Hi Folks,

This is not a simple question about pulling old tubes and exchanging
for new ones. I'm trying to establish a "cook book" routine for
electronically changing output tube types. For example, suppose I want
to change a single ended amp that uses a 6L6 output tube to an EL84
output tube?

I know there are some considerations regarding the output transformer
impendance, but if we assume this is okay, what are the basic
equations? To simplify, let's assume cathode bias.

My first thought is that I have a known HT voltage. To simplify even
further, let's leave that voltage alone, assuming it is ok for the tube
in question. My next thought is that I know what idle current I want
for a particular tube to bias it to some percent of output power for
the specific voltage. This is where I get stuck. It seems that it would
be some simple math involving the bias resistor dropping a certain
voltage to bias the tube, but for the life of me I am stuck! Can it be
that simple, or are there a bunch of other things I need to worry about
(tube impedance, preceding stage impedance, etc.)?

Any step by step advice on this?

I know all about Yellow Jackets and all that. I am trying to establish
the math, so that if I build a DIY amp, I can easily design for a
different output tube.

Many thanks in advance,
Mick

"Mick"

This is not a simple question about pulling old tubes and exchanging
for new ones. I'm trying to establish a "cook book" routine for
electronically changing output tube types. For example, suppose I want
to change a single ended amp that uses a 6L6 output tube to an EL84
output tube?

I know there are some considerations regarding the output transformer
impendance, but if we assume this is okay, what are the basic
equations? To simplify, let's assume cathode bias.

My first thought is that I have a known HT voltage. To simplify even
further, let's leave that voltage alone, assuming it is ok for the tube
in question. My next thought is that I know what idle current I want
for a particular tube to bias it to some percent of output power for
the specific voltage. This is where I get stuck. It seems that it would
be some simple math involving the bias resistor dropping a certain
voltage to bias the tube, but for the life of me I am stuck! Can it be
that simple, or are there a bunch of other things I need to worry about
(tube impedance, preceding stage impedance, etc.)?

Any step by step advice on this?


** Step 1. Look up the published data for the tube in question.

Step 2. Find the class A amp conditions.

Step 3. See if a cathode resistor value is given - use it.

Step 4. If a negative grid bias voltage is given, divide this by the plate
current to get a resistance.


Eg:

The GE book gives Rk = 135 ohms for a 6BQ5 with 250 volts on the plate &
18 volts @ 55 mA ( Rk = 333ohms) for the 6L6G with 350 volts on the plate.

The tube will not likely fry with these values - but impedance mismatching
will mean very poor power output.




......... Phil

Mick wrote:
Hi Folks,

This is not a simple question about pulling old tubes and exchanging
for new ones. I'm trying to establish a "cook book" routine for
electronically changing output tube types. For example, suppose I want
to change a single ended amp that uses a 6L6 output tube to an EL84
output tube?

I know there are some considerations regarding the output transformer
impendance, but if we assume this is okay, what are the basic
equations? To simplify, let's assume cathode bias.

My first thought is that I have a known HT voltage. To simplify even
further, let's leave that voltage alone, assuming it is ok for the tube
in question. My next thought is that I know what idle current I want
for a particular tube to bias it to some percent of output power for
the specific voltage. This is where I get stuck. It seems that it would
be some simple math involving the bias resistor dropping a certain
voltage to bias the tube, but for the life of me I am stuck! Can it be
that simple, or are there a bunch of other things I need to worry about
(tube impedance, preceding stage impedance, etc.)?

Any step by step advice on this?

I know all about Yellow Jackets and all that. I am trying to establish
the math, so that if I build a DIY amp, I can easily design for a
different output tube.

Many thanks in advance,
Mick


A simple question with hidden depths. Here is a suggested workflow:

1. Generally, the most expensive part of the amplifier to change is the
output transformer. However, most tubes can be operated over a very
wide range of primary load impedances. In SE, for instance, Rp
multiples from 2 to 8 are in use. The thing to look out for is not the
primary impedance so much as the current handling capability of the
transformer. Divided and accessibly terminated secondary windings can
be used to reflect whatever impedance you like via the primary onto the
plate

2. Commonly the most expensive part of the amplifier to build is the
power supply. It is however, once standing, also the most flexible
element of the design. Most tubes can be operated somewhere between
120V and 500V. Most power supplies can be configured as either full
wave rectified, bridge rectified or as a voltage doubler by simply
changing a piece or two of cheap silicon and rearranging the caps.
Resistors are also cheap for voltage dividers, adding bigger bleeds (a
good thing anyway) to leech off some current, or inserting as ballasts
to tune the voltage. A centretapped power supply can be used across
only one half, and some can be reversed.

3. Changing sockets is major work. However, where two tubes stand, it
is simple to rewire so that one tube provides power and the other is
wired as a shunt regulator. That is how in the beginning I kept my T199
Millennium's End 75W SE amp within the desired operating parameters
when switching from 75W PSE to 25W SE. (I didn't like the sound of
regulated power and soon stopped doing it. It's a super idea though for
small signal stages, where regulated power can sometimes make an
improvement.)

4. We have now dismissed or taken control of two out of the three
elements of control of the tube's operation: the load impedance and the
power supply. The final one is the amount of current the tube is
permitted to consume and is in fact the dangerous parameter.
Dissipation is VA. But again, the required alteration, if any, is
merely a resistor in the cathode circuit to control the amount of
current permitted at that plate voltage and negative grid bias
settings.

5. An amp designed from scratch to be instantly alterable between tubes
should have, in order of importance which is fortuitously also the
order of cost:
a) variable cathode resistor to control current delivered to the plate
(look at the 2-5W 100-220R wirewounds you use as humbusters on DHTs...)
b)
b) switchable or tapped or otherwise configurable DC power supply (or
it could of course be permanently attached to the variac)
c) sectioned windings on the secondary of the OPT trx in order to
reflect any desired primary impedance onto the plate of whichever tube
is plugged in

6. A subtlety easy to overlook is that an amp with an unknown power
tube should in the driver position boast a beefy tube arranged to have
substantial current (at least 8mA and preferably 20mA) on the plate,
otherwise Miller could screw the bandwidth of whatever experiments are
conducted as surely as Murphy would if he were into tube amps.That lets
12AX7 and other wimpy, useless little tubes right out of consideration.
This is more important than the consideration of impedance matching
below.

7. Unless the driver stage is built to give a very low output
impedance, SRPP or better still a cathode follower, better not swap
driver tubes at the same time as power tubes or really gross impedance
mismatches could occur.

HTH.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

Phil and Andre,

Many thanks for both of your replies. How about a "simple" example?

My '69 Super Reverb uses 6L6GC output tubes in PP. Voltage on the
plates is about 460 VDC. Current cathode resistors are 150ohms. Recipe
steps to change to EL84 operating in Class A/B (could do class A if
easier to describe)? Schematics are he

http://www.charlestonarea.com/Fender...b568_schem.gif

Again, many thanks,
Mick

"Mick"
Phil and Andre,

Many thanks for both of your replies. How about a "simple" example?

My '69 Super Reverb uses 6L6GC output tubes in PP. Voltage on the
plates is about 460 VDC. Current cathode resistors are 150ohms. Recipe
steps to change to EL84 operating in Class A/B (could do class A if
easier to describe)? Schematics are he



** That ain't nothing like your original question about SINGLE ENDED amps.

IMO - the 450 volt screen supply voltage in that amp will kill EL84s.




......... Phil

True, true, Phil. Funny how we start simple and move toward complex.

So...how about converting a Champ AA764 circuit (6V6GT output tube) to
an EL84 output tube. ;-)

Plate and G2 voltage ~350 VDC. 470 ohm cathode resistor. Schematic is
he

http://www.charlestonarea.com/Fender...champaa764.gif

Cheers,
Mick

"Mick"
True, true, Phil. Funny how we start simple and move toward complex.

So...how about converting a Champ AA764 circuit (6V6GT output tube) to
an EL84 output tube. ;-)

Plate and G2 voltage ~350 VDC. 470 ohm cathode resistor. Schematic is
he

http://www.charlestonarea.com/Fender...champaa764.gif



** Just change the cathode resistor to 270 ohms.

Should have 12 volts at the cathode.


BTW

That previous Fender amp used grid bias - the 150 ohms cathode resistors
were a gimmick.



.......... Phil

With no mods to the grid bias pin?

"Mick"
With no mods to the grid bias pin?



** Try again.

This time quote some previous text so I know WTF you are on about.




......... Phil

Mick wrote:
Phil and Andre,

Many thanks for both of your replies. How about a "simple" example?

My '69 Super Reverb uses 6L6GC output tubes in PP. Voltage on the
plates is about 460 VDC. Current cathode resistors are 150ohms. Recipe
steps to change to EL84 operating in Class A/B (could do class A if
easier to describe)? Schematics are he

http://www.charlestonarea.com/Fender...b568_schem.gif

Again, many thanks,
Mick

This isn't a simple example at all. It requires major surgery on the
power supply, for a start.

Andre Jute

Mick wrote:
True, true, Phil. Funny how we start simple and move toward complex.

So...how about converting a Champ AA764 circuit (6V6GT output tube) to
an EL84 output tube. ;-)

Plate and G2 voltage ~350 VDC. 470 ohm cathode resistor. Schematic is
he

http://www.charlestonarea.com/Fender...champaa764.gif

Cheers,
Mick

This is more like it. In the cathode circuit, parallel another 470 ohm
resistor to the one already there, or replace the 470R with 270R.

Andre Jute

Andre said:
"This isn't a simple example at all. It requires major surgery on the
power supply, for a start."

But Andre, I can purchase Yellow Jackets and run EL84s with absolutlely
no mods of any kind. So why can't I add a dropping resistor, switch
sockets, change a few other wires and resistors, and go?

Mick

Mick wrote:
Andre said:
"This isn't a simple example at all. It requires major surgery on the
power supply, for a start."

But Andre, I can purchase Yellow Jackets and run EL84s with absolutlely
no mods of any kind. So why can't I add a dropping resistor, switch
sockets, change a few other wires and resistors, and go?

Mick

What are Yellow Jackets? -- AJ

Yello Jackets are tube adapters. They fit in the 6L6 sockets and then
take an EL84 tube. So you quite literally unplug the 6L6, plug in the
Yellow Jacket with EL84 attached, and you have a cathode biased EL84.
Try a quick Google search.

Mick

On Tue, 17 Jan 2006 11:01:14 -0800, Mick burbled:

Yello Jackets are tube adapters. They fit in the 6L6 sockets and then take
an EL84 tube. So you quite literally unplug the 6L6, plug in the Yellow
Jacket with EL84 attached, and you have a cathode biased EL84. Try a quick
Google search.


Yep, you can connect a valve that way, but it won't correct for a mismatch in
transformer primary impedance and this is fairly critical for pentodes
such as the EL84 if you want low distortion. Note that the Yellowjackets
are only designed for putting EL84s (in class A only) into 6V6, 6L6 and
EL34 amplifiers. They will give some protection against damage to the EL84
from high voltages but in many cases you will sacrifice output power so
there isn't really that much to be gained by using them (unless you are
just looking for a change of "tone"). It costs far more for an
EL84+Yellowjacket than it does for a reasonably good 6L6 or EL34 so there
*really* isn't much point!

An amplifier is designed "in reverse". You start with the output
transformer & work back, making sure that each stage is matched for
impedance & gain, so changing to a different output valve isn't really a
good idea - it messes up the whole design.

Even something as simple as swapping an EL84 into an EL34's position can
bring major problems. The anode and heater currents are lower so the HT
rail voltage goes up because there is less load on the supply. If the amp
hasn't been designed with this possibility in mind you can then get
overheating or even exploding capacitors. If you want to experiment then
replace the output valve with one that can handle the same (or more) anode
current - although putting, say, an EL34 in a 6V6 amp can burn out the
heater winding on the mains transformer if there isn't enough spare
capacity! You may get away with swapping 6L6GC (but not 6L6 or 6L6B) and
EL34 in some amps. Likewise you may get away with a 6V6 swapped into a 6L6
(but perhaps not 6L6GC) amp with a loss in power. The reverse direction
may cook the heater winding...

So, basically, swapping output valves isn't really a good idea (if you
want the amp and valves to have a reasonable life, anyway). The amp has
been designed around the correct devices.

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk

"mick" wrote in message
news
Yep, you can connect a valve that way, but it won't correct for a mismatch
in transformer primary impedance and this is fairly critical for pentodes
such as the EL84 if you want low distortion. Note that the Yellowjackets
are only designed for putting EL84s (in class A only) into 6V6, 6L6 and
EL34 amplifiers. They will give some protection against damage to the EL84
from high voltages but in many cases you will sacrifice output power so
there isn't really that much to be gained by using them (unless you are
just looking for a change of "tone"). It costs far more for an
EL84+Yellowjacket than it does for a reasonably good 6L6 or EL34 so there
*really* isn't much point!

He's talking about guitar, not "hifi" amplifiers and any such mismatches may
in fact be seen as desirable in achieving certain overloaded/distorted
sounds.

Even something as simple as swapping an EL84 into an EL34's position can
bring major problems. The anode and heater currents are lower so the HT
rail voltage goes up because there is less load on the supply. If the amp
hasn't been designed with this possibility in mind you can then get
overheating or even exploding capacitors. If you want to experiment then
replace the output valve with one that can handle the same (or more) anode
current - although putting, say, an EL34 in a 6V6 amp can burn out the
heater winding on the mains transformer if there isn't enough spare
capacity! You may get away with swapping 6L6GC (but not 6L6 or 6L6B) and
EL34 in some amps. Likewise you may get away with a 6V6 swapped into a 6L6
(but perhaps not 6L6GC) amp with a loss in power. The reverse direction
may cook the heater winding...

So, basically, swapping output valves isn't really a good idea (if you
want the amp and valves to have a reasonable life, anyway). The amp has
been designed around the correct devices.

All fair points in the case of "conventional" valve amps, but how do you
explain this:

http://www.thdelectronics.com/products/univalve.htm

He's talking about guitar, not "hifi" amplifiers and any such mismatches may
in fact be seen as desirable in achieving certain overloaded/distorted
sounds.

"He" certainly is. ;-)

Distortion is an important part of the instrument. But really, I'd like
to know how to design an output tube. The example of switching tubes
was based on seeing several "redesigns" of Champs and other classic
amps with different output tubes.

Cheers to all,
Mick

"Andre Jute"

What are Yellow Jackets?



** A very * fishy * kind of scam.



........ Phil