pulling two tubes in a 100w amp?

[Randall]

Is there an argument against pulling two of the four tubes (either the two outside or two inside) in a Marshall 100 watt or Fender Twin to lower the power output?

[g1]

Not arguments, but considerations, the impedance settings will no longer be correct as labelled. By pulling 2 power tubes, you double the impedance on the primary side, so for proper matching, you should also double it on the secondary side. This means running into a load double the impedance stated on the amp (or double what the original load impedance was).
A second point is that you are reducing the current drawn from the supply, so heater and HT voltages may rise. This should not be a problem unless filter caps have voltage ratings too close to the supply voltages. So for example, if you had 450V on 450V rated caps, removing a couple tubes could result in having more than 450V sitting on those caps.

[Leo_Gnardo]

Is there an argument against pulling two of the four tubes (either the two outside or two inside) in a Marshall 100 watt or Fender Twin to lower the power output?

Ideally you'd double the speaker impedance the amp's driving when you do that. In Marshalls, run 8 ohms from the 4 ohm tap, 16 ohms from 8 tap. Power a little more than half what it was. And practically, only a little less loud. You can get a better power cut by running the outputs in triode instead, typically loses about 2/3 power, also less sensitive so you have to push the preamp harder, also rounds off the high frequencies so the amp's not so bright, iow triode mode gets that "warm tube tone" we're s'posed to expect from tubes.

No argument against. Lots of "100W" Mesa Boogies have a half power switch that does lifts 2 cathodes off ground. They don't say much about compensating by speaker load adjust, but that doesn't mean you can't do it, say run 8 ohms off 4 tap.

[g1]

To further complicate the issue, some will leave the impedance mismatched for further power reduction. Maybe that's why Boogie does it that way. Depends on the particulars I guess, I wouldn't chance it with a Marshall running full tilt (or beyond).

[The Dude]

......I wouldn't chance it with a Marshall running full tilt (or beyond).

Especially a Marshall with Dagnall transformers. Those things run hot as hell as is.

[Randall]

So would a way to remember this be, "half the tubes, half the impedance selector"? I need something simple to remember for my customers who ask me this question.

[The Dude]

I think "half the impedance selector" leaves too much room for interpretation. It could be taken to mean set the impedance selector to half of the speaker load, or it could be taken to mean load the amp to 1/2 of what the selector is set to. Maybe "double the load" instead?

Also, it should be noted (and Leo alluded to it), this is not going to make the amp 1/2 as loud. The difference will be only slightly noticeable.

[DRH1958]

If it's too loud you could sub in a 12AY7 for a 12AX7. This will reduce preamp gain so the sound might be different than you want but it's the cheapest and most easily reversible mod. Might be worth a try.
For a 100 watt amp to be half as loud, you would need to go down to 10 watts. Mostly, reducing to 50 watts just lowers clean headroom, not much overall loudness.

[Enzo]

Don't say vague things like half into half to customers. Just tell them, "Plug the 8 ohm cab into the 4 ohm jack."

[mhuss]

I've always wondered about lowering the attached speaker impedance. The fact that you're also halving the current going through the OT (more or less, depending on the actual load) means it will be under less stress from a heating point of view. If the OT output side reflected impedance is double the load, I imagine the peak voltages could be higher than normal, but is 2x or 4x normal VAC an issue for the wire normally used? (As opposed to open circuit which would be a lot higher?) The advantage of mismatching would be to reduce the output power even further, of course.

Naturally it's a personal taste thing, but I never liked the sound of overdriven output tubes strapped as triodes as much as the normal sound of pentodes/beams.

[Leo_Gnardo]

Naturally it's a personal taste thing, but I never liked the sound of overdriven output tubes strapped as triodes as much as the normal sound of pentodes/beams.

I've gotten some good reviews of going triode, especially from players who entertain in small clubs in venues. Even a RI Deluxe Reverb, dropped to 6 watts at clipping & softer tone when overdriven has gotten "saved my career" and "guys in the band aren't going to throw me out now" statements. So it's courses for horses, those who like it that's cool, and those who don't that's cool too.

[mhuss]

"guys in the band aren't going to throw me out now"

Heh, yeah, I understand that. I've given up on 'natural output distortion' in small clubs and just use pedals now.