Running an amp on one tube

[alvodicka]

About a year ago or so I scored a Friedman be50 for a steal of a deal because it wasn't working right.

Dave F. walked me through the replacement of a bad screen resistor for one of the power tubes. Now it works proper and is great, super clean clean.

When the amp was running only one power tube it roared on the clean channel. Really amazing full tube distortion at lower volumes... One tube...

My question:
Is there potential amp damage if I pull one of the power tubes to experiment and take advantage of the lower volume distortion?

Yes this amp has two further gain channels. But they are not the same quality because they are component based not full tube distortion.

[martin manning]

Is there potential amp damage if I pull one of the power tubes to experiment and take advantage of the lower volume distortion?

Removing one of the two tubes turns the amp into a single ended type, with the remaining tube running at an off-center operating point. That's where the distortion is coming from. Half of the waveform is severely clipped. You also eliminate the standing (idle) current from half of the OT primary, so it is no longer cancelling the magnetic flux produced by the current on the other side. A push-pull OT is not made for that, and the core may take on some magnetization and loss of performance. Probably not a good idea.

[Stevem]

You can get within 70% of that same effect by greatly unbalancing the PI section .
The amp will still be louder though since both output tubes will be running.

[FUCHSAUDIO]

Peavey had a knob on their amps (I honestly forget what it was called, although "suck knob" would have be a logical choice), that did exactly that.
Basically a volume control with an additional coupling cap, which turned down one side of the PI output. Like a PPI master, only half-assed.
The one benefit is the power tubes are both still pulling current, so you get a somewhat better use of the output transformer, since both sides draw current.
It sounded pretty bad tho... We actually had a repair customer with a Crate that he ran on a single power tube, and swore it was "the perfect tone"...
It fell out of the amp on the way to a gig, broke and he loved the end result. Much like the stories about the creation of Fuzz in the 60's to "model the sound of a broken amp)...

[Colossal]

A push-pull OT is not made for that, and the core may take on some magnetization and loss of performance. Probably not a good idea.

+1

Push-pull OTs are smaller and cheaper than SE OTs because they do not suffer magnetization current/DC offset and not (usually) intentionally gapped to deal with this. But sonic beauty is in the ear of the beer-holder, so use at your own risk/discretion.

[alvodicka]

Awesome, thanks for the info. I'm going to leave it as is. It's broken but it sounds so good, was mysteriously fun for the short while. And I have been curious ever since. Cheers.

[10thTx]

London Power "The Ultimate Tone" books have a design called a Soma 84. I cannot remember which book? It uses a concept with a concertina PI to simulate a PP to SE idea.

With respect, 10thx

[10thTx]

Perhaps this would be reasonable to consider as a "half of a PPIMV" ?

With respect, 10thtx

[Colossal]

That's not how Peavey did it, but you can run one grid to ground, like that.

[roberto]

Brunetti did something similar on some of his amps. One system is the "powerlimit": https://www.brunetti.it/wp-content/uplo ... an-ENG.pdf
The other one was a PPIMV on one of the two antiphase signals of the PI, plus other switches.
It was then possible to unbalance the output while keeping the idle current.

[bepone]

mesa 5:25 also, but i never liked