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previous: liam I've been having very similar probl... -- 5/5/1998 3:36 AM

Re: Not your typical bias problem!

5/5/1998 12:30 PM
Doc	Re: Not your typical bias problem! I was recently "tuning up" a 100w 1959 SLP reissue. I noticed that the tubes were biased way cool, like around 22ma (465v plate). I thought this was too low, so I raised it to about 32ma. Seemed reasonable. However, after the amp was warmed up with playing for about a half hour, the idle current was up above 36ma, so I set it back to ~28ma when hot. Maybe this drift-up is a characteristic of certain brand of tubes. I didn't fit new tubes, as the Marshall-branded ones still tested good, and sounded good. I guess I'm sort of spoiled when it comes to measuring bias current. The meter I use is a clip-on probe type that reads DC current. As long as I can get the probe around the OPT's leads, I get an accurate reading. With amps that have cathode standoff resistors installed, the DC millivolt meter gets used. (The clip-on unit is an old tube powered unit that has a case as big as a car battery, and has to be warmed up some prior to using.) I've not had to rely on the OPT shunt method. It's probably a good idea to install 1ohm or 10 ohm cathode standoff resistors on the tube sockets, in place of the ground wire coming off pin 8. This makes future biasing a breeze. I haven't done it, but I don't see why a 1ohm resistor couldn't be placed between pin 3 and the transformer primary lead to read anode current, but I prefer to include the screen current on EL34s and therefore use the cathode resistor.

5/5/1998 12:30 PM

Doc

Re: Not your typical bias problem!

I was recently "tuning up" a 100w 1959 SLP reissue. I noticed that the tubes were biased way cool, like around 22ma (465v plate). I thought this was too low, so I raised it to about 32ma. Seemed reasonable. However, after the amp was warmed up with playing for about a half hour, the idle current was up above 36ma, so I set it back to ~28ma when hot. Maybe this drift-up is a characteristic of certain brand of tubes. I didn't fit new tubes, as the Marshall-branded ones still tested good, and sounded good.

I guess I'm sort of spoiled when it comes to measuring bias current. The meter I use is a clip-on probe type that reads DC current. As long as I can get the probe around the OPT's leads, I get an accurate reading. With amps that have cathode standoff resistors installed, the DC millivolt meter gets used. (The clip-on unit is an old tube powered unit that has a case as big as a car battery, and has to be warmed up some prior to using.) I've not had to rely on the OPT shunt method.

It's probably a good idea to install 1ohm or 10 ohm cathode standoff resistors on the tube sockets, in place of the ground wire coming off pin 8. This makes future biasing a breeze.

I haven't done it, but I don't see why a 1ohm resistor couldn't be placed between pin 3 and the transformer primary lead to read anode current, but I prefer to include the screen current on EL34s and therefore use the cathode resistor.

Replies:

Derrick I will check the Z of my meter... -- 5/5/1998 2:01 PM