Fender Bias Cap

[Underwood]

I always love reading threads about this. I have some Sprague 25 uf 150V caps and were wondering if they would be good for my bias section on my Super reverb. The uf rating is stock, but the voltage is higher. I sizzled out one of my tubes yesterday. I have a new sprague 25uf 50V in there now. I actually think either the tube was bad or the grid resistors burned out. I have not opened it up yet. Bu,t when I do, I am changing all the 470 ohm reistors and the 1500 resistors and wanted advice on the bias cap.  thanks!!!

[pompeiisneaks]

you'll never go bad putting in a higher voltage rated cap.  You've just given more breathing room for the capacitor.  NEVER go lower.  If you do you can have some fireworks, at best, and major damage at worst. 

[Underwood]

Great. Thanks! I will give it a go then.

[HotBluePlates]

... I sizzled out one of my tubes yesterday. ... I actually think either the tube was bad or the grid resistors burned out. I have not opened it up yet. Bu,t when I do, I am changing all the 470 ohm reistors and the 1500 resistors and wanted advice on the bias cap.  thanks!!!

The 470Ω resistors could not have burned open, or the tube's plate current would drop near zero.

A burned-open 1.5kΩ could equal bias-loss and melted tubes, but I've not seen that happen in 30 years of looking at tube amps.  I can't grok a realistic scenario where the 1.5kΩ grid-stopper (1/2w) would burn open.


I can envision realistic scenarios where:
  -  A bad/failed tube could redplate or pop fuses, in spite of good bias voltage being present, or
  -  A failed bias supply component causes redplating, or
  -  A leaking coupling cap leading to the output tube grids causes redplating, or
  -  Loose tube socket contacts results in no connect from socket to tube pin 5 causing redplating, or
  -  A loose bit of wire/solder-blob/liquid/etc floating around inside the amp creates a short-circuit, or
  -  Excessive dust/dirt buildup inside the chassis creates a path for arcing/short-circuit.

I'd be looking to verify the tube is not loose in the socket, that the amp is generally clean (no stray bit of wire/solder), that the bias supply presents proper voltage to pin 5, and that the tube is not defective.