
message m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC
over a period of 10 seconds. It's a 'feature' worth considering.
A similar feature might turn the bias supply on FIRST, then ramp up
the HV side.

Anyway, this kind of thing gives me a reason to consider that sort
of stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a
regulated DC supply to power it using the ancient Fairchild uA723
regulator with a pass transistor. I set the current limit to 50%
more than the normal filament draw. When first turned on, the
supply regulates the current for about 10 seconds with the voltage
rising slowly. Once the current starts dropping, the supply
switches to regulate the output voltage. This supply not only
protects the tube during startup but it also increased the stability
of the oscillator about ten fold.

every time someone confirms the idea of proper voltage regulation in
tube amps I raise the project's priority level. If I put such a
beast into hardware I suppose I oughta 'open source' the board layout
at least (after getting ideas from this newsgroup). Start filaments
before B+, ramp up filaments, bias on first, and sbort-circuit
protection like a computer P.S.. Yeah.

/me pondering how to do all this with a single toroid.

The time I went 'whole hog' with DC heaters I put a CCS under each
tube so there isn't any 'surge' at all.

I'd guess about an extra $1 per tube?

All the parts, including heatsinks, were junk bin and, so, 'free'.

CCS was a pass transistor and current sense resistor under each heater
with a 324 opamp closing the loop. The one 324 holds 4 for 4 tubes.

how was your warmup time?

That was a long time ago and I don't recall. But I don't remember it
being 'much' longer.


I suppose you could just use a LM317& 1 resistor for each heater now.
Those things are stupidly cheap and no op-amp supply needed. Allow 1.25v
across the resistor and another 2-3v across the 317 to let it regulate
properly. Although where you get a 4.16666666666666667 ohm resistor for a
300mA heater from I've no idea!


lucky you it's not a KT88 - 1.6A or something like that.

[mick]

On Thu, 14 Apr 2011 02:38:31 -0700, Big Bad Bob wrote:

On 04/13/11 14:23, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:42:13 -0700, Big Bad Bob
wrote:

On 04/13/11 13:32, flipper so wittily quipped:
On Wed, 13 Apr 2011 13:05:54 -0700, Big Bad Bob
wrote:

On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:
"Big Bad wrote in
message m...

perhaps a switching power supply for tubes _could_ have a 'warmup
cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC
over a period of 10 seconds. It's a 'feature' worth considering.
A similar feature might turn the bias supply on FIRST, then ramp
up the HV side.

Anyway, this kind of thing gives me a reason to consider that sort
of stuff in any future designs.

I have a signal generator with a rather exotic (read as nearly
unobtanium) oscillator tube. To protect this tube, I built a
regulated DC supply to power it using the ancient Fairchild uA723
regulator with a pass transistor. I set the current limit to 50%
more than the normal filament draw. When first turned on, the
supply regulates the current for about 10 seconds with the voltage
rising slowly. Once the current starts dropping, the supply
switches to regulate the output voltage. This supply not only
protects the tube during startup but it also increased the
stability of the oscillator about ten fold.

every time someone confirms the idea of proper voltage regulation in
tube amps I raise the project's priority level. If I put such a
beast into hardware I suppose I oughta 'open source' the board
layout at least (after getting ideas from this newsgroup). Start
filaments before B+, ramp up filaments, bias on first, and
sbort-circuit protection like a computer P.S.. Yeah.

/me pondering how to do all this with a single toroid.

The time I went 'whole hog' with DC heaters I put a CCS under each
tube so there isn't any 'surge' at all.

I'd guess about an extra $1 per tube?

All the parts, including heatsinks, were junk bin and, so, 'free'.

CCS was a pass transistor and current sense resistor under each heater
with a 324 opamp closing the loop. The one 324 holds 4 for 4 tubes.

opamp - 'high tech' solution, heh.

how was your warmup time?

That was a long time ago and I don't recall. But I don't remember it
being 'much' longer.

good enough assessment - that suggests that a rampup wouldn't take 'much
longer' either by limiting current during the initial phase through
limiting voltage.

/me ponders microcontroller running the power supply. At ~$17 per
controller board (unassembled), it would make things VERY interesting.


Should be fairly reasonable to do - with a PIC at least.
Use a PWM output, RC filter to give Vref into op-amp/transistor current
source. Then ramp or step the PWM signal to give whatever warmup speed
you like. You don't need precision so software-driven PWM would be ok.
You could use an 8-pin, something like 12F675, and write the software in
GCBASIC ('cos it's free). You don't need a clock crystal, it's all on-
board. Just stick it on a piece of veroboard with the other bits.

Of course, once you have a chip in there you start to get other ideas
too... ;-)

You can do a rampup with a LM317 (or similar) regulator - it's shown in
the data sheets. It's constant voltage though, not current. Should be
easy enough to add another transistor/resistor to get a fixed max current
limit though.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

[Big Bad Bob]

On 04/14/11 11:13, mick so wittily quipped:
/me ponders microcontroller running the power supply. At ~$17 per
controller board (unassembled), it would make things VERY interesting.


Should be fairly reasonable to do - with a PIC at least.
Use a PWM output, RC filter to give Vref into op-amp/transistor current
source. Then ramp or step the PWM signal to give whatever warmup speed
you like. You don't need precision so software-driven PWM would be ok.
You could use an 8-pin, something like 12F675, and write the software in
GCBASIC ('cos it's free). You don't need a clock crystal, it's all on-
board. Just stick it on a piece of veroboard with the other bits.

Of course, once you have a chip in there you start to get other ideas
too... ;-)

You can do a rampup with a LM317 (or similar) regulator - it's shown in
the data sheets. It's constant voltage though, not current. Should be
easy enough to add another transistor/resistor to get a fixed max current
limit though.


yeah all kinds of possibilities exist. total cost of hardware +
long-term reliability would be the driving factors. If micro-C is cheap
but fails before tubes then it's kinda dumb. On the other hand if a
simple CCS charging a cap makes your rampup work then it's under $1. I
would actually sequence more than that since you want bias first, then
heaters, and after heater rampup, a quicker rampup for B+ lines (maybe 1
second). That way you don't get a 'pop' when you apply B+, you don't
apply B+ without reasonably warm cathodes, and you don't apply B+ unless
there's a bias supply already charged. It's a bit more than I
considered before looking at the 'Gettimg the Most Out of Vacuuum Tubes'
(Tomer) book I recently found on-line.

[mick]

On Thu, 14 Apr 2011 14:55:54 -0700, Big Bad Bob wrote:

On 04/14/11 11:13, mick so wittily quipped:
/me ponders microcontroller running the power supply. At ~$17 per
controller board (unassembled), it would make things VERY interesting.


Should be fairly reasonable to do - with a PIC at least. Use a PWM
output, RC filter to give Vref into op-amp/transistor current source.
Then ramp or step the PWM signal to give whatever warmup speed you
like. You don't need precision so software-driven PWM would be ok. You
could use an 8-pin, something like 12F675, and write the software in
GCBASIC ('cos it's free). You don't need a clock crystal, it's all on-
board. Just stick it on a piece of veroboard with the other bits.

Of course, once you have a chip in there you start to get other ideas
too... ;-)

You can do a rampup with a LM317 (or similar) regulator - it's shown in
the data sheets. It's constant voltage though, not current. Should be
easy enough to add another transistor/resistor to get a fixed max
current limit though.


yeah all kinds of possibilities exist. total cost of hardware +
long-term reliability would be the driving factors. If micro-C is cheap
but fails before tubes then it's kinda dumb. On the other hand if a
simple CCS charging a cap makes your rampup work then it's under $1. I
would actually sequence more than that since you want bias first, then
heaters, and after heater rampup, a quicker rampup for B+ lines (maybe 1
second). That way you don't get a 'pop' when you apply B+, you don't
apply B+ without reasonably warm cathodes, and you don't apply B+ unless
there's a bias supply already charged. It's a bit more than I
considered before looking at the 'Gettimg the Most Out of Vacuuum Tubes'
(Tomer) book I recently found on-line.


I just grabbed that book. Good isn't it?

Let's see...
opto-isolator fed from bias supply: input into PIC
No further action without bias
PIC ramps up heater(s)
Allow some warm-up time
Close a relay to allow B+ via a resistor
Another delay
Close second relay to short out B+ resistor & allow full supply

owzat?

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

[Big Bad Bob]

On 04/14/11 15:13, mick so wittily quipped:
On Thu, 14 Apr 2011 14:55:54 -0700, Big Bad Bob wrote:

On 04/14/11 11:13, mick so wittily quipped:
/me ponders microcontroller running the power supply. At ~$17 per
controller board (unassembled), it would make things VERY interesting.


Should be fairly reasonable to do - with a PIC at least. Use a PWM
output, RC filter to give Vref into op-amp/transistor current source.
Then ramp or step the PWM signal to give whatever warmup speed you
like. You don't need precision so software-driven PWM would be ok. You
could use an 8-pin, something like 12F675, and write the software in
GCBASIC ('cos it's free). You don't need a clock crystal, it's all on-
board. Just stick it on a piece of veroboard with the other bits.

Of course, once you have a chip in there you start to get other ideas
too... ;-)

You can do a rampup with a LM317 (or similar) regulator - it's shown in
the data sheets. It's constant voltage though, not current. Should be
easy enough to add another transistor/resistor to get a fixed max
current limit though.


yeah all kinds of possibilities exist. total cost of hardware +
long-term reliability would be the driving factors. If micro-C is cheap
but fails before tubes then it's kinda dumb. On the other hand if a
simple CCS charging a cap makes your rampup work then it's under $1. I
would actually sequence more than that since you want bias first, then
heaters, and after heater rampup, a quicker rampup for B+ lines (maybe 1
second). That way you don't get a 'pop' when you apply B+, you don't
apply B+ without reasonably warm cathodes, and you don't apply B+ unless
there's a bias supply already charged. It's a bit more than I
considered before looking at the 'Gettimg the Most Out of Vacuuum Tubes'
(Tomer) book I recently found on-line.


I just grabbed that book. Good isn't it?

Let's see...
opto-isolator fed from bias supply: input into PIC
No further action without bias
PIC ramps up heater(s)
Allow some warm-up time
Close a relay to allow B+ via a resistor
Another delay
Close second relay to short out B+ resistor& allow full supply

owzat?


not bad but I'd eliminate the relay in favor of an 'and' gate in series
with the oscillator that drives the switchers. In effect, like a series
relay except you're just going to 'deny it' any signal until all of the
conditions are met. So same basic idea, minus the physical contacts.
Typical switcher will conduct one cycle through the toroid whenever it
'needs more voltage' and filter caps discharge while voltage is 'too
high'. If the RLC time constants are long enough, you get the effect of
a nice continuous DC voltage with no ripple. Opto-isolators of course
needed since the input will be a bridge-rectified 110 or 220 AC. You
could still have the regulator circuits on the secondary side and the
oscillator on the primary, then use the opto-isolators as 'on/off' for
driving the transistor or MOSFET that puts current into the toroid. A
lot of these ideas are covered by quite a number of patents. One of the
more prominent ones (as I recall) was filed in the early 90's. You can
always do an online patent search on switching power supplies, and would
find a few sample schematics in the process (all patented, of course).
At least onepatent (in 2005) covers part of what I described above.
http://www.freepatentsonline.com/7199649.html

[Big Bad Bob]

On 04/14/11 16:30, flipper so wittily quipped:
On Thu, 14 Apr 2011 14:55:54 -0700, Big Bad Bob
wrote:

On 04/14/11 11:13, mick so wittily quipped:
/me ponders microcontroller running the power supply. At ~$17 per
controller board (unassembled), it would make things VERY interesting.


Should be fairly reasonable to do - with a PIC at least.
Use a PWM output, RC filter to give Vref into op-amp/transistor current
source. Then ramp or step the PWM signal to give whatever warmup speed
you like. You don't need precision so software-driven PWM would be ok.
You could use an 8-pin, something like 12F675, and write the software in
GCBASIC ('cos it's free). You don't need a clock crystal, it's all on-
board. Just stick it on a piece of veroboard with the other bits.

Of course, once you have a chip in there you start to get other ideas
too... ;-)

You can do a rampup with a LM317 (or similar) regulator - it's shown in
the data sheets. It's constant voltage though, not current. Should be
easy enough to add another transistor/resistor to get a fixed max current
limit though.


yeah all kinds of possibilities exist. total cost of hardware +
long-term reliability would be the driving factors. If micro-C is cheap
but fails before tubes then it's kinda dumb. On the other hand if a
simple CCS charging a cap makes your rampup work then it's under $1. I
would actually sequence more than that since you want bias first, then
heaters, and after heater rampup, a quicker rampup for B+ lines (maybe 1
second). That way you don't get a 'pop' when you apply B+,

A cautionary tale, that is not always the case. Avoiding a 'pop' or
'thump', I mean.

I did a lot of sims on that issue with an amp I never seem to get
around to finishing but, nonetheless...

If you're using "well regulated" CCS, and the heaters are warmed up,
then, when you apply B+, no matter how 'slow' it's ramped, you get a
surge in the output tubes.

The reason is the CCS wants to regulate but can't during the initial
rise of B+, so current in whatever stage the CCS lives rises *until*
current reaches regulation, at which point the rise abruptly stops.
Plate voltage does a similar thing, the rate of rise changes abruptly
when CCS regulation kicks in, and this abrupt 'knee' in it's rise
becomes an in band 'pulse' that propagates through the amp.

I.E. If the B+ ramp is slow enough then interstage DC blocking caps
will suppress that but the 'knee' has high frequency components that
pass right on through.

you could use an RC charge for Vref to give you a 1-e^(-n) effect,
making the time constant around 0.2 seconds, for a "1 second" ramp-up
with an asymptotic shape. Ideally you block frequencies below 10hz
anyway since the output transformer won't handle them well, so ramp-up
should be relatively 'pop-proof'. As a side note, the pilot should come
on only when all of the voltages are up to 100%.

you don't
apply B+ without reasonably warm cathodes, and you don't apply B+ unless
there's a bias supply already charged.

chicken/egg principle applies here. as long as both are present, then
B+ is allowed (otherwise, no B+), but having bias use the same PS as
filament only works if you have enough volts and current. If your bias
supply is -50V (like mine have typically been) it wouldn't work so well
for a DC filament supply that needs only 6.3 unless you like using PWM
(which would still work but is over-complex). Anyway, having a
switching reg with built-in protection like this, specifically for tube
amps, would have plenty of merit.

worthy of mention, there are a lot of patents on switching power
supplies. it's doubtful that such a thing could be built without
forking over some royalties.

[Big Bad Bob]

On 04/13/11 12:51, flipper so wittily quipped:
and according to Tomer the reduced heater
voltage may actually damage the tube.

That section was about indirect heat types operated beyond what the
electron cloud can handle.

Besides them not being indirect heat types Steve Bench is operating
the DHTs as small signal tubes and, so, exceeding the 'electron cloud'
would not be an issue even if they were.

reasonable.

That's also why you should be able to run small signal tubes with low
heater voltage to increase (filament/heater) life: your operating
point is (usually) way below what the electron cloud can handle so
reducing it, within reason, doesn't expose the cathode.

long term testing would determine that. I would have hoped the original
manufacturer would've done that already, for the rated values anyway.

/me wonders how you can measure all of that

[Tabby]

On Mar 27, 7:45*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. *Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). *Those
tubes typically fail emissions tests. *I've never once seen a heater
burn out. *Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. *And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

When heaters are run off a transformer, the transformer's resistance
limits initial current a fair bit. When run off mains via dropper, the
dropper will. Highest currents are seen on series heaters with no
dropper.


NT

[Big Bad Bob]

On 05/01/11 17:17, Tabby so wittily quipped:
On Mar 27, 7:45 pm, Big Bad BobBigBadBob-at-mrp3-
wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while.
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). Those
tubes typically fail emissions tests. I've never once seen a heater
burn out. Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

When heaters are run off a transformer, the transformer's resistance
limits initial current a fair bit. When run off mains via dropper, the
dropper will. Highest currents are seen on series heaters with no
dropper.

good point. Also a transformer's magnetic flux limitations will limit
current in the secondary, but on a large multi-winding power transformer
that could still be several times the rated current on the filaments.
hard to say what the primary limiting factor would be (probably
resistance like you said).

series heaters were really popular in the 70's on TV sets, particularly
the "instant on" transformerless variety that used a voltage doubler on
line current for the power supply, and series heaters on all of the
tubes (including the picture tube, at least on some models). They also
had a series resistance in line with the heaters when the device was
'off' (saving the tubes from the really high surge). As I recall the
instructions on one such set said something about plugging it in for a
minute before turning it on for the first time.

Not sure if any radios did this in the 50's or 60's but it wouldn't
surprise me if they did. You'd need polarized plugs and double
insulation to keep them electrically safe.

[Big Bad Bob]

On 05/01/11 18:38, flipper so wittily quipped:
When heaters are run off a transformer, the transformer's resistance
limits initial current a fair bit. When run off mains via dropper, the
dropper will. Highest currents are seen on series heaters with no
dropper.

Well, mightn't that explain a different level of concern since series
string was the predominate operation in radio sets like the AA5 and
variants?

ah, there were some radios that did that then.

[Tabby]

On May 5, 11:31*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 05/01/11 17:17, Tabby so wittily quipped:



On Mar 27, 7:45 pm, Big Bad BobBigBadBob-at-mrp3-
*wrote:
On 03/26/11 12:10, John L Stewart so witilly quipped:

Anyway, the turn on surge measured 16 Amps, so 11.2 A RMS for a while..
Measured across 0.1R, 50 Watts. Steady state after warmup would be 2.4
A.

Beginning a few years ago I started using NTC Thermistors from
Thermometrics-

http://www.thermometrics.com/assets/images/cl.pdf

With the cost of tubes ever increasing it is cheap insurance.

I've never had issues with heaters or supplies. *Nearly always the tubes
themselves just wear out, characteristics altered enough that things
just don't work properly any more (or else the glass breaks). *Those
tubes typically fail emissions tests. *I've never once seen a heater
burn out. *Still, the thermistor idea isn't bad.

I would expect warmup times to increase with any series resistance,
however. *And I think a 16A surge when 2.4A steady state is expected
MIGHT do more damage to the transformer than to the tubes themselves, if
you power cycle the unit a lot.

When heaters are run off a transformer, the transformer's resistance
limits initial current a fair bit. When run off mains via dropper, the
dropper will. Highest currents are seen on series heaters with no
dropper.

good point. Also a transformer's magnetic flux limitations will limit
current in the secondary, but on a large multi-winding power transformer
that could still be several times the rated current on the filaments.
hard to say what the primary limiting factor would be (probably
resistance like you said).

series heaters were really popular in the 70's on TV sets, particularly
the "instant on" transformerless variety that used a voltage doubler on
line current for the power supply, and series heaters on all of the
tubes (including the picture tube, at least on some models). *They also
had a series resistance in line with the heaters when the device was
'off' (saving the tubes from the really high surge). *As I recall the
instructions on one such set said something about plugging it in for a
minute before turning it on for the first time.

Not sure if any radios did this in the 50's or 60's but it wouldn't
surprise me if they did. *You'd need polarized plugs and double
insulation to keep them electrically safe.

Live chassis sets had neither double insulation nor, in lots of cases,
polarised plugs. Nice modern extras, but not really needed.


NT

[Big Bad Bob]

On 05/07/11 00:44, flipper so wittily quipped:
Well, with a radio and TV you don't usually have a user touch
connection, like the guitar in your hands.

heh, when I was 14 I tried making a transformerless SE tube amp using
parts stolen from an ancient TV set (horizontal amp tube became the
power tube, hah). I used (hold breath) a VOLTAGE DOUBLER using 2
silicon diodes, so no matter how you plugged it in, there was at least
60VAC floating on the chassis. After taking it to my friend's house I
discovered that I drew a small arc when I touched guitar strings. As a
result, I stopped using it. Fortunately I wasn't barefoot at the time.

mad science at an early age

[Big Bad Bob]

On 05/10/11 01:35, flipper so wittily quipped:
I plan on polarizing the plug but that wouldn't have helped at my
previous house because the idiot builder randomly wired the AC socket
hot and cold connections. I mean, it's only TWO WIRES, and COLOR
CODED, for heaven's sake.


I saw something like that a number of years ago at a 'duplex' I'd rented
(basically it was a house that got "walled" in half with a simple
kitchen on my end where the back porch used to be). I wanted to do some
recording, and when I plugged in all my stuff I got tons of noise that I
couldn't get rid of. I checked the grounds, and there was no
connection. I opened up several outlets (to find a REAL ground) only to
discover that in SOME cases, the hot and neutral leads were BACKWARDS,
but in (fewer) cases it was correct [within the same room even]. And it
was done on GROUNDED outlets in which the 'ground' was actually
free-floating. "Not to code" indeed. I showed the property owner, but
I don't think he ever fixed it [at least not while _I_ was there].
Later I found out why, when his handyman was trying to cut a door to fit
the old, smaller frames... by using a square against the SIDE HE JUST
CUT to draw the next line to cut on. I saw him ruin 3 doors before I
said something like "you should measure everything first, from the same
side". No doubt THIS guy did the wiring, too.

[John L Stewart]

Originally Posted by Big Bad Bob

On 05/07/11 00:44, flipper so wittily quipped:
Well, with a radio and TV you don't usually have a user touch
connection, like the guitar in your hands.

heh, when I was 14 I tried making a transformerless SE tube amp using
parts stolen from an ancient TV set (horizontal amp tube became the
power tube, hah). I used (hold breath) a VOLTAGE DOUBLER using 2
silicon diodes, so no matter how you plugged it in, there was at least
60VAC floating on the chassis. After taking it to my friend's house I
discovered that I drew a small arc when I touched guitar strings. As a
result, I stopped using it. Fortunately I wasn't barefoot at the time.

mad science at an early age

Full or Half-wave Doubler? Half-Wave gets around part of the problem. I used selenium rectifiers, then grounded the supply lead to a water pipe. Only one lead connected on the power plug. It ran PP 6U6GT's (not a typo). The 6U6 heaters had a transformer.

All done around 1948.

Cheers to all, John

[Tabby]

On May 10, 9:43*pm, John L Stewart John.L.Stewart.
wrote:
Big Bad Bob;931114 Wrote:

On 05/07/11 00:44, flipper so wittily quipped:-
Well, with a radio and TV you don't usually have a user touch
connection, like the guitar in your hands.-

heh, when I was 14 I tried making a transformerless SE tube amp using
parts stolen from an ancient TV set (horizontal amp tube became the
power tube, hah). *I used (hold breath) a VOLTAGE DOUBLER using 2
silicon diodes, so no matter how you plugged it in, there was at least
60VAC floating on the chassis. *After taking it to my friend's house I
discovered that I drew a small arc when I touched guitar strings. *As a

result, I stopped using it. *Fortunately I wasn't barefoot at the time.

mad science at an early age

Full or Half-wave Doubler? Half-Wave gets around part of the problem. I
used selenium rectifiers, then grounded the supply lead to a water pipe.
Only one lead connected on the power plug. It ran PP 6U6GT's (not a
typo). The 6U6 heaters had a transformer.

All done around 1948.

Cheers to all, John

--
John L Stewart

Its a lot easier to just run with a neutral chassis. It has its risks,
but I think less than using an external earthing lead that will be
unconected sooner or later. Add a voltage monitoring relay and its
almost safe.


NT