Bottlehead Forum

General Category => Technical topics => Topic started by: Strikkflypilot on July 23, 2015, 09:14:12 AM

Title: Re-biasing the Mainline
Post by: Strikkflypilot on July 23, 2015, 09:14:12 AM

Hi.

My Mainline curcuit has now been running since, oh, february.
Decided to rebias it.
Left it on for 30 min to be sure of stable Voltages.

On terminal 20, it has dropped to 134V, while terminal 30 still shows slightly more than 145V.
I have a very rudimentary idea of what this means, like that the headroom for amplification might be different for the two tubes...

Would someone like to elaborate on what 10v difference means, the short time it took to have this difference, what does it tell me about the tubes?

Title: Re: Re-biasing the Mainline
Post by: Paul Joppa on July 23, 2015, 10:36:34 AM

The bias is optimized for maximum peak power, i.e. equal clipping on positive and negative peaks when just overloaded. Peak output is +/-75v at the output transformer primary.

The downward drift in plate voltage is probably a result of the cathode stabilizing in a new tube. Eventually the cathode will weaken, which will result in the plate voltage increasing.

Title: Re: Re-biasing the Mainline
Post by: Strikkflypilot on July 23, 2015, 10:45:35 AM

Thanks.
Understanding is slowly forming.