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Author Topic: Disconnecting one output tube in a push pull amp  (Read 15612 times)

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Offline RobBozic

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Disconnecting one output tube in a push pull amp
« on: August 14, 2008, 11:50:34 pm »
NOTE: the drawings for this thread can be found here>>>  http://www.el34world.com/Forum/index.php?topic=4437.0

I remember reading somewehere on this forum that you could disconnect one of the inputs to a output tube in a push pull amp, thereby reducing the power of the amp. I think that the Carr Viceroy has the same arrangement. Has anyone confirmed this?
Rob
« Last Edit: February 22, 2009, 09:57:06 pm by da Geezer »

Offline tubenit

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Re: Disconnecting one output tube in a push pull amp
« Reply #1 on: August 15, 2008, 05:34:28 am »
I've read something like this would work, but have not tried it myself.  It was described as sort of getting a psuedo
single ended sound from a push/pull amp.  One of the forum guys a few yrs ago did it and stated it worked.

With respect, Tubenit

Offline RobBozic

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Re: Disconnecting one output tube in a push pull amp
« Reply #2 on: August 15, 2008, 05:52:25 am »
Ironically i just found a toneqeust report interviewing Mr Carr and he said the following regarding the power drop from 33 watts to 7 watts in his cathode biased 6L6 amp, here's some of the interview;

"The Vincent is a cathode biased, Class A amp. In most cathode biased amps you find the two (or more) output tube cathodes being tied together and run to a common cathode bias resistor.This resistor is usually bypassed by a cap. In this configuration, the bypass cap gives the amp a little more power and a faster feel. but it does not really change the output. In preamp tube circuits there is a giant difference in gain between a bypassed and non-bypassed cathode - times three or more gain difference. When the cathode bias resistor is shared in the output section, you don't see this difference with the bypass resistor. The Vincent uses independent bias resistors for each output tube, so, when the two resistors have capacitors bypassing them (in parallel with the resistor) you get full power at 33 watts. When the caps are switched out of the circuit, the output drops to 7 watts. The other nice thing besides the different f'eel from the Rambler's shared cathode resistor and the Vincent's double resistor style is with the Vincent, the need fbr rnatched tubes is not so important.
They tend to settle in near the same no matter the match. I have tried 6L6 tubes that are on the two extremes of our matching scheme - say 8ma draw and 35ma draw - and in
the Vincent they settle to within 3 ma of each other! We will use matched tubes in the amp, but it is not necessary."

So I just found the answer to my own question an hour after I posted my post
Rob

Offline Twanger

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Re: Disconnecting one output tube in a push pull amp
« Reply #3 on: August 15, 2008, 05:55:47 am »
Check out the larmar type master volume, instead of using a dual ganged pot use 2 pots, this will allow you to completly mute one output tube.

John

Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #4 on: August 15, 2008, 06:01:09 am »
Quote
When the caps are switched out of the circuit, the output drops to 7 watts.

No way!  Do you think he meant to say turn off one tube AND switch the cap out??
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Offline tubenit

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Re: Disconnecting one output tube in a push pull amp
« Reply #5 on: August 15, 2008, 06:10:12 am »

I agree with LooseChange ....... I don't think separate resistors with caps being switched out will change it from 33 to 7 watts.  Something else is going on. Like LC said, maybe a tube is being completely shut off?

I have used a spst "smooth" switch on a Dano Centurian which removed the 47uf cathode cap on the 6V6 power tubes
shared resistor.  It had very little difference in volume. It removed a little grit and bass and that was pretty much it.

With respect, Tubenit

Offline tubesornothing

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Re: Disconnecting one output tube in a push pull amp
« Reply #6 on: August 15, 2008, 08:17:00 am »
I reverse engineered a Carr Vincent to see how they did the 33W to 7W, and it is as described above.  Here is the thread with the schematics etc.

http://www.el34world.com/Forum/index.php?topic=2060.msg15633#msg15633


Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #7 on: August 15, 2008, 03:00:17 pm »
Quote
When the caps are switched out of the circuit, the output drops to 7 watts.

No way!  Do you think he meant to say turn off one tube AND switch the cap out??

Still don't believe it after I looked at the thread.  I have lifted Cathode bypass caps many times in the power amp stage with no tremendous loss in gain.  In fact the only way to really hear the difference is if the amp is pushed hard.
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Offline tubesornothing

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Re: Disconnecting one output tube in a push pull amp
« Reply #8 on: August 15, 2008, 06:29:18 pm »
Next time you are in the music store check it out - works suprising well.  The reviews of the vincent are well based, it is a nice amp.

Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #9 on: August 16, 2008, 07:00:51 am »
I am sure the amp works as advertised. I am still doubting the exhibited electronics behind it. Can we reverse engineer it again.  Got some pictures??

Everybody wants to cut the wattage of their amps down.  It's the latest fad.  If this really works (as spec'ed) it would be all over the web. BTW, I would try it but just don't have the time.

Hey Tubenit... Got the time to try this?  You are always on the cutting edge.
« Last Edit: August 16, 2008, 07:08:54 am by LooseChange »
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Offline tubesornothing

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Re: Disconnecting one output tube in a push pull amp
« Reply #10 on: August 16, 2008, 08:48:04 am »
LC I don't have the time either to investigate your concerns.  I have already done a lot of work on this and am happy to share what I have.  I will leave it as is.

With all due respect, if you doubt it, try it.  If not...


Offline Tiny_Daddy

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Re: Disconnecting one output tube in a push pull amp
« Reply #11 on: August 16, 2008, 10:27:18 am »
I converted a HRD by paralleling the output tubes, converting to cathode bias and using half of the OT primary. It's great for small gigs. Did not measure the power though.

Offline tubenit

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Re: Disconnecting one output tube in a push pull amp
« Reply #12 on: August 16, 2008, 11:28:10 am »
"I have lifted Cathode bypass caps many times in the power amp stage with no tremendous loss in gain."

If I am looking at the schematic correctly ........ it looks to me like the cathode caps are NOT being switched in and out. They look like they're in series with the cathode resistor not parallel.  It looks like the cathode resistor is being bypassed by the spst.  If I am reading that correct ....... maybe that would work as described?

I don't think (from what I've experienced) that using a spst to engage or disengage a cathode cap will make that much of a difference ......... HOWEVER ........ that's not what the schematic is indicating.

With respect, Tubenit
« Last Edit: August 16, 2008, 11:30:13 am by tubenit »

Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #13 on: August 16, 2008, 12:09:40 pm »
The way I'm seeing it is that the 100R resistors (R6 & R7) should be much larger (100k, or 10k at least), & are used as "anti-pop" bleeders for the caps, so that the switch is silent when thrown. (I believe this is referred to in the linked thread)

The 500R/15watt resistors are the actual bias resistors, & the 100k's (or even 10k"s) have little to no effect on the bias when they are paralleled with the 500R's.

I, too, find it hard to believe that you can go from 30w to 7w by just switching out the cap(s), but I will try it on one of my 6L6 cathode biased amps & report back as soon as I can.

Carr sez that the dual bias resistors/caps are the secret mojo.......
If it is indeed true, then it will be a great additional option for larger wattage cathode biased amps.
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Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #14 on: August 16, 2008, 03:41:20 pm »
OK....just wired it up to a cathode biased amp running 2x 5881's. It's exactly as the drawing shows, except that I didn't use a switch....I have the neg of the caps connected to an alligator clip wire.

There is no difference either way....with the caps connected or not, the volume/power is the same.

There must be something else going on in the Carr to drop the power to 7w(?)
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Offline punkykatt

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Re: Disconnecting one output tube in a push pull amp
« Reply #15 on: August 16, 2008, 04:24:37 pm »
Try removing the short jumper wire between the 47uf and .1 caps.

Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #16 on: August 17, 2008, 07:45:34 am »
OK....just wired it up to a cathode biased amp running 2x 5881's. It's exactly as the drawing shows, except that I didn't use a switch....I have the neg of the caps connected to an alligator clip wire.

There is no difference either way....with the caps connected or not, the volume/power is the same.

There must be something else going on in the Carr to drop the power to 7w(?)

THANK YOU GEEZ!!! Makes you want to crack that Carr open again... Who's got that amp???

My guess is they are turning off one tube and lifting the bypass caps. (Looking at the actual amp circuit and sleuthing the switching may have caused a bit of confusion.)
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Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #17 on: August 17, 2008, 12:49:40 pm »
Whoops.....uh-oh.....hold it!!!!

UPDATE!!!!

The schematic must be wrong (something is shorting something else out) or maybe (quite possibly) I wired it up wrong, BUT:

I tried just doing a straight connect/disconnect of the individual bypass caps using clip lead wires (without the 0.1uf & 100k bleeders) and IT WORKS!
AND, the cool thing is that you can disconnect each cap separately (instead of both at the same time) to get (I'm guessing) ~20 watts output that has some "single-ended" characteristics.

** Both caps connected = full power (~30w) normal operation
** one cap disconnected/one cap connected = approx. 2/3 power....maybe 20w(?)
     It even gets a slight SE hum with just 1 cap connected & seems to be very touch responsive & "single-ended" -like.....(unbalanced output, I'm guessing)
** Both caps disconnected = maybe 5w to 7w....it's not as loud as the "Little Wing" 6BM8 amp.

So, it really does what it's supposed to.....I'm going to put a 3-way switch in to have all 3 powers available.

Geezer


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Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #18 on: August 17, 2008, 01:22:57 pm »
I'll eat crow... No problem. Never figured it!

Thanks again Geez!!!
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Re: Disconnecting one output tube in a push pull amp
« Reply #19 on: August 17, 2008, 02:28:29 pm »
I think about it like this. If the cathodes are tied together the current of both tubes is going through that one resistor. Each power tube is out of phase then when one tube is drawing more current the other is drawing less. So the equal and opposite IR drop across the cathode resistor keeps the bias voltage +/- the same.  But with seperate, unbypassed cathode resistors for each power tube you do get the negative feedback drop. As more current is drawn through the resistor the bias voltage increases, decreasing gain. So adding the caps will keep the bias voltage constant(more gain than no caps).
« Last Edit: August 17, 2008, 02:55:17 pm by jeff »

Offline phsyconoodler

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Re: Disconnecting one output tube in a push pull amp
« Reply #20 on: August 17, 2008, 05:12:59 pm »
So did you use the 100R resistors and the .1 bypass?Or did you just switch out the bypass caps?
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Offline bnwitt

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Re: Disconnecting one output tube in a push pull amp
« Reply #21 on: August 17, 2008, 05:33:30 pm »
I'm following this thread with great interest.
Guides on your quest for tone.
 Oh yeah, and I'm usually just kidding so don't take me too seriously.

Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #22 on: August 17, 2008, 05:37:21 pm »
So far I have just "manually" switched the caps using clip lead wires.

When I wire in the switch, I will use the 100k resistors (or 10k, whatever value stops the cap charge "pop"), but will not use the 0.1uf caps....they seem to be what caused the problem with my 1st attempt. I don't think they are needed.

BTW, the tone seems to stay very much the same at all levels of attenuation (!)
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Offline RobBozic

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Re: Disconnecting one output tube in a push pull amp
« Reply #23 on: August 17, 2008, 06:19:05 pm »
Good work gentlemen!

I'm going to implement this into my homebrew AB763, I've got a spare hole where the extension speaker jack was.

Rob

Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #24 on: August 17, 2008, 06:36:08 pm »
I jumped in too.
I mangled my 18Watt.  Works great.

Got even an easier method using a SPST. Same scenario with separate cathode resistors and caps.. BUT:

If you disconnect both grounded ends of the cathode bypass caps and connect them together, but not grounded, you get full volume! Try that!!

I used two 250R resistors and two 20uf caps.
What did you use?
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Offline phsyconoodler

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Re: Disconnecting one output tube in a push pull amp
« Reply #25 on: August 17, 2008, 06:44:06 pm »
Very interesting!Wonder how that works?No ground source for the bypass caps should be no different than lifting them.
   
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Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #26 on: August 17, 2008, 06:49:41 pm »
I used 2x 500R/10w, since my amp has 2x 6L6/NOS Tung Sol 5881's, and I used 8uf/150v caps....the 47uf made it a bit mushy.
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Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #27 on: August 17, 2008, 07:48:23 pm »
You guys are gonna love this... My 18 watt has a few controls that made this easy to experiment.
Totally variable from full on Push-Pull to low 5 watt single ended... Relates right back to the first post.
See the attachment.  Geez, you'll like this one!

OR put a 1M pot between the two cathode caps and get variable Push-Pull power. Didn't draw that.

I'm really waxin' it up tonight!!   ;D
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Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #28 on: August 17, 2008, 08:43:17 pm »
.sch & gif of the switch I implemented
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Offline Ritchie200

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Re: Disconnecting one output tube in a push pull amp
« Reply #29 on: August 17, 2008, 09:17:43 pm »
Hey Loose, how does it sound in SE mode?  Does it have some fizz to it when you drive it?  WE NEED SOME SOUND CLIPS!!!

Thanks,
Jim

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Offline Fresh_Start

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Re: Disconnecting one output tube in a push pull amp
« Reply #30 on: August 17, 2008, 10:12:42 pm »
You guys are gonna love this...

Totally variable from full on Push-Pull to low 5 watt single ended...

I'm really waxin' it up tonight!!   ;D

You're right about us loving it! :D  I had to look at your drawing a couple of times before I saw the 1 meg pot in parallel with the 470K resistor to get 330K.

I jumped in too.
I mangled my 18Watt.  Works great.

Got even an easier method using a SPST. Same scenario with separate cathode resistors and caps.. BUT:

If you disconnect both grounded ends of the cathode bypass caps and connect them together, but not grounded, you get full volume! Try that!!

I used two 250R resistors and two 20uf caps.
What did you use?

LooseChange - is your drawing missing one of the 20uf caps?

LC & Geezer - thanks so much for sharing the results of your experiments!

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Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #31 on: August 18, 2008, 05:59:22 am »
The SE sounds good. It still has a full tone as compared to the P-P.  Interesting note: Before this mod, turning that pot didn't do much unless you went to both extremes of the pot. This is a HUGE improvement.

I had to look at your drawing a couple of times before I saw the 1 meg pot in parallel with the 470K resistor to get 330K.
If you want the standard 220k values in there use the 1M pot in parallel with a 280k.

LooseChange - is your drawing missing one of the 20uf caps?

The drawing is for the P-P to SE idea, it only requires one cap.
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Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #32 on: August 18, 2008, 09:16:07 pm »
I've been playing with the "modded" amp all nite....this is one the most useful features I've ever put on an amp!
Three (3) definate, distinct power levels that do not significantly affect the "tone" of the amp.

I also tried LC's idea of a pot between the negatives of the bypass caps for "infinately adjustable" power....it works, too. I found a 5k linear pot worked best, but most of the useable sweep is at the last 25% or so of the dial....maybe a reverse audio pot would work better(?) UPDATE: After experimentation, I believe a 2k pot is best....see posts below.

Here's how to do the variable pot>>> (UPDATED to show switched pot)
« Last Edit: August 19, 2008, 05:56:14 am by da Geezer »
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Offline phsyconoodler

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Re: Disconnecting one output tube in a push pull amp
« Reply #33 on: August 18, 2008, 10:16:07 pm »
So tell me please how this works?The 5k pot seems to be a  better way than the DPDT switch,unless the pot fails.The ground reference for the bypass caps goes backwards?
 No comprende???
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Re: Disconnecting one output tube in a push pull amp
« Reply #34 on: August 19, 2008, 05:52:20 am »
Quote
So tell me please how this works?

I honestly don't know, I just know that it does work!
When the cap negatives are tied/connected together (NOT grounded, just connected to each other) the amp has full power as normal. If any amount of resistance is placed between the negative ends, then attenuation begins.
I experimented with different values (100Ω, 220Ω, 470Ω, 1kΩ) and each value gave an incremental step of attenuation.
At 2kΩ, there is only a slight noticeable increase in volume over the total disconnection of the cap negatives. Therefore, I think a 2kΩ pot with a spst switch would be perfect (Mouser actually sells one).

I envision it working like this:
*With the pot completely CCW & the switch "off", the circuit would be "open" with maximum attenuation (lowest volume).
*Turning the pot slightly CW as to turn the switch "on" would "close" the circuit & bring the pot's max 2kΩ resistance into play, giving a very slight increase in volume.
*As the pot is turned further CW, resistance is decreased, thus increasing the volume.
(I have updated the drawings above to reflect this change)

I have found the PPIMV on this amp is no longer needed....I just leave it all the way up. That way the power tubes are receiveing the max signal from the PI & the tone is unaffected.

Quote
unless the pot fails.The ground reference for the bypass caps goes backwards?

No...if the pot fails "open", then the volume will be at max attenuation. If the pot fails "short", then the amp will be at normal full volume w/ no attenuation.
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Offline Twanger

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Re: Disconnecting one output tube in a push pull amp
« Reply #35 on: August 19, 2008, 06:14:08 am »
Looks like this trick is very promissing... but does anybody really understand what it does ?? it's quite confusing to me.

John

Offline phsyconoodler

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Re: Disconnecting one output tube in a push pull amp
« Reply #36 on: August 19, 2008, 10:19:37 am »
What about disassembling the pot and scraping off a small area of the carbon trace?That would act like a 'switch' when fully CCW.
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Offline HotBluePlates

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Re: Disconnecting one output tube in a push pull amp
« Reply #37 on: August 19, 2008, 11:22:57 am »
Looks like this trick is very promissing... but does anybody really understand what it does ?? it's quite confusing to me.

Jeff detailed the basic action. The excerpt from ToneQuest is talking about how a single tube using an unbypassed cathode resistor has negative feedback due to the current flow through the resistor. The voltage developed across the resistor is directly proportional to the amount of current flowing, and that current is wobbling up and down during a signal. That means your bias voltage is also wobbling up and down, with the end effect of counteracting the current the tube is passing due to an input signal. Hence it is "negative feedback" but at one given tube stage.

Normally, we counter this effect in a preamp tube by bypassing the cathode resistor, which bypasses the alternating current around the resistor and keeps the voltage across the resistor steady. It restores gain lost due to negative feedback.

In a push-pull output stage, you could use a shared cathode resistor, and not need a bypass cap. If you assume somewhat matched tubes, then during a signal swing one tube is conducting more current while the other tube is conducting less current by more or less the same amount. The total current passing through the resistor remains unchanged, and so the voltage across the resistor remains unchanged. Therefore, there is no negative feedback effect, even without a bypass cap.

All I have just written is a restatement of exactly what Jeff already said.

So tell me please how this works?

Well, if we don't use a common cathode resistor, then there will be negative feedback at the cathode due to the current flow during a signal. We would have to bypass each resistor to eliminate the negative feedback and have full power. If we did not bypass the resistor, the negative feedback due to the resistor reduces the current swing through the tube due to signal, but that also reduces the voltage swing at the plate of the output tube, because the voltage swing is caused by a current swing through the primary impedance of the output transformer.

In the end, less current swing times less voltage swing equals less power output.

What about the bypass caps? If we bypass, we get full power. But what's up with the funky effect of connecting the negative ends together??

Look again at Geezer's drawing.



If the negative ends of both bypass caps are grounded, are they not effectively connected together? Yeah, they are connected to true ground too; however, if you disconnect the negatives from ground, but keep them connected to each other, they have a "virtual ground" at their junction and have the same end effect. In fact, 2 caps connected in this manner are essentially in series, and the net value is half of the value of each individual cap. The pair of (maybe 22uF) caps could be replaced with a single 10uF cap. This idea (a pair of bypass caps being replaced by a single cap of half-value connected between the 2 non-grounded points to bypass) has been used before by Quad amps and was explained by Morgan Jones in Valve Amplifiers.

If you wanted to replace the pair of bypass caps, you'd want a non-polarized cap, but you'd need ~10uF which might be bulky in a film cap, and a non-polarized electrolytic is basically 2 back-to-back electrolytic caps in one package (and much more expensive). If you did replace with a single cap, I'm not sure how effective a single pot would be from one leg of the cap to a single output tube cathode, but it could be tried out for the sake of experimentation.

Anyway, a single pot between the 2 caps as Geezer shows destroys the bypassing effect due to a virtual ground as the resistance is increased. In the end, this is atypical, fairly unusual, but not voodoo or make-believe. Pretty cool rediscovery/re-application if you ask me.
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Offline The Radium King

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Re: Disconnecting one output tube in a push pull amp
« Reply #38 on: August 19, 2008, 11:33:26 am »
what i figure is happening is the bypass caps are actually acting as coupling caps to scrape dcv off the signal at the cathode of one power tube and use it to limit bias at the other power tube. the 2k linear pot and the cathode resistor of the other tube form a voltage divider - when the pot is out of the circuit,  bias voltage is the same at each cathode. you put the pot in the circuit and you impose a less negative bias voltage due to the ratio of the 2k pot and the cathode resistor - less output. it works due to the fact that the tubes are out of phase class ab operation (as previously stated) - one tube is idling while the other conducting. with no signal both tubes are at idle, so i figure the caps are also there as bypass caps to allow you to do your bias calcs for each tube individually.

HAH! HPB entered an essay while i was typing this ... i'll leave mine anyhow; i think we are saying the same thing.

Offline Twanger

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Re: Disconnecting one output tube in a push pull amp
« Reply #39 on: August 19, 2008, 12:39:51 pm »
thank you both, i think im starting to get it... it's probably gonna take me the rest of the day to digest HBP post  ;D

John

Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #40 on: August 19, 2008, 02:39:15 pm »
I have posted the drawings in the schematics section to have quicker access for those wanting to try this attenuation system.

Geezer

http://www.el34world.com/Forum/index.php?topic=4437.0
« Last Edit: August 19, 2008, 03:01:36 pm by da Geezer »
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Offline panhead

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Re: Disconnecting one output tube in a push pull amp
« Reply #41 on: August 19, 2008, 03:10:02 pm »
Does it work well with Class AB or just Class A?
Panhead

Offline Fresh_Start

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Re: Disconnecting one output tube in a push pull amp
« Reply #42 on: August 19, 2008, 03:32:06 pm »
Does it work well with Class AB or just Class A?

Good question.  Re-reading HBP's post, it almost seems that this approach would work better with Class AB than with Class A.  My theory, if you can call my mutterings a "theory", is that in Class AB the tubes take turns handling the signal and generating current.  However, in Class A both tubes would be drawing the same amount of current all the time.  Does that mean the plan won't work as well with a Class A push-pull amp?  Don't know.  LooseChange's drawing showed only one cathode resistor being bypassed and one tube being gradually "turned off" IOW swamped with negative feedback.  (That sounds bad but is "good" in this situation.)

Chip
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We have proven once again no plan survives contact with the enemy, or in this case, with the amp.

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Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #43 on: August 19, 2008, 07:53:33 pm »
I like this mod so much that i am going to install it on at least 2 other amps.....a 2x EL34 amp that is currently fixed bias (but soon to be cathode bias) and my Sluckey November.
Both amps sound fantastic, but I never use them because they are just too loud for most of the venues that I play.
I'm ordering a few of the 2.5kΩ pots to have the "infinately variable" version on these amps....can't wait to see if it works as well on these as it did on the 1st one!
   Cunfuze-us say: "He who say "It can't be done" should stay out of way of him who doing it!"

Offline tubenit

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Re: Disconnecting one output tube in a push pull amp
« Reply #44 on: August 19, 2008, 08:56:42 pm »

DaGeezer,

THANKS for your contribution to the forum and developing the schematic information on this and sharing your experiment!

I am very appreciative of the creative ideas and innovative thinking/approaches you bring here. The variable version (pot) sounds like a cool idea.

Keep coloring outside the lines!

With respect, Tubenit

Offline VMS

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Re: Disconnecting one output tube in a push pull amp
« Reply #45 on: August 20, 2008, 06:35:03 am »
Hey guys!

This is VERY interesting thread and it got me thinking that since we have individual bias resistors could we use mixed pair of tubes (for example 6L6 and EL34)?

Also LCs drawing gave me an idea of a balance pot. I have no idea if this would work or not but i drew it anyway. All comments are welcome.

Offline LooseChange

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Re: Disconnecting one output tube in a push pull amp
« Reply #46 on: August 20, 2008, 08:04:39 am »
Disconnct the bottom leg of that pot and that is exactly what I did.  It works really well and after doing that Blackheart conversion I'm liking the single ended EL84.
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Offline rzenc

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Re: Disconnecting one output tube in a push pull amp
« Reply #47 on: August 20, 2008, 08:13:31 am »
HI there!!!!!

After reading this topic....it took me to realize that this would be well regarded in a 100w amp....which could actually be switched to cathode bias and then through the attenuation steps when in cathode bias mode....actually I thought abouth it since I`m building a mean machine and this could be a very useful to bring it to acceptable volume levels in a small club...even though this could still spill blood....
Any ideas of how to do it?? I`ve been trying to figure out....should i use ONE RESISTOR PER CATHODE? and swithces to engage them one at a time? or could I use ONE RESISTOR for each opposite pair??? may sound dumb but 4 tubes always screws things a bit....

I should mention that this well be a 4 - 6550 with a switch to go from Pentode to U.L. and a switch to fixed/cathode bias...and i would like to install it...what you guys think??

Thanks for sharing this info

With Respect
Best Regards

Rzenc

Offline Geezer

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Re: Disconnecting one output tube in a push pull amp
« Reply #48 on: August 20, 2008, 08:34:17 pm »
While still experimenting with the attenuator, I found a better way to wire the switch which (1) eliminates the "Single-ended" hum, and (2) allows you to choose any power level you wish for the "medium" power setting.

The 330Ω resistors shown on the following "improved" drawing gives ~1/2 power. By increasing or decreasing that value, you can choose slightly higher or lower power output.
For example, increasing the value to 470Ω would yield slightly less output, while decreasing the value to 220k would give slightly more power. The more the caps are isolated from ground, the less output power is generated.

I'm also posting these in the schematics thread......
   Cunfuze-us say: "He who say "It can't be done" should stay out of way of him who doing it!"

Offline Fresh_Start

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Re: Disconnecting one output tube in a push pull amp
« Reply #49 on: August 20, 2008, 09:28:04 pm »
It is so cool that you guys are thinking outside the box and sharing your ideas.  Many thanks!  :D

Chip
Quote from: jjasilli
We have proven once again no plan survives contact with the enemy, or in this case, with the amp.

Quote from: PRR
Plan to be wrong about something.

 


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