Disconnecting one output tube in a push pull amp

[mister ed]

I tried this on my cathode-biased AB763/reverb.... 560 ohm resistor/25@50 cap on each 6L6, ( 2 6L6s' )... I found that this mod DID drop the output, but in a way that made it sound thinner. The tone was not the same. The effect is like switching-out pre-amp bypass caps. Possibly I'm missing something ?

[rzenc]

Hi...

I came up with this schem for 4 POWER TUBES

What you guys think?

With Respect
Best Regards

Rzenc

[CraigB]

Here's a layout of Geezer's improved 3-selection switch.  Looks better if you zoom in.  Craig

[phsyconoodler]

I just tried the attenuation on an 18 watt lite I just finished and it indeed works.It's hard to say how much it drops the wattage down because the 18 watt is low wattage already,but it dropped it quite a bit.The tone is in debate because I couldn't crank it up yet,but the cleans seem to be alright.
  I'll reprt back later today with a real report on the tone differences.

[phsyconoodler]

I tried it at all volume levels and it definitely works fine.I just used a switch for the caps and it doesn't pop even with no resistors.The low watt sound is good but not outstanding.For bedroom gigs it gets an A.
   It works very well with a strat and not bad with humbuckers.I have a PPIMV on this amp too and that rounds it out nicely.

[theundeadelvis]

Hey everybody! I'm finishing up a fairly straight forward 5E3 and would like to implement this attenuation method. I currently have a 25uF 50v cap and 270ohm 5watt resistor in place. Do I just separate the cathodes of the 2 tubes and add a second cap and resistor of the same values for the other tube? Or do I need to change the values of those components? Thanks in advance!

[phsyconoodler]

You need to double the resistor value with separate ones.Thw same value cap will be fine,but usually you halve it when using two.

[tubenit]

Does the attached drawing of a DPDT switch look correct here??  I have actually wired one up on a friend's amp and it worked but not sure I have drawn up how I wired it????

Please confirm or correct the drawing, thanks. If it's incorrect can you post a DPDT drawn up in this style to show what it would look like.

Would both of these work as I have drawn them???

I'm trying to draw the one on the top right to be the same as DaGeezer's on the bottom.

With respect, Tubenit

[VMS]

Hi, I think you are missing the caps to ground connection on the full-1/4power switch.

The other one seems to be ok.

[Geezer]

VMS (& his added drawings) are indeed correct.

T, how did the mod sound on your friend's amp?

G

[tubenit]

VMS ........ THANKS for your help it is appreciated!

DG, the amp sounded GREAT with the mod. As I heard others say, there was little to no tone difference which I liked alot. It was on a Carolina Blues Special (Bogen conversion) that belongs to my friend (incredible guitarist) who lives up there within a few miles of you. He bought a condo and needed even lower volume to continue playing without the neighbors complaining.

While the wiring values I used were called 2/3 & 1/8 ....... it sounded more like 2/3 and 1/4 to me.

Very useful mod. I'm grateful that you shared your success with it on the forum.

With respect, Tubenit

[Frankenamp]

This idear has really got me curious: I see it implemented with a pair of EL84's- will it work with 6L6's (such as my Red-Headed Stepchild)? It looks simple enough that a ham-handed tube novice such as myself. How would that affect the adjustable bias mod? (Or is this too complicted fer my pea-brain?) ;)

[sluckey]

This circuit applies to a cathode biased amp. I think your stepchild is a fixed bias amp.

[VMS]

Glad to help!

This mod is something I'm definitely going to try in some future build.

Maybe 6bm8 amp with even lower wattage.  :)

[Frankenamp]

Hmmm, (doin' my best Arsenio Hall impersionation)

http://www.el34world.com/charts/fenderservice5.htm

I think the above link details how to change from fixed to adjustable bias. Is this the same thing? Or is my profound lack of knowlege rearing its ugly head (again)?  Got my money on 'ignorance'- but I could be wrong! ;D

(Yer gunna have to think of me as that kid in the corner with the thick crayons)
 ;)

[sluckey]

I think the above link details how to change from fixed to adjustable bias.

Yes. But it doesn't change to CATHODE bias.

Fixed bias applies a negative voltage to the control grid in order to bias (control) cathode to plate current.

Adjustable bias is a form of fixed bias with a pot to fine tune the negative voltage applied to the grid, thus providing fine control to the cathode to plate current.

Cathode bias uses a resistor between cathode and ground to develop a positive voltage on the cathode that will control the cathode to plate current.

Your amp is fixed bias (even if you can adjust it) and this circuit applies to cathode bias circuits ONLY.

(Upper case used for emphasis, not shouting)  :)

[Frankenamp]

OK, that makes sense :)
So, Fixed = PLATE bias
Now, the sixty-four dollah question: is there a way to do something similar to a fixed (plate) bias amp? or is there an adjustable bias configuration that will work with my iron?

I wish I'd bet real money on my ignorance... coulda hada winnah.

(Upper case used for emphasis, not shouting)  :)

('s OK, I'm used to gittin' shouted at, wife does it all the time)  ;D
She sez it's fer emphasis too... but I don' think so unless it's followed up by the fryin' pan, now THATS emphasis!

[sluckey]

So, Fixed = PLATE bias
I've never heard of PLATE bias.

Now, the sixty-four dollah question: is there a way to do something similar to a fixed (plate) bias amp?
May have to think outside the box.

or is there an adjustable bias configuration that will work with my iron?
Sure.

[Frankenamp]

So, Fixed = PLATE bias
I've never heard of PLATE bias.

Don't mind me I'm either low on caffiene, or stuck on stupid... same difference, just tryin' to hone my 4H electrical skills ;)  Just making a gallant attempt to follow the bouncing electron with my thick crayolas.

[tubescent]

just wanted an update:
how many of you guys are still using this mod and happy with the sound?
also any problems with wear on the transformers? (someone mentioned to me it might be an issue with this mod)
thoughts much appreciated before i go ahead and drill my 5e3

[chocopower]

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

In the cathode biased amp i tested coneccting to ground one of the tube grids and it work REALLY WELL. Even better thant split cathode resistor/cap with the caps switch.

How could it be done in a fixed bias amp?
I suposse that bypassing one of two 220k bias resistor will work, but i´m afraid about bias voltage going crazy.

any idea?

[passaloutre]

Is there any chance of recovering the images from this thread? I'm very interested in this topic. I think I understand the concept well enough to figure it out on my own, but I'd like to see the different implementations.

Also curious about the original question. If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?

[sluckey]

Is there any chance of recovering the images from this thread?

Not likely. Hoffman periodically purges forum images from his server in order to make room for new images.

If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?

No problems as long as you don't disturb bias. You can directly ground the grid of a cathode biased amp, but not so with a fixed biased amp. For those you'll have to kill the input signal some other way.

[passaloutre]

Is there any chance of recovering the images from this thread?

Not likely. Hoffman periodically purges forum images from his server in order to make room for new images.

If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?

No problems as long as you don't disturb bias. You can directly ground the grid of a cathode biased amp, but not so with a fixed biased amp. For those you'll have to kill the input signal some other way.

Just to make sure I'm understanding this correctly: the output transformer only becomes mismatched when both tubes are pulling different amounts of DC? That's why we ground the grid instead of lifting the cathode (or why we can't just pull one tube out of its socket)? This way both tubes are still drawing the same DC current, but one has the additional AC riding on it, which doesn't matter because the OT only cares about keeping the DC balanced between both sides? We would of course lose the hum cancelling effect of being push pull?

[sluckey]

We would of course lose the hum cancelling effect of being push pull?

No. The high B+ ripple will still cancel as long as each tube is drawing the same DC current.

[HotBluePlates]

... If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?

No problems as long as you don't disturb bias. You can directly ground the grid of a cathode biased amp, but not so with a fixed biased amp. For those you'll have to kill the input signal some other way.

Envision a Volume control ---> Coupling cap ---> Bias supply + Grid
Simply "turn down the volume" on one side of the push-pull output.  That side still idles, but has no AC-input.

... the output transformer only becomes mismatched when both tubes are pulling different amounts of DC? ... one has the additional AC riding on it, which doesn't matter because the OT only cares about ...?

Be careful with the word "mismatched."  Mismatched to what?

At lower power-output levels, each side of the push-pull output sees 1/2 of the total "plate-to-plate impedance" of the primary.  At higher output levels in Class AB, one side is cut off and the remaining side sees 1/4 of the total plate-to-plate impedance.  That allows the side that is on to flow more current for the same AC-voltage-drop (which is most of how Class AB delivers higher power output that Class A).

Turning off the drive to one side of the push-pull power section but retaining the DC idle current effectively never "shuts off" that side of the transformer primary.  The driven side always sees the "1/2 of the total plate-to-plate primary impedance."



Now whether that load is "optimal for max power output" or not probably doesn't matter, because the user wanted less power anyway.  And the tube doesn't really care what its loading is, it is simply attempting to pull current through the primary in response to an input signal.  Therefore "mismatching" is an arbitrary and/or academic notion, and best forgotten (it's common due to the way people first learn about transformers, but the concept is over-emphasized past a point where it stops being useful).

[passaloutre]

  Therefore "mismatching" is an arbitrary and/or academic notion, and best forgotten (it's common due to the way people first learn about transformers, but the concept is over-emphasized past a point where it stops being useful).

This is probably where I'm at. I was under the impression that push-pull transformers required some sort of equal-but-opposite current running through each side, and this was what made them different and more compact than single-ended transformers (something about an "air gap"...). Given that understanding, and reading through this thread, it started to sound like that requirement only applied to the DC, and not the AC.