How can I reduce B+ from 460VDC to 420VDC in the power supply of a tube guitar amplifier?
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 .