Pulling tubes for lower output power

[MonkHouse93]

A silverface Twin Reverb with the master volume has it's 4 6L6's in the power amp. Sometimes I pull out a pair of tubes so I can get a little more drive out of it without being so loud. I never thought about it until now, but it's a class AB amplifier right?
If i pull out a certain pair of tubes, could it become a class A amp?
http://www.ampwares.com/schematics/1...F_Twin_Rev.pdf
like if I pullsed out tubes 7 and 8 and left 9 and 10 in would that happen? is it better if i took one out from each side, like 7 and 9?

[Chuck H]

Operation class is a bias condition that is partly dependant on drive voltage. With the two left, or right tubes pulled you can get the amp to operate class A on very low volume settings because the drive voltage is low enough to NOT send the tube into cutoff, a needed aspect of class AB1 operation that the amp is designed for. Past that the drive to the power tubes, relative to bias condition, will cause the amp to operate in AB1 again but with only one side conducting and could possibly damage the amp.

When pulling tubes for lower power it's best to observe the amps designed operating class, which means pulling either the two inner or the two outer tubes AND making an impedance adjustment. IIRC a Twin Reverb uses two 8 ohm speakers in parallel presenting a 4ohm load to the amp. When using only two tubes you'll want to use only one speaker presenting an 8ohm load.

FWIW cutting your wattage in half DOES NOT cut your volume in half. Far from it actually. But you should find that using only one speaker helps reduce volume as well for an overall greater decrease in volume.

If you want "bedroom" level from that amp I would suggest a good attenuator.

[Gtr_tech]

Right. Running only one side of a push pull output transformer ain't a good idea. They're not airgapped to avoid DC saturation like a single ended OPT is. Don't do it......

[MonkHouse93]

Hmmm, good points. I was intending to use it for small gigs. I was hooking a champ up to a 2x12 cab, which just barely cuts it, so I figured I'd try this instead. I know this wouldn't half the volume, but it does give it a little more drive at a low volume. and what's the reason for bringing the impedance up?

Thanks for these responses, I woulda tried cranking it like that :|

[Chuck H]

what's the reason for bringing the impedance up?

Two tubes require twice the impedance of all four to stay within the designed parameters of the circuit. You could run the amp safely with only two tubes into both speakers (as in, nothing will blow up) but it does present a 100% impedance mismatch to the output tubes and reduce tube life. Also, IIRC, two speakers together are 3dB more efficient than just one. The 3dB reduction in efficiency would be like halving your watts again. A Twin running on two output tubes with one speaker disconnected is about half as loud as the stock amp.

[pops]

If you like the sound of your champ, make a line out off the speaker jack and line it into he P.A. or your twin in the no 2 input jack. Sounds like a champ, but with either it can be loud as you need.

[MonkHouse93]

Couldn't that potentially harm the PA or the twin coming from the speaker jack?

[MonkHouse93]

I tried it anyway, it gets REALLY loud

[Chuck H]

No... A speaker driven line out is a voltage divider circuit that reduces the signal you'll send to the PA or a slave amp. Any potential harm from wattage is pretty much abated by the circuit, but even if it werent the high input impedance of a PA or slave amp would keep current to a minimum and no harm would should result. However... Plugging an amps output directly into a PA or slave amp without a speaker load will very likely damage the first amp because there is no proper load for the output tubes.

You can learn a good bit about speaker driven line out circuits by searching here and Google. Keep in mind that a speaker output is a current dependant circuit where the peripheral line out is a voltage dependant circuit. This will make understanding the application much easier.

[MonkHouse93]

Oh I misunderstood at first. . . Whoops! So it's like the jack is in series with the speaker?

[Chuck H]

The circuit is in parallel with the speaker jack. But the impedance of the circuit is such that it does virtually nothing to the actual amplifiers speaker load. A good basic line out circuit would be a 2.7k resistor right off the speaker +jack to the ungrounded end of a 500ohm pot. The CTR of that pot goes to the +terminal on the line out jack and of course the other pot lug is grounded. This allows you to adjust the output for most speaker loads and settings via the pot. IME most PA inputs aren't that sensistive and this circuit works just fine. It has the disadvantage that the host amp must be playing in order to power the line out. It has the advantage of fairly accurate representation of the actual tone of your amp. Works great on Champs and can indeed be plugged into either a PA channel or a "slave" amp (like your Twin).

[MonkHouse93]

Oh alright, I looked it up and the diagrams appeared to be parallel, but voltage divider made me think series :| series wouldn't have made sense though, I understand it now. It was a little late when I last responded, but thank you!