Fender Vibro Champ

[stokes]

Got a 66 Vibro-Champ on the bench. Owner said he got it new in 66. Hadnt used it in years and it was stock. When I opened it up the first thing I see is that the cathode cap had been changed on the 6V6 and the resistor looked none too well. So I call him and ask about it and he says "oh yeah, I kind of remember it having a break down over 30 years ago". He bought it to me for a routine check up. I was gonna recap and give it a good cleaning etc. So looking further I see the output tranny was changed too and they used a small PP tranny. So I get a new one, change the cathode resisitor and cap on the 6V6. The schem on these amps shows a 25v cap here, I put in a 50v as I have seen at least 3 of these amps with blown caps here. Get it all together and I have it on the scope watching the signal and suddenly the trace goes flat on the scope and I hear a hum I reach for my other probe and start to signal trace and the cathode cap goes bang. Just looking for opinions here,put the tube on my tester and its shot. My question is, do you think the tube went first and fried the cap or vice versa. I am thinking since the signal disappeared first the tube went first, I did check the resistance on the windings on the OT and they seem fine. If you think its the other way around, what would be some issues that would cause that cap to blow? Cap was oriented correctly and the resistor was very hot, its a 5 watt resistor.

[The Dude]

You tested the tube on your tester and said it is shot. Was it shorted. no gain, leakage, other, etc.? What value resistance was the cathode resistor- 470Ω? Did you measure cathode voltage when you fired it up? Aside from all that, it's a chicken or egg question, and may be difficult to answer. Next time you fire it up with new parts, I'd certainly check cathode voltage before running it for very long.

[stokes]

The tube tester I use is a modern computer controlled "U-tracer". It doesnt show shorts, if there is a short it wont show where, it just doesnt give any of the readings, pl current, gm, pl res etc remain blank on the test. I suspect grid to cathode shorted? There is a snubber cap between the grid and cathode, maybe that went south. Cath res is correct. Did a voltage check before connecting it to the scope and everything looked fine. I suspect the tube, its an old stock that tested new but you can never tell. The trace looked good for the minute or so and then went flat about 30 secs later and the cap let the smoke out. I do plan to monitor the voltage on the cathode when I fire it up again. Yeah, chicken or the egg,exactly what I thought looking at this. Figured it would make a good discussion is all, not looking for def answers.

[g1]

Coupling cap from the previous stage leak or intermittent leak?
I think it would take a fair bit more than 50V to blow the 50V cap, maybe 70V? At 470R, 5W cathode resistor might be problematic. Will allow 50V there before it is over-rated.

[stokes]

Gotta go back and check, I did check when I did the voltage check before connecting the scope and signal gen. I get what you are saying about the 5watt cath resistor, but what cause the over voltage on the cap is the mystery. A big clue that it was the tube that blew first is that the output signal went flat before the cap blew, a good 30 secs before.

[Mick Bailey]

You have to wonder if there's something going on that was the original reason for the OT being replaced. An over voltage condition that would blow the cap must stress the transformer a fair bit.

[stokes]

So I got it working good today. I went and checked the coupling caps for dc leakage, all good again. I mentioned a small cap between the grid and cathode of the 6V6. As I was doing the voltage check today I saw that it had been removed. I know from working on a few of these and building a similar circuit for my son that this cap is necessary to snub a serious parasitic. So I'm thinking, for this current failure the oscillation drove the power tube so hard it blew. Still doesnt answer the chicken/egg thing. I replaced the snubber cap, new tube and cathode cap and clipped my meter to the cathode and monitored the cath voltage as I applied a signal and watched the scope for about an hour, raising and lowering the volume control now and then and everything looks good, cathode voltage never went above 26 v was 22v at idle and increased a little as I turned up the volume.As for how the amp got this way to begin with is a mystery. When I got it, the owner told me there was nothing in particular wrong with the amp, just wanted a maintenence thing since its so old. Said it was still stock. When I found the sloppy work I sent him a picture and asked about it, he said he kind of remembers the amp "blowing" some 30 yrs ago but didnt have a lot of memory about it, I suspect it was more than 30 yrs. This guy is an older jazz player and I suspect he probably hasnt used it since it was "repaired". I'm guessing its been in a closet forever, said he got it for his 13th b'day and it had sentimental value. He got my number from a local jazz guy that I did work on a bunch of his amps and I guess he just pulled it out for the first time since it blew. Cant figure for sure what caused the OT to blow. I know the cathode cap value is marginal and they blow often,at least IME, I've seen it in at least 3 of them.Have no idea why he pulled that snubber cap off. Or if the amp blew as he said, and it was pulled off during that repair, or if it was removed and then the amp blew. I'll have to send it to MIT for a forensic exam.

[Helmholtz]

The grid-to-cathode cap is used to prevent oscillation.
With oscillation the cathode current will go up and the cathode voltage will increase.

[stokes]

So, I spoke too soon. Everything looked good on the scope, no problem with the cathode voltage. I just connected a speaker to it to plug a guitar into it before I put it back in the cab. Its fine with the volume control between 3 and 9, above or below and I'm getting a loud squeel. Thats with nothing plugged into the input. Put a jack in it and with the guitar volume all the way down and I can turn it to 10 and it takes a minute for the squeel to start and its much quieter.From 1 to 3 still the same but a bit quieter. Pull V1 and it stops, assume thats where I gotta look.This is really pissin me off cause the guy told me the amp was stock. Every time I get an amp that has some doubt to its past, turns into a nightmare. From now on, I dont care who it is, when I open an amp and find it has been messed with,when I was led to believe otherwise, I'm closing it up and sending it right back. Any ideas on this one?

[The Dude]

Since the OT was changed, it could be wired incorrectly so that you have "PFB" instead of NFB. Try reversing the primary wires.

[stokes]

I put this tranny in. It is a Heyboer made for BF Champ. Red to B+ blue to plate, unless it is wired backwards internally. And I would think it would still be there with the V1 pulled, no?

[Chuck H]

unless it is wired backwards internally.

That does happen.

I don't recall seeing the plate voltage indicated. I'm thinking of a VC I worked on many years ago that was actually bone stock and had ridiculously high voltage.

[stokes]

A couple volts higher than the schem, normal for todays higher line voltage.

[The Dude]

I put this tranny in. It is a Heyboer made for BF Champ. Red to B+ blue to plate, unless it is wired backwards internally. And I would think it would still be there with the V1 pulled, no?

That is often the case with new transformers. It's certainly worth a try. In the wise words of Enzo, "Never think up reasons not to check something."

[stokes]

That is often the case with new transformers. It's certainly worth a try. In the wise words of Enzo, "Never think up reasons not to check something."

While it was on my list of things to check, I was gonna start at the input and work my way to it. As I was staring into the chassis I decided to start with the primary leads. Bam! That was it. What was really throwing me was the fact that it only oscillated with the volume all the way down or all the way up. Plus the fact that I have no idea what somebody did to this amp and the owner really couldnt remember. Theres just so much head scratching things in there. A PP OT that had the secondary wires connected with electrical tape, sorta half twisted bare wire ends with tape wrapped around them, the cathode cap on the 6V6 was two 25 uf 25v caps paralleled, the volume pot and the bass pot were replaced and all the solder joints on the right end of the tag board associated with the 1st stage had big sloppy gobs of solder but no components replaced and that snubber cap on the 6V6 was removed. Gonna tell the owner if he ever remembers who did those repairs he should kick him in the balls. Thanks to all for the contributions. Oh, Greg, I did change that 470ohm 5w with a 2 watter.