HT capacitor balancing Rs , necessity ?

Well I call them HT balancing Rs anyway, in valve amps. Where 2 HV caps are used in series to avoid using extra high voltage single caps. I always thought they needed 100K to 200K across each to balance out likelihood of midpoint voltage drifting up or down and exceeding the rating of one cap of the pair. Hiwatt AP CP103 ,2005 bouncing back blowing mains fuse , different set of output bottles from before. Worked for about 6 hours in total and over an hour before problem re-emerged. Amp lost sound, owner turned round in time to see the mains neon fail , ie after sound failure. This amp does not have balancing Rs , unlike its 1970s versions. I will add a couple of 220K/1W but anything else to ponder? These amps always seem to have had 3A mains fuses and 3A HT fuses for some odd reason - anyone know why? so the mains fuse sees more current than the HT fuse in normal use, same in abnormal sistuations?

You could try having a good old sniff around, trying to locate any burning/arcing smells, particularly around/underneath the output valve sockets, and the transformers. (Making sure, of course, it is not plugged in and all HT voltages have subsided)

If this is a contemporary re-issue, check to see if it has inrush limiter devices and that they are not dry jointed - along with anything else in the mains supply to the transformer including switches, pcb fuseholders, 13A mains plug wiring etc. Any kind of dodgy connection here can blow a mains fuse.

Gareth.

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I thought about putting a telltale 2A fuseholder in line specifically with the caps and then thought better of it. A high voltage in the output Tx whan a fuse blows could induce very high voltages across windings but loosing the smoothing could mean an even higher peak voltage if the HT or Mains fuse should blow.

N_Cook-

I think equalizing resistors are a good idea. However, you must use high voltage resistors.

A 220K resistor would dissipate 1 Watt at 469 Volts. A higher value resistor might be appropriate if DC leakage current through the capacitors is low. But you will find that ordinary resistors may fail due to dielectric breakdown, even if their power rating has not been exceeded!

Fred

"N_Cook" <

** Not needed in most cases, the electro caps will simply balance themselves at a mutually satisfactory leakage current.

The idea come from series connected, high voltage film caps which DO need them or it is snap crackle pop time.

... Phil

Before adding the balancing R in the first 10 minutes the difference between the 2 caps voltage went from 25V to 35V and still increasing 1V / min or so before I decided that was evidence enough for some balancing.

The 4/8/16R switch is made by SWP with logo of a linearised S in a rectangle. I would not want to rely on them for their supposed 250V/8A rating. The fixed contacts are just punched holes in the tinplated terminals and the moving contact, just a dome, path has 4 crude sprung surface contacts along the way

I've just realised the mains voltage selector is the same make and design just different markings. That too is unreliable I now find concentrating on it, so now permanently wired for 240V , no bad thing, in case someone mistakes this selector for changing oputput for different cabs and in the UK.

At least 1970s and earlier loop-thru-tag and solder assembly has gone by the board by 2005