DIY Bias probe test idea ?

[fikri]

I dont know if anyone had done this before, but it just come to my mind to use 2 guitar jacks to "switch" between the tube socket pin no 3 and the DMM when the "plug" is inserted. And the "plug" here, is connected into a DMM.

So when the plug is not connected, the tube socket pin no.3 is connected directly into the amp circuit, and when the plug is connected, it break the connection between the tube socket no.3 and the amp circuit and it is "switched" to the plug that is connected to a DMM. What do you guys think ? (sorry if my writing is confusing !)  

[RicF]

You say pin 3, do you mean an octal tube or a 9-pin power tube? For an octal that would be the plate, which you probably don't want, but for a noval power tube that's the cathode, which would be a better idea. Seems like it would work just fine, so long as the jack is able to handle the current that will run though it. On smaller amps this might be fine but I'm not sure I'd trust a shorting jack to handle the plate current of anything very large.

Better question to ask over at AX84.com probably.

[aron]

Well, but you usually measure across a 1ohm or 10ohm resistor so I'm not sure how much of an improvement this would be.

[R.G.]

Dynaco did that with some of their tube amps. The used a series resistor in the cathode circuit of their power stages and brought out both ends of the resistor to an RCA jack.

You may be meaning that you'd like to use a pair of guitar jacks to insert a DMM as a current meter in the circuit. That's OK, it will work, but the glitches that happen as you switch will sound nasty in the speakers, and this approach then potentially makes your meter leads at dangerous voltages, unless you do it only in the cathode circuit.

[fikri]

You may be meaning that you'd like to use a pair of guitar jacks to insert a DMM as a current meter in the circuit. That's OK, it will work, but the glitches that happen as you switch will sound nasty in the speakers, and this approach then potentially makes your meter leads at dangerous voltages, unless you do it only in the cathode circuit.

Well, maybe i can turn the amp off first and insert the plug and then turn it on again. But is it okay if i implement it in an octal tube ? (bringing the plate leads out)

Maybe i can save money to buy bias plug with this method !

[RicF]

Yeah, I assumed we were talking about inserting the jack so that the DMM could be used to measure current directly. Sounds more dangerous than I'd like to mess with; you'll have several hundreds of volts (DC!) directly on the jack and probe leads. I much prefer the method of inserting a 1 ohm resistor on the cathode. You can put a probe point on that if you really need to rebias w/o opening the amp up.