Intermittent problem with PAB

[brads]

Hopefully someone can shed some light on this:

I have Bluesmaster-style HRM amp that has an intermittent PAB issue.

When PAB is engaged, on one or two select notes there is bit of congestion, then a slight volume spike, which drops again after a few seconds.

The builder thought it might be a bad relay, and after swapping it with the MID-boost relay, everything seemed fine for a day or two. Until today.

The problem is back, on a different note now (B-string, 7th fret, hit pretty hard and then the note is bent up). Same little burst of congestion, and short-lived volume spike.

The amp is a head, and it is not sitting on the speaker cab. If I don't hit the note hard enough, it won't exhibit this issue. It happens whether I use the footswitch or the toggle on the amp.

In all other respects, the amp sounds great.

Anyone care to venture what this might be?

Thanks!

[Bob-I]

That's a really odd one. No I don't know what it is, but here's how I'd go about trying to locate the problem.

It seems to be related to the level of the input signal. Hook up a dummy load and a signal generator set to maybe 100Hz, or even lower. Crank the input signal and watch with a scope for the output to change. Once you can force the issue with the signal generator, start probing the areas around the PAB and tone stack to see where the problem starts.

Once you locate the problem you can go about finding the cause.

[tubedogsmith]

Sounds like it's caused by vibration. My guess is a cold solder joint or bad connection. It shouldn't be hard to find with a chopstick.

[brads]

Thanks, I'll give these a try.

I wondered about vibration, but since it is a head isolated from the cab, I didn't think there would be much movement.

[Funkalicousgroove]

Have you tried different tubes or speakers?

Have you checked all your solder joints?

[greiswig]

I was going to recommend trying a different speaker, too. I just chased my tail on an issue with a Bad Cat amp that sure sounded electronic in nature. Similar to yours, it was only on a few notes, but in this case it didn't matter much what the volume was. But before I tore into the amp, I tried a different cabinet. Sure enough, the problem went away.

[Structo]

So did it turn out to be a sympathetic vibration type rattle in the cab or was the speaker defective?

[greiswig]

So did it turn out to be a sympathetic vibration type rattle in the cab or was the speaker defective?

Tom, I should have been more specific.

It's something stupid. I'd put a favorite speaker in this combo, an old 18W Weber Blue Dog that just sounds great...chime like a Celestion Blue, but with friendlier high-mids. But the amp is rated at 30W. I didn't want to blow out the speaker, so I tried an old trick I used to use to protect horns in monitors and cabinets: I put an auto light bulb in series with the speaker to act as a limiter. But there is something about the bulb itself that generates some noise...possibly the filament rattling at certain frequencies. I've since bought a speaker with a higher wattage rating for that amp.

My main point to the OP is that the speaker is an easy thing to check, and could be the source of the sound.

[Structo]

Ah, thanks for that. I seem to remember that trick as well.

[brads]

Have you tried different tubes or speakers?

Have you checked all your solder joints?

Have switched speakers and cables, as well as preamp tubes, but it's still there.

Will swap the power tubes and the tube in the D-lator. If it's still there, then on to the solder joints.

It's just so odd that it occurs on the B string bending up to a G. It's more prevalent on the neck pickup but I can get it to do it sometimes on the bridge pickup. It's like the frequency overloads everything somehow.

Thanks all.

[Structo]

Are your relays plugged into sockets?
Are they firmly pushed in (I imagine you have tried all the obvious things).

Have you tried pushing the front end harder with an overdrive pedal or fuzz to see if it is related to how strong of a input signal there is?

Have you tried banging on the head cab when it does it?

One time I had a tube that would rattle at a certain frequency.
Filament rattle I believe.

[brads]

Yes, I swapped relays around.

Will try the OD pedal and hammer.

Thanks!

[Structo]

Ah I was half way joking about banging on it.
What I was suggesting if it is an intermittent or bad connection, hitting the top of the cabinet with your hand might re-create the problem, or not.
NO HAMMER!
Although I'm sure it has crossed your mind in frustration.

These intermittent things can drive you crazy!

[brads]

Ah I was half way joking about banging on it.
What I was suggesting if it is an intermittent or bad connection, hitting the top of the cabinet with your hand might re-create the problem, or not.
NO HAMMER!
Although I'm sure it has crossed your mind in frustration.

These intermittent things can drive you crazy!

Maybe just a ball peen.

Well, I think I figured it out: Good news bad/news scenario. The only thing I hadn't tried switching were the power tubes.

Good news: That seemed to do the trick.

Bad news: The bad tubes were RCA blackplates.

Thanks everyone.

[Funkalicousgroove]

99% of tube amp issues are tube related, sorry it was your black plates man!