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In Reply to: RE: Lots of vintage components! posted by gusser on July 18, 2012 at 15:11:11
Its hard not to try and use everything one has but the old caps keep biting me so I broke down and spent some money
ckeck out the pic this is how I made the 11.2
Get rid of that WW pot in the cathode circuit and just use the 10 ohm resistor. That pot is a liability. Better yet, use a separate 10 ohm resistor per tube so you can match each one independently.
The reason for the 11.1 ohm resistor is a bit interesting. In the early 1960s a precision volt meter was expensive. $10 digital meters were science fiction. The cheap hobbyist analog meters were rather inaccurate at such low voltage readings. So Mr. Hafler made the proper bias current work out to be 1.56 volts. That just happens to be the terminal voltage of a D flashlight battery so you could calibrate your meter to a fresh battery and the Dynaco bias should be the same!
Today we have precision volt meters that can be bought at an auto parts store for less than $10. So by just using the 10ohm resistor, you can easily calibrate your bias.
thanks I will do that next time I have it open
If you do put a 10ohm cathode resistor on each output tube, the voltage reading across EACH 10 ohm resistor should be HALF of what it was when both tubes shared the same resistor.
This is important as you could grossly over bias the tubes if you don't remember this.
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