Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: playindabluz on December 18, 2019, 09:28:35 am
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I just finished a 20 watt JCM800 via Rob Robinette's design. I cannot understand why the bias balance pot has no effect but the bias pot is functioning properly. As an aside, I also cannot get the grid reading below -47V.
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I don't see a bias balance pot on the layout you posted
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Sorry, my bad. Here is the correct one.
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I also cannot get the bias reading below -47mV.
What is your procedure for measuring that?
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Sorry, I meant the grid voltage below -47V. I check the bias reading at external test points off of pin 8.
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Sorry, I meant the grid voltage below -47V. I check the bias reading at external test points off of pin 8.
Increase the value of the 470Ω resistor feeding the bias diode. I'd start with 1K and go up/down in value until you have a suitable voltage.
I doubt the balance is going to have much range. I would check it by connecting the voltmeter between pins 5 of the two output tubes. You should be able to adjust the balance and see the meter reading swing positive and negative. The difference in voltage between pot extremes is the amount of swing you have. IE, if you get -4V with pot at max and get -4V with pot at min, then you have a swing of 8V. I'd set the reading to zero initially, then monitor the bias test points for final tweaking. If you connect a probe to each bias test point you can simply adjust the balance for 0mVDC.
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Sluckey,
Thank you for the advice. I was wondering about the value of the 470ohm resistor. You have confirmed my thoughts.
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Sluckey,
Thank you for the advice. I was wondering about the value of the 470ohm resistor. You have confirmed my thoughts.
sim hints at 470R > 2.2K to get -35V
--pete