Hall VVR in Selmer T&B 50

[Willholmania]

Hello. I want to install a Hall VVR3 board in my Selmer Treble & Bass 50. My question is, how hot does the large Mosfet get? I've ordered a ally heatsink for it but if I install inside the chassis, where all the amp's guts are, will it get enough airflow? Or do I need to mount it on top of the chassis, where the valves are?

Has anyone fitted one of these in a fixed bias amp?

Thanks

Will

[Stevem]

I have used a Vvr kit from London power and in aMarshall major head I had room to mount the heat sink on top of the chassis,if you mount it inside it had better be on the cooler preamp tube side of the amp, but than you need to watch out that MOSFET switching noise does not get into the preamp stages.
Also in regards to switching noise it must be 3 inches away from any preamp tube.

[Willholmania]

Thanks Steve. I think I'll play it safe then, and mount the heatsink and Mosfet on the top of the chassis so it gets some decent airflow.

[ampgeek]

I have used the Hall fixed bias VVR in 4 X 50'ish watt builds (5F6A, JCM-800'ish, Dumble 102'ish and Express'ish) and they have performed flawlessly in all.

I solder the mosfet to the PCB and bend the leads to allow it to sit flat on the chassis surface when the pot is mounted to the control panel (e.g., no dedicated heat sink). I also use a mica isolater between the mosfet and chassis and attach it with non conductive hardware (nylon IIRC).

The huge surface area of the chassis "plate" provides adequate heat transfer area even when it is mounted in a head cab. I can just sense a higher than ambient temperature out to ~3-4" of the mounting point with my fingertips.

Cheers,
Dave O.

[TUBEDUDE]

Instead of mica, and that clothes ruining thermal grease, try using those viscoelastic silicon insulators.
They are a great electrical insulator and thermally they are a better conductor than the grease, and much better if you are just using the mica without thermal grease. It comes in sheets if you can find it, but is readily available in TO-220 and TO-3 sizes. Allied Newark or Digital are good sources.

[Richie]

I have used a small heat sink and fan once. It was on a marshall 50 watt with a steel chassis. This was a few years ago. Its still working fine. I know it may be overkill. But I just wanted to try it and see how it worked.

[ampgeek]

Instead of mica, and that clothes ruining thermal grease, try using those viscoelastic silicon insulators.

Yes! I have also used those in a handful of builds with great success. They just happend to all be in amps with cathode biased power sections or other unique, regulated DC power supplies. But...I can't see any reason why the wouldn't work equally as well in this specific application either.

Rock on,
Dave O.

[Willholmania]

Thanks guys.

Ampgeek - were the chassis you installed them in aluminium? Mine is shell, which obviously doesn't conduct heat as well as ally.

[ampgeek]

Both painted steel and aluminum.

I do wire wheel (Dremel tool) away the paint in the area where the mosfet will ultimately land before pinning it down to eliminate that heat transfer resistance.

I will admit that I haven't gone back to check for rust formation in those areas though. It has been ~4 yrs (IIRC) without failure.

Cheers,
Dave O.

[dynaman]

Haven't tried in a fixed bias amp, but I successfully used a Hall VVR in a Rocket clone. I squeezed the mosfet against an externally mounted heatsink that looked like the attached pic. I did a tidy job of it by drilling and tapping the heatsink to stand vertically on the chassis so the chassis itself would dissipate heat as well. I soldered the mosfet leads to a board inside the chassis. I can't remember if I used compound or a mica spacer. Regardless, it worked well because the heatsink got awfully hot so there was definitely some efficient transfer of heat going on. I was surprised at how hot it got.

[Willholmania]

Thanks to all for the tips. I think I'll go for a heatsink similar to that finned one in the last reply as the chassis on these amps is pretty cramped and I want reliability.

[Structo]

If you do put the mosfet on the exterior of the chassis, make sure the leads/ wires are not accessible to the end user.

These amps will most assuredly live longer than we do and guarding against shock hazards should be a priority.

If you do mount it externally, I like the idea of using a heat sink with the mosfet mounted underneath of it to protect the user from shock hazards.

Guitarists are particularly prone to electrocution since the guitar strings are grounded through the amp.

So if you grab something energized with your free hand, the current will go through your body to the other, grounded hand.
Your heart is between your hands.

/ soapbox

[Willholmania]

Don't worry Tom, I plan to insulate the Mosfet's legs with heat shield.

[Willholmania]

I've finally got this fitted.[/img]

[Willholmania]

I mounted the board within the chassis but had to fit the pot remotely as it's all a bit cramped.