[Cipher]

Peter Wieck wrote in
:

On Dec 7, 3:47*pm, cipher wrote:
Paul G. wrote
innews:ncemj4h4mdppegc81l7n7nion5rine
:





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
solde
r
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 gr
id
(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/le
ad,
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 r
un
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


Good deal, Sir.

just shoot your mailing address to

luxaeternaaudio AT gmail DOT com

--
Ray
luxaeternaaudio AT gmail DOT com