DVD-Software.info

[Lord Valve]

Sander deWaal wrote:

[Lord Valve]

Pooh Bear wrote:

[Ryan Stalter]

No, they don't. It's when it's up around 5 that the pair starts glowing.
Thanks.


-Ryan

[Ryan Stalter]

I thought I should add after Lord Valve's reply about parasitic's that when
the kid who owns this amp first got it, it blew a screen resistor. He took
it back to the shop he purchased it at where they replaced it and it was
only after this that the current problem started. I don't have a scope yet.
Should I try swapping those out for good measure? Currently there are 470
ohm 1 watters. Thanks.


-Ryan

[Sander deWaal]

Ryan Stalter <ryangobie@hotmail.com> said:

[Ryan Stalter]

Sander, yes I tested those resistors. They were all the correct value. I
mentioned that the screen resistor was changed because I recall reading
that they can have an effect on parasitic oscillations. Thanks!


-Ryan



Sander deWaal <nospam@wanadoo.nl> wrote in
news:ctlcm19f43q3qs22k5nq4la5cofe9hqql8@4ax.com:

[Sander deWaal]

Ryan Stalter <ryangobie@hotmail.com> said:

[Ryan Stalter]

I sure hope it's not the OT. Thanks Sander!


-Ryan

[Guest]

A couple of statements first:
1. As you are getting some output, the OPTs are very most likely good,
and the Power-supply is also good, at least the iron.
2. From the symptoms you describe, it is unlikely that the tubes are
bad. Mostly they simply give up before they slow down to ~25% of
expectations. That you can get variations in volume supports this
contention. BUT, and at the same time, subbing in some other tubes is
not a bad test.

So: Some obvious questions:

a) Have you checked the bias? Even guitar amps have bias. And if it is
off, you will get exactly the symptoms you describe. This is the FIRST
place you should go before doing anything else. Every time the output
tubes are changed, and about every 200 hours of use, or so the bias
should be checked. Even more often if the amp plays at high volume all
the time and/or if the tubes are driven at their maximum. From what you
describe, I expect that amp uses either 4x6550 (KT-88) outputs and is
driven conservatively, or 6881/7027s driven to their theoretical
maximums. If the latter, check your bias each time out. Really. Even if
the bias is theoretically automatic, STILL check it in-situ. Be very
careful when you do so!

b) Is the Amp based on printed circuits, or point-to-point wiring? With
these tubes (any of the three above), and PC boards, often the traces
are thermally stressed and break. When you describe the 'red plate'
condition, that is AKA "Runaway". Usually from a poor/no bias setting,
or a broken trace. Check ALL your connections, and most especially
CHECK THE ACTUAL CONNECTIONS physically. I have often enough seen a
connection held in place by the insulation, but the conductor has
broken and is either open or barely touching.

c) Have you actually checked the tube sockets for good connections? On
occasion, I have had to tighten them up with a very fine pair of
needle-nose pliers. Clean the individual pin-sockets as well. A (fresh,
round) toothpick is good, even with a bit of contact cleaner on it.

d) What sorts of tools and/or instruments do you have? Do you have an
isolation transformer? Which leads me to.....

e) Always remember that there are potentially lethal voltages inside
this amplifier, and that the output tubes are quite capable of frying
eggs. So, you can 'fry' yourself in any of several ways. Keep in mind
that the human body may be likened to a 10,000 ohm, 1/4 watt resistor.
Anything capable of putting more than 1/4 watt of current through
10,000 ohms will do you damage. Next time you are out, look and see how
small a part that really is.

f) And Finally: You appear to have a schematic? This will tell you the
correct bias, the way of measuring and setting the bias, and all sorts
of other useful information, including the ability to troubleshoot each
stage.

Now, it could also be that this design is not tolerant of grossly
unmatched output tubes. Do you have a tube tester capable of matching
the tubes in any meaningful way? Have you even tested the tubes?

I am shying away from this solution, as your descriptive seems to
indicate that the tubes are on the verge of runaway... But anything is
possible if all else has been eliminated.

Good luck,

Peter Wieck
Wyncote, PA

[Pooh Bear]

Ryan Stalter wrote:

[Pooh Bear]

Ryan Stalter wrote:

[Pooh Bear]

Fro this reply I guess you're not much familiar with tube amps for rock 'n
roll.

It's *extremely* rare for any to have a bias adjustment for starters.

A bit of output tube mismatch increases the distortion nicely which is one
reason tube amps are so popular for guitar use.

Graham


pfjw@aol.com wrote:

[Guest]

Yikes!!

So, those several Fender et.al. tube amps that have crossed my path DID
have bias adjust. Thus, 100% of my random samples (5) were so equipped.
No, I am not intimately familiar with guitar amps. But I am at least
reasonably familiar with how tube amp circuits work, whatever the
signal source. And from what was described by the original poster,
there is some considerable risk to the system.... with specific
reference to the output transformers. And, again, from symptoms
described, that risk is immediate and on the face of it _possibly_ due
to bad bias. Also _possibly_ due to gross mismatch.

And, why would ANY maker wish to introduce potentially fatal conditions
into an amp for the sake of distortion....

"Gee whiz, boss, the Amp sounded just GREAT until the output
transformer melted down".
"Gee whiz, boss, the Amp sounded just GREAT until I melted down four
$30 tubes AND both output transfomers."

Whether or not the bias is adjustable it MUST be checked. That is my
point. The tubes mentioned to make the purported 50 watt output are
driven at their absolute limits. Even if all kinds of distortion is the
goal, running them at that level requires pretty-much that everything
else be right on target, or the next *POOF* you hear will be very
expensive.

"bit of output tube mismatch".... Clearly YOU are not familiar with how
tubes work in some circuits. That "bit" gets exaggerated depending on
other factors until something goes *POOF*.... see above.

Straight question, no prejudice, bias (pun intended) or sarcasm
intended: Do you have a tube checker capable of making 'actual'
matches? I do. I would be glad to match up a set of tubes for you. Try
for yourself and see what difference that makes. Consider that it takes
~5 random new tubes to get a nearly matched pair, and ~20 of the same
to get a nearly matched quad. You might easily double that for *exact*
matches. (Nearly = w/in10%, all measures, no burn-in; 'exact' - w/in
5%, all values and inclusive of burn-in).

Peter Wieck
Wyncote, PA

[Pooh Bear]

pfjw@aol.com wrote:

[Guest]

No argument on the "meant to distort"... but there are limits to that,
presumably below the failure threshold of expensive equipment. What
Ryan is describing is something that is indicative of being perilously
close to that same failure point. Even cheap tubes are expensive these
days, other parts more so. So he had better make sure that the problems
he is facing are not due to equipment failure, gross mismatches, and
other non-design sources of distortion. Once he has established what is
NOT a problem, he can go ahead and work on the volume problem.... his
original reason for posting, as I remember.

If you want an analogy, once upon a time an individual asked Picasso if
he painted the way he did because he actually could not draw. During
the conversation, Picasso did a near-photographic sketch of the person
on a napkin. My general approach to solving tube equipment problems is
to first make sure that the very obvious stuff is OK... So, maybe make
it sound like an audio amp for a few minutes... just to make sure that
everything is working as it should. Otherwise, and unless one is very
good at calculating and/or has a good scope, one is never quite sure
whether the problem is fixed or not.

One tiny example drawn from long experience with vintage radios.... if
a radio is consuming say... 8 watts more than it should (about the same
amount of power as a conventional night light), that is barely
measurable with most forms of instrumentation, and within most
allowances for error in calculations. But if that same amount of heat
is dissipated within an output transformer, it is only a matter of a
few hours before that transfomer fails. Just a few hours. THAT is the
point. And we are not even going to get into what might happen if a
plate melts in an output tube.

Peter Wieck
Wyncote, PA