Scott Dorsey wrote:

Wayne wrote:
This may be a little OT, but it still has to do with caps.

I've got a Fender '65 Twin thats got old GE paper electrolytics in the B+
circuit.
Most are 20uf @ 450 volts. I'm a relunctant to replace those with NOS for the
obvious reasons the NOS probably are not gonna be an improvement over whats in
there now.

NOS caps will probably be worse, since they will have been sitting on the
shelf for a long time.

First. Is it advisable to replace with something else, since technology
changes and this is 40 year old design.?

If you change them with a modern technology cap, the amp will sound different.
This could be good or bad. If you replace them with a new cap of old
design (Antique Electronics Supply has a nice selection), it will sound
much the same.

Second, I'm having problems finding new replacements because of the 450v
requirement.

Panasonic makes some modern ones. Mallory I think makes some old ones.
Are these metal can types? Personally I would go to a 600V one for
somewhat longer life.

Third, should I also replace all those .1mfd and .047mfd while I'm at it and
with what?

If they are paper caps, putting anything else in there will change the
sound but improve reliability.

BTW, I'm really not interested in changing the "sound" of the Twin. It's a
great amp as is!!! Just wanting to clean up some of the hum, etc.

Anything you do will change the sound, which is the problem.

OTOH you may be well pleased with the changed sound !

I had a few original Vox AC30s in for repairs many years ( decades ) back.

Most of the problems were 'burnt out' 100k anode / plate resistors and horrible
'Hunts' 0.01uF coupling caps that degrade with time to the extent that the plastic
case would literally 'crumble' like dry bread !

Relaced those parts with new stock of modern characteristics - e.g. metal oxide /
film Rs and plastic film caps.

Also found a poor output stage biasing problem ( used cathode bias alone ) that
meant that the 2 paralled EL84s on each side of the push-pull arrangement ( 4 x
EL84s total ) shared -ve bias and therefore there was no way of assuring any remote
chance of matching Ia. Changed this to a proper individual bias arrangement.

The owners were well pleased with the results. It was like giving the amps a new
lease of life and a breath of fresh air.


Graham