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Unbeknownst to me until after all this occurred, my buddy loaded in my
Silvertone 1484 upside down as we travelled 45 minutes to a show. It
powered up fine and even when I switched from Standby to operate it
stayed on. However, I got no sound out of it, not even the much
dispised 60 cycle hum I am so accustomed to hearing. I thought this
was strange and I turned volume up on both channels to see if maybe my
ears were still messed from an extra loud rehearsal the night before.
No sound. I eyeballed the tubes and they were all operating
properly-lit up as they should be. When I unplugged the cable from my
guitar and touched the tip with my hand I heard a very faint high
pitched squeal. Shortly after that I started to smell something
burning and the pilot light went off. I checked the fuse and it had
popped. I replaced the fuse and hit the power switch-immediately
popped that fuse. At that point I realized my problem was deeper than
a fuse and I left it alone until after the show. When it had cooled
off, I pulled the 1st power tube (6L6) (as the smell seemed to be
strongest near it) and saw that the bottom tip of it was black.

When I got it home and pulled the chassis I could see that the socket
had been burned at pins 2 & 3 and that the wires actually fused
together. I'm not sure what caused these wires to heat up to melting
point but I'm thinking that the tube may have wiggled itself a bit
loose during our drive and therefore been making weak contact.
Everything else in the amp seems to be ok, it's just that one socket
that is affected as far as I can tell.

I suspect that the wires making contact with each other caused the
short as pin 3 is the plate and pin 2 is the filament---Is this right?

I think the OT is fine as the job of the fuse is to protect anything
major from being damaged---Is this right?

I plan to add a 3 prong power cable and I may replace the tube sockets
while I'm in there. All the output caps are original and look great,
no leakage and the amp sounded amazing before this. I've never had a
problem with it prior to this. Should I replace the output caps
anyways? Also, do I need to replace the tubes? They're Svetlanas;
about 2 years old. I'm going to try to get some pix so those that
decide to respond can see what I can see.

Thanks for reading!

[Peter Wieck]

Barry:

A couple of things could have happened, including what you suggested.
But:

Something inside the tube could have worked loose and shorted a tube
internally.
Yes, it could have arced from a poor connection and caused the damage
you saw.

It is unlikely you were getting signal to the OPT such that it would
have been damaged... or if it did, you would have smelled it pretty
quickly. From the symptoms, it "sounds like" a local short of some
nature.

BUT: Before you apply power, check the tubes (all of them) for shorts.
Clean up any possible carbon tracks generated by the fault (it could
even be 100% unrelated to the inverted transport), and check all the
wiring and circuit boards (if any) carefully for bridges or breaks.

Good luck and report back, please.

I will spare you my rant on fuses.... Phil Allison might rise from his
crypt.

Peter Wieck
Wyncote, PA

[barry casebeer]

I also forgot to ask...does anyone know how can I build a device to
slowly charge an amp without ruining the caps? I know of the technique
of putting a lightbulb in series with the amp, but how is this
specifically done? I'd prefer not to buy a variac and I've heard
they're not the best option anyways.

Thanks for the info Peter, I'll post a follow-up hen I get into the amp
and I'll do my best to let sleeping fuse rants lie.

[Peter Wieck]

barry casebeer wrote:
I also forgot to ask...does anyone know how can I build a device to
slowly charge an amp without ruining the caps?

Barry:

There are multiple answers to that question based on what you wish to
achieve, as follows:

a) "Forming" capacitors in amps that have been sitting for many years,
are more than (arbitrary cut-off here) 20 years old, or are of unkown
antecedents:

Don't. Unless it is for short-term testing only based on the full,
honorable and complete intention to replace all electrolytic caps
anyway (at least). More to follow below.

b) You are looking for a soft turn-on, and have a solid-state
rectifier: There is a device called a Thermistor that you may install
in series with your power-cord that will soften the turn-on to the amp.
It should be purchased to the correct size for the load, installation
is pretty simple but DO NOT DO IT unless you know exactly what you are
doing. If you have a tube rectifier, theoretically, it starts as slowly
as the rest of the tubes, and so softens the blow when B+ hits a nearly
cold tube.

The "more later" part:

Forming HV capacitors takes time and attention. Theoretically it should
be done with an external DC supply, a meter, time and close attention,
and according to a simple but persnickety process. And a good
percentage of the time the it either does not work or the effects are
temporary. If your unit has a SOLID STATE rectifier, you can somewhat
do it yourself by using a Variac and time and attention.

You cannot use a Variac (despite rumors to the contrary) with a tube
rectifier as the it will not even pass current until somewhere between
60% & 75% of voltage anyway. So the caps get slammed with 60% - 75% of
full B+ anyway, hardly a slow-forming process. Mini-Rant: Variacs
*must* be equipped with proper metering (voltage and current) or they
are dangerously worse than useless. Whether this metering is on board
or outboard, it must be there.

On the other hand, if you have _ANY_ reason to suspect _ANY_
electrolytic cap inside your unit, replace it RIGHT AWAY. The results
could be spectacular and unhappy otherwise.

CAUTION: Lethal voltages exist in most tube equipment. Do not mess
around inside unless you know what you are doing *and* are familiar
with working around such voltages.

Peter Wieck
Wyncote, PA

[barry casebeer]

Thanks Peter,

I've been using the amp for a couple of years now and I discharged it
last night before poking around in it so it most definitely has not
been sitting for long periods of time. I installed a 1/4 " speaker
jack prior to learning that the power should be brought up slowly and
while doing that I discharged the caps and just turned it back on when
I finished. That was about a year ago and didn't seem to have had a
negative effect on the amp. The reason I am asking now is that I would
like to bring them up properly because I may have been lucky with that
one. The rectifier is solid-state in this one, btw.

[Peter Wieck]

barry casebeer wrote:
The reason I am asking now is that I would
like to bring them up properly because I may have been lucky with that
one. The rectifier is solid-state in this one, btw.

As long as the discharge did not go through you... that would have made
you sit up and take notice!

A thermistor might be your best solution then. But caps discharge
within a very few hours (or much less depending on design) anyway, so
near-as-makes-no-difference, you would be getting only a minimal
benefit from that treatment. But a benefit none the less.


Peter Wieck
Wyncote, PA