3 tubes push pull

[Colas LeGrippa]

In order to raise the power of a pp amp, I plan to add a tube on one side .  So, two tubes in parallel on one side and one on the other side. Both sides cathode biased separately.
It would be more or less like an amplifier with two different power tubes. I know  both trannies are strong  enough for 3 tubes, at least. Right now, the amp is a 50w copied from a Marshall 2204.
The unbalanced created by such different output loads would be manageable ???


Thanks for comments..


Colas

[HotBluePlates]

In order to raise the power of a pp amp, I plan to add a tube on one side .  So, two tubes in parallel on one side and one on the other side. ...
The unbalanced created by such different output loads would be manageable ???

In theory, the output transformer's core can only handle a certain amount of "unbalanced DC," as that reduces the transformer's ability to transmit audio from the primary to the secondary.

Single-ended transformers cope with this by adding an air-gap, and by using a bigger & heavier core for a given amount of power through-put.
Push-pull transformers are built with smaller/lighter cores for the power through-put, because they assume DC on one side of the primary will be roughly balanced by DC on the other side.

Will this matter in your case?  You'll have to try it & find out.
  - It could be that bass is shaved at high power outputs.
  - You might encounter some other undesirable results... or not.

[Raybob]

I built an amp using 3 power tubes but I used 1x6L6 and 2x6V6.  The Two 6V6s are parallel push-pull against 1 6L6. I used fixed bias with two bias pots.  Came out good.  You can see mismatch on the scope but I believe it has slightly more headroom than a similar amp using 2x6L6s.

[sluckey]

In order to raise the power of a pp amp, I plan to add a tube on one side .  So, two tubes in parallel on one side and one on the other side. Both sides cathode biased separately.

Don't be surprised if you get a lot of power supply hum.

The unbalanced created by such different output loads would be manageable ???

Easy. Just add one more tube to the other side.   

[Colas LeGrippa]

Yeah that's what I'm expecting, a lot of hum...
But I got to find out.


Thanks for comments


Colas

[tubeswell]

load resistance will be funky. In Class B when the side with the single power tube is in cutoff, you’ll have the pair of power tubes on the other side conducting 2x as much plate current into 1/4 of the load, and when the cycle is reversed you’ll only have 1 tube’s worth of plate current going into 1/4 of the load, making every other amplitude cycle different in size. And the rest of the time in Class A when all tubes are conducting you’ll have uneven current on opposite sides skewing the output signal. So I think there will be some weird Fourier transformy thing going on. Will it sound ‘good’? Ummm… 

[HotBluePlates]

In order to raise the power of a pp amp, I plan to add a tube on one side .  So, two tubes in parallel on one side and one on the other side. Both sides cathode biased separately.

Don't be surprised if you get a lot of power supply hum.

The hum is what we assume, but I tried a severe mismatch intentionally in a Super Reverb.  One side was pulling 6.7mA while the other pulled 67mA.



The amp sounded normal when I played through it at moderate volume:

https://soundcloud.com/hotblueplates/mismatched-tubes

[Colas LeGrippa]

Hbp  the tone is cristal clear..
A certain unbalance sounds good, even severe unbalance as I can hear.  I never buy matched tubes, the extra.money  spent is useless.


Thanks for interesting comments.


Colas

[Willabe]

I had a BF Dual Showman years ago and a single 6L6GC went out, amp sounded very strange to my ears. Strange type of distortion.

It was at a gig, turned the amp on and I heard it as soon as I started playing. I didn't know what was wrong with it until I took it in to be fixed. There was no problem with hum, just sounded terrible.

[pdf64]

I had a BF Dual Showman years ago and a single 6L6GC went out ...

To be able to assess whether your scenario was applicable here, the exact failure mode of the valve that went out would need to be known.
As how the amp sounded with the bad valve still in there might not be the same same as how it would sound if it was removed.

[Willabe]

The tubes heater died.

[pdf64]

Thanks, fair enough, in the likely absence of any other weird faults, I agree that should be electrically equivalent to operating with that valve removed.