1977 Deluxe Reverb. Schematic calls for a power transformer input of 120VAC, and a secondary HT winding producing 330VAC (x2, center-tapped). This gets rectified by a 5U4GB (which consumes 50VDC in the process), and is filtered to produce approximately 420VDC B+, and 415V on the power tube plates.
My original power transformer appears to have been wound 'hot'. At approximately 120VAC wall voltage, my HT winding is at 377VAC, not 330. The downstream effect is that my B+ is closer to 465VDC. At the same time, the heater winding is right where it should be, 6.3-6.4vac, at 120-121vac input. So it seems isolated to the HT winding.
The amp works fine, and has for years, but in the interest of learning and experimentation, I'd like to bring the B+ down to spec levels. I'd like to drop 35-40VDC from the B+.
I've googled and come up with shunt regulator, bucking transformer, etc. I don't know what these mean in practical terms, in the context of a vintage tube amp. I'm hoping there's some component, or small analog circuit, I can place after the rectifier and filtering, and before the standby, to reduce the voltage.
I'm aware there are other ways to reduce the B+. I don't want to apply a variac to reduce the input voltage, because it affects the heaters, too. And for this discussion, I don't want to swap the 5U4GB rectifier for a 5R4GB. While feasible, it would only drop a further 10-15V. Also, it's my understanding that this solution would introduce more 'sag' in the power supply. Finally, the capacitor input voltage of the 5R4 is only 20uf, and my (spec) primary filter caps are (total) 32uf. Probably not an issue, but still. In short, I'm trying to get the amp closer to spec, not farther.
Best Answer
There are many ways of reducing the plate voltage on valve equipment .I have done this on radios but not on a guitar amp .The most common way I have done this is to place experimentaly 2 100 ohm 5 watt wirewound resistors in the HT transformer secondary .One in series with each leg before the rectifier .You will notice the HT drop .Now you can ballpark calculate your final resistor value which in general wont be 100 ohms .The resistors get pulsating DC currents with a high peak to average ratio which is why the correct value is lower than most people think.The total power that the resistors waste is the same as a resistor in the DC side or a series Zener or a linear reg .If your total filter cap is generous which is the expectation of high Quality Audio and you are doing a lot of class A then your concerns about droop need not be concerns .This AC HT resistor approach does have the advantage of providing additional protection .Although I have not seen this I have heard about rectifier tubes flashing over and ruining HT transformers .The AC resistors limit fault current and hence Arcing and they will fail open circuit .The AC resistors make the rectifier current pulses slightly broader making the transformer copper slightly cooler .