Overvoltage Protection

[Smitty]

I'd like to provide overvoltage protection for the preamp power supply node on old fender amps. It's node 'D' below



Is there some simple circuit that I can use like a shunt regulator that will allow no more than 450 volts during the transition from standby to operate. Will the 4700 ohm resistor provide enough current limiting so that I can use a smallish-current, high-voltage regulator?

Thanks in advance for your help!

Smitty

[Leo_Gnardo]

I'd like to provide overvoltage protection for the preamp power supply node on old fender amps. It's node 'D' below

Is there some simple circuit that I can use like a shunt regulator that will allow no more than 450 volts during the transition from standby to operate. Will the 4700 ohm resistor provide enough current limiting so that I can use a smallish-current, high-voltage regulator?

Millions of old Fender amps have survived decades without any voltage regulation. As long as your tube filaments are warmed up the voltage should ramp up to the expected value and level off without overshoot after you switch into play mode. (Assuming you have a standby switch.) If you want to do it anyway my quick & dirty method would be to stack up some zener diodes at that node, to whatever voltage you want them to clamp. Zeners are cheap, and the voltage adds in series so the math is E-Z.

[Smitty]

I was hoping it was going to be that easy! Thanks, Leo_Gnardo!!

[trobbins]

Smitty, your concern highlights the need for caution when swapping tubes, or inadvertently leaving them out of sockets, or servicing an amp without a variac (and pulling tubes to track a problem) - or people powering up an old amp to see if it 'works' (ie. to sell) even if not all valves are in their sockets.

Because, I can, I prefer to restore an amp so that leaving out any tubes will still keep capacitor working voltages below or near their rating - but that said, its pretty rare for an electrolytic to be noticeably damaged if taken overvoltage for a short period when its downstream of a few dropper resistors. I've sometimes used series connected 33V zeners, and also used a 'mosfet' zener type clamp. Older choke input power supply filter amps can be such a concern if tubes are pulled.

[Leo_Gnardo]

I was hoping it was going to be that easy! Thanks, Leo_Gnardo!!

I'm surprised nobody's offered some LM317 voltage regulator circuit which would be the next level of complexity. Try the zeners if you'd like, and rock on Smitty!

[Smitty]

As trobbins pointed out it is that I'll be experimenting with 450 volt electrolytics and if I like the way they sound (not because they're 450 volts) I'll just leave them in. The no-variac/no-tube condition worries me. The zener approach will be sonically transparent when the amp is operating.

Perfect. Thanks, again guys.