BF Fender Vibrochamp questions

[Chuck H]

The trem voltage lead also connects there according to the layout. But there is what looks to be a thin, green lead there also. And I can't even see if that cathode lead is connected there and then to the cathode or obscured by the photo.?. And that cathode resistor lead is flirting with that pin 6 connection via the bare component leads.

I still think it looks like the trem plate is soldered to the filament supply. Maybe not. It looks like there may be a break there, but it's real close. I'd definitely be investigating all the wiring in these areas.

[nosaj]

Pretty sure I see some miswiring. It's hard to see everything well in the pics, but I'll open with seeing pin 6 being soldered to pins 4/5 (filament) on the tremolo circuit (V2b)

That particular one I took the pic then I pulled the voltage on that pin and it broke off. But voltages did not change after that I powered it off then on to see if it would.

nosaj

[nosaj]

If still have that much idle current with a 1.5k, I would say there is definitely something wrong. I would double check all wiring to the output tube socket. Make sure each pin is wired where it's supposed to be. Does it look like it's been messed with?

Well of course it's been messed with by me.

I'll do that and get back

nosaj

[Chuck H]

That particular one I took the pic then I pulled the voltage on that pin and it broke off. But voltages did not change after that I powered it off then on to see if it would.

nosaj

Leave it broken then. It's not supposed to be connected there

In fact, clean it up so it's not so dangerously close to connected.

[nosaj]

Leave it broken then. It's not supposed to be connected there

In fact, clean it up so it's not so dangerously close to connected.

Ok I put it back to stock minus the 1.5k and 47uf.

Pin3 344 pin4 358 pin 8 was 77 then down to 50ish.

When I turn it on the voltages looks good but as soon as rectifier starts conducting then voltages drop to these.
Im going to try a new octal socket tomorrow maybe when the OT bit it it did something to the socket?
New pics.

[Chuck H]

So the cathode voltage was 77, then 50 (ish). Did that correspond with the reported changes after rectifier warm up? And did you measure the plate and screen voltages with the cathode at 77V or 50(ish)V???

These details are important because a 1.5k cathode resistor with 50V and 358 plate volts represents about 12 watts. That's exactly what you want. Well, except that it shouldn't take a 1.5k resistor to get there.

Incidentally there is absolutely ZERO chance that the addition of a grid stop resistor, a screen grid resistor and newer filter caps caused any problems such as you were experiencing UNLESS something undetected and incorrect was hiding in the work. My position is that by returning the amp to stock you have inadvertently undone what was wrong, though it was none of those things.

FWIW, though I've never seen a VC with 358 plate volts I have seen mid 80's era Champs with that voltage. If your amp is about that vintage it's probable I just never encountered a VC from that era. Certainly the Champ and the Vibro Champ would have used the same power transformer.

I think you may have accidentally fixed that thing. But report back about the actual test numbers and circumstances in detail. We need to know what the stabilized voltages are.

It's still a good idea to use a grid stop resistor, a screen resistor and new filters regardless of what was wrong before you took those things out. Because it wasn't those things.

[nosaj]

So the cathode voltage was 77, then 50 (ish). Did that correspond with the reported changes after rectifier warm up? And did you measure the plate and screen voltages with the cathode at 77V or 50(ish)V???

These details are important because a 1.5k cathode resistor with 50V and 358 plate volts represents about 12 watts. That's exactly what you want. Well, except that it shouldn't take a 1.5k resistor to get there.

Incidentally there is absolutely ZERO chance that the addition of a grid stop resistor, a screen grid resistor and newer filter caps caused any problems such as you were experiencing UNLESS something undetected and incorrect was hiding in the work. My position is that by returning the amp to stock you have inadvertently undone what was wrong, though it was none of those things.

FWIW, though I've never seen a VC with 358 plate volts I have seen mid 80's era Champs with that voltage. If your amp is about that vintage it's probable I just never encountered a VC from that era. Certainly the Champ and the Vibro Champ would have used the same power transformer.

I think you may have accidentally fixed that thing. But report back about the actual test numbers and circumstances in detail. We need to know what the stabilized voltages are.

It's still a good idea to use a grid stop resistor, a screen resistor and new filters regardless of what was wrong before you took those things out. Because it wasn't those things.

Will do but it will be tomorrow before I can. I just took the 2 resistors out just to go back to earlier.

Something Enzo or some body said about not modding before it's fixed. So once we are sitting ok with where we should be I'll put them back in. Tomorrows a half day so I should get more time on it.

Thanks,

nosaj

[The Dude]

I agree with Chuck as far as what's happened here. I'll just add: I have seen VC's with lower plate voltage, but that's not as big of an issue as the extreme idle current in this case. IMO, I wouldn't be as concerned about the plate voltage, WITHIN REASON and where ever it ends up, as I would the bias.

[nosaj]

I agree with Chuck as far as what's happened here. I'll just add: I have seen VC's with lower plate voltage, but that's not as big of an issue as the extreme idle current in this case. IMO, I wouldn't be as concerned about the plate voltage, WITHIN REASON and where ever it ends up, as I would the bias.

Pin 5 when I probe it will have anywhere from 16 to 10v but goes down to 0 while im probing. Which makes the pin8 go from about 70 to 40ish VDC. The amp doesn't sound bad at all. And i did change the octal socket to rule it out.

nosaj

[Tom Phillips]

Pin 5 when I probe it will have anywhere from 16 to 10v but goes down to 0 while im probing. Which makes the pin8 go from about 70 to 40ish VDC...

I suggest that you check that the 220k resistor from 6V6 pin 5 is connected to ground. See the attached photo for where to check with the amp off and all filter caps completely discharged. Check both from the eyelet to chassis ground and from the resistor lead to ground without touching the eyelet with the probe. Also measure the resistance from 6V6 pin 5 to chassis ground and tell us the reading.
Tom