Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on August 07, 2023, 11:26:02 am

Title: EL34 vs 6L6 bias supply
Post by: fossilshark on August 07, 2023, 11:26:02 am

I just read the valvewizard page on designing a bias supply and I feel I have a pretty good understanding of how to design one that fits my needs. I am just wondering how come the bias circuit for a mesa boogie dual rectifier drops the voltage by 10v if the mu of an EL34 and 6L6 is the same? Wouldn't they both want a max bias voltage of around 50v when running off 400v HT?

Title: Re: EL34 vs 6L6 bias supply
Post by: tubeswell on August 07, 2023, 01:29:53 pm

We had a good old discussion about that a while ago


https://el34world.com/Forum/index.php?topic=22519.0 (https://el34world.com/Forum/index.php?topic=22519.0)

Title: Re: EL34 vs 6L6 bias supply
Post by: PRR on August 07, 2023, 02:39:11 pm

Quote from: fossilshark on August 07, 2023, 11:26:02 am

....if the mu of an EL34 and 6L6 is the same?.....

Mu is a voltage relation, and is not constant over current.

The short answer is that EL34 has higher GM, transconductance, current per volt. Although the Mesa plan implies the opposite. A deeper answer arm-waves about the highly curved plots on EL34 so that no single-number 'constant' really works.