Hello. This is my first time posting to this group and I hope some of you
might be able to provide me with some direction. I'm a college kid who's
trying to learn as much as I can about tube amps and after so much reading
I wanted to get my hands dirty. I've fixed a few amps for my friends and
one of them had a broken Ampeg V4 that was giving him extremely low volume.
Needless to say, I took it off his hands to see if I could get it up and
running. First I just played it to see what was happening. On 10 it wasn't
at all uncomfortable to listen to. Maybe slightly louder than a television.
Both channels were doing this so I figured it was after the channels
converged. Then I took this Sunn Coliseum I have and ran the preamp out
into the power amp in on the Ampeg. No appreciable difference in volume. I
opened it up and took voltages and everything was within a few volts of the
schematic I have so I swapped the tubes out. No change still. I found some
resistors that drifted so I swapped them out and still nothing. I don't
have a scope yet but I tested the AC across the primary of the output
transformer and got a reading of about 20 volts which according to one
books power table is about right for a 100 watt amp into a 4 ohm load. So
just the other day I had some free time on my hands so I went through and
heated up some joints in the power amp and it seemed to get a little bit
louder so I played for a minute and just looked at the tubes and while
idling, the power tubes have a normal blue glow about them but when I hit a
note and hold it the plates start to turn red and when I let go they go
back to that blue glow. I'd really just like to get this thing up and
working and back to my friend. Any suggestions? I'd really appreciate it.
Thanks!

-Ryan

"Ryan Stalter" wrote in message
.196...

Hello. This is my first time posting to this group and I hope some of you
might be able to provide me with some direction. I'm a college kid who's
trying to learn as much as I can about tube amps and after so much reading
I wanted to get my hands dirty. I've fixed a few amps for my friends and
one of them had a broken Ampeg V4 that was giving him extremely low
volume.
Needless to say, I took it off his hands to see if I could get it up and
running. First I just played it to see what was happening. On 10 it wasn't
at all uncomfortable to listen to. Maybe slightly louder than a
television.
Both channels were doing this so I figured it was after the channels
converged. Then I took this Sunn Coliseum I have and ran the preamp out
into the power amp in on the Ampeg. No appreciable difference in volume. I
opened it up and took voltages and everything was within a few volts of
the
schematic I have so I swapped the tubes out. No change still. I found some
resistors that drifted so I swapped them out and still nothing. I don't
have a scope yet but I tested the AC across the primary of the output
transformer and got a reading of about 20 volts which according to one
books power table is about right for a 100 watt amp into a 4 ohm load. So
just the other day I had some free time on my hands so I went through and
heated up some joints in the power amp and it seemed to get a little bit
louder so I played for a minute and just looked at the tubes and while
idling, the power tubes have a normal blue glow about them but when I hit
a
note and hold it the plates start to turn red and when I let go they go
back to that blue glow. I'd really just like to get this thing up and
working and back to my friend. Any suggestions? I'd really appreciate it.
Thanks!

-Ryan

Hi Ryan,

According to your long description, I do not think anyone will be able to
really help you. I might suggest to describe your problem as a completely
non-technical person would. For example, " Here is my ----, model # ----
amplifier and the sound out of both speakers is about 25% of the normal
volume. I know the problem is not my preamp because when I play my CD and
preamp with another amplifier, all is OK." Forget for now all the technical
talk about voltage, load, etc. Remember ... layman. Then someone might
suggest to try something more specific. I'm not trying to be rude, but a
little knowledge can be dangerous, especially with the LETHAL voltages in a
tube amp. Good luck.

Cordially,
west

Ryan Stalter wrote:

Hello. This is my first time posting to this group

A suggestion.

Learn about paragraphs. A large solid block of text is very uninviting - and
actually unnecessarily difficult - to read.

Keep your life history to minimum.

Concentrate on the relevant facts and ask a specific well-considered ( not
vague ) question ( s ).

Graham

"Ryan Stalter" wrote in message
.196...

Hello. This is my first time posting to this group and I hope some of you
might be able to provide me with some direction. I'm a college kid who's
trying to learn as much as I can about tube amps and after so much reading
I wanted to get my hands dirty. I've fixed a few amps for my friends and
one of them had a broken Ampeg V4 that was giving him extremely low
volume.
Needless to say, I took it off his hands to see if I could get it up and
running. First I just played it to see what was happening. On 10 it wasn't
at all uncomfortable to listen to. Maybe slightly louder than a
television.
Both channels were doing this so I figured it was after the channels
converged. Then I took this Sunn Coliseum I have and ran the preamp out
into the power amp in on the Ampeg. No appreciable difference in volume. I
opened it up and took voltages and everything was within a few volts of
the
schematic I have so I swapped the tubes out. No change still. I found some
resistors that drifted so I swapped them out and still nothing. I don't
have a scope yet but I tested the AC across the primary of the output
transformer and got a reading of about 20 volts which according to one
books power table is about right for a 100 watt amp into a 4 ohm load. So
just the other day I had some free time on my hands so I went through and
heated up some joints in the power amp and it seemed to get a little bit
louder so I played for a minute and just looked at the tubes and while
idling, the power tubes have a normal blue glow about them but when I hit
a
note and hold it the plates start to turn red and when I let go they go
back to that blue glow. I'd really just like to get this thing up and
working and back to my friend. Any suggestions? I'd really appreciate it.
Thanks!

-Ryan

Do you really mean primary ??

20 volts on the Primary is very low.

20 volts on the Secondary is correct for 100 Watts into 4 ohms. If it does
not sound loud then there is something wrong with your speaker. ( or your
hearing )

Pooh Bear wrote:

Ryan Stalter wrote:

Hello. This is my first time posting to this group

A suggestion.

Learn about paragraphs. A large solid block of text is very uninviting - and
actually unnecessarily difficult - to read.

Keep your life history to minimum.

Concentrate on the relevant facts and ask a specific well-considered ( not
vague ) question ( s ).

Graham


Yeah - who do you think you are, George Gleason or something?

LV

Thank you for your reply West. I cut it down as short as possible. I have
an Ampeg V4 that has low volume and the left two output tubes glow red
after a few seconds if a signal is applied.

I admit my first post was a bit long winded but in many ventures, it seems
like the more information provided the easier it is to get a good answer.
And paragraphs? I passed all my college writing courses and I'm not writing
a formal letter. Sorry. Just trying to learn.

Thanks

Ryan

Ryan Stalter said:

Thank you for your reply West. I cut it down as short as possible. I have
an Ampeg V4 that has low volume and the left two output tubes glow red
after a few seconds if a signal is applied.


That could be a number of things, varying from a leaky coupling
capacitor to the grids of both output tubes (assuming they have one in
common), to an intermittent or permanent broken wiring in/to the
output transformer, causing the other 2 tubes to supply a (distorted)
signal on their own to the transformer.
Check the voltages at the tube's electrodes, especially at the grid
and plate connections of the failing tubes.
Come to think of it, failing grid-grounding resistors may cause such
problems as well (value going up, therefore causing tubes to draw too
much current).
I can't be any more specific because I don't have a schematic at hand,
but the above things I'd check at first.

I haven't seen your initial post, so I don't know if there was any
useful information to add to the above.

You can try to ask Lord Valve, he might have a better idea about this
amp. He posts regularly on this newsgroup, but he can be found for
sure over on alt.guitar.amps.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

Lord Valve wrote:

Pooh Bear wrote:

Ryan Stalter wrote:

Hello. This is my first time posting to this group

A suggestion.

Learn about paragraphs. A large solid block of text is very uninviting - and
actually unnecessarily difficult - to read.

Keep your life history to minimum.

Concentrate on the relevant facts and ask a specific well-considered ( not
vague ) question ( s ).

Graham

Yeah - who do you think you are, George Gleason or something?

LV

Uh ? Where are the spelling and grammar errors ? ;-)

The above was my response to being totally put off from reading or responding by
the style of the post.

At times like this you realise what those English teachers were getting at.
Inability to express yourself concisely and accurately has its drawbacks.

Graham

Ryan Stalter wrote:

Thank you for your reply West. I cut it down as short as possible. I have
an Ampeg V4 that has low volume and the left two output tubes glow red
after a few seconds if a signal is applied.

Glowing red means they're getting hot.

That normally only happens when there lots of volume.

The suppresor grids glow blue when quiescent though ?

You may have a shorted turn in the output transformer. That'll also restrict
the maximum output ( a *lot* ).

Graham

That's exactly it. It's only after you play for a minute or so that they
heat up to the point that they glow. I was in it before sending a signal
through till they just started to glow and the grids on the two glowing
were around -70 whereas the other two were around -90...at idle they're all
around -60

I checked the DC resistance across the primary and it was 99ohms but
between one side and the center tap I got two different readings. One
higher than the other by about 10 ohms. Is that suspicious? Thanks.


Ryan



Pooh Bear wrote in
:



Ryan Stalter wrote:

Thank you for your reply West. I cut it down as short as possible. I
have an Ampeg V4 that has low volume and the left two output tubes
glow red after a few seconds if a signal is applied.

Glowing red means they're getting hot.

That normally only happens when there lots of volume.

The suppresor grids glow blue when quiescent though ?

You may have a shorted turn in the output transformer. That'll also
restrict the maximum output ( a *lot* ).

Graham

in article , Ryan Stalter
at wrote on 10/30/05 11:39 AM:

That's exactly it. It's only after you play for a minute or so that they
heat up to the point that they glow. I was in it before sending a signal
through till they just started to glow and the grids on the two glowing
were around -70 whereas the other two were around -90...at idle they're all
around -60

I checked the DC resistance across the primary and it was 99ohms but
between one side and the center tap I got two different readings. One
higher than the other by about 10 ohms. Is that suspicious? Thanks.


Ryan



Pooh Bear wrote in
:



Ryan Stalter wrote:

Thank you for your reply West. I cut it down as short as possible. I
have an Ampeg V4 that has low volume and the left two output tubes
glow red after a few seconds if a signal is applied.

Glowing red means they're getting hot.

That normally only happens when there lots of volume.

The suppresor grids glow blue when quiescent though ?

You may have a shorted turn in the output transformer. That'll also
restrict the maximum output ( a *lot* ).

Graham




Sure sounds like bad (leaky) output tube coupling caps to me . . . .

Jon

Ryan Stalter said:

That's exactly it. It's only after you play for a minute or so that they
heat up to the point that they glow. I was in it before sending a signal
through till they just started to glow and the grids on the two glowing
were around -70 whereas the other two were around -90...at idle they're all
around -60


There you have it.
Meanwhile, I found 2 schematics of this amp, one Ampeg and the other
one a Magnavox.
The 2 sxhematics resemble each other very much:

http://www.drtube.com/schematics/ampeg/vt22-v4-70.gif
http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif

Check C11 and C13, and R35 and R36.
Also the resistors in the anode circuits R39, R40, R46 and R49.

In the latter circuit, bias hovers around -62V.

I checked the DC resistance across the primary and it was 99ohms but
between one side and the center tap I got two different readings. One
higher than the other by about 10 ohms. Is that suspicious? Thanks.


Usually, there is a difference between windings DC resistance, but 10%
ranks a little high IMO.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

Jon Yaeger wrote:

Sure sounds like bad (leaky) output tube coupling caps to me . . . .

I beg your pardon ?

Graham

Ryan Stalter wrote:

That's exactly it. It's only after you play for a minute or so that they
heat up to the point that they glow. I was in it before sending a signal
through till they just started to glow and the grids on the two glowing
were around -70 whereas the other two were around -90...at idle they're all
around -60


The point is...... Do they glow at relatively low volume ? That's a fairly
likely sign of a shorted turn.

I checked the DC resistance across the primary and it was 99ohms but
between one side and the center tap I got two different readings. One
higher than the other by about 10 ohms. Is that suspicious? Thanks.

Not horribly so.

It's actually quite difficult to exactly match the DC resistance of both halves
of the primary.

Graham

in article , Pooh Bear at
wrote on 10/30/05 12:16 PM:



Jon Yaeger wrote:

Sure sounds like bad (leaky) output tube coupling caps to me . . . .

I beg your pardon ?

Graham




Oops! I guess I overlooked the qualifier, "at idle they ALL are around -60."

Ignore my advice, which might be right if one or two of the grid voltages
were lower at idle.

J

Sander deWaal wrote:

Ryan Stalter said:

That's exactly it. It's only after you play for a minute or so that they
heat up to the point that they glow. I was in it before sending a signal
through till they just started to glow and the grids on the two glowing
were around -70 whereas the other two were around -90...at idle they're all
around -60

There you have it.
Meanwhile, I found 2 schematics of this amp, one Ampeg and the other
one a Magnavox.
The 2 sxhematics resemble each other very much:

http://www.drtube.com/schematics/ampeg/vt22-v4-70.gif
http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif

Check C11 and C13, and R35 and R36.
Also the resistors in the anode circuits R39, R40, R46 and R49.

In the latter circuit, bias hovers around -62V.

I checked the DC resistance across the primary and it was 99ohms but
between one side and the center tap I got two different readings. One
higher than the other by about 10 ohms. Is that suspicious? Thanks.

Usually, there is a difference between windings DC resistance, but 10%
ranks a little high IMO.

Not for a guitar amp. ;-)

LV

Pooh Bear wrote:

Ryan Stalter wrote:

Thank you for your reply West. I cut it down as short as possible. I have
an Ampeg V4 that has low volume and the left two output tubes glow red
after a few seconds if a signal is applied.

Glowing red means they're getting hot.

That normally only happens when there lots of volume.

The suppresor grids glow blue when quiescent though ?

You may have a shorted turn in the output transformer. That'll also restrict
the maximum output ( a *lot* ).

Graham

I vote for parasitics.

Either way, he needs a scope to find out.

LV

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


-Ryan

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

Ryan Stalter said:

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.


Can you determine what's glowing, the plates or the screens?
Did you try to measure the resistors I mentioned in my earlier post?

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

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 wrote in
:

Ryan Stalter said:

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.


Can you determine what's glowing, the plates or the screens?
Did you try to measure the resistors I mentioned in my earlier post?

Ryan Stalter said:

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!


That is indeed possible.
If they're the wirewound kind, try replacing them with 2 watts carbon-
or metal film resistors.
Also, try to add grid stopper resistors as close at the grids as
possible, one for each power tube.
Value about 2.2 kohm.

If that doesn't solve the problems, you'll have to find an
oscilloscope somewhere to observe whether your problems are rooted in
parasitic oscillations or still somewhere else.

Good luck! (and let's hope the OT is still OK!)

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

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


-Ryan

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

Ryan Stalter wrote:

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

Unfortunately 'around 5' tells me absolutely nothing about the power output.

A 'hot' signal will produce more power for any given volume setting than a
quiet one.

The volume control *isn't* a power control It sets the *gain* of thne
amplifier.

Graham

Ryan Stalter wrote:

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

Can you measure the output voltage when that happens ?

What load impedance are you driving ?

Graham

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


wrote:

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

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

wrote:

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).

You're talking ********. On just about every count. Guitar amps are *meant*
to distort you idiot ! That's their 'reason d'etre'.

How do you think those amps fly off the production line ?

I've seen Fenders without bias adjust too ( esp Twin Reverb ).

The *only* guitar amp I saw with bias adjust was an Orange.

Try looking at some guitar amp schematics !

Graham

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

I know it's been a while since anythings happened in this thread but I've
had some time to tinker with it. On the suggestion that it might be
parasitics I replaced all the screen resistors with 2 watters and replaced
the resistors that feed the bias. I was also suggested to put some 2.2k
gridstoppers on the pins. I did all that and it's still happening.

When playing, the bias on the 2 seemingly good tubes shifts from -60 up to
around -90 whereas the 2 that glow only shift to -68 or so.

To anwser your question Pooh, I'm running it into a 4ohm load and when it's
on "5" I'm getting about 200VAC across the transformers primary


-Ryan

[Bob Perilstein]

You are a jerk.

[Bob Perilstein]

I beg your pardon. My reply was directed to Graham. He seems to have quite a chip on his shoulder.
Regards,Bob

[Bob Perilstein]

Ryan,
Please don't let such a longwinded, wretch of a man intimidate you. It's obvious he thrives on such behavior.
Respectuflly,
Bob