VVR Install Problems

tictac · Post by **tictac** » Wed Nov 26, 2008 3:13 pm

the middle lead of your Mosfet looks awfully close to that chassis nut

bclick · Post by **bclick** » Wed Nov 26, 2008 4:59 pm

Thank you all for your help. Really would like to get this working!
Here's some more voltage readings:

VVR.Pot__V4.p1__V4.p3__V4.p5___V5.p1__V5.p3__V5.p5__B+ Out
Max_____33mV__420V___-38V___39mV __420V___-39V___424V
half_____17mV__244V___-17V___19mV __243V___-21V___260V
Min____ 2.4mV___40V___-3.5V__3.6mV __40V____-3.5V___38V

Good eye tictac. It is close, but I turned out the lights and did not see any arcing, or any evidence of arcing. The 'buzz' sure sounds like an arcing noise.

One other thing I have tried is bypassing the 10K dropping resistor in the preamp B+ circuit, which brought the preamp voltages back up close to normal. No effect on the VVR issue though.

New schematic with Dana's VVR connections and Metro's bias resistors.

UR12 · Post by **UR12** » Wed Nov 26, 2008 5:11 pm

Can you try one more thing to see if it helps the Buzzing? Try twisting the wires together going from the pot to the board. If you loosen the pot from the front panel and carefully rotate it I think you might be able to put a twist in them without unhooking anything.

Your voltages look good BTW

bclick · Post by **bclick** » Wed Nov 26, 2008 8:50 pm

When I tested this time I turned the two channel volumes up to 5 with MV at 10. At this level I CAN hear a volume reduction

as I turn the VVR pot! Which I could not hear when channel vol were at 2.

But the buzz is still very evident,

particularly between 40% & 60% rotation on the VVR pot

I removed the VVR pot from the chassis, twisted the wires & pointed it up away from everything. Installed Presence pot back in chassis. Unbolted mosfet from chassis and raised VVR board up a bit, with homemade 'shield' between it and the transormer.

Only played a few seconds as mosfet is only air-cooled right now. Really made little difference at all in buzz.

UR12 · Post by **UR12** » Sat Nov 29, 2008 12:47 am

I will try and get you out another Mosfet in tomorrows mail.

iknowjohnny · Post by **iknowjohnny** » Sat Nov 29, 2008 1:07 am

I don't know if anyone saw my thread. But i was getting a ton of buzz as soon as i turned the VVR knob down about 1/8 of the way all the way to the lowest setting. read a post at 18 watt in the archives saying someone with the same problem found that if he loosened the screw that months the mosfet to the chassis, the buzz completely stopped. Before i loosened it to see if i had the same problem, I checked the conductivity between the metal part of the mosfet and the chassis to see if it was shorting, but it was totally open. I tried l and wham bam, it stopped totally. So you may want to tru that before you do anything else. I don't know why this happens, nor do i know how to keep the mosfet cool if you can't mount it. But i suspect some other form of insulation may do it like another insulator on top of the mosfet or a nylon washer or such. I never tried because once i got it working right i decided i didn't like VVR anyways so i never went forward and tried to figure it out.

bclick · Post by **bclick** » Sat Nov 29, 2008 1:27 am

Thanks Dana. I guess you think it might be the mosfet even tho the voltages are tracking.

iknowjohnny - thanks for the thought . Last thing I did was completely detach from chassis for testing..

--------------------------------------------
Bil

iknowjohnny · Post by **iknowjohnny** » Sat Nov 29, 2008 6:34 am

Ok, well, i had to mention it because i was amazed when i tried loosening the screw to find the same problem i had read about. In any case, good luck with it...

Structo · Post by **Structo** » Sat Nov 29, 2008 4:32 pm

Is it ground looping?

I suppose you need to mount a heatsink to the mosfet and if you mount that to the chassis you need a insulator type flat piece between the heat sink and chassis.

UR12 · Post by **UR12** » Sat Nov 29, 2008 4:58 pm

Structo wrote:Is it ground looping?

I suppose you need to mount a heatsink to the mosfet and if you mount that to the chassis you need a insulator type flat piece between the heat sink and chassis.

The back of the mosfet is connected to the drain. If there wasn't a insulator between the chassis and mosfet he would be shorting B+ to ground. He would have a lot more problems than a ground loop. I am guessing we have either a bad mosfet or a bad zener.

iknowjohnny · Post by **iknowjohnny** » Sat Nov 29, 2008 5:33 pm

Whoa !!!! I had no idea the insulator was keeping B+ from ground ! In that case i wouldn't even consider using one of these unless i used a seperate heatsink that is not touching circuit ground. Maybe i'm over reacting, but no matter how reliable those insulators are, just the idea that it's all that seperates B+ from ground wouldn't allow me peace of mind no matter how convincing an argument i was given. If i were to use one i would now float a heatsink on a terminal strip or such and use that. tho it would be tough in a cramped amp.

Structo · Post by **Structo** » Sat Nov 29, 2008 6:24 pm

Ah ok, yeah it's been a while since I messed with any mosfets.

I had forgotten that the tab is connected to the drain.

dave g · Post by **dave g** » Sat Nov 29, 2008 7:01 pm

iknowjohnny wrote:Whoa !!!! I had no idea the insulator was keeping B+ from ground ! In that case i wouldn't even consider using one of these unless i used a seperate heatsink that is not touching circuit ground. Maybe i'm over reacting, but no matter how reliable those insulators are, just the idea that it's all that seperates B+ from ground wouldn't allow me peace of mind no matter how convincing an argument i was given. If i were to use one i would now float a heatsink on a terminal strip or such and use that. tho it would be tough in a cramped amp.

...yet you trust the dielectric material in, say, a preamp coupling cap or a filter cap?

Those insulators are completely safe.

UR12 · Post by **UR12** » Sat Nov 29, 2008 8:09 pm

iknowjohnny wrote:Whoa !!!! I had no idea the insulator was keeping B+ from ground ! In that case i wouldn't even consider using one of these unless i used a seperate heatsink that is not touching circuit ground. Maybe i'm over reacting, but no matter how reliable those insulators are, just the idea that it's all that seperates B+ from ground wouldn't allow me peace of mind no matter how convincing an argument i was given. If i were to use one i would now float a heatsink on a terminal strip or such and use that. tho it would be tough in a cramped amp.

Not only are they safe they are made for just that purpose by the manufacturer of the mosfet. They are made to be an electrical insulator but also be a good conductor of heat. Even if you were to use a seperate heat sink, if you didn't use the insulator your heat sink would become the same potential as your B+. Since most heat sinks are on the outside of the chassis this would present a safety hazzard for anyone sticking their hand back in the amp with power on. You didn't seem to mind having B+ on a control on the front panel that was grounded. If your that afraid of the voltages in an amp I would suggest you find another hobby. There are mosfets out there that have the drain not connected to the back of the mosfet and these can be directly mounted to the chassis without the need of an insulator. They usually start out around $35 and go up if that would make you feel better.

iknowjohnny · Post by **iknowjohnny** » Sat Nov 29, 2008 8:25 pm

i realize they are made for that purpose, but like i said it just doesn't give me peace of mind having B+ that close to chassis.
As for finding another hobby, how bout i just don't use VVR? Pretty simple really. Point being, i am not afraid of voltages, i'm just not particularly enamored with the idea of having a thin piece of material between 4 or 5 hundred volts and my guitar strings. Whether there are any possibilities for shorting such as vibration loosening the screw and the insulator moving over time or not. i just don't feel it's a great idea for me. Thats all.

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