: capacitor working voltage  ( 1846 )

TC Chris

  • Hero Member
  • *****
  • : 2816
capacitor working voltage
« : January 16, 2019, 10:45:56 PM »
I started to recap the GE radio tonight.  I picked one that looked toasted--a big paper cap from the plate of the 6L6 output to ground.  .01 mfd, and then "1600."  ????  So I found the schematic and it didn't give a voltage value.  The parts list did--1000 VDC.  Again, ???  Seems like a high rating, especially for an old paper and wax cap. The plate voltage is shown as 290 V.  I was wondering if I could get away with a modern film cap of 600 or 630 WVDC.  I've got those on hand.

My only guess about the voltage is that it has something to do with the output transformer primary, maybe acting like a choke or something.

Any advice?

Chris Campbell

ed from Baltimore

  • Sr. Member
  • ****
  • : 486
Re: capacitor working voltage
« #1 : January 17, 2019, 02:02:51 AM »
                If the 6L6 grid is overdriven by turning up the audio and disregarding the resulting  distortion, when the tube is driven to plate current cutoff, there will be a large positive voltage spike just like when the points open in the primary of an automobile ignition coil circuit. This spike can be many times the DC supply voltage.
                If it wasn't for the capacitor, the voltage spike would get high enough to arc inside the transformer winding. Even the load of the speaker on the secondary won't prevent this, especially with an old-style transformer  with a lot of leakage inductance. A modern amplifier with a high quality transformer can just put the capacitor, or maybe a capacitor in series with a resistor, across the speaker winding.
                You could always put two 600 volt rated capacitors of .02 uF in series to make the equivalent of one .01 uF capacitor, but if you do parallel each cap with a high value resistor like 1.0 meg or so to equalize the DC voltages and to let them drain off the otherwise isolated junction of the  two cap leads. You  probably won't be cranking the sound into gross distortion, but if the speaker voice coil should open circuit you could still need the high voltage rating because the load on the output tube plate could go so high at certain frequencies that the pentode would have a gain of several hundred.

Bill

  • Hero Member
  • *****
  • : 2055
Re: capacitor working voltage
« #2 : January 17, 2019, 08:42:38 AM »
I have the same problem Chris.  I have been recapping a RCA table radio, model 1R81.  My last cap to do is .0033 @ 1000 volts.  I checked with my local go to guy, but he did not have anything, but offered to order it for me.  I'm guessing I should put together a little order of high voltage caps and get them coming.   

Bill


TC Chris

  • Hero Member
  • *****
  • : 2816
Re: capacitor working voltage
« #3 : January 17, 2019, 07:56:38 PM »
Is this a place where a ceramic cap would be superior?  It's not in the signal path so wouldn't be objectionable from that standpoint.

Chris Campbell