removing 2 pwr tubes in a 4 tube amp

[Mars Amp Repair]

Say,
I dare say this controversy has come up before, but I'm still a little unclear on how this works. I've drawn it out & still can't come up with the answer.

When you removed 2 power tubes on a 4 tube output stage & just use 2...which way do you move the impedance selector on the OT?

I understand that with 2 tubes loading the OT instead of the 4, you are now mismatched upwards by a factor of 2 (tubes are presenting 2wice as much impedance to the OT as with 4). Does this mean you have to DROP the output matching to try & compensate...IOW, with 8ohm speaker, now set the impedance selector to 4?

I know there have been other threads on here about this, but I can't figure a good keyword to find them.

I'm confused. thanx, g.

[Bruce / Mission Amps]

Yes, the speaker load is what changes the primary impedance.
The tranny is really only transforming the load...
If the four power tubes (you didn't say which kind) were looking for a 4Kohm primary, the OT has taps for 2, 4 and 8 ohms, and you have an 8 ohm speaker load now, then only two power tubes with the same OT would like to see 8Kohms and you would run the 8 ohm load on the 4 ohm tap now so the primary impedance would be doubled.

[Mars Amp Repair]

thanx Bruce,
That's were my logic led me. g

[Amp Kat]

This is true however, usually the main reason for pulling two tubes is to attenuate the amp which you only get -3dB at the very most and now switching to 4 ohms makes it louder so you've almost achieved nothing by pulling the two tubes.

[Mars Amp Repair]

earlier breakup

HMMM,
I thought the whole idea was that only 2 pwr tubes would break up at lower volumes than what you'd get with 4...Hey, whatta I know...I'm just a bass player living in a guitarist's world! g