I just posted a schematic for active protection of a tube amp,
and B+ turn on delay at ABSE.

At turn of the amp, the DC heater voltage from a silicon diode rectified

supply is quickly established.
This is applied to the circuit to give a rail at +12v.
The R2 and C2 form a time constant circuit with a slow rise in C2
voltage.
It takes 25 seconds for the C2 voltage to rise to about +10v, when the
ZD
begins to conduct current to the high impedance base input of Q2,
connected to Q1
to form a darlington pair.
After 25 seconds after turn on the heaters in the amp have all heated
the cathodes
to be able to conduct current.
When Q1/Q2 turn on, the relay closes, thus allowing the
HT winding to begin delivering power to the power capacitors, which
should be large as possible, to give a slow rise to full B+ potential.

If the amp is turned off for longer than 0.5 secs approx, the heater
voltage will fall very quickly,
and the 12v rail will fall.
D8 allows the fast discharge of C2, so that if the amp is turned on when
still hot, the
delay circuit is forced to re-commence a 25 second delay to the B+ turn
on.


There is active protection against excessive cathode current in
one or all output tubes.
If the normal idle voltage at any cathode is +20v,
and it rises to 27v, the idle current x B+ voltage will
make the tube dissipation excessive, but will not cause a fuse to blow.
Only the direct voltage changes are registered at the tops of C3-C6.
Adjustments for different amps may be made with variations to
R11 to R14.


So if the dissipation is 25 watts per tube with Ek = 20v,
then if Ek rises to 27v, the dissipation would be 33.75 watts,
which would be excessive in the case of a 6CA7 or EL34.

When the voltage at Q3 base rises to +6v, current flows through Q3
D3,4,5,
and over comes the forward voltage of the diodes of about 4.5v.
The voltage at R6 rises to +0.72v, latching on the SCR.
This turns on the red LED, D2, indicating a fault with the amp.

With the SCR turned on, the voltage in C2 is drained down through R3 and
D3,
thus cutting off current in the base of Q2, since ZD does not conduct
below its threshold voltage.
The relay is then allowed to spring open, and the HT is interupted, and
the B+
falls to 0V, but the amp is still turned on, but with only the heaters
on.

Its stays like this until the owner attends to the amp, and works out
what problem caused
excessive Ek.
Was it a shorted speaker lead?
Too much volume?

He can try to reset the amp by turning it off, then back on again
after a couple of seconds, and the SCR then unlatches, and the whole
delay
process restarts.
If the Ek rises yet again and turns off the HT, he knows he has a
problem
and he takes the amp to Mr Fixitkwik who will find out what's the cause
of the problem,
but who will not be faced with an amp which has melted down due to tubes
being allowed
over conduct for any length of time, possibly wrecking a precious OPT.

The rail voltage of the circuit isn't critical. I generally use a 6.3AV
winding in a doubler circuit
to make +15v, then C1 and R15 are not required, since the current in the
12v rated relay coil
won't be excessive if the rail is up to 16v.

Q2 and Q3 are common varieties of NPN signal bjts worth 10c each,
so almost anything will do.
Q1 is BD139, which will cope well with the current via the relay coil.

The SCR is a sensitive gate type that draws a tiny gate current, but
still requires
the gate voltage to rise to +0.7v to latch on the SCR.

The D3 to D5 could be replaced with a zener diode but I choes leds,
because
they are cheap, and give a good threshold effect for when voltage
exceeds about
1.4v in each.

The circuit only reacts to DC changes to the cathode voltages.
I have used the same circuit to monitor DV at cathodes where there is a
large cathode feedback signal
present. The C3-6 220 uF filter out any signal voltages, and the R11 to
14 are such high values as to not
waste any power from an active cathode circuit.

Patrick Turner.

Hi Patrick,

now, that's an extremely useful circuit to protect
the tubes (and ourselves and wallets...). Thank
you for sharing it!

I think most of us forget about such "paraphenalia"
all too often.

I for myself only came up so far with a very basic and
simple bias control circuit using the 4x comparator
IC LM339 (so 4 tubes could be monitored) that shows
"too much bias deviation" by a LED per output tube,
but it has no active protection facilities (for example
cutting B+ or mains).

But finding a suitable relay capable of reliable
switching the B+ rail (and all that stored energy in
the filter caps) at, say, 350V-500V and some hundred
mAs continous current running seems to be no easy
task for the electronics parts shop around the corner...

Tom

--
Falling in love is a lot like dying.
You never get to do it enough to
become good at it.

I can't see your binary posts on ABSE......
my bloody ISP .......

--
.................................................. ........................
Choky
Prodanovic Aleksandar
YU

"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
.................................................. ...........................
"Patrick Turner" wrote in message
...

I just posted a schematic for active protection of a tube amp,
and B+ turn on delay at ABSE.
*/**********************

Appreciate it!

but ABSE?
where in other words,

thanks
KC

ABSE for
alt.binaries.shematics.electronic

--
.................................................. ........................
Choky
Prodanovic Aleksandar
YU

"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
.................................................. ...........................
wrote in message
...
Appreciate it!

but ABSE?
where in other words,

thanks
KC

Whoa. I just make the simplest crkt possible- EZ80 in series with a relay
coil, 1N4001 and a current limitting resistor. When the relay acts, heater
(also connected in series with a resistor) is switched off and the anode is
connected directly to the cathode of EZ80. The whole thingie is powered from
20V mains transformer winding (I've got a vintage tranny with many windings,
one was 20V and I decided to use it).

Tom Schlangen said:

But finding a suitable relay capable of reliable
switching the B+ rail (and all that stored energy in
the filter caps) at, say, 350V-500V and some hundred
mAs continous current running seems to be no easy
task for the electronics parts shop around the corner...

For those applications I use a heavy duty 12V loudspeaker relay with
both contact sections in series, and a snubber cap across both.
Each contact section is specified as 250V max. (Omron).
Several amps using that circuit are running for more than 8 years now,
so this method apparently works.

--
Sander deWaal
"SOA of a KT88? Sufficient."

Tom Schlangen wrote:

Hi Patrick,

now, that's an extremely useful circuit to protect
the tubes (and ourselves and wallets...). Thank
you for sharing it!

I think most of us forget about such "paraphenalia"
all too often.

I for myself only came up so far with a very basic and
simple bias control circuit using the 4x comparator
IC LM339 (so 4 tubes could be monitored) that shows
"too much bias deviation" by a LED per output tube,
but it has no active protection facilities (for example
cutting B+ or mains).

Bias should control itself, with cathode bias, or when fixed bias is
adjusted. But tubes drift, or give trouble sometimes,
so some sort of B+ interupter is needed...



But finding a suitable relay capable of reliable
switching the B+ rail (and all that stored energy in
the filter caps) at, say, 350V-500V and some hundred
mAs continous current running seems to be no easy
task for the electronics parts shop around the corner...

The HT winding is usually up to 350 vrms and since its AC, there is no
problem finding
a relay to switch it. Heavy duty relays for 240v rated for 10 amps are
usually fine.
I have a cap across the contacts to stop arcing.

In my 300 watt amps the HT is 200vrms, less than the mains, since I use
a doubler.

The 240v mains relays are having it easy there.

Maybe a triac could also be used.


Patrick Turner.



Tom

--
Falling in love is a lot like dying.
You never get to do it enough to
become good at it.

I just sent you a copy

Patrick Turner.

Choky wrote:

I can't see your binary posts on ABSE......
my bloody ISP .......

--
.................................................. .......................
Choky
Prodanovic Aleksandar
YU

"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
.................................................. ..........................
"Patrick Turner" wrote in message
...

I just posted a schematic for active protection of a tube amp,
and B+ turn on delay at ABSE.
*/**********************

Krzysiek Slychan wrote:

Whoa. I just make the simplest crkt possible- EZ80 in series with a relay
coil, 1N4001 and a current limitting resistor. When the relay acts, heater
(also connected in series with a resistor) is switched off and the anode is
connected directly to the cathode of EZ80. The whole thingie is powered from
20V mains transformer winding (I've got a vintage tranny with many windings,
one was 20V and I decided to use it).

Why not post a schematic?

In some amps I have used a small 7VA auxilliary tranny to create the +12v rail
for the
circuit, and switched off the mains voltage to the main PT.
Its a little more complex, but it is also the rolls royce method.
The relay is then working with no more than the mains voltage.



Patrick Turner.

"Patrick Turner" a écrit dans le message de news:
...

[ . . . ]
Maybe a triac could also be used.


Patrick Turner.

Oh! Not a such noisy device

What about a source follower with an HV MOSFET between the rectifier
and the first smoothing cap ?
With a properly adjusted RC to feed to gate, may be made so that HV rise
slowly, at the same rate that toobs start to conduct.

Could also help in filtering or even regulating HV !!

Yves.

Yves wrote:

"Patrick Turner" a écrit dans le message de news:
...

[ . . . ]
Maybe a triac could also be used.


Patrick Turner.

Oh! Not a such noisy device

What about a source follower with an HV MOSFET between the rectifier
and the first smoothing cap ?
With a properly adjusted RC to feed to gate, may be made so that HV rise
slowly, at the same rate that toobs start to conduct.

Could also help in filtering or even regulating HV !!

Yves.

The circuit I just posted is for a 35 watt SE amp.
The amp has Si rectifiers throughout.
It sent out a "whoop", or short burst of audio oscillation during warm up, and
I discovered the
whoop noise began just as the 4 output tubes just began to conduct, and then
stopped
after 1/2 a second during the charge up of the cathode caps.
This happened even with the grids all grounded, so it was a phenomena
that generated in the output stage only, which has a CFB winding on the OPT.

I tried all sorts of slowed down screen voltage rise, but nothing helped,
and I am not sure why I got the noise burst, and it could be a string of RF
pulses.

With the heaters on, and able to conduct, the problem went away.
I the 4 screens are fed from a shunt regged 272v supply, with a 3k feed R,
and I increased the C across the zener regs from 100 uF to 470 uF,
and that slowed the screen voltage rise when the HT winding turned on,
and along with the other 701 uF in the B+ supply, the rise in
B+ from HT turn on to +380v takes about 5 seconds, which is slow enough,
and does not generate any unwanted LF transients at the output of the amps
in excess of about 1/3 of a volt.

So I don't need that HV mosfet source follower, although its a nice idea.
My experience has not always been positive with HV regulators, sooner or later
a
voltage spike gets in there somewhere, and poof!, no more reg.
I'd rather use heaps of capacitance, with enough series R to slow a B+ rise,
and have the amp turn its HT off when it gets ill in some way.
Even better is the use of the small slave tranny, which turns off the mains,
but space in the SE35 is a bit cramped.

In my amp, which has a fixed voltage applied to the screens,
It'd be best to have a slow rise HV mosfet SF or HV emitter follower applied to
the screen voltage,
because its the screen voltage which controls the DC flow more than the anode
supply,
and the currents and voltages involved are less with the G2 supply than with
the anode supply,
so a BU208/MJE340 darlington pair would be a nice way to slow the rise of the
anode current.

The warm up oscillation probably won't occur in a PP amp, since its a balanced
circuit.
Its only in this amp I noticed the problem.

Once warmed up, and ready for use the SE35 with 4 x 6CA7 have excellent
stability, with
BW from 16 Hz to 65 kHz at 30 watts, 5 ohms, and able to
power any value of capacitance without any R at the output.
BW is from 7 Hz to 65 kHz 5 ohms at 2vo.
With 4 uF as the load at 2vo, the response rises only 1 dB at 20 kHz.
There is not the slightest sign of any RF instabilities.
The other advantage of a delayed B+ turn on is that
the inrush current to cold heaters does not coincide with
the inrush current to the B+ rails, because of all that capacitance I use.
So the mains fuse value can be smaller, so the amp has better
fuse protection than usual.

Patrick Turner.

Patrick Turner wrote:

I just posted a schematic for active protection of a tube amp,
and B+ turn on delay at ABSE.

Patrick's delay circuit does not show up here.

However, see a very simple circuit that does the job at ABSE.
I did this one in Glass Audio, Issue5, 1999.

Cheers, John Stewart

John Stewart wrote:

Patrick Turner wrote:

I just posted a schematic for active protection of a tube amp,
and B+ turn on delay at ABSE.

Patrick's delay circuit does not show up here.

I just sent you a copy.



However, see a very simple circuit that does the job at ABSE.
I did this one in Glass Audio, Issue5, 1999.

I can't see that here, but maybe it appears tomorrow.

Patrick Turner.



Cheers, John Stewart

Hi Krzysiek,

could you please make available a simple schematice
somewhere, since I couldn't follow your text
explanantion?

Tnx2u!

Tom

--
To err is human - to purr feline.
- R. Byrne

OK, maybe I'll mail it to you- but I gotta draw the schematic and scan ti
(remember Trini Lopez sing: "if I had a scanner..." ? ) or draw in
paintbrush (which I dislike).

Krzysiek Slychan wrote:

Whoa. I just make the simplest crkt possible- EZ80 in series with a relay
coil, 1N4001 and a current limitting resistor. When the relay acts, heater
(also connected in series with a resistor) is switched off and the anode is
connected directly to the cathode of EZ80. The whole thingie is powered from
20V mains transformer winding (I've got a vintage tranny with many windings,
one was 20V and I decided to use it).

That sounds quite similar to the circuit I posted at ABSE under
the heading "Another B+ Delay Citcuit". Have a look.
Cheers, John Stewart

Hello Tom,
I´m very intersted in the circuit you mencioned about using 4xICLM339 to
monitor 4 tubes. Would you mind to share this information and send me some
schematics ( if possible) about that circuit? I´m planning to project a tube
tester for matching up to four audio tubes.
Thanks a million in advance.

Juan Segui




"Patrick Turner" escribió en el mensaje
...


Tom Schlangen wrote:

Hi Patrick,

now, that's an extremely useful circuit to protect
the tubes (and ourselves and wallets...). Thank
you for sharing it!

I think most of us forget about such "paraphenalia"
all too often.

I for myself only came up so far with a very basic and
simple bias control circuit using the 4x comparator
IC LM339 (so 4 tubes could be monitored) that shows
"too much bias deviation" by a LED per output tube,
but it has no active protection facilities (for example
cutting B+ or mains).

Bias should control itself, with cathode bias, or when fixed bias is
adjusted. But tubes drift, or give trouble sometimes,
so some sort of B+ interupter is needed...



But finding a suitable relay capable of reliable
switching the B+ rail (and all that stored energy in
the filter caps) at, say, 350V-500V and some hundred
mAs continous current running seems to be no easy
task for the electronics parts shop around the corner...

The HT winding is usually up to 350 vrms and since its AC, there is no
problem finding
a relay to switch it. Heavy duty relays for 240v rated for 10 amps are
usually fine.
I have a cap across the contacts to stop arcing.

In my 300 watt amps the HT is 200vrms, less than the mains, since I use
a doubler.

The 240v mains relays are having it easy there.

Maybe a triac could also be used.


Patrick Turner.



Tom

--
Falling in love is a lot like dying.
You never get to do it enough to
become good at it.

[rubli]

another alternative:

http://nervenet.info/HT_delay/

cheers

[joebajoe]

Originally Posted by Patrick Turner

I just posted a schematic for active protection of a tube amp,
and B+ turn on delay at ABSE.

At turn of the amp, the DC heater voltage from a silicon diode rectified

supply is quickly established.
This is applied to the circuit to give a rail at +12v.
The R2 and C2 form a time constant circuit with a slow rise in C2
voltage.
It takes 25 seconds for the C2 voltage to rise to about +10v, when the
ZD
begins to conduct current to the high impedance base input of Q2,
connected to Q1
to form a darlington pair.
After 25 seconds after turn on the heaters in the amp have all heated
the cathodes
to be able to conduct current.
When Q1/Q2 turn on, the relay closes, thus allowing the
HT winding to begin delivering power to the power capacitors, which
should be large as possible, to give a slow rise to full B+ potential.

If the amp is turned off for longer than 0.5 secs approx, the heater
voltage will fall very quickly,
and the 12v rail will fall.
D8 allows the fast discharge of C2, so that if the amp is turned on when
still hot, the
delay circuit is forced to re-commence a 25 second delay to the B+ turn
on.


There is active protection against excessive cathode current in
one or all output tubes.
If the normal idle voltage at any cathode is +20v,
and it rises to 27v, the idle current x B+ voltage will
make the tube dissipation excessive, but will not cause a fuse to blow.
Only the direct voltage changes are registered at the tops of C3-C6.
Adjustments for different amps may be made with variations to
R11 to R14.


So if the dissipation is 25 watts per tube with Ek = 20v,
then if Ek rises to 27v, the dissipation would be 33.75 watts,
which would be excessive in the case of a 6CA7 or EL34.

When the voltage at Q3 base rises to +6v, current flows through Q3
D3,4,5,
and over comes the forward voltage of the diodes of about 4.5v.
The voltage at R6 rises to +0.72v, latching on the SCR.
This turns on the red LED, D2, indicating a fault with the amp.

With the SCR turned on, the voltage in C2 is drained down through R3 and
D3,
thus cutting off current in the base of Q2, since ZD does not conduct
below its threshold voltage.
The relay is then allowed to spring open, and the HT is interupted, and
the B+
falls to 0V, but the amp is still turned on, but with only the heaters
on.

Its stays like this until the owner attends to the amp, and works out
what problem caused
excessive Ek.
Was it a shorted speaker lead?
Too much volume?

He can try to reset the amp by turning it off, then back on again
after a couple of seconds, and the SCR then unlatches, and the whole
delay
process restarts.
If the Ek rises yet again and turns off the HT, he knows he has a
problem
and he takes the amp to Mr Fixitkwik who will find out what's the cause
of the problem,
but who will not be faced with an amp which has melted down due to tubes
being allowed
over conduct for any length of time, possibly wrecking a precious OPT.

The rail voltage of the circuit isn't critical. I generally use a 6.3AV
winding in a doubler circuit
to make +15v, then C1 and R15 are not required, since the current in the
12v rated relay coil
won't be excessive if the rail is up to 16v.

Q2 and Q3 are common varieties of NPN signal bjts worth 10c each,
so almost anything will do.
Q1 is BD139, which will cope well with the current via the relay coil.

The SCR is a sensitive gate type that draws a tiny gate current, but
still requires
the gate voltage to rise to +0.7v to latch on the SCR.

The D3 to D5 could be replaced with a zener diode but I choes leds,
because
they are cheap, and give a good threshold effect for when voltage
exceeds about
1.4v in each.

The circuit only reacts to DC changes to the cathode voltages.
I have used the same circuit to monitor DV at cathodes where there is a
large cathode feedback signal
present. The C3-6 220 uF filter out any signal voltages, and the R11 to
14 are such high values as to not
waste any power from an active cathode circuit.

Patrick Turner.

Hello Patric,
I am very interested in this schematic. Would you mind email me a copy of it?
Thanks