Need help with VVR installation

[rynugz007]

I'm installing a (fixed bias) VVR3 board from hallamplification on a Allen Hot Fudge (6G3 Brownface deluxe clone). After reading instructions, I'm still a little confused as to where to insert the VVR.

There is a red wire connecting directly from the standby switch to the first large filter cap. I'm assuming this is where I break the signal? I've read some conflicting comments on various forums and I just want to make sure that I'm not gonna ruin the amp or the VVR.

Also, what about the bias tracking? there is a white wire coming from the trem depth pot that connects to the board at which point two wires head from there to the grids on the power tubes. Is the white wire where I break the signal here?

I've built a few amp kits but I'm fairly new to modding and tweaking. Any help would be appreciated. Thanks!

[rynugz007]

ok, I think I've got it working right. I broke the connection going from the standby switch to the OT/plates. I tested it according to the instructions I found online.

Can someone at least confirm that my measurements are where they need to be? Also, I replaced 2 resistors on the VVR board from 100K to 330K to limit how low the voltage drops (I don't need a whisper quiet bedroom amp).

Rectifier pin 8 (418v VVR full on - 356v with VVR full counterclockwise)
B+ in (418v - 356v w VVR full counterclockwise)
B+ out (412v - 90v w VVR full counterclockwise)
bias in ( -40mv - -40mv w VVR full counterclockwise)
bias out (-37mv - -8mv full w VVR full counterclockwise)
bias test points from the amp (22mv 100mv w VVR full counterclockwise)

I've found that unlike the VVR in my 65amps Ventura, instead of behaving like a volume control, it seems to just be reducing headroom and adding distortion as I turn it down. The ventura is cathode biased, maybe that's why they're behaving different? I like being able to set the amp for the breakup I want, then adjust the VVR to knock a little volume down. I'm still not 100% if I have it hooked up right.

[martin manning]

It goes in like this. I'm assuming you want to scale the whole amp. If you did not put the choke on the other side of the VVR you are only scaling the power tube plates. That would account for the overdrive distortion at low settings.

[rynugz007]

Boom! Got it! I had the choke before the VVR. It's working great, now!

Thanks a bunch, Martin!

[rynugz007]

Well, it didn't last long. The B+ out is now reading 345v at its lowest pot setting and the chassis is no longer warm where the mosfet is. I'm assuming MOSFET stopped working?

[Structo]

I don't see the MOSFET in your pictures.

Do you have a heat sink on it?

Those devices can be sensitive to heat, static, location among other things.

I don't see a lot of success stories using VVR type circuits.
Probably due to lack of experience with them or wrong application specs.

[rynugz007]

Those are 'before' pics. The mosfet is insulated and installed on the back wall of the chassis. It worked fine for a bit, I moved the amp across town (without a head case) and it wasn't working next time i fired it up.

[rynugz007]

No heat sink, just the insulator (which is what Dana recommends)

[Structo]

Yes if it is using the chassis for a heat sink, it should be fine.

[rynugz007]

I did just read a post from Dana himself stating that he recommends a heat sink if using a steel chassis but not so with an aluminum chassis. Mine is steel.

I also plan to use thermal compound once I get the replacement mosfet. should I be looking into a heat sink as well?

[martin manning]

An aluminum heat sink is a good idea. You probably read this, but replace the G-S Zener diode too, and use non conductive heat sink goop.

[rynugz007]

Will be replacing the diode and using non conductive thermal compound. But I'm a little confused on where to put the heatsink and what kind to get. Can I get one from any computer store? Does it go under the mosfet or the other side of the chassis? Or just as close as I can get it?

Btw, thanks for all the help, fellas!

[martin manning]

Under the FET... You don't want the thermal resistance of the steel chassis in between. I suspect you are limited in space, so get the biggest one that matches the FET package style that will fit.

[rynugz007]

I have it working again, heatsink seems to be doing its job.

One last question- I noticed the pad on the mosfet reads the full B+ voltage (430v) regardless where the VVR dial is set. Is this normal? I was under the assumption that the pad would read the remainding voltage that was discarded from the VVR. ???

[martin manning]

The tab (drain) is the input side of the FET in this application, so it should have the full B+ voltage on it.