Express with 1 output tube

[Mont]

Is it possible to run an express with only one output tube? I have a gig coming up at a small club and I don't want to be banned for life.

[Jana]

No.

Even if you could, pulling one tube isn't going to drop the volume from "banned for life" to "we love you."

[Bruciep07]

Here is a direct quote from Kevin O'Conner...

Removing tubes from a multi-tube fixed-bias output stage is never a problem. You can remove any number of tubes, and yes, that means you can take one tube out of a two-tube amp; one, two, or three out of a four tube stage, et cetera. This sounds heretical to techs stuck in the mire of convention, but it is something that has been known since tubes were invented.
The even-number tube extractions reduce power symmetrically. Neither the tubes nor the transformer will be damaged. Power will be reduced and so will frequency bandwidth - you will lose some bass and some treble. This is the point that switching the impedance selector to a less-than-load setting is supposed to correct, but it is completely subjective whether you should. The only 'should' of the matter, is do I like it this way, or do I like it that way?
In the uneven tube extractions, asymmetric power reduction occurs. Conventional thought says "the one tube on one side of the circuit will be trying to match the output of the two tubes on the other circuit half". This is wrong. The single tube can only produce so much power, and that's all it does. It doesn't melt down. The transformer does not blow up.

Again, this is not my explanation... However, volume wise, like Jana said, pulling one tube wont give you a dramatic drop in volume, best bet is to try an in-line attenuator for now, though there is a price to pay on tone...

[M Fowler]

For this up coming gig try 6V6 tubes rebiased and work the volume control on the amp and guitar. Use a single 1-12 cabinet rather than 212 or 4x12 and you should be fine. Use open back 1-12 and sound will disperse out the rear as well. Also raise the amp nearer to your ear shot which makes you turn down as well.

[dave g]

What KOC doesn't mention is that by pulling one tube in a push pull amp, you no longer have equal and opposite currents flowing out of both sides of the primary, so the core is well on its way to being saturated even at idle. That's not a good thing.

Oh, and not to mention that in a push-pull amp each output tube is biased really close to cutoff because it's only meant to amplify one half of the incoming waveform.

I wouldn't do it.

[Bruciep07]

Agreed with both posts above, just thought the KOC read was interesting... I've had good luck with the Airbrake I built, though too much ATT and it makes my Express sound "raspy"...

[Structo]

Yeah, I read that O'Connor comment before and was like, WTF?

I don't think I would try that......

[Mont]

Thanks for the heads up, I think the amp will be facing the wall, and be in a box.

[Cliff Schecht]

What KOC doesn't mention is that by pulling one tube in a push pull amp, you no longer have equal and opposite currents flowing out of both sides of the primary, so the core is well on its way to being saturated even at idle. That's not a good thing.

Oh, and not to mention that in a push-pull amp each output tube is biased really close to cutoff because it's only meant to amplify one half of the incoming waveform.

I wouldn't do it.

This! Push-pull transformers have no air-gap in the core and therefore will saturate much quicker than a core with an air gap (SE transformers are gapped to support DC without saturating). A push-pull amp with one of the output tubes pulled is no longer getting cancellation of the DC in the core and therefore will saturate much quicker. While you may not blow anything up (you're pushing less power), you are still saturating the core and dramatically reducing the bandwidth of your amp. I can't say that I would recommend this method either!

[funkmeblue]

what if you were to just disconnect the output of one of the tubes?

[dartanion]

You can do a SE/PP control, which grounds out the input to one power tube through a pot. Both tubes are still in the circuit, so you don't have any issues with core saturation or impedance mismatching. Quite simple to implement, yet it is more of a tone shaping tool than reduction of output.

I would also reiterate that it's not a good practice to just pull one tube in a push-pull amp.

[guitardude57]

Nope ...Nope ...Nope............I wouldn't do it. Yank a tube. Nope.

6V6's like Mark said.

Or Yellow Jackets with smaller tubes, maybe would slightly reduce volume. But in any case, will change the tone.

[Cliff Schecht]

An L-pad attenuator at 4-6dB is about perfect for the Express IMO. You lose a bit of speaker distortion but the amp itself still sounds great. You can get the full on distortion without causing hearing loss to your bandmates.

[paulster]

One word: attenuator. Or two: Faustine Phantom!

[dave g]

I still think VVR is the best method of reducing the volume in these amps.