Stepped Power Scaling?

[tbryanh]

PS1 is the power supply in my Peavey Triumph PAG 120. It is a 120W amp.

PS2 is the proposed modification.

The goal is to have a choice between 120W and 30W. I need 120W to drive 1 or 2 4x12 cabinets, but I only need 30W to drive a 1X12 cabinet.

PS1 and PS2 are the same except PS2 has the F5 fuse and the K1 Relay added to the circuit.

It appears that when the K1 Relay is in the B position, the power supply will put out 1/2 the voltage. This will cause the amp to deliver 1/4 the power.

I have some questions.

1. Will PS2 work as described?

2. Will there be more or less sag when the K1 Relay is in the B position instead of the A position? It appears there will be less sag because the power supply can deliver the same amount of current at either voltage, but the amp uses less current at the lower voltage.

3. How will the preamp and splitter tubes be affected? Will they work well at the lower voltage?

4. I need to change the bias voltage when the K1 Relay is in the B position. I think I switch in a zener diode or do something similar, so I do not see this as a big issue.

5. Are there any other issues?

[Enzo]

I don't see why the B+ half voltage relay wouldn't work.

I think of sag as largely the result of series resistances (in the rectifier tube and in resistors. yes, I know there is no recto tube here) in the B+ supply, moreso than just the transformer winding dropping. And since there isn't any, I'd think sag would be limited anyway. But that is just me thinking, the transformer winding might be a major contributor. PV tends to use pretty robust PTs in their stuff.

A simple experiment would be to monitor B+ in the amp as stock, just to see how the transformer reacts now. At half voltage I would agree there would be less sag in the transformer, but have no idea what the amount is to start with.

The small tubes would all also be running at half voltage. Got a 6V transformer? Disconnect the heaters from the power transformer winding and heat them from this external heater tranny. Now dial the main PT down to half voltage with a variac and see how the circuits react.

Bias: I guess a zener could be used somehow, but why not just switch a resistor in and out to alter the voltage division in the supply? You could even create a second little supply off the same source, and have a relay contact just select which supply it prefers. There is no current needed for the bias, so there would be no particular drain on the raw bias supply. Another diode, couple caps and a couple resistors and ther you are. Both could be adjustable even.

Peavey on their more recent amps that have a 6L6/EL34 switch simply switch a resistor in and out of the bias supplies there. See for example their Triple XXX amp. One resistor, with a relay contact that shorts across it or not.

Two other thoughts:

Have you considered just making it a 120/60 watt amp instead of going for 30? That would be as simple as switching two power tubes off, easily done. And the difference between 30 and 60 watts is only 3db of loudness. Then the B+ stays constant, the preamp is no longer an issue, and teh bais stays the same.

Have you considered maybe switching two power tubes off AND switching the remaining two over to triode mode?

[tbryanh]

I don't see why the B+ half voltage relay wouldn't work.

I think of sag as largely the result of series resistances (in the rectifier tube and in resistors. yes, I know there is no recto tube here) in the B+ supply, moreso than just the transformer winding dropping. And since there isn't any, I'd think sag would be limited anyway. But that is just me thinking, the transformer winding might be a major contributor. PV tends to use pretty robust PTs in their stuff.

Yeah, ther might not be much sag at either wattage, so it might be a non-issue.

[tbryanh]

Bias: I guess a zener could be used somehow, but why not just switch a resistor in and out to alter the voltage division in the supply? You could even create a second little supply off the same source, and have a relay contact just select which supply it prefers. There is no current needed for the bias, so there would be no particular drain on the raw bias supply. Another diode, couple caps and a couple resistors and ther you are. Both could be adjustable even.

Yeah, that sounds cool.

[tbryanh]

I don't see why the B+ half voltage relay wouldn't work.
Peavey on their more recent amps that have a 6L6/EL34 switch simply switch a resistor in and out of the bias supplies there. See for example their Triple XXX amp. One resistor, with a relay contact that shorts across it or not.

I'll check that out. Simplicity is best.

[tbryanh]

Have you considered just making it a 120/60 watt amp instead of going for 30? That would be as simple as switching two power tubes off, easily done. And the difference between 30 and 60 watts is only 3db of loudness. Then the B+ stays constant, the preamp is no longer an issue, and teh bais stays the same.

Well, from that I am thinking I might have 15W, 30W, 60W, and 120W steps.

I have heard removing tubes can change tone some. 2 tubes sounds different than 4, but I have heard both ways are desirable.

[tbryanh]

Have you considered maybe switching two power tubes off AND switching the remaining two over to triode mode?

I have heard of the triode mode before. I am wondering how different that would sound?

[tbryanh]

http://www.blueguitar.org/new/schem/fender/the_twin.gif

This Fender circuit is similar, but it does not reduce the voltage to the preamp or splitter.

Plus, the capacitors keep their voltage so they don't get damaged or have to be reformed.

This info came from Kevin O'Connor and stoo at powerscaling.com where I posted the same question.

[mooreamps]

It would have to do for now, since this is a push-pull amp with fixed bias. I have a new power brake circuit for Class A/B push-pull, but I have not tested it as yet.

-g