HK citation 12 MOSFET (HOT)

[jeepy1260]

Hello All,

In the process to adjust the bias around (100mV) as per Mr. Nelson Pass recommandation, I cannot go below 600mv! is it because I am using TO3 Exicon 10N20 and 10P20?. The bias transistor fixed on the heatsink is a BD139 instead of a MPSL.
It is really hot (i cannot leave my finger on the heatsink more than 4-5 sec but the bandwith is huge on the scope and the sound is great on my JBL 100 Century.
Many thanks,
jp

 

[woody]

The amp was not designed for those mosfets. Those exicon mosfets require a lot less bias voltage and I seriously doubt that adjustmant pot will allow you to turn the voltage down low enough to use them without making some change to that veb bias circuite.

 

[jeepy1260]

Oh I see! Is there any chance to find some TO3 IRF 1xx somewhere?
Many thanks,
Jp

 

[Nelson Pass]

The original can use IRFP240 parts, and the complementary version can use
IRFP9240. Many Mosfets can be used here, and you can adjust the range
of bias downward by removing R15 and possibly lowering the value of R14.

 

[jeepy1260]

Thanks a lot Mr. Pass, I will lower the R15//R17 value. Thank you for sharing your knowledge!
Jp