Power Reduction Circuit Opinions

[c.stoffel]

Hey guys, I've been looking at gut shots of an amplifier from a well respected builder trying to understand how his high/low power switch works. After some time drawing tentative schematics I think I finally understand what is going on.
Apparently the center tap of the power transformer is attached to the junction of two series filter caps, instead of ground, in a centre-tapped bridge configuration. Then the plates and screen grids of the push-pull amplifier would get either full voltage (from top of capacitors) or half voltage (from the juntion) working like a fixed VVR. The PI and preamp nodes always see full voltage. The other side of the DPDT switch is used to change the bias circuit so it is adjusted for the 'new' lower voltage.
What do you guys think? Would this work as drawn on the schematic? Is it a good idea? Thanks!

[sluckey]

Looks good to me. It will work as shown. Good idea.

The only thing I'd change is eliminate the two 220K resistors on the first filter caps. The center tap already forces the voltage to split equally across those two caps and you have resistors across the screen caps so no need for bleeders on the first caps.

Note that this simple circuit works well with a bridge rectifier and PT with CT but will not work so well with a conventional two diode rectifier.

[92Volts]

Looks like it should work. Might not sound the same, but that doesn't mean it will sound bad.

"Normal" operation at lower voltages involves lower OT primary impedance, but this is partly to maintain high power output, so it makes sense you wouldn't bother changing that for this application.

I would worry about that switch. Breaking high voltage DC is tough. If an arc forms in a switch carrying AC current it will be broken when voltage/current drops to 0 which it does 120 times per second with 60 Hz AC. If an arc forms with DC it may continue until something fails catastrophically-- or at least, significant wear on the switch has occurred. 

[bnwitt]

very interesting circuit

[PRR]

One of the enormous Fenders does the same thing (slightly different details).

IIRC, it was drawn buy a guy on acid so very hard to suss out the drawing.

[Willabe]

One of the enormous Fenders does the same thing (slightly different details).

IIRC, it was drawn buy a guy on acid so very hard to suss out the drawing.

                               I knew it, those guys out in Cali!!!!!     

[shooter]

those guys out in Cali!!!

IIRC the pharmacy students at UofM did good work

[c.stoffel]

Looks good to me. It will work as shown. Good idea.

The only thing I'd change is eliminate the two 220K resistors on the first filter caps. The center tap already forces the voltage to split equally across those two caps and you have resistors across the screen caps so no need for bleeders on the first caps.

Note that this simple circuit works well with a bridge rectifier and PT with CT but will not work so well with a conventional two diode rectifier.

Thanks sluckey. When I was studying this centre-tapped bridge circuit I've read there's no need for balancing resistors exactly like you said. I just decided to put them on the schematic since they are actually installed on the amplifier I was tracing.
So if I wanted to implement this circuit on my amplifier I would need a center-tapped power transformer that gave me half the AC voltage I would want if it used a simple full wave rectifier, am I right? For example: if my full wave rectifier power supply needed a 300-0-300VAC PT, with the centre-tapped bridge configuration I would need a 150-0-150VAC PT to keep the same DC voltage, correct?

Looks like it should work. Might not sound the same, but that doesn't mean it will sound bad.

"Normal" operation at lower voltages involves lower OT primary impedance, but this is partly to maintain high power output, so it makes sense you wouldn't bother changing that for this application.

I would worry about that switch. Breaking high voltage DC is tough. If an arc forms in a switch carrying AC current it will be broken when voltage/current drops to 0 which it does 120 times per second with 60 Hz AC. If an arc forms with DC it may continue until something fails catastrophically-- or at least, significant wear on the switch has occurred. 

Thanks 92volts! I've no real experience with this amp but based on reviews and videos on the internet it looks like it sounds pretty good in low power position, but the volume is not THAT MUCH lower, but that is expected isn't it? I think it should saturate quicker since only the power tubes are working at lower voltages and the preamp would hit them harder in that case, right?

very interesting circuit

Isn't it? I was pretty intrigued since everybody was saying it sounds so good... that's why I tried to trace the circuit.

One of the enormous Fenders does the same thing (slightly different details).

IIRC, it was drawn buy a guy on acid so very hard to suss out the drawing.

Thanks PRR! Would you mind telling me which Fender are you referring to? Maybe some day with the right chemicals I could have a look at it? 

[sluckey]

For example: if my full wave rectifier power supply needed a 300-0-300VAC PT, with the centre-tapped bridge configuration I would need a 150-0-150VAC PT to keep the same DC voltage, correct?

That's correct.

Thanks PRR! Would you mind telling me which Fender are you referring to?

The Twin, aka the evil Twin (red knob)

     http://el34world.com/charts/Schematics/files/fender/Fender_the_twin.pdf

[92Volts]

Thanks 92volts! I've no real experience with this amp but based on reviews and videos on the internet it looks like it sounds pretty good in low power position, but the volume is not THAT MUCH lower, but that is expected isn't it? I think it should saturate quicker since only the power tubes are working at lower voltages and the preamp would hit them harder in that case, right?

Yep, in typical circuits max power is when grid voltage hits 0v, because the capacitor-coupled phase inverter can't provide current to drive it higher.
At low voltage, required bias is closer to 0v, therefore the signal which will hit 0v and max output is smaller.

Since your preamp can provide the same size signal, it's easier to get power tube clipping without preamp clipping.

This is just one way to look at it, there are multiple reasons lower voltage equals lower power.

Our ears don't perceive volume linearly, so it's not surprising this power reduction (maybe 1/3 or 1/4 original power) doesn't sound like much. But if you want "pure" power tube distortion without your preamp introducing distortion first, this would be a very effective way to achieve that.

[c.stoffel]

The Twin, aka the evil Twin (red knob)

     http://el34world.com/charts/Schematics/files/fender/Fender_the_twin.pdf

Thanks! Well, they are REALLY similar. Even the 220k resistors across the filter caps are there! The bias circuit is almost the same except the amp I was tracing has a second trimpot to adjust the "low power" bias voltage. Maybe The Twin was the "inspiration" for this amp's low power circuit? 

Our ears don't perceive volume linearly, so it's not surprising this power reduction (maybe 1/3 or 1/4 original power) doesn't sound like much. But if you want "pure" power tube distortion without your preamp introducing distortion first, this would be a very effective way to achieve that.

So it looks like a good idea to try someday when I order a custom transformer, we'll see...

[PRR]

> the evil Twin

I thot there was an uglier drawing, but that will do.