Super Medalist Getting Hot

[dorrisant]

I have this up and running, biased right, passing audio.

I wonder about this power supply. In particular, the 14k 15w resistor to ground, at the screen supply filter cap. It certainly drops the voltage. I can't remember exactly how hot it was getting, but it was at least 360° F. I'm pretty sure I was seeing a peak of about 390°. 14k at 350v would be dissipating 8.75 watts. Don't know if it should be left to get that hot. Seems ridiculous.

Screenshot_20220919-000116-218.png

I'll dig back into it soon. Voltages didn't seem crazy, but I didn't jot them down. I will post voltages when I go back in. Any ideas on how to get this heat down? I can't believe that it is that hot inside the chassis!

Maybe I should increase the 1k dropping resistor between the plate and screen nodes. I'm just curious about that being 1k, if it does anything to promote the screens tracking with the plate. Maybe someone else sees something cool about this setup. It just wreaks of a cheap, bandaid way of doing this from the factory.

[sluckey]

I would not worry about the resistor or the heat. It has an adequate power rating. Hopefully it has plenty of air space around it. Output tubes should last a long time.

If you are really really concerned you could buy this 14K 25W and bolt it to the chassis, maybe even on the outside. Then the chassis could act as a heat sink.

[Stevem]

Yes those types are made to handle that level of heat.

If you have enough room then step up to a 20 or 25 watt type and it will run a tad cooler and not get other local components as hot either.

On old cathode biased amps I always go in and double the wattage of the resistor in the output stage.

It really helps to stabilize the outout stage since most times these amps are played cranked.

[TUBEDUDE]

Good advice from Sluckey and Stevem.
High heat shortens component life.
If you have a 400° oven, anything you can do to shed heat will extend your amps life. Chassis mount the resistor, add a few holes to aid convection, or even a small personal fan from the dollar store blowing air on the chassis will help.

[Phil_S]

I like the fan solution. An old small 12V computer fan running on the 6V supply will turn at half speed and do a good job. The problem, as I see it, is finding an "exhaust vent." If this is an original '60's amp, cutting a hole isn't a realistic option. Just blowing the hot air around inside seems useless. I'd expect there is room inside to use a 25W chassis mount resistor, so that's what I'd do if I was concerned. Just my 2¢. Good luck.

[TUBEDUDE]

I incorporate fans in all my builds, but I was talking about a one of those $5 personal fans. Plugs in externally and clamps to the back panel, to blow air at the chassis to remove some of the air in the cabinet. Sheds heat with no mod to the amp.
For combos. If this is a head nevermind.
Though in a chassis, I recommend a fan have its own supply, transformer or switching.

[dorrisant]

Thanks guys. I realize that I could just leave it, but I talked with the customer and I'm gonna put the resistor from the link above on the topside if the chassis. Great info. Btw it was dropping the node down to 380. Dissipating about 10 watts. Thanks again for all the info.