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Ok, just wanted to get some ideas before I go to test my Bassman later. Was playing on it the other day and got some smoke. Burnt out on of the 1 Watt 470 ohm plate load resisters. I only had a 5 watt ceramic laying around to replace it with but when I got it going again no matter what I do the current is 80ma or so. One tube acts just fine but the one that had the PL resister go bad is way high no matter. It is new matched tubes in there now and not a balancer issue. I am thinking a bad cap in the power supply. Any other ideas?
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The new tube is most likely shorted. Pull both tubes out and measure the voltage on pin 5 of each socket to make sure your bias voltage is present. -
That's a screen resistor, not plate load resistor.
I agree with Twist, sounds like a tube short. It can happen right out of the box.Comment
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I'll echo the other respondents and note that replacing the screen current limiting resistor is perhaps the commonest repair on '60s Fenders - also that both resistors should be replaced if you didn't do and that 1K, 5W resistors will give you the ability to work with more variety of tubes.
Your matched pairs are only matched until they are put in circuit - assuming that one/both aren't dropped - and if you've got a failed bias supply on one side that would keep one redplating. But sometimes tubes only like to get hot once and from then on you have problems with a bottle - best to make sure you've got bias voltage at pin 5 of each output tube socket and try another pair of tubes. How are you measuring plate current?
RobComment
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Ok, I did some probing and got -44 one pin 5 and -37 on the socket with the problem. The difference I'm sure is the balance pot, right? Anyway you guys got any other ideas? No matter how I adjust the bias pot I get 80 to 90ma on one tube and 59 or so on the other. Tried another set of tubes since the set I have in there is a GT grade 7 and still same result.Comment
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I would pull the output tubes out and turn on the amp then set the output tube balance control so that you have the same negative voltage on pins 5 of each socket. I'm assuming you have a matched set of tubes. Then unsolder the leg of the 15K resistor going to ground on the balance control and solder a 25K pot in series to ground. This will allow you to adjust the idle current of the output tubes together. While your at it you might as well install a 1 ohm resistor between pin 8 and ground on each socket. This way you can monitor the current of each tube by reading the millivolts across the resistors. Once you have it set, remove the pot and read the resistance of the pot. Then find a resistor that value and put it in series with the 15K resistor. I guess you could leave the pot in there if you could fasten it down.
Then you can fine tune it with the balance control.Comment
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