Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on November 04, 2021, 12:11:33 pm
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In the pic below there are (2) resistors. A 470ohm and a 27k. The common suggestion is to lower the value of the 27k to around 15k to 22k. I was getting ready to do that when i came across someone recommending to change the 470ohm. Why do that one and what do i change it to? Max setting on my pot gets me to 32ma on these 6L6's. My bias tap is pretty high 66v so this is why i need to change the resistor
(https://i.ibb.co/DG0xqmC/biasres.png)
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Either lowering the value of the 27K or increasing the value of the 470Ω will increase your tube current. I prefer to change the 470Ω resistor. Hoffman even calls this resistor the "Bias Range" resistor on all his AB763 amps. I would increase to 1K and re-evaluate. Increase/decrease again if necessary.
It would be very useful if you tell us what the range of negative voltage on pin 5 of the 6L6s. A range of -45 to -60 is usually good enough to make most AB763 6L6s happy.
Look at this for reference...
https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf