Bias Pot

[gearhead]

Heh all,


I had a bias pot go bad on an amp (not one I built), and grabbed an alpha pot to quickly replace. Only used the center and right hand tap, to get a variable resistor.

It hums like a mother now, and don't know if that is because of the pot, or if original going out took something with it.

Is there something else I should have done with the left leg? Grounded it, or attached to the center tap?

Thanks

[UR12]

If the amp was left on and the bias was lost you may have a power tube that has redplated and needs replaced. (this will cause a LOUD hum) Was the fuse blown when you got the amp?

[gearhead]

I've replaced the preamp tubes for that channel and power tubes and biased with the new pot. It's definitely better with the newer tubes, but only by about 25%. Still hums. I didn't replace the PI, but that seemed to check out ok (have got a VTV tester).

More details on what happened:

I got the amp and biased it myself, using an Allessandro bias meter. Have had this amp 2+ years, with same tubes/bias. It had been a looong time, so was going to do a rebias to make it a teeny bit hotter. Looked around for a slot-head screwdriver to figure out which one would fit. Tested it by trying to turn the pot. It was stuck, so gave it a gentle push and it broke free, and returned it to original position. What I didn't figure out till later was that this probably completely broke the bias pot into an open circuit.

Tried to fit the probes with the tubes on top onto the chassis, and had forgotten I can't do it in this amp, not enough head room- have to remove chassis. OOps. Ahh, will do it later.

Played amp. Pooof went the fuze. Replaced, popped again.

Removed chassis and took out the bias pot. It was supposed to be a one turner, but it rotated endlessly. Measured, and it was an open circuit.

Thanks!

[UR12]

Did the fuse pop instantly or does it take a while for it to blow? What kind of am are you working on?

[gearhead]

It took a little while for it to blow;5-10 secs?, not a minute. Didn't blow till when I strummed a chord, but that might have been a coincidence. It is a marshall-type circuit, EL-34 based.

Thanks Dana!

[UR12]

It took a little while for it to blow;5-10 secs?, not a minute. Didn't blow till when I strummed a chord, but that might have been a coincidence. It is a marshall-type circuit, EL-34 based.

Thanks Dana!

It just sounds to me that there is stil something wrong in your power amp stage. Have you checked your screen resistors? It sounds like you are drawing excessive current AFTER your heaters warm up (5-10 sec) What is your B+ and screen voltage? what is your bias voltage?

[gearhead]

Is there any tell-tale way to check the screen resistor without desoldering one leg? (edited-doh, just pull the tube)
By B+, I take it you mean maximium B+ (as in B+1 on the Twreck).
How do you measure the screeen voltage?

Thanks so much,

Dave

[lastwinj]

How do you measure the screeen voltage?

on the tube socket. can also measure the voltage drop across. stick the blakc lead on the socket, and red lead on the screen tap.

germ

[gearhead]

Is screen voltage measured from socket/pin 4 to ground?

What's the screen tap?

[gearhead]

Voltage from socket 4 to ground was about 430V.

Voltage -across- the bias pot/resistor was about 14V. Forgot to check if this is directly to ground or goes to some more circuitry.