Odd Ball Happenings from Miswired Mains


I've seen this happen on more than one occasion,
typically with a 60s or 70s Fender amp where the
first pair of mains caps was incorrectly installed.
I assume someone who didn't know what they
were doing got a hold of the wrong schematic
and went to town. or someone didn't double
check their work.

Either installed in series incorrectly
or in parallel incorrectly, that is either
the wrong polarity or with both caps
wired in parallel as follows:

Red - Neg cap lead (opposite lead tied to other cap).
Black - Neg cap lead

Red - Pos cap lead (opposite lead tied to other cap).
Black - Pos cap lead

What amazes me is that these amps seem to still
for a while but sound like ****.

and then

Strange things noises etc start happening within the amp.

Can anyone say what would the likely failure
modes and symptoms be?

Along with componants to replace?

It is obvious, the mains filters, yes but
I'm wondering what other electrical stresses
this causes.



hi... I don't know much, but I felt sorry you'd had 21 views but no replies...

seems to me that this fault will only have resulted in a lot of ripple on B+ and (while the amp will have sounded terrible) nothing else will have been damaged. Replace (don't re-use) the wrongly-connected filter caps and fire it up.

If this is wrong, maybe someone else will help you by correcting me! best wishes!


bruce egnater

The caps will most likely eventually explode....really. The wrong polarity is well....just plain not a good thing. Putting them in parallel in a typical Fender amp will exceed the voltage rating of the capacitors. Either mistake is "bad" and should be corrected right away with new caps. The incorrectly installed ones are probably damaged. If they were to short, there is a possibility it would damage a rectifier tube or diodes and even a power transformer.


John Phillips

What Bruce said. The usual symptom of a cap short is a boiling-kettle noise followed by a loud explosion and foul-smelling smoke and other residue going everywhere. Do NOT allow this to happen, it makes a real mess of the inside of the amp as well as possibly being hazardous to your health (I'm not sure, but it smells bad enough). I've only had one or two go - on old amps which had been in storage for years where I 'optimistically' (ie unwisely :)) powered them up to see what else needed fixing, without first changing the caps.

If you suspect the caps have been installed wrongly, don't even run the amp - you're risking really major damage. If they're just very old, or slightly over the voltage rating, they may run OK but probably will fail sooner or later. (Some early JMP Marshalls run the 450V caps slightly over the rating, BTW - they're fine with modern 500V ones.)

Don't risk reinstalling wrongly-polarized caps or ones which have been run significantly over the rating, they're an accident waiting to happen. If they're not original to the amp anyway, just throw them out.


Thanks SP, Bruce, John:

I replace the caps as a rule on anything
I find like that. What really surprises me
the most was that these amps worked.
That is about all you can say though,
they sure don't sound good.

Then you replace the caps and check things out
and all is fine. Other times it isn't.

That BF Bandmaster had caps installed incorrectly,
then ened up with some sort of over all polarity
problem, ending up being inphase with itself.

I just got in a Super Reverb with the same thing.
Caps again installed incorrectly. No one seems
to know who's doing this mis-work, unless
these folks are doing the work them selves
and when the amp doesn't sound right
after their incorrect cap job they come in
to be repaired.

Stranger things have happened.


How is the RM4 stuff coming along?
Great concept, I hope it really catches on.
I for one would rather plug in a real pre amp
then model something digitally.

I wish you the best and a lot of success!


How did your 6L6 swap in your Blue Angel turn out?
Are you happy as it is, or going to open her up and
make a few mods?

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