Crackling and fading Admiral HS-550

[gsl]

Hi All!

I have a several years old but unused Russian JCM 800 clone, an Admiral HS-550 Tube Combo. 2x 12AX7 in the Preamp, a 12AX7 in the Phase inverter and 2 Sovtek 5881WXT in the PP Power Stage.

The problem is twofold:
1) The amp crackles intermittently and unpredictably. Not very loud, it is inaudible when playing along with drums, but very annoying when it's quiet. The source of the crackle must be in the power stage because it also occurs with no input signal and the volume pots down to zero.
I have cleaned the tube contacts with a brass brush and alcohol and also the sockets. I think it is better now but the problem still persists.

2) The amp fades in volume intermittently and unpredictably. Say I play with the master volume at 2-3 and out of nothing the volume drops to almost inaudible. Then I have to turn the volume pot up to 9-10 to get my previous volume. And then suddenly it kicks back in very, very loud.

I have opened the amp, inside it looks OK, no visible evidence of sparking or cold solder joints. The chop stick test was also without any notable outcome except that when I chop a little bit harder on the PCBoard the amp pops very loud. It's a DC pop that lasts for about 1 second and then it pops "back".

I would appreciate any opinions or pointers as to where to look further. Thanks,
Michael.

[Steve Conner]

The Magic 8-ball says it looks like a bad phase inverter tube, or maybe your master volume pot might be worn out.

[gsl]

The Magic 8-ball says it looks like a bad phase inverter tube, or maybe your master volume pot might be worn out.

Thanks for your reply Steve, I was already starting to despair thinking that nobody loves me ;-)

I do not think that the volume pot can be worn out. The amp is unused as I said. I had bought it about 4 years ago because it was a curiosity and very cheap but I didn't need it till now. So it has been switched on for maybe 10 hours in it's entire life.

Same goes for the tubes. I will try another splitter tube as soon as I put it back together but it will be hard to test since the behavior cannot be induced at will (it appears only incidentally) and the band is already fed up with it so I can't play it at rehearsals.

Any other suggestions that I can check while the amp is dismantled and I can look at and reach the bottom side of the PCB?

Thanks,
Michael.

[mac1amps]

I don't know to what degree your amp is a JCM 800 "clone", but if it has the same input jack configuration (a "high sensetivity" input and a "low sensetivity" input), your problem could quite likely be from a tarnished/ corroded shorting contact on the "low sensetivity" input, which [in the case of the JCM 800's] is notorious for causing the exact symptom's you are describing (crackeling and signal drop-outs). The signal path relies on this shorting contact in order to "pass" properly when you're USING the "high sensetivity" input. I would also check that all your controls are physically tightened firmly....as far as your master volume control goes, it's unlikely that it's "worn" unless it is CONSTANTLY being turned up and down like a MADMAN (as if it was being used as the pot in a wah-wah pedal, for instance). And, of course, it could ALWAYS be some of that lousey "factory" P.C. board soldering.........good luck!

[gsl]

Wow! mac1amps. Thanks a lot. That sound like very useful information.

Since I also own a 1982 JCM 800 Model 2204 I can vouch that the Admiral is an *exact* clone of this amp (except for the 5881s instead of the EL34s) and of course has a "high" and "low" input wired "in series" relying on a not-very-high-quality-looking plastic jack to make the contact.

So I am refloating/resoldering all the joints on the PCB, cleaning the contacts or replacing the whole jack, putting the whole thing together, and reporting as soon as I know.

Thanks man. That's the kind of information that saves the day. Thank God for the internet.

Michael.

[mac1amps]

FWIW, I've had reliable success on many occassions (I repair/service amps for a living) just by CLEANING the aforementioned contacts, and perhaps a tiny spurt of Deox-it. It's RARE to have to actually need to replace the jack.....'just thought I'd pass that along........(Mac)

[Albert Kreuzer]

What mac1amps said.
The Admiral is the same as a Sovtek 50H Midget, which is a 2204 clone. I had one in for repair recently with a broken input jack. Looked OK when I opened it, but fell apart as soon as I touched it. The amp did all kinds of crazy oscillations and stuff. IMO these jacks are utter crap, replace them by all means.
But otherwise, these amps were a good buy (I have one myself)

HTH
Albert
www.albertkreuzer.com

BTW I'm not far from Vienna, contact me if you need help

[gsl]

I did as I said and refloated all solder joints on the upper&underside of the pcb and all the others too. I actually filed the contacts on the input jacks and tightened the oval tube sockets with a strong pin.

After testing this amp for about 10 hours at home with CD-Walkman input (no incident) I was confident enough to take it to the rehearsal. The first 4 hours Sunday before last, no problem whatsoever. Sweet and tight - as a matter of fact, I think it's ever tighter at the bottom than it used to be. Signal fade out completely gone.

Last Sunday alas the crackle reappeared twice but much lower in volume and only for a few seconds so that nobody but myself noticed. If it doesn't get any worse I consider the amp cured.

Thanks again Mac.

@Albert: Thanks for your input and encouragement. I visited your website and I'm gonna give you a shout via mail if I may.

[gsl]

Update

Hi all,

Here is an update after 4 months:
After cleaning the contacts and refloating all the solder joints the amp is as well behaved and silent as a 10W SS amp even when at high volumes.

That did the trick!!!

Thanks to everyone for your help.