help for a volume scalling for a twin reverb

[Bruce Night]

sorry for the same thread but i figured that it could be better to post it here too, anyway,

i'd like some help, i have a twin reverb 100w and i want to add a swich to scale it at 50w and 25w something like the thread for orange tt.

could anybody, please, give me directions about how to do it? some schematics or part that could help etc. thank you very much...

[Bruce / Mission Amps]

A very easy thing to do is, take one speaker lead off a speaker (using just one 8 ohm speaker) and pull two outside 6L6s using just two center 6L6s instead of all four.
If the power tubes are not biased too hot you can pretty much do this at will.
If you are looking for a mod to drop B+ plate and bias voltage, then there are other sites to show that using high power Mosfets to control the B+ rail and other circuitry to run in parallel with the voltage control to scale the bias voltage.

[Steve A.]

A very easy thing to do is, take one speaker lead off a speaker (using just one 8 ohm speaker) and pull two outside 6L6s using just two center 6L6s instead of all four.
If the power tubes are not biased too hot you can pretty much do this at will.
If you are looking for a mod to drop B+ plate and bias voltage, then there are other sites to show that using high power Mosfets to control the B+ rail and other circuitry to run in parallel with the voltage control to scale the bias voltage.

Bruce:

That is a real neat trick for Twin Reverbs because the single speaker matches the impedance of the two output tubes.

Just wondering what would happen if you switched in six 9 volt 5watt zeners to ground right at the Standby Switch- that should drop the 460vdc on the schematic down to maybe 406vdc. But would that put the fixed bias out of whack? I guess you could use a double pole switch and use the other side to correct the bias (if necessary).

Thanks!

Steve Ahola

[Bruce / Mission Amps]

Bruce:

That is a real neat trick for Twin Reverbs because the single speaker matches the impedance of the two output tubes.

Just wondering what would happen if you switched in six 9 volt 5watt zeners to ground right at the Standby Switch- that should drop the 460vdc on the schematic down to maybe 406vdc. But would that put the fixed bias out of whack? I guess you could use a double pole switch and use the other side to correct the bias (if necessary).

Thanks!

Steve Ahola

The amps don't really run a super high B+ so an amp with a set of four 6L6s running cooler bias might be fine with two 6L6s running slightly hot.
I put together an odd harp amp for a guy using four JJ6V6s in a Twin with the power tubes idling at about 10 watts each... it sounded great even with the OT mismatch... To be honest, later I did use an after market TMI inc., OT with 2, 4 and 8 ohm taps and ran the 4 ohm, 2x12 (two old P12Ns) load on the 2 ohm tap. It sounded almost as loud as the damn 6L6s amp with two C12Ns so, of course he started running the thing with only two of the 6V6s and idling kinda cool but ended up liking it best with the OT mismatched at 8K with the 4 ohm load on the 8 ohm tap! Go figure.

But with respect to the zener diodes, what I would consider is using a good grounded power cord and use a ground switch in a different way.
Hold the high voltage center tap up from ground with some zeners to lower the B+ and then use the ground switch to bypass them all when using all four power tubes.

[Steve A.]

The amps don't really run a super high B+ so an amp with a set of four 6L6s running cooler bias might be fine with two 6L6s running slightly hot.
I put together an odd harp amp for a guy using four JJ6V6s in a Twin with the power tubes idling at about 10 watts each... it sounded great even with the OT mismatch... To be honest, later I did use an after market TMI inc., OT with 2, 4 and 8 ohm taps and ran the 4 ohm, 2x12 (two old P12Ns) load on the 2 ohm tap. It sounded almost as loud as the damn 6L6s amp with two C12Ns so, of course he started running the thing with only two of the 6V6s and idling kinda cool but ended up liking it best with the OT mismatched at 8K with the 4 ohm load on the 8 ohm tap! Go figure.

But with respect to the zener diodes, what I would consider is using a good grounded power cord and use a ground switch in a different way.
Hold the high voltage center tap up from ground with some zeners to lower the B+ and then use the ground switch to bypass them all when using all four power tubes.

Bruce:

Hey, thanks! One more question- in regards to the fixed bias would it make a difference with the zeners inserted in the hi-vac center tap to ground or in series with the rectified plate voltage?

Also, if you scale down the plate voltage, should you also scale down the bias voltage, and in the same proportions?

I get a headache thinking about all of that math- and I used to be a math major!

Thanks

Steve Ahola

[Pedro Vecino]

Yes. Scaling bias is necessary.
For example, in a Dual Rectifier with the solid rect. (440 volts typically) and with 38 mA. in the plates, on having activated the tube rectification the voltage goes down to 400 volts approx. and the flow falls to 26/27 mA. If this relation is not automated it does not sound well.
Probably the first amplifier (more former that I know) with two different selectable HT voltages (460 and 230 approx.) that incorporates this automation (though badly done) is the Fender The Twin (red knobs).
Regards

[Bruce / Mission Amps]

Bruce:

Hey, thanks! One more question- in regards to the fixed bias would it make a difference with the zeners inserted in the hi-vac center tap to ground or in series with the rectified plate voltage?

Also, if you scale down the plate voltage, should you also scale down the bias voltage, and in the same proportions?

I get a headache thinking about all of that math- and I used to be a math major!

Thanks

Steve Ahola

Yes, but using a simple DPDT switch could do that by using the other half of the switch to slightly load the bias node down with an additional resistor across the bias supply's setting resistor.... and that would drop the -B a few volts when the switch is thrown to lower B+.

[mac1amps]

Twin Reverbs are probably one of the worst amps around IF you are looking for ANY kind of "natural" break-up (occasionally, I may hear a nice "blackface" that can break-up nicely at a "reasonable" volume, but they are few). The lower powered Fenders ALWAYS produce a much better sounding break-up (and without having to be as loud). IMO, there are MANY players that own Twin's that shouldn't. They would be much happier with a Super Rev., a Pro Rev., a Vibrolux Rev., or a Deluxe Rev. (depending on the ideal volume level). Don't get me wrong, Twin's are great amps IF you want to sound clean, or if you're the type of player who's content with the "processed" break-up you can get from a stomp box. There's also the issue of transporting the amp. Twin's are a royal b*tch to move, even with "stock" speakers, which tends to tempt many owners to install casters (ugh!). Casters are a tube's worst enemy. The vibrations that result from rolling an amp over concrete (or similar surfaces), literally "beats the sh*t" out of your tubes (they're not too hot for the reverb springs either). If the amp is still warm from operation, the vibration's are even more damaging.....last but not least, it costs twice as much to re-[power] tube a Twin.
I understand that the "pull two tubes and undo one speaker" trick is somewhat effective, but a Pro, or a Super, etc. would still sound better in most cases.