To tube or not to tube, that's my question

[soundmasterg]

Good suggestion as far as waiting for the warranty to expire before working on the amp.

Sounds like you will have to open it up to set the bias unless it is cathode biased. I would assume that since it is a bass amp, it is fixed bias, but you never know. Cathode bias amps basivcally set their own bias and you don't need to adjust them.

The easist way to bias is to use a tool like you mentioned where the tube goes into a socket and that socket plugs into the socket on the amp. Then a switch is used to change from tube to tube. This would be like the Weber Bias Rite I mentioned above. Weber stopped making and selling these last year, but there are other brands out there also. The key is that they have to measure not only the cathode current, but they also have to be able to measure the plate voltage, in order to accurately set the bias. In all of these, there is a 1 ohm resistor arranged in the cathode circuit inside the socket, and this allows a voltage drop across the resistor to be measured, which corresponds to the cathode current since you measure across the resistor. They also measure the plate voltage, and you use these figures that you get to determine if the bias is correct or not.

EXAMPLE: Multiply plate voltage times cathode current, which will be in milliamps, so example >> 450v * .030 = 13.5 watts. max power dissipation for a 6L6GC is 30 watts, and for a fixed bias Class AB amp you typically shoot for between 60 to 70% of the max, so 30 *.7 = 21, and 30 * .6 = 18...so 13.5 would be biased quite cold. So you decide to adjust it up to a hotter level. Now the plate voltage goes down as a result, so you have 440v * .040 = 17.6 watts...so that is a bit closer to the range. These are just examples btw...I just made up the numbers for the most part, but you get the idea.

I'll add more on your other thread.

Greg

[doctorwho]

I just realized that the title of this thread could have been "Tu-be or not tu-be" ...

[cjj]