CEBergheim

Hello,

I've just received my multimeter and test probe. Now I want to check the bias of my 1959 Super Lead. I've never done this before, so I want to be on the safe side.

Adjusting the bias with the trim pot seems to be pretty easy, but I'm not sure how to measure plate voltage. I have no experience with grounding equipment (something which I see many people do).

I also wonder which multimeter settings I should use. I guess "DC 200m" would be ok to check the bias. Is this the same for plate voltage? The inputs say "10A", "COM" and "V&mA". Which one is the correct input?

If this is important: The voltage is 220V where i live.

I hope you can help me!

Best wishes,

Chris

PaoloJM

Chris, is it just a multi-meter that you have or do have a quiscent current mearurement tool such as a Weber Bias Rite?
There is no safe, practical way to measure quiescent current in the 1759 series that I am aware of without using a unit like this.
Some people mod the circuit to include a high precision, power resistor of about 1ohm on the cathode of the power valves (which are connected straight to ground normally). By measuring the voltage across it you can infer the current. In fact this is exactly what the Bias Rite and Tube Amp Doctor unts do.

In the meanwhile you can measure the plate voltage by the following procedure:

• Put on a pair of rubber soled shoes and make sure that your work area is as clear and debris free as possible.
• Remove power completely froom the amp. Double check that it is unplugged.
• Remove the chassis and place on a worktop. Again make sure that there are no potential obsticles in the area .
• Place the chassis, circuit-board-up, on the worktop ensuring that it is firmly seated and not liable to move. Some people like to use a homemade frame which I would recommend.
• Connect the black test lead to com on the meter and use the crocadile clip connection.
• Connect the red test lead to V&mA and use the probe connection.
• With power still removed firmly attach the black test lead to the chaissis of the amp. Clip it somewhere where it can make a firm connection and is not likely to slip.
• Plug the speaker output into the cab and reattached the power cable.
• Turn on the power in the usual fashion and allow the usual time for stand-by. After turning the stand by allow about 2 mins for the amp to settle.
• Now put one hand (your bad hand, left if you're right handed) firmly in your pocket and, using your "good" hand take the red test lead and carefully probe pin 3 of the power valve with your other hand.
  Your hand is in your pocket so that you do not provide a path across your heart if you do accidentally short across yourself. Make no mistake it still will not be fun if you do get shocked but there's less chance of you dying. Sorry for the drama but this is dangerous work and should not be attempted if you are not confident of what you're doing. A human body typically offer about 300/500ohms of resistance to electrical current. In a valve this amp this could send 1 to 2 amps DC through you in an instant which is more than enough to kill you.
• This reading gives you the value of V for the formula Iq = (P/Vq) X desired bias percentage where Iq = quiescant current and Vq = Quiescant voltage and P equals the max plate dissipation of the valve (in you case I'm guessing it's an EL34 and therefore 25 Watts) and desired bias percentage between 65% and 85% depending on your preference. If you measure 480Vdc then 37mA is a good starting point.

Hope this helps.

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CEBergheim

Thanks! I'll probably buy a Weber Bias Rite from eBay within the next few days.

Chris