Hi/Low Power Switch ?

[tbryanh]

What are some of the ways to reduce the B+ to the power tubes in about half in an existing amplifier build?

The attached Fender 75 schematic shows a low-voltage tap off the filter caps.

The schematic appears to have an error. It appears the RED/YEL center-tap on the transformer should be connected to ground, not to the node between the filter caps (the low-voltage tap off the filter caps).

Is there an error in the schematic?

What other schematics are out there that have Hi/Low power switches in them? Any links to them?

What are some of the other ways to reduce B+ besides having a filter cap tap?

(I know Power Scaling exists, but I think the effort needed to have a knob to adjust B+ is not worth it.)

[cbarrow7625]

The schematic does not have an error. That looks to be a voltage doubler on the HV power supply. Since a bridge rectifier is used and has one end connected to ground already, the CT would not be used at all unless it was used like this in a voltage doubler setup.

The thing that is different about the low power switch in this setup that is not found in most is that the lowered voltage is applied only to the power tubes. The power tubes are switched to the 250V node while the rest of the amp continues to run off of the 500V node. That should help keep the overall feel and tonality of the amp similar even in low power mode.

Also, most "low tech" voltage reduction schemes rely on having the power tubes cathode biased so that the power tube bias just follows the B+ voltage. On this amp they actually modify the fixed bias voltage to the power tubes when the B+ is switched.

The simplest way to lower B+ with a typical Fender (FW rectified, grounded CT) is to put a high voltage zener in series with the CT to drop a set amount of volts from the B+. That will reduce all of th HV in the amp though, not just to the power tubes. Unless you also modify the bias, it would require cathode biasing of the power tubes as I mentioned above.

[tbryanh]

Yeah it is a voltage doubler. It appears any existing build with a center-tapped transformer can be modded into a voltage doubler setup for a Hi/Low power switch.

I checked into the zener diode approach some time ago and found it was difficult to use zeners to drop the voltage in half. I think it was because the zeners that would handle the current were large and had avalanche voltages of maybe 50v at most. This is ok if you just want to drop 50v, but if you want to drop 250v, then 5 zeners are needed. I've kind of ruled out the zener diode approach.

[Enzo]

It is not a voltage doubler. A doubler is a circuit that alternately charges one cap or another and adds them for a total larger than the orignal voltage rectified.

This is a plain old bridge rectifier circuit. This is the exact circuit one would use to make a split supply, in this case +/-250v. Most times you see the center tap grounded and the ends of the bridge would form the +250 and the -250. But those voltages are with respect to the center tap, not NECESSARILY ground.

SO instead of grounding the CT and having +250 and -250, they grounded the -250 point. SO now we have +250 and +500.

[tbryanh]

Yeah thats right, with doublers, one half cycle charges one of the capacitors, and the other half cycle charges the other capacitor. Yeah this is a centertapped bridge rectifier where both capacitors are charged by both half cycles. Good catch.

[tbryanh]

It doesn't appear that existing builds can be modified to have the Hi/Low power switch like the Fender 75 because the B+ would be doubled by the modification.

[Enzo]

All depends on the transformer. if you are using a CT PT with two diodes, then you are right. Can't do it with the same transformer But amps that use the full winding with a bridge could instead use a CT transformer of the appropriate voltage. The CT circuit is invisible to the rest of the circuit unless you tap into it.

[cbarrow7625]

Thanks Enzo. That makes more sense.

[Joss]

Hi

I have a fender bassman AA371, the PA ( PI included ) was modded to AA864 specs. After seeing the schem of fender 75 i thought to implement the High/Low power switch in my bassman.

I don't Know if can take the voltage ( half of B+ ) from the junction of 70uf caps in series ( first filter cap ). The rectifier topology is different but at this point i have half B+

Sorry my english

[tbryanh]

There would probably be regulation issues. You might try it and see what happens.

Fender Bassman Amp AA864 Schematic - KBapps.com

[Enzo]

Hi Joss, welcome to the forum.

The problem with your idea is that if you took the voltage from the junction of the series caps, the current to operate the tubes would then have to flow through that 220k resistor.

[Joss]

Thanks guys.

The fender 75 shows 100K bleeder resistors instead 220k on bassman but with PT CT at junction providing current from it ( CT ) ¿ right ?

[tbryanh]

Also, you would probably wind up with intermodulation distortion issues. Might be an interesting special effect, or might even be the next holy grail.

[pdf64]

'but with PT CT at junction providing current from it ( CT ) ¿ right ?'
Yes, it's having the transformer winding with a CT that is the crucial thing.
There have been a lot of fenders since then which have a 1/4 power switch, you might find 1 that has a power transformer which is a good substitute for the one in your bassman. The 75 has really big transformers, the size of a twin.
However, 1/4 power is not enough for a gig (for me, anyway), and still way too loud for power tube overdrive at domestic levels.
Check out the tone lizard for some other ideas on dropping power
TALES FROM THE TONE LOUNGE: TABLE OF CONTENTS
Using a zener and / or sag resistor to lower the B+, or switching to cathode bias, are useful ways of losing a bit of output power. Peter.

[loudthud]

The London Power "Power Scaling" trick will accomplish the B+ reduction and you can go lower than 1/4th power. If you don't want the pot, use fixed resistors and a switch.