Help! Amp problem

Bill Moore · Post by **Bill Moore** » 23 Jul 2000 4:50 am

I have a old Music Man 210/65 watt amp that seems to have a problem. This amp has a hi/low switch, it seems to work fine on low, when switched to high, it begins to develop a loud hum, heat up drasticly and one of the tubes glows brightly. This only happens when switched to high. Obviously this is a problem, how serious? Any guesses what might cause this? Thanks.

Bill

Donny Hinson · Post by **Donny Hinson** » 23 Jul 2000 5:06 am

Bill, if that's an output tube (6L6, 5881, etc.) that's lighting up like 25-watt bulb, then the tube is bad. Sometimes, these tubes will develop an internal short, and draw way too much current. This causes the plate (that big piece of grey metal in the tube) to glow cherry red...not a good sign! Sometimes the short can be "broken loose" by hitting the amp with your fist (no kidding!) when it's humming. But it will usually come back at some point. At any rate, replace the tube as soon as you can, it could cause other problems if you don't!

Carl Dvorcek · Post by **Carl Dvorcek** » 23 Jul 2000 6:52 am

I've had similar problems with Fender Twins. If it was me, I'd swap the tube with an adjacent tube first. If the problem moves to the other position then it's the tube. If it stays in the same position, then you have a problem either with a shorted capacitor or a biasing resistor. That's the easiest way to determine where the problem is before spending money on something you might not need. I just retubed my Twin for the first time since I bought it 28-years ago. More often than not, the problem is internal.

JMTCW,
Carl

Lefty · Post by **Lefty** » 27 Jul 2000 5:52 pm

I had the exact same problem in one of my musicman HD-130's. I thought it was a cap, or something, but my amp tech replaced all the tubes, and rebiased it, and did a little tweeking and it sounds great (about $100.00 with labor and tubes).