Quiet fizzy 2204 -VVR2 issue

[bcmatt]

It's being unable to solve things like these that make me feel really incompetent. I'd really appreciate some help and advice.

I've had a Ceriatone JCM800 2204 for 3 or 4 years now, but almost for the whole life of it, it felt like I had occasional but rare power problems until recently when it went quiet and fizzy after working great for a while.

My most recent change was to add VVR2, which worked great for about a several months. (This is not my main amp though; so it doesn't see heavy use).

I'm wondering if there is a component like the VVR Mosfet that exhibits these symptoms when failing or if it could be something else. I took some voltages yesterday and put them in blue to compare to the red ones M Huss has on this schematic. These are done with the VVR knob turned all the way up (off), but I get voltages mostly between 1/2 and 2/3 of his.

THanks so much for any suggestions. I thought I would come first here, because you guys are the best as well as most others aren't familiar with VVR in the first place (not that I'm saying that's the issue).

Should I try by passing the VVR altogether first, or is this obviously some other issue like a filter cap or PT?

My chassis is aluminum and I have a large heatsink mounted opposite my mosfet. I've used VVR on all my amps without issue, but I suppose this is the most powerful out of any I have it on.

[Structo]

Looks like you are about a 100v light everywhere.

What do you have the voltage selection switch on?

[bcmatt]

Looks like you are about a 100v light everywhere.

What do you have the voltage selection switch on?

Haha! That's a neat idea. I did just check that and it is on 110 voltage. I switched it to 220 and back again just now and started it up and it is still the same.

It's got to be putting out less than a watt of total power right now. It's that sort of sound like if you were to switch off the amp and keep playing for a few more seconds and you still get signal but it is quiet and fizzy. Makes me wonder if it is the resister from the + of the filter cap to ground that drains them has failed or something and is not resisting enough.... Maybe I shall check that...

[David Root]

In that case check the caps and the diodes prior to the plate B+. And your wiring of them. Something is seriously wrong.

[sliberty]

I think you may have misunderstood what Structo was saying.

Please measure your voltage at the filter caps right after the standby switch. Mark shows his voltage as 468V, but we don't know what you are seeing there.

Looks like you are about a 100v light everywhere.

What do you have the voltage selection switch on?

Haha! That's a neat idea. I did just check that and it is on 110 voltage. I switched it to 220 and back again just now and started it up and it is still the same.

It's got to be putting out less than a watt of total power right now. It's that sort of sound like if you were to switch off the amp and keep playing for a few more seconds and you still get signal but it is quiet and fizzy. Makes me wonder if it is the resister from the + of the filter cap to ground that drains them has failed or something and is not resisting enough.... Maybe I shall check that...

[bcmatt]

ok, thanks so far guys. That would be more helpful had I finished taking voltage readings. My band had showed up and was waiting for me to practice when I was in the middle of it last time. So, this should be revealing:

B+ voltage at first filter cap after diodes: 465v
Then we have the VVR2 B+ in: 462v
VVR2 B+ out: 300v

Then the B+ rail goes into the 200s from there.

\so, the fact that we drop 160v in the VVR2 when it is supposed to be full voltage is probably a pretty big deal...
So... does this mean that some piece in the VVR has failed?
Or is it still possible that one of these filter caps is failed?

[sliberty]

I'd recommend removing the VVR from the circuit as a final test, but it certainly sounds like there is a problem with the VVR.

[bcmatt]

I'd recommend removing the VVR from the circuit as a final test, but it certainly sounds like there is a problem with the VVR.

OK, so I finally did this and the amp works fine. So it was something with the VVR that failed. Anyone know which component failed and why? Is this what happens when the Mosfet fails? I do have a giant heatsink mounted to the chassis side opposite the Mosfet. Is there an even heavier duty Mosfet that would work for this amp? Or is it some other component that would cause this?

[bcmatt]

So, since I need money real bad, I'm thinking of selling this amp off. If this amp is too powerful to keep from burning out Mosfets, I'm going to need to do something more reliable to reduce the volume. I don't imagine people these days want a full power 50 (60??) watt 2204; or do they?
"Well it sounds really great if you are willing to make your ears bleed".
So, I'm thinking a PPIMV may be the way to go for better tone at lower levels than stock. Thoughts?

The general public doesn't seem to understand how great VVR is anyways, so why put them in danger of overheating a Mosfet?

If only I didn't have to order away to the States for a single dual-ganged pot...

[sliberty]

The general public doesn't seem to understand how great VVR is anyways, so why put them in danger of overheating a Mosfet?

Agreed. If you were building amps and wanted a feature that puts you ahead of the rest, the VVR would be a real plus. If the amp was for you, same thing. But if you are just trying to unload the amp, a good MV should be good enough.

[stubbyfex]

I had an odd problem with my 6V6 Marshall Hybrid amp. (Front end of the 2022, into the 2204 driver and Phase Inverter) I installed the VVR and it dropped the B+ from 400v to about 285 volts. I tried many things, but ended up replacing the 12v zener, and now it works from 400v down to about 50v. I scaled the entire amp. It sounds great. The tone changes a little at 50v, which surprises me that the amp will even run at that low of B+. I also scaled the bias. I'm using Mr. Hall's VVR2 board. (Wonderful product!). Even with a shorted Zener, it did not blow the Mosfet.