help diagnosing crackling in Ampeg B-25B
Richard Chapman
5AR4 rectifer (magnavox)
2 x 12AX7A (Yugoslavia)
7199 (Ampeg)
2 x 7027A (just has 7027A in an Octagon)
I bought this last week. Sigh. After working fine for several days,
yesterday morning, after having had the Amp on for about 3 hours, a
serious crackling noise (sounds like bacon frying or something)
started. It's in both channels, independent of volume settings or
whether anything is plugged in. This means something is arcing, right?
Probably because it got too hot for too long. I've heard that
tubes can arc internally. I tried replacing both the 12AX7's,
the 7199, and the 7027's (with 6L6GC), to no avail. Now the tubes I
replaced the 7199 and the output tubes with were of unknown quality,
so I've ordered some new Sovtek spares for those. The only tube I couldn't
swap was the 5AR4 rectifier, so I ordered a spare for it also.
I cleaned the sockets and pins of all tubes. The amp is clean with no
dust inside. No capacitors or resistors are obviously bad. Poking everything
with a chopstick causes no change in the output, but I did find the 7199
and the spare to be somewhat microphonic.
Can anyone suggest other things to try/do? I'm going to resolder all the
connections (not all that many). If replacing tubes and resoldering doesn't
fix it, that means it has to be either a
- bad resistor
- bad capacitor
- bad xformer
that causing the arcing, right? My questions
1) Where can I get capacitors like 0.1uf, 400V?
2) Where can I get big capacitors for the power supply, like 30uf,40uf,600V?
3) Please tell me that the output transformer couldn't be the source of this
crackling, due to being run at the wrong impedance (this amp wants a 16ohm
speaker, no idea what previous owners may have done)
Thanks,
Richard
kmcc...@flexstornet.com
www.tubesandmore.com
www.triodeelectronics.com
www.angela.com
There are probably tons more as well. I've personally had good luck with
Antique Electronic Supply (tubesandmore).
-Kevin
SkrewBall
greg z
Someone up there is sure to have experience with this
amp and point you in the right direction.
Greg Z
to thine own sound be true
Richard Chapman
: This is a 2-channel electric guitar/bass amplifier using ...
: I bought this last week. Sigh. After working fine for several days,
: yesterday morning, after having had the Amp on for about 3 hours, a
: serious crackling noise (sounds like bacon frying or something)
: started.
Thanks to everyone who replied. Here's an update on this. I got some
new replacements for all tubes. That didn't make any difference. I
did, however, learn that the Sovtek 7199 is no good in this
application due to excessive hum. An second, this time NOS, 7199
didn't have the hum problem, but also didn't solve the crackling
problem. I replaced all of what I think are the coupling capacitors (I
replaced all the capacitors except the big ones in the power
supply). I replaced the standby switch. I poked all the wiring a LOT
with a chopstick. I resoldered all the connections. There's not much
left to replace!
- resistors
- tube sockets
- output transformer
- PS filter capacitors
I'm betting it's the output transformer. The only thing that puzzles me is
that it went bad all of a sudden, with a perfectly good speaker connected.
I'd expect the output xformer to start shorting due to playing the amp
w/o a load or some kind of abuse like that, not from just sitting there.
Nothing to do but bite the bullet I suppose. I've ordered the tube sockets.
Still looking for an output transformer (if anyone has any, I need
to handle two 6L6GC's, 55 watts, with output taps of 8 and 16 ohms, but
I'd be happy with any set of output taps that included 8 ohms. Some of
these multicaps are no longer made. This amp was a "good buy" at first, but
the total cash expended is now upwards of 2x the original cost. At least
I'm learning something, right? Experience is what you get when you don't
get what you want.
Dave Matthews
Check the grid resistors on the output tubes. Look for cracks, or signs
of burning.
I carried that monster amp around the country for a couple of years
while working in a lounge band. In the end I did lose the output
transformer I believe from a combination of road abuse and bad tubes. I
still have a picture of me tilting the amp into a dumpster behind some
club somewhere, sacrilege I know, but it was the best decision at the
time and place...
--
Dave Matthews, http://www.lostfrogs.com
SkrewBall
ThreeMile
transformer is fine.
Mike Rippe
thre...@aol.com
Richard Chapman
: Years ago (20 years, yikes!), I had a similar problem in a V4-B.
: Check the grid resistors on the output tubes. Look for cracks, or signs
: of burning.
I replaced the grid resistors. No improvment.
: I carried that monster amp around the country for a couple of years
: while working in a lounge band. In the end I did lose the output
I used to have a V4-B. Man, that thing was heavy, esp. in proportion to the
amount of sound it put out.
Richard Chapman
: Sounds like coupling caps,or maybe resistors are starting to go south..
I replaced the coupling caps and the output tube grid resistors. No
improvement.
: Richard Chapman wrote:
: >
: > This is a 2-channel electric guitar/bass amplifier using
: > yesterday morning, after having had the Amp on for about 3 hours, a
greg z
>I replaced the coupling caps and the output tube grid resistors. No
>improvement.
-------------------------------------
have you tried pulling tubes to isolate what stage the problem is ocurring?
Richard Chapman
: have you tried pulling tubes to isolate what stage the problem is ocurring?
: Greg Z
: to thine own sound be true
Yes. I can pull out both of the 12AX7's with no effect (still crackles).
If I pull out the 7199 (leaving only the rectifier and the output tubes)
the amp goes silent (not just no crackle with a tiny bit of hiss, the
way it does when working normally, but dead silent). The only other tubes
are the rectifier and the two 6L6GC output tubes. I didn't know if it
was safe (really don't want to blow the output xformer if it isn't already)
to pull either output tube.
To me, all this says is that it isn't in the preamp section. That is
consistent with the crackle being independent of what channel is in use
or the knob settings. It crackles plenty with both volume knobs turned
down to zero.
So, there are two more stages, the one with the 7199, and the one with
the output tubes. I've read that
the 7199 is used here as a "phase inverter" -- I don't understand that
part of the schematic.
I've replaced every component in the output tubes
section except the power tube sockets and some wires at this point. I have
sockets on order and will replace them when they get here.
Lord Valve
LV
SkrewBall
you.) if it goes away when you pull the 7199,chances are it is in that
part of the circuit,or somewhere before that..
Sounds like the circuitry around the 7199 to me..
Warren Lane
Richard Chapman <cha...@pecan.eng.auburn.edu> wrote in message
news:9grokc$1av$1...@aunews.duc.auburn.edu...
> greg z (gm...@aol.comnospam) wrote:
>
> : have you tried pulling tubes to isolate what stage the problem is
ocurring?
>
> : Greg Z
> : to thine own sound be true
>
>
> Yes. I can pull out both of the 12AX7's with no effect (still crackles).
> If I pull out the 7199 (leaving only the rectifier and the output tubes)
Hi Richard,
I've just happened across your dillema, and would suggest if the output
tubes are still in cct. and therfore pulling heaps of current, then the
tranny is almost 100% OK as far as I can see...one of your other respondents
has suggested the 7199 stage has a fauly component and that sounds pretty
right to me...just my 2c.'s worth!
Cheers,
Warren.
Dave Matthews
SkrewBall wrote:
>
> Wheres the club at,and do ya think the dumpster is still there? ;-)
Somewhere between Richmond, Virginia and Casper, Wyoming!
We were a sober band, but I can't place the exact time or location.
>
> Dave Matthews wrote:
> >
> > Years ago (20 years, yikes!), I had a similar problem in a V4-B.
> > I
> > still have a picture of me tilting the amp into a dumpster behind some
> > club somewhere, sacrilege I know, but it was the best decision at the
> > time and place...
Dave Matthews
Richard Chapman wrote:
> I used to have a V4-B. Man, that thing was heavy, esp. in proportion to the
> amount of sound it put out.
Yes, but the tone was the thing. It was the best sounding amp I ever
used with my bass, and even with the size and weight...
Richard Chapman
: Replace the plate load resistors in the PI stage.
: LV
Yes. That did it. Thank you. Damn. I'm impressed. Could you explain your
reasoning?
Thanks to all the others who replied, helping to narrow it down. It was a
$1.29 resistor. But I've got enough spare parts to keep this amp running
until well after the apocalypse now :-)
Vince Rhea
Vince
"Richard Chapman" <cha...@pecan.eng.auburn.edu> wrote in message
news:9gtlnq$gcm$1...@aunews.duc.auburn.edu...
Richard Chapman
Okay, in the absence of a reply from LV, here's my supposition about how
I should have reasoned:
1) I know it's not in the pre-amp since I can pull both 12AX7's and the
problem persisted. Also problem persisted with both volume knobs down
to zero.
2) I had replaced everything in the output section except the tube sockets,
which I had cleaned and which looked okay.
3) The circumstances of the onset of the problem didn't suggest the output
transformer.
4) In the power supply, I had replaced the rectifier tube and cleaned
the socket. I read that it is usually the case that power supply
transformer failure is catastrophic. Power supply capacitor failure
more often results in hum than crackle. (I'm not sure about the
deductions in this section).
5) If it isn't in the PS, pre-amp, or output,
now we know it is in the phase inverter.
5) The coupling capacitors and cathode capacitor
in the phase inverter had been replaced.
6) The tube in the PI stage had been replaced and the socket cleaned.
All wires ahd been inspected and poked, and all solder connections reflowed.
7) Now we're down to just resistors in the PI stage. They all looked
fine on inspection by poking with a chopstick. How did LV conclude it
was the plate load resistor?
Lord Valve
Richard Chapman wrote:
Richard Chapman (cha...@pecan.eng.auburn.edu) wrote:
: Lord Valve (detr...@ix.netcom.com) wrote:
: : Replace the plate load resistors in the PI stage.
: : LV
: Yes. That did it. Thank you. Damn. I'm impressed. Could you explain your
: reasoning?
: Thanks to all the others who replied, helping to narrow it down. It was a
: $1.29 resistor. But I've got enough spare parts to keep this amp running
: until well after the apocalypse now :-)
>Okay, in the absence of a reply from LV, here's my supposition about how
>I should have reasoned:
>
>1) I know it's not in the pre-amp since I can pull both 12AX7's and the
>problem persisted. Also problem persisted with both volume knobs down
>to zero.
LV: Right.
>2) I had replaced everything in the output section except the tube sockets,
>which I had cleaned and which looked okay.
LV: There isn't much there besides screen resistors and swamp
resistors, as I recall. Replacing them doesn't hurt,
since they're cheap. If they're on the tube sockets, they
get cooked after 20 years or so, so it's not a bad idea.
>3) The circumstances of the onset of the problem didn't suggest the output
>transformer.
LV: Right.
>4) In the power supply, I had replaced the rectifier tube and cleaned
>the socket.
LV: I'm curious...what did you use to clean the sockets?
>I read that it is usually the case that power supply
>transformer failure is catastrophic.
LV: Depends on whether you get a short or an open.
Shorts are more exciting. ;-)
>Power supply capacitor failure more often results in
>hum than crackle. (I'm not sure about the deductions
>in this section).
LV: Most of the time, even if the cap(s) is/are completely
open, you won't hear much hum on the output.
The reason is twofold; one, the CMRR of the OPT
in a push-pull design tends to null out any ripple
in the output stage, even if it's pretty large.
Two, the NFB also reduces any artifacts generated
in the output stage itself. These two things work
together to make the output fairly silent even
with bad caps. Of course, if they are shorted,
the fuse will blow. Open or substandard caps will
manifest themselves by producing "ghost notes"
when the guitar is played; these notes are sum or
difference frequencies caused by the guitar beating
with the 120Hz ripple on the supply. You can also
see it on the scope; the worse the caps are, the
earlier (in amplitude) the output waveform will
display hash or ripple on the peaks/valleys.
>5) If it isn't in the PS, pre-amp, or output,
>now we know it is in the phase inverter.
LV: Yep.
>5) The coupling capacitors and cathode capacitor
>in the phase inverter had been replaced.
LV: Worth a shot, although I'd have left 'em for last.
>6) The tube in the PI stage had been replaced and the socket cleaned.
>All wires ahd been inspected and poked, and all solder connections reflowed.
LV: Good technique. Still curious about what you put in the sockets.
>7) Now we're down to just resistors in the PI stage. They all looked
>fine on inspection by poking with a chopstick. How did LV conclude it
>was the plate load resistor?
LV: Well, I wish I could tell you I was a hotshit rocket
scientist or something, but I'm just going to have to
chalk it up to experience. Plate load resistors are
*always* the number one suspects for crackle in guitar
amps, and they are the *first* thing I take a shot at
when I have a noisy amp on my bench. The old Ampegs
are especially subject to this problem, since the
tubes are upside down on PC boards, with the resistors
right next to 'em where they can get cooked. Being
sandwiched between the PCB and a steel plate doesn't
help any, either. If you're going to mess with tube
guitar amps, invest in a couple of cans of freeze spray;
when you have noise, shoot the plate load resistors in
the suspect stage(s) with the freeze spray and see what
happens. The noise will either get *really* bad or it
will stop completely, and come back gradually as the
resistor returns to normal temperature. Using the
freeze spray will often save you from having to sub
in a mess of resistors, too. BTW, don't get any on
hot tubes, they tend to crack if you do. (More
"experience." ;-) Sorry for the delayed reply; I'd
forgotten that I posted to this thread, and I've (as
usual) been fighting numerous flamewars on AGA, where
I am feared and hated. ;-)
Lord Valve
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