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previous: Steve K Yes, I use a fan in all my class A ... -- 8/9/1999 4:08 PM view thread

Re: questions about Spitfire

8/9/1999 4:38 PM
Mike B
Re: questions about Spitfire
The Vox AC15, which the Spitfire circuit is based on, used a 130 ohm Rk with a B+ around 330V. So your circuit is not far off. I have built several cathode-biased amps using EL84's and have had no reliability problems running the tubes in the 14W area. I typically use a 130 ohm Rk and run the B+ around 345V.  
As far as tone is concerned, I have found that increasing the value of Rk thins the sound out. Increasing the bias current, within reason, seems to beef up the tone, at least to my ears. I think Matchless came to a similar conclusion and that is why they use 120 ohms Rk. They tend to run all of their amplifiers very hot. I was concerned with reliability, so I eventually settled on 130 ohms for my DIY amps. Something to keep in mind - these amps run class A without a feedback loop, so if they are too lightly biased crossover distortion will occur. Running a high bias current is a necessary evil in order to keep the tubes conducting throughout 360 degrees of input signal.  
Mike b

dave OK, a 150 ohm Rk gives me 14 watts ... -- 8/10/1999 2:01 AM