Vibro Champ rising DC leakage problem

[Fretts]

What's going on?
Vibro Champ shows slowly rising DC voltage on pin 5 of the 6V6, that's where the output of the preamp comes in. DC is supposed to be blocked by a ceramic .02. I hooked up a meter, and powered up. As the amp sits, the DC voltage there slowly rises, starting out around 25 millivolts and gradually rising. When it gets to around 250-300, the 6V6 plate starts glowing a little and the famous hum begins. Ceramic caps are pretty stable, aren't they? Why would steadily rising DC appear at the 6V6 grid? If I shut down, wait 30 sec or so and start over, it behaves the same... starts out at 25 or 30 mv, settles down at around 500 after 2 or 3 minutes, after 15 or 20 minutes, it shows about 300mv and stays there.
Gurus - any clues?

[Enzo]

Pull the 6V6 from the socket and measure resistance to ground from pin 5. Is it open, or do you get the resistance of the grid resistor?

Power up the amp. Now monitor the voltage at pin 5. Does it still climb? If so, replace the cap. If the voltage remains stable, I'd have to say your 6V6 was bad.

[Fretts]

Thanks Enzo, I guess it has to be the cap. Pin 5 to ground = the grid resistor, yes, 3 totally different 6V6s behaved similarly, so that pretty much leaves the capacitor, or space aliens. I'm going with Capacitor.
Realistically, what is "normal" leakage for a typical cap used this way?
Thanks
Jeff

[Fretts]

new clue

I unscrewed the eyelet board and lifted it slightly off the deck.
The problem 100% went away.
It's the board, or the insulator underneath. Now what?
I can't just tear it down, much of the amp's value is its original vintageness, and all the solder joints are factory and its a real showpiece.
What can a feller do to address this, without Invasive Surgery?

[R.G.]

Thanks Enzo, I guess it has to be the cap. Pin 5 to ground = the grid resistor, yes, 3 totally different 6V6s behaved similarly, so that pretty much leaves the capacitor, or space aliens. I'm going with Capacitor.
Realistically, what is "normal" leakage for a typical cap used this way?

Normal leakage is 0.00000000000000000000000000000000000000000000001A.

That is to say none at all. The old standard used to be you would lift the downstream end of the cap and use your voltmeter to measure the voltage between the free end of the cap and the "grounded" end where it went into the board. Any voltage at all meant a leaky cap needing replacement.

However, you say:

I unscrewed the eyelet board and lifted it slightly off the deck. The problem 100% went away.
It's the board, or the insulator underneath. Now what?
I can't just tear it down, much of the amp's value is its original vintageness, and all the solder joints are factory and its a real showpiece.
What can a feller do to address this, without Invasive Surgery?

You need to decide what's worth more - an amp with a huge amount of vintageness, the only defect of which is that it won't work, or a working amp with a bit less vintageness. I already have all the expensive bits of sculpture sitting around that I need, so I always decide that working and less vintage is better than pristine but nonfunctional.

I guess that's just my way of saying that one element of real vintageness is that the piece operates. Nonfunction is a big flaw even if no changes to the solder joints have been made.

The thing you need to look at really carefully is what connection was made or unmade when you lifted the circuit board. Is there a solder ball under there that shorts the coupling cap get opened? Did the cap wire ends contact a deposit of conductive dust and solder rosin, or bar funk? Did the flexing of the board when lifted cause the broken grid resistor ends to contact again and make the open-grid problem go away because the grid can now leak?

[Fretts]

mea culpa

I am an idiot. I had set my meter to AC volts for a different test I did before I got distracted by the phone and got involved in other things. I came back and didn't even check. The leakage is still there. If I owned this amp I would not hesitate to do the correct fix, along with a couple other things like balanced heaters and better grounding too. But I don't, and it is a trophy piece, so I have to tiptoe around. A mint 57 Chevy Bel Air shouldn't have electronic fuel injection even though it is so much better. I think the real fix is the replacement of a capacitor that I cannot easily obtain, in cosmetic terms anyway. I better talk to the owner and see what his preciousness level is regarding originality. I mean the amp works, it just isn't "right". I'll have to let him make the call.
Thanks Enzo!
Jeff

[Jag]

How does your cutomer expect you're going to fix the issue without changing anything?

[Fretts]

I know, it's ridiculous. I think he was hoping it would just be a cleaning, or tubes, or something that didn't permanently change any soldered parts.
I'll have to see where this amp fits into his world. It might just be a trophy that he doesn't expect to play much. If it's supposed to be a gig-ready working amp, then we're gonna have to talk.

[Enzo]

Well you COULD just unsolder one end of that cap and lift it away from the circuit. THEN watch to see if that DC voltage rises at the grid. If it still does, then some other mystery remains. if it stops, then you know the cap is bad.

HAving that sort of information leads to a much better conversation with the owner.

I can't imagine someone would beef if the solder had been redone somewhere, my god, what would he do if a solder connection was cracked, live with it?

How many of those mint 1957 Chevy owners really expect the original tires on it? Original oil in the tranny or differential? Not to mention in the engine. Original spark plugs? MAybe. All light bulbs original? Belts?

Keep the bad parts in a baggie and keep them with the amp. Someone ever wants it original again, they can solder the bad parts back in.


On the other hand, there is a guy in the area who horse trades and restores old Fender amps. He came looking through our parts drawers one day and found our stash of genuine NOS chrome top straps - the metal strips the chassis screws go through on top the cab. We had a bunch of them in various lengths and they were mint. They really were from the old days. He didn't disbelieve us, but he didn;t want them. Said no one would believe they were old parts. He went for the old and discolored stuff

[Fretts]

The relic thing shows that the market has gone to that extreme. The reissues look so cosmetically correct that the badge of honor is now for amps to show their age. Guitars have been like that for some time, to the point that you can now buy fake oldness albeit at a premium.
I have to admit, that's the tack I took for my 64 Super Reverb, I liked the weathered, been-through-hell look, so I made no attempt to make it look new. I did use Rolite on the shiny parts and they came up with a nice, old, cared-for look, but the discolored grill, and slightly injured Tolex stayed as I found them.
With CNC machining, carving and everything else, a perfect looking amp or guitar is so familiar as to be boring. I do get it, but at the same time, half of me sees the absurdity of it!
If you guys read Tonequest, you know there is at least one guy who ages amps in the "relic" way. He has a secret formula for dust bunnies that he hides inside the wood cabinets, a scuffing and ageing technique and so on. I guess it's no different than paying $90 for a pair of Levis that look like they have been in an industrial accident, but that's the fashion, and they do sell plenty of them.
Life is funny.

[Steve Conner]

Stick some extra insulator under the eyelet board and screw it back down. (after cleaning the area thoroughly to remove "bar funk" etc) I like to use pieces cut out of the clear plastic blister packs that some electronic gizmos come in, but probably even a couple of layers of PVC electrical tape would do.

It's not permanent, so if the guy gets upset, you can remove it and restore his amp to genuine non-working order.

[Fretts]

Thanks, that's a great suggestion. I was considering something similar, using "fish paper" which I have in a drawer. As it happens, the guy has taken the amp back to think it over. We'll see where it goes from here.
Thanks everybody for the great thoughts...
Jeff