VVR type 2 in a Rocket Reverb amp

[angelodp]

Hi I have a VVR type 2 in my Rocket Reverb. I am having some issues with my amp at the moment. The B+1 is at about 340v and then it drops quite a bit down to 40v at B+2. As I read the incoming B+ at the VVR I see the 340v but even with the VVR full up I only get 40v at the output of the VVR. Is it possible that the VVR unit has failed?

A

[angelodp]

Here is the amp in question.

[angelodp]

The amp plays but at quite low volume. When I engage the 1/2 power switch it gets a bit louder (which is odd), but nowhere near the appropriate volume for this amp. Have swapped tubes and reflowed various connections. With the lower B+2 none of the plate voltages are correct.

[dorrisant]

I thought the VVR3 was for fixed bias amps... with bias tracking.

[angelodp]

Correction I have the VVR2 for cathode biased amp.

[angelodp]

Here is the VVR I have. So with the incoming B+ at 340v and the output B+ locked at 40v, no change with the pot, is this unit kaput? Do I simply exchange the Mosfet?

[xtian]

Yeah, that VVR2 is a simple circuit; only the FET and one diode. You can test them with multimeter for shorts and replace if needed.

[angelodp]

Can you expand on testing the Mosfet. Wondering why it would fail? Do I need to replace with this exact FET? I don't see this exact one now avail.

[angelodp]

I do not see a short on the Mosfet? Probing the leads but i am not sure if I am doing this correctly.

[angelodp]

if I want to test that the VVR is the problem, I am thinking to just take it out of circuit and make the standby connection directly to the B+ as in the original version without the VVR. Make sense?

[brewdude]

How close is the underside of the PCB to the chassis?
Are the mosfet’s leads as close to the chassis as it appears in the photo?

[angelodp]

There is a clear shield under the Mosfet that came with the kit. The board is a bit close for comfort. At any rate, I bypassed the VVR and made the connection to the standby, as in the original and the amp is fine and working. I guess I'll get a replacement Mosfet STW12NK90Z looks like it will work fine. Truth is I don't often use the VVR. I have seen where an additional cap can be installed for further protection on the VVR.

[xtian]

There is a clear shield under the Mosfet that came with the kit. The board is a bit close for comfort. At any rate, I bypassed the VVR and made the connection to the standby, as in the original and the amp is fine and working. I guess I'll get a replacement Mosfet STW12NK90Z looks like it will work fine. Truth is I don't often use the VVR. I have seen where an additional cap can be installed for further protection on the VVR.

Just FYI, the function is not meant to protect the VVR. The caps I mentioned are to block DC from the tone stack and from going out to the guitar from V1a.

[pompeiisneaks]

Testing a mosfet is usually pretty easily done in diode mode, The diode should read normal one direction and OL the other, in other words, nothing. In circuit you may have issues, though as the other diode might cause a false reading. Usually you just test each pin to the other, they behave just like diodes and if there's a built in protection diode across the drain and source it will also test well one way and not the other, etc.

It does sem a bit close to the chassis with those legs and no isolation though.

~Phil

[angelodp]

Yup Mosfet is toast. After 10+ years of service, I wonder what caused the failure. I know the set up looks close to the chassis, but I double checked and there is a clearance of 1/8" or more. I am gonna put a shield on that once I get it squared away. Still do love the tone of this amp.

Thanks all