Carvin V3 Strange Oscillation

[The Dude]

I'm working on a Carvin V3 with (I think) a strange problem. The output is going into low frequency oscillation. It starts when you get the level up to a reasonable level- not at power up. It's very low frequency. You can't hear it, but you can watch the speaker bounce up and down even after signal is removed. I pulled V4 after I got the oscillation started, and it continues to oscillate, so I'm thinking it must be in the output section. I tried new tubes- same thing. I clipped in new filter caps at all of the PS nodes thinking maybe a decoupling problem- no change.

A couple other notes: The amp has a 2/4 tube switch. When you have it in 2 tube or 1/2 power mode the oscillation is not there. Also, if you remove signal once the oscillation starts, then put your finger on the speaker cone, you can make it stop. Otherwise, it will continue to oscillate until the amp is shut off.

I've also noticed that this amp has much more low frequency response than typical guitar amps- maybe too much and that's the problem? I'm almost out of hair to pull out.

http://carvinimages.com/schematics/3...ocked_full.pdf

[g1]

I guess lifting the NFB would stop it too?

[The Dude]

I thought about NFB. I haven't disconnected it, but I did temporarily reverse the OT primary wires to see if the feedback was wrong. The amp didn't like that. It screamed at me loudly to let me know.

[The Dude]

After some Google searching, I've found this is a known issue with the V3, although I haven't yet found a solution. Worse yet, Carvin tech support is no longer available.

[m1989jmp]

I'm probably way off here, but feedback is fed from a 8ohm OT secondary, right? Does that mean it "becomes" a 16ohm secondary when two tubes are pulled, increasing the neg. feedback, making the oscillation stop?

I know I'm cpt. Obvious, but you'd either have to move the feedback loop to the 16ohm tap or mess with the cap/resistor combo R91/C70. 10k seems like a crazy low value. Carvin Legacy for example has 390k/10pF.

[The Dude]

Thanks for the suggestion. I got parts in for some other projects, so this is temporarily sidelined. I will investigate and update when I get back into the amp.

[The Dude]

Well, I got back into this today. On m1989jmp's suggestion, I tried changing the NFB resistor to 390K to see what happened. No more oscillation, but the amp sounded like a$$. After much experimenting with different values and compromising between no motor boating and a decent sound (at least to my ears), I arrived at 47K. The amp is stable, and I actually think it sounds better, but it wasn't working right, so it's bound to be better. So thanks for the help from Croatia!

[m1989jmp]

Well, I got back into this today. On m1989jmp's suggestion, I tried changing the NFB resistor to 390K to see what happened. No more oscillation, but the amp sounded like a$$. After much experimenting with different values and compromising between no motor boating and a decent sound (at least to my ears), I arrived at 47K. The amp is stable, and I actually think it sounds better, but it wasn't working right, so it's bound to be better. So thanks for the help from Croatia!

Nice. 47/100k are standard values.

I wonder how Legacy model gets away with that resistor/cap combo; different PI design?