Vibroverb gremlins

[mdoolin]

Hi All, this is my first time posting, and I'm hoping for some repair advice. I have a 64 Vibroverb that's making some odd noises. When I turn it on, just as the amp starts making sound, there's always a soft pop, not very loud, and when I turn it off there's another pop, somewhat louder. These pops happen even with both channel volumes off, and if the reverb is on the pop does go through the reverb. What happens in between power up and power down gets more interesting. Usually within the first minute of operation, there will be a soft crackling sound that builds in volume over a few seconds, and then another pop happens and the noise stops. Then a minute or so later, a soft warbling oscillation appears, with a pitch that wanders up and down within about a half step, and again a pop happens and that noise stops too. And then sometimes, there's just a random pop, every few minutes. None of these noises are very loud, and they aren't affected by the settings of any of the controls. The pops are the loudest. And there's no effect on the guitar signal when I'm playing, the noises just come and go in the background.

Here's what I've done so far to try to track this down:
1) All new tubes, mostly JJ except the two 12AT7's are JAN NOS
2) All new 100k plate resistors
3) All new electrolytics, all Sprague Atoms
4) Cleaned all the pots
5) Retensioned and cleaned the tube sockets
6) Re-soldered every joint on the eyelet board
7) Pulled the brass ground plate under the pots and scoured off the oxidation

And none of that had the slightest effect.

So, I'm stumped. This Vibroverb is the crown jewel of my modest BF collection, and I would gig and record with it more if I could just banish the gremlins!

[Enzo]

Well, never overlook the possible conductive eyelet board material.

meanwhile, we start by isolating the problem. You know the pop is before the reverb, and after the volumes. Do the tone controls affect the tone of the pop?

Remove one tube, does the pop remain? Return it and pull the next tube, pop remain? etc. Even if the tube itself isn't the pop, knowing which stages are involved helps.

[mdoolin]

That was very helpful. It's V2. I replaced it with another JJ ECC83s and so far it's behaving itself. I doubt the problem is really fixed, more likely the new tube just has a little less gain so it doesn't incite the problem. Looking at the schematic there isn't a whole lot between the volume control and the reverb, and I've already replaced most of that. But there are two coupling caps that might be leaky, so I'll replace those. It may indeed be the conductive eyelet board problem. But if that's the case maybe using a 12AT7 in that position would keep the gain under the threshold. I use this amp for clean jazz playing so I don't need maximum gain.

Anyway, thanks for the troubleshooting suggestion!

[Enzo]

Gain is not usually the cause of erratic noises. Gain causes hiss, a steady noise. And conductive eyelet board is also not gain related. A lower gain tube might reduce the overall noise by amplifying it less, but that is not the same thing as a cure.

[mozz]

Think you know what the problem is, coupling caps.

[Chuck H]

That was very helpful. It's V2. I replaced it with another JJ ECC83s and so far it's behaving itself.

then

I doubt the problem is really fixed, more likely the new tube just has a little less gain so it doesn't incite the problem. Looking at the schematic there isn't a whole lot between the volume control and the reverb, and I've already replaced most of that. But there are two coupling caps that might be leaky, so I'll replace those.

None of that makes much sense from a repair perspective. You should continue to use/test the amp and continue trouble shooting properly if the problem persists. FWIW new, noisy tubes aren't uncommon anymore. It's not at all out of the realm of modern possibility that when you originally replaced the tubes, you put another noisy one in the V2 position. Noisy, crappy new tubes have become the bane of our existence as amp guys. JSYK

Caps can be tested for leakage. Arbitrarily replacing parts is a recipe for failure. The more you burn into that old board with an iron and the more work you do, the greater the possibility that a secondary and confusing issue will manifest. IT HAPPENS ALL THE TIME!!! You shouldn't fix things that you don't KNOW are broken.

It may indeed be the conductive eyelet board problem. But if that's the case maybe using a 12AT7 in that position would keep the gain under the threshold.

Just backing Enzo here... A lower gain tube doesn't do anything for such a problem other than reduce it's volume in proportion to your playing volume. Not only is that NOT a repair, it leaves you with exactly the same level of problem at equal volume levels relative to gain. Please re think the logic in your reasoning for this "solution".

Now. Since I've been less than graceful you have already decided that you hate everything I say. Don't care much. If it were "MY" amp I would heat the circuit board with a hair dryer for a couple of hours while I watched a movie. Then I would jab probes right into the board and test for voltage in the fiber board itself. Even a large fraction of a volt is enough to ruin things if it's near a preamp tube bias resistor! If I didn't find voltage on the board I would move forward by replacing any original electrolytic capacitors in the amp. They are ALL past their expiration date. Then I would test resistors for drift and replace anything out of spec. If I continued to have trouble I would TEST film caps for leakage rather than arbitrarily replace them. Any arbitrary replacement of parts is utter frivolity and most often causes more problems than it fixes.

Whether you already dislike me or not, ignoring this advice will put you on a common path that often leads to disappointment. I am rudely doing you a big solid right now by diverting you from the course you intend.

[mdoolin]

Thanks for the extensive reply. No, I don't hate you, I very much appreciate your advice. If I were looking for tact, I wouldn't have posted a question on an electronics forum.

Points all taken. I've been recording with the amp all evening and it's quiet as a church mouse. I'm going to chalk it up to a bad tube.

[bob p]

regarding the intermittent pops -- did you look at the power tube screen resistors?

[mdoolin]

Thanks, I did check those and they're both good. Again, the pops have reverb on them so they have to be coming from earlier in the circuit.

[bob p]

How do you determine that a power tube screen resistor is good? As in not the source of crackling?

[mdoolin]

I looked at them, they're not charred or cracked. I measured them with an ohmmeter and they read within 10% of value. And I tapped on them with a wood stick while the amp was running and they didn't make any noise.

[Enzo]

I may have mislead you. The pop gets reverberated, so I said it comes in before then. However, a noise on the power supply could inject itself many places. I do think it is before the reverb, but we must not overlook the other possibility.


Noisy parts won't usually read off value. Crackles and pops may not have any mechanical relationship.

[mdoolin]

All true. But at this point I'm pretty sure it was just that particular ECC83S tube. They don't make 'em like they used to! And FWIW I did put a 12AT7 in V2 and it makes the amp's gain much more manageable.

[Chuck H]

If you like the lower gain of the 12at7 you might also like an 12ay7. To my ears, in a typical type circuit for a 12ax7 triode the 12ay7 sounds really solid and tube-y. I know that sounds a little like lore rubbish, but just try it and see for yourself. A little lower gain than the 12at7. The 12at7, to my observations anyway, is sort of a "function" tube that can be put into audio service if necessary. They tend to microphony and sound sort of blah in a circuit designed for a 12ax7 to me. YMMV. A more signal path intended tube with similar gain to a 12at7 would be the 5751. I've never used them, but they seem popular with players.

[Tom Phillips]

...there are two coupling caps that might be leaky, so I'll replace those...

I'm adding my comment to the others that parts should not be replaced unless you verify or strongly suspect that they are bad after proper troubleshooting. I'll also add that the blue molded tubular caps used by Fender when your amp was manufactured are extremely reliable and long lived. Any time I see an amp with a bunch (or all) of the blue caps replaced I know that someone shotgunned a repair or fell prey to the hype about "orange drops" or such being an upgrade.