Is This a Bias Trim Pot?

[cudamax2343]

Ok, I just saw your previous thread on this amp regarding the wiggling around of the power tube to get it to work. Have you tried to tighten the pins in the power tube sockets with a pointy instrument with the power off of course? Also, you might have a cracked solder point on the power tube sockets at the pc board. you'd have to drain the amp, pull the board and use a magnifying glass to find the crack or cracks on the pin solder joints on the board. Can you solder? If one of the pin 5 solder joints is cracked you'd lose the negative bias voltage at the tube and it would redplate and blow a fuse. you're still gonna need a set of bias probes to bias the amp.

No I was wrong the tube is dead. It just seemed that way at first.

[cudamax2343]

Here is a dual bias probe on eBay:

http://cgi.ebay.com/AMP-HEAD-Dual-Bi...lenotsupported

Thanx's for all your help. I'll have to get some Bias Probes. P.S. what do they tap into pin# 5 for live readings? What do you think a good value sweep is, maybe 38-42 for EL34's? Couldn't one just solder an extension wire/leg on pin#5 on the board side for a quick reading or permanently attach small gages on the back for extended live full-on reading check?

[bnwitt]

All power tubes are different per set. Your bias range goes from zero (affectively) to -50 volts which is plenty for the hottest or coldest EL34's. Typical bias voltage on pin 5 for EL34's is -30VDC when your plate voltage is around 400 volts.

On some amps a 1 ohm 1% resistor is placed between pin 8 (cathode) and ground. When you measure millivolts across a 1 ohm resistor you can convert directly to milliamps via ohms law. From that you get the milliamps of current going through the tube and when that is multiplied by the plate voltage you get watts of dissipation. It's a little more complicated and that is for after you get your probes. I'd definitely pull that power tube board and check those solder points on the power tube sockets.

[bnwitt]

attached is a picture of a bias test point setup. The jacks are called test point jacks. The black one is connected to your chassis ground. The two Red jacks are connected individually to each of the power tubes pin 8 and route the cathodes thru the 1 ohm resistors. When you put your black meter lead in the ground jack you can then test each tube's millivolts across the 1 ohm resistors via the two red jacks to get milliamps. You have to know your plate voltage from pin 3 so you can multiply volts times milliamps to get the watts. You set your bias so that with zero on the volume controls and no signal you get 70% of max dissipation for the tubes. For EL34's that would be 70% times 25 watts or 17.5 watts. If your plate voltage was 400 VDC you'd need 43.8 milliamps on each tube to be at 70%.

[defaced]

Bob has some videos on biasing at Eurotubes:
http://eurotubes.com/euro-video.htm