VVR running hot

[CraigB]

It's installed on a cathode bias 2xEL84 amp and scaling power amp only.  The chassis is light gauge aluminum like the Hammonds.  It hangs in the top of the head cab, tubes pointed downwards.  The pot is mounted to the bottom, with the mosfet bolted with a nylon nut and bolt to the back panel with the mica insulator in between (not heatsink grease).  I really like how it works in conjunction with the dual pot PPIMV, which is a must since I did not scale the PI.  Sounds nasty if the PI drives too much into the scalled back '84s.  Great tones at many different volume levels with a little adjustment.  My favorite spot to set it seems to be about 100v below 100%.  Full power is 330Vp.  I played it for about a half hour over the weekend set in that spot, roughly at 70%.   I can touch the back panel in the spot where the mosfet is bolted to the other side and it gets pretty hot.  Not too hot to touch and hold my fingers on.  Is this OK?

[LooseChange]

It's okay... They do get hot.  I added a Radio Shack heat sink to the outside of the chassis... That gets hot too.

[FYL]

with the mosfet bolted with a nylon nut and bolt to the back panel with the mica insulator in between (not heatsink grease).

Thermal grease allows for much better coupling between the transistor and the chassis. You shoud either use some or replace the mica with a modern silicon or thermal change isolator which offers better thermal transfer and doesn't require grease.

My favorite spot to set it seems to be about 100v below 100%.  Full power is 330Vp.  I played it for about a half hour over the weekend set in that spot, roughly at 70%.   I can touch the back panel in the spot where the mosfet is bolted to the other side and it gets pretty hot.  Not too hot to touch and hold my fingers on.  Is this OK?

The mosfet shouldn't go above 80°C or so, if the chassis is at app. 45°C and the mosfet at 75°C your rig is OK.

[LooseChange]

Thermo Pads...
http://mouser.com/ProductDetail/NTE/TP0010/?qs=xTaouvGDdgD4jkxGzXgEvg%3d%3d
Heat sink...
http://www.radioshack.com/product/index.jsp?productId=2102857&tab=summary

[PRR]

If spit boils, it is too hot.

If you can hold it forever, it is super-cool.

If you can hold it a few seconds, that's not bad.

FYI: I have seen a transistor -melt the solder on its legs- and still be working. Life might be hours instead of centuries, but modern silicon/epoxy can get HOT and still keep some control of the current.

On a GOOD heatsink, thermal grease is important.

> light gauge aluminum

Then thermal grease is probably a pointless mess. Like putting bigger nails in undersized floor beams, you get a better connection to flimsy support.

> Not too hot to touch and hold my fingers on.

You are fine.

If you were not fine (blisters, spit-sizzle), you would have to find some thicker metal to spread the heat to more air.