Hot running Reverberocket

[01redcobra]

Well, I'm new to this forum but I thought I'd give it a shot.
I'm trying to help a friend out with his Ampeg Reverberocket that started to run hot for him. Problem is, it's a long distance diag since he lives about 350 miles from me.
He said it stared to run hot after doing a show out in the sun on a hot day. The one 6v6 was running with red plates and started to sound bad and loose power. He replaced the tubes. Now it sounds ok, but he said it's still running hot enough to fry an egg on the side with the power tubes and rectifier. he says nothing looks melted or burnt inside but it's just extremely hot. What do we look for next? I'm thinking a bad resistor or cap on the cathode bias.

[CaseyJones]

Well, I'm new to this forum but I thought I'd give it a shot.
I'm trying to help a friend out with his Ampeg Reverberocket that started to run hot for him. Problem is, it's a long distance diag since he lives about 350 miles from me.
He said it stared to run hot after doing a show out in the sun on a hot day. The one 6v6 was running with red plates and started to sound bad and loose power. He replaced the tubes. Now it sounds ok, but he said it's still running hot enough to fry an egg on the side with the power tubes and rectifier. he says nothing looks melted or burnt inside but it's just extremely hot. What do we look for next? I'm thinking a bad resistor or cap on the cathode bias.

Make sure all your grounds are solid.

I'd bet on a shorted cathode bypass cap on the power tubes. It would also be a simple matter even for a non-tech to measure the bias resistor while he was in there. Either way I'd eventually replace the bias resistor for reliability. It's shown on the schematic as a 250 ohm 10 watt which should be adequate but check it anyway.

The one red plated 6V6 was probably the better one from a mismatched pair so it ended up doing most of the work when the cathode network began to fail.

Reverberockets are a little light in the power supply so it's essential that everything is up to par inside. A transformer is a terrible thing to waste!

[rfgordon]

I second that emotion on the cathode cap. Lots of builders of yore (and some still today) put the cathode bypass cap and the resistor next to each other. Yeah, that's convienent, but it's not sound building practice. That cathode bias resistor is the hottest thing inside the chassis!

I'd say replace it with a 25 watt aluminum housed resistor and mount it away from everything. Put the bypass cap on the board somewhere. Their grounds don't even have to go to the same place, though the purists may disagree.

[CaseyJones]

I had another thought... perhaps some tech at some point with the best of intentions replaced the cathode network and installed the cap in direct contact with the resistor, that's a quick way to cook the cap. I've seen the bias resistor get hot enough to melt the jacket off the cap.

[01redcobra]

Thanks for the input guys! I seem to have been on the right track at least. I explained the cap situation to my friend and he's decided to UPS the amp to me for repair after all. He said he doesn't want anything to do with capacitors!
So I'll let you know what transpires once I get it. Thanks again!

[01redcobra]

Well, I got the amp the other day and found that indeed the cathode cap was bad. Replaced it (mounted it on the board away from the resistor) and all voltages were back to normal range. But I'm still getting a lot of heat from that area and from the transformer. looks like they tried to jam too many heat producing parts into the same area. The chassis by the on-off switch does get pretty hot. Definitely not any area to rest your hand. It really seems like there's too much heat confined to that area and very little air circulation. Any ideas?