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Hi All,
My amps seen to be working ok but when I wen't to ckeck the bias today, I can not bias them. I get a reading of around 10-30 millivolts and no matter which way I turn the bias pots, the voltage reading stays the same. This is happening on both amps. what may have caused this or am I doing something wrong? I test from pins 2-3 for negative probe and 5, 8 for the positive probe. Can not get 700 millivolts. Any help would be appreciated.
Should I not use the amps untill figure this out? The seem to work and sound fine allthough there is a little hum at idle. No hum noticed before.
Yellow_Light_Colorz_PDT_14

Hi,

To check the bias, assuming you have a 10 ohm resistor to ground from the power tube cathodes, you need to place the positive probe (carefully since there is HV underneath) on pin 8 and the negative probe connects to ground. You should be measuring the voltage across the cathode resistor. If that is 10 ohms then 700 millivolts equals 70 mA. Check it under no signal conditions.

Hope this helps! Since you've been adjusting the pots I would check it this way right away before you play them any longer. You may have set the bias too high inadvertently.

Regards,

WA4SWJ wrote:Hi,

To check the bias, assuming you have a 10 ohm resistor to ground from the power tube cathodes, you need to place the positive probe (carefully since there is HV underneath) on pin 8 and the negative probe connects to ground. You should be measuring the voltage across the cathode resistor. If that is 10 ohms then 700 millivolts equals 70 mA. Check it under no signal conditions.

Hope this helps! Since you've been adjusting the pots I would check it this way right away before you play them any longer. You may have set the bias too high inadvertently.

Regards,

Yes, the resistor is in place. I have been using these amps for a year now and everything checked out before hand. This problem was just now discovered since I haven't biased them for about 7 months. They were stable the last time I checked. I use the SDS Cap board as well as the Weber SS Rect. tubes. I t appears that I have a bias supply problem. No parts were fried or anything like that. Can't figure out why this in now happening. Especially on both amps! I don't where to go next. Maybee Mr. McNally can help?

In the stock bias supply, there is a diode, a pot, 2 electro-caps and 2 resistors. You could try to pin-point the defective part, by taking voltage readings or by swapping each part in one mono-block, one part at a time, until the correct bias voltage appears. This tactic would give you a good idea what is wrong with both mono-blocks.

Barring the above, you could just rebuild both bias supply circuits.

If the amps are working, and the tubes aren't glowing bright red,
I'd suspect a problem with your meter or connection.
You should put the meter on a 2 volt DC scale and measure a AA, AAA, C or D battery and see if you get about 1.5 volts. Something
is funny there. You may have blown a fuse in the meter at some point,
or it has dead batteries or something ?

Also, are you using a good DC ground connection? That happens to me occasionally.

separks wrote:Also, are you using a good DC ground connection? That happens to me occasionally.

Thank you all for your advice. As it turns out the problem was me. I haven't biased these in so long that I forgot the test point pin config. The wiring config. is upside down from what it should be. After I opened them up I saw what I did wrong. "Brain Fart"!!! All of this agravation over a simple oversight. Amps are biased and working perfectly.
Thank You all and MERRY CHRISTMAS ! Yellow_Light_Colorz_PDT_27

Excellent! There are those who have done it, and those who will. :)