HD 212 130 ,12ax7 . Bias findings.

Hi , is this the correct way to set the bias in my amp . I measure across the 10k resistor on pin 8 ,and adjust the trimpot until i get .25v . I say .25v because the .5v setting is not attainable with 4 tubes present . So i assume .25v is the correct measurement because .5v is for a single tube . Is this correct.[IMG]http://i303.photobucket.com/albums/nn160 /WRGra/P7310312.jpg[/IMG]

Hello there.
The 0.5V is the same for both 2 and 4 tube configurations. Each tube pair has a 10 ohm cathode resistor. Remember you have to be in high power mode for this measurement to be accurate.
Cheers,
Lars Verholtwww.captain-foldback.com

O.K. Then ,even with the trimpot turned fully clockwise ,i am still only getting .32v . And i am adjusting the trimpot furthest to the right .Cheers!

What is the actual bias voltage you are getting at the junction of the two 330k resistors and the 150uF/50V cap? The trimmer should give you a range of approx -30V to -45V. If this voltage is not right (for whatever reason) you will not get the right bias point. Obvious question: Have you measured the voltage for both tube pairs? Have you checked that the 10ohm cathode resistors are within spec? Are you getting ~715V on the plates in 'hi' power mode?
Sincerely,
Lars Verholtwww.captain-foldback.com

Sounds like i am out of my depth depth at this stage . Think i'll drop it off at my local amp repairer ,Rock Dog, http://www.rockdog.com.au/
Thank's for the reply's .