Bassman won't stay fixed!

[Danlampton]

I have a bassman in the shop for the third time. I fix it, test it, customer takes it home, it works for a while (a day or so), and then it comes back. Customer is being patient but I'm at my wit's end.

It's a '68 AB165. I've replaced the filter caps, bypass caps, bias cap and resistor, screen and grid resistors, and replaced the tone stack in the normal channel. The last time he brought it back one of the power tubes was coming apart physically and arching. Put a new set of JJ's, measured everything, and tested - sounded great.

He took it home and played about 40 minutes then started getting loud noise. I went in and replaced resistors in the PI (they measured a bit high) and redid most of the solder connections around the PI and the power tubes. I fired it up and it sounded great again, but this time I left it on for over an hour. Sure enough it started this noise again.

Where do I need to look now? I don't want to see it again - for a while anyway.

[MWJB]

Describe the noise.

Did you check the board for conductivity?

You say evrything measures up OK, humour me, what are the measurements?

[R.G.]

Per'aps it's cursed, or haunted. 8-)

I had an issue with an amp once where new Bad Things happened repeatedly. Turns out the genius that fixed on it before me had left dropped balls of solder in the chassis that rolled about under the board.

[Chuck H]

Have you ever cleaned and retensioned the tube sockets??? are they worn to the point of replacement??? Intermittent faults can cause all manner of wierdness. Loss of bias and crackling noises, etc.

[Tom Phillips]

I've got a 1968 Bassman headed back to my shop with a similar problem.
After carefully listening to the customer's most recent description of the noise I now realize that the amp never made the noise he was complaining about when it was last on my bench. However, it did make other very bad noises which I isolated to the PI tube. I did verify that it was the tube by multiple swaps with a good tube and the noise always followed the tube. Result = a real problem fixed but the other much worse problem remained in hiding. My case is not "Amp won't stay fixed" it just needs something else fixed. The amp was left powered on for 2 days after the last repair and operated normally when I was there to listen.

I did a very thorough service of the amp last year so I now need to troubleshoot the problem and be sure I find the true cause. The good news is that the customer has better clues about how to force the amp to fail. The bad news (for you) is that whatever I find it is unlikely that it will be the same thing that is causing your problem. Sorry. But...I will report back to close the loop. It's just interesting that this amp is a 1968 AB165 Bassman. At least it started out life as an AB165. It has been converted to an AA864.

Intermittent failures can be most frustrating.

Best of Luck,
Tom

[R.G.]

My amp-tech friend found a Fender with intermittent clicks, rumble, noise, and so on. When he opened it up, he found that all of the wires to the output tube pins had been properly prepped and had at least one turn of bare wire on the lugs - but they'd never been soldered, from the factory. The amp started failing after over 20 years of intermittent use.

[Danlampton]

Thanks for running to my rescue. I appreciate it.

The noise is like the static on a radio - I'm terrible at describing noises. I let the amp cool down and fired it up again and it sounds fine, so I'm thinking something is heating up and changing shape or values. It isn't affected by either channel's controls.

When I say things measure correct I mean the resistor/cap values. I haven't measured voltages, but I will. Didn't thing to check the board - that sounds like my next plan.

Thanks again.

BTW, I did retention the tube sockets.

[Billy R aka DynaFreak]

a couple of things come to mind. Plate resistors on the preamp make statiky noises when they go bad and often dont reveal themselves until warmed up. A good diagnostic tool for eyelet board Fenders is to pull out the bottom board and examine it (with amp powered down of course!). It will often reveal arcing or other problems on the side that faces the eyelet board. I had an evil Bandmaster once that had very similar symtoms to yurs. Turned out a component lead had actually punctured the bottom board and intermittently grounded to the chassis! It happened only when the amp was warm and heat expanded and flexed the eyelet board.

[R.G.]

Good one.

I once found a solder joint that looked perfect, but was "tented" on top of the component lead, but not attached. It was a perfect conical tent, and when cold made contact fine. When hot, the solder tent expanded and no longer contacted the component lead. It had its own, built in and completely unintentional thermal cutout.

I found it purely by accident. I suspect that solder joints like this may be why shotgun resoldering works in many cases. Well, OK, that and mechanical stress cracks.

[Chuck H]

It's very true those black fiber boards can sometimes become conductive. Especially after extensive mods or rework. I've had to can three conductive boards of the five black fiber boards I've worked on. My own experience may be disproportionate to the actual occurence of the problem but I refuse to build an amp or suggest a kit that uses a black fiber board. Today, as it has always been, 99% of black pigments are made from "lamp black" which is basically that soot that builds on the indide of flame lamp glass. It's carbon and it can become conductive if enough voltage is applied or it becomes very hot. Two conditions not uncommon to tube amps. Once a conductive path starts to develope it seems to get worse as the conductive path exacerbates the problem.

Just stick the black lead on the chassis and poke the red lead into the black board at several locations. Even a small voltage can be very problematic. Imagine an extra half volt of bias on a preamp tube that typically has only 1.2V of bias anyway.

Down with black boards.

[jrfrond]

In a Fender amp that age, you can have manifold issues. Most have already been mentioned. Apparently, what you have going on seems to be thermal, and the way you describe the noise, it sounds like a bad plate resistor. If you want to save yourself headaches and returns on this one, "shotgun" all of the plate resistors with new carbon comp resistors, and then cook the amp into a load and get it nice and hot and see if the noise occurs again. You need to start eliminating commonly suspect components. You can also wait until the noise happens and scope individual stages, but this will take more time.

BTW- when troubleshooting weird issues in ANY tube amp, always do it with a fresh set of tubes, or you may very well wind up chasing phantom component issues.

[Jag]

Yeah, plate resistors are my first thought too. I always shotgun the plate load resistors with new metal film resistors when I get this complaint. 99.9% of the time it fixes the issue.

[Enzo]

OK when cold, but starts making noise when it warms up? That is what freeze spray is for. My first thought is noise plate resistors somewhere also. Hit them with some freezit.

[Ronsonic]

My amp-tech friend found a Fender with intermittent clicks, rumble, noise, and so on. When he opened it up, he found that all of the wires to the output tube pins had been properly prepped and had at least one turn of bare wire on the lugs - but they'd never been soldered, from the factory. The amp started failing after over 20 years of intermittent use.

Ha ha. I did that to myself on my tele. Took fifteen years to find that I had forgotten to solder one lead on the volume pot.

[Bruce / Mission Amps]

Thanks for running to my rescue. I appreciate it.

The noise is like the static on a radio - I'm terrible at describing noises. I let the amp cool down and fired it up again and it sounds fine, so I'm thinking something is heating up and changing shape or values. It isn't affected by either channel's controls.

When I say things measure correct I mean the resistor/cap values. I haven't measured voltages, but I will. Didn't thing to check the board - that sounds like my next plan.

Thanks again.

BTW, I did retention the tube sockets.

I've been doing this for many years.... I can't tell you how many hundreds and hundreds of vintage Fender amps I've worked on, where the eyelet board was defective, conductive or shorting out somewhere as others have described... because the answer is: So few that I can't remember any of them, and that's if there were any at all.
I think that's a red herring this time.
However, the bad solder joint sounds like a good bet and I have also (RG's point) found old solder joints that looked good yet I could gently tug the lead right out of the solder!!
The #1 culprit I've found is the grounding lead(s) from the eyelet board to the brass plate... yours sounds like it could be the one like the bare wire from the phase inverter section off the NFB grounded 47 or 100 ohm resistor.
Since it is unaffected by the volume or tone pots, it has to be after the two channel mixer stage.
Check them all and resolder them anyhow.