[Peter Wieck]

On Dec 7, 3:47*pm, cipher wrote:
Paul G. wrote :





On Sat, 6 Dec 2008 05:18:46 -0800 (PST), fryzz
wrote:

....snip!.....
I'm guessing you have an intermittent bias problem. *Your amp has a
separate bias supply, so the 68 ohm resistors are mostly just a
convenient way of adjusting the idle current. *If those bypass
capacitors on the 68 ohm jobs are leaky, it would actually have the
effect of increasing the negative voltage on the grid, which reduces
the idle current, so that's not why the tubes would draw too much
current.

The first thing I would look at is the bias potentiometers, pots do
tend to be trouble prone due to the moving parts. *You could try
cleaning them with something like Caig Deoxit. *You might also try
feeling those two filter caps in the bias supply after the amp has
been on for a while. *They'll pick up some heat just from being in a
tubeamp, but if one is warmer than the other it's a sign that cap is
leaking. *Another thing you might try is touching up all of the solder
joints in the bias supply. *You might also just leave your multimeter
hooked up, to see if the output of the bias supply is dropping when
the amp gets that "close encounters" look. *When you measure the
output of the bias supply, it should be stable and noise free.

* *The fact that just ONE tube goes red, and not the pair, suggests
that the bias supply & adjustment is not a problem, since both tubes
on each channel run off the same connection. If it is just the one
tube that goes, and it's always the same socket (even if you switch
tubes around), then I'd zero in on the socket, or the traces on the
circuit board. That will require very careful inspection, resoldering,
or possibly checking the springiness of the socket contacts. Since the
socket gets alternately hot and cold with the accompanying
expansion/contraction, it is very common for the socket contacts to
break away from the circuit board. This is one of those applications
where point-to-point wiring can be more reliable than circuit boards.
I worked at RCA when they made tube sets..... there was no end of
troubles with printed circuits and tube sockets that were soldered
into them. I would also solder over the solder traces in case there
was a crack in the board. That might not be practical, since most
mfr.'s put stuff over the traces so when the boards are soldered, it
won't adhere and waste solder.
* *Very carefully inspect all the connections and traces between grid
(pin 5) of the EL34's and the bias control. Make sure nothing is
loose.
* *If the caps in parallel with the 68 ohms were to get real leaky and
short, the tube might get red, but the resistors would not cook (the
caps would shunt all the current). My guess is your problem is in the
grid circuit of the tube that cooks.
* *If the 0.68 uF caps were to fail and break down, you would see
symptoms similiar to what you're experiencing. Also if "R1K" and
"R220K" in the grid circuits were to open up, you would get similiar
behaviour. That's not very common for resistors, unless they have been
mechanically abused. "Opening up" by a poor solder connection, thermal
expansion, or bad trace would do the same (and more common).
* *I also assume that this unit unit used common solder with tin/lead,
and not the lead free (ROHS) kind. The lead free solder is not as
reliable (critical equipment can duck out of the lead-free
requirements!). How old is this unit? Did the manuals make any remark
about being lead-free?
* *A possible problem (but not likely, since the mfr. should have run
into this) is that the system could be oscillating at some very high
frequency. There are already grid stoppers, I'd be paranoid and put
stoppers (100 ohms) in the screen circuit. If it did oscillate,
probably both tubes would run red, and from your symptoms, that hasn't
ocurred.
* *I don't think you need this, but I would solder a resistor (100K)
from the wiper of the bias pot to the (-) end where the cap is. If the
wiper failed to make contact (as they do when dirty), you will still
maintain a bias that won't destroy anything.

* Once you got mad in a previous post, and put down all the symptoms,
it became a lot easier to troubleshoot. Just to make sure, is it just
ONE tube that goes at a time? Does the same resistor cook each time?
If so, it's probably a circuit issue. If the problem moves around, it
MIGHT be a tube problem.
* *Usually when I have a customer bringing in a naughty piece of
equipment, it takes a few minutes of careful questioning to pin down
the problem. Just a list of symptoms from the customer aren't
enough..... I usually run them through what is for me a
"troubleshooting tree". Knowing the history of the unit, what it does
when naughty, what it does when you wack it (that's inevitable) helps
enormously. The fact that it misbehaves intermittently makes it very
difficult to service, and even more dependent on knowing the unit.
Chances are, that on a bench, this thing could run properly for years!

-Paul G.

thanks for that info sir..

weird behaviour.... i hooked it up yesterday for a bit, with the tubes
that I had used the day It went assed on me.... and now everything is
running ok, or seems to be.... the same tube that was glowing bright red
before is fine now, and the sound from both channels seems to be fine..
I am going to be sending this thing far away for someone to do it
proper, hopefully one o you gents can take it on for me. I do not feel
comfortable working on this thing.. it is the same resistors, and the
same tube that goes up every time.

is someone willing to take this on??- Hide quoted text -

- Show quoted text -

If you don't mind waiting. You are third behind two other amps - and I
just finished a receiver and a tuner that were with me for ~90 days.
This is a hobby, so I get to it when I get to it.

But, I do good work, or so I am told.

How did those tubes work out, by the way?

Peter Wieck
Melrose Park, PA