Dropping B+ substantially

[1ampman]

Maybe stay with 6L6s and use cathode biasing to lower the output power. Or install a giant choke.....

[Mickey]

Dropping resistors are a bad idea, a mosfet circuit connected to the centre tap of the power transformer can allow for various HT voltages and as the 6V6s will be drawing less current heat dissipation will probably be lower than expected and arc over issues from high voltages will also be reduced. Mount the mosfet away from the power tubes and isolate wires as necessary.

[Mickey]

If you look at the schematic for the "AK-9 Guitar Amp" you will see that the power was lowered by the use of zener diodes in the centre tap circuitry of the power supply. With a little experimenting I'm sure that you can lower the B+ on your amp as desired.

[Enzo]

A choke is another option. Put a choke between the rectifier and the first filter cap. That makes it a choke input filter, which ought to charge the caps to 0.9 times the AC peak instead of 1.414 times for cap input.

[potatofarmer]

Brilliant! Don't know how that got past me and others.

EDIT: It occurred to me that there may well be TOO MUCH voltage reduction by changing rectifier arrangements. Probably something around 250Vp in this case.



I proposed that sort of circuit here and took some heat for it. I know it'll cause uneven ware to each cap, but so what! I can't remember what else may need to be considered about this circuit. Another good call IMHO.

EDIT: It also occurred to me that this would produce about the same voltage as the scenario above. So how about a three totem arrangement with the HV taken from one node down.?. All this assumes some modification to the power supply.

Yeah, you typically can't draw current from the junction of totem-pole caps. You can in this instance because of the center tap. It's safe, effective, easily reversible, doesn't add heat, and practically free. Wish I'd caught him earlier.

And yeah, the plate voltage drops to roughly half of what it was. With a pair of 6L6s he'd still be putting out somewhere in the ballpark of 15W, and through a pair of 12" speakers that seems like the level of loudness the customer is looking for. It'll still be louder than a Deluxe Reverb through a single 12. Probably should keep taking the preamp supply from the top filter cap to keep those voltages from dropping too.

It's like a built-in, fixed value, "all tube" VVR.

[gui_tarzan]

I'm still here, still looking at the suggestions.

[gui_tarzan]

Something else that's weird with this power supply, it has a FWB rectifier yet the PT has a center tap on the HV. But the center tap doesn't go to ground, it goes to the junction between the first two caps in series.

So I have 259v on the junction between the first two 220 mfd caps in the power supply, but I still have 440v to the screens so that's not going to work unless I take that connection and swing it over to the same point as the OT, then the 470 ohm resistors will drop the voltage a few volts for the screens. Before I swing that connection over, am I right?

[Mickey]

Placing a choke in the circuit will not achieve your desired outcome. Filter chokes are exactly that, they filter the rectified DC for a smoother DC voltage.
You will also have to consider that the 6V6s will draw less current so your B+ voltage will rise.
Re biasing your valves also won't solve your issue which I perceive is that you want to substantially lower the B+ coming from the power supply which supplies voltage to the output stage. Lowering the overall B+ will affect the other stages in the amplifier as they have been designed with a high B+ voltage.
Thinking about your dilemma I suggest that the most practical and cost effective solution is to fit the correct original replacement output transformer and show your customer how to use the volume control.