Proper tube amp bias?

[Radioboy950]

Question about plate voltage and bias adjustment for the Rivera Quiana 112 combo.
The specs indicate Tube voltage at 430 DC. When it’s time to retube with new 6L6s, where do I want the bias (mA) set so it maintains the gain and tone structure??
I tried a setting of 38mA, and I'm thinking it's a little hot for the amp. The cleans are still wonderful, but the overdrive channel got a tad harsh.
Where's the nice happy middle ground for average breakup...not too hot/not too cold?

 

[tunghaichuan]

According to what you've written here, 38mA is a little cold. The general rule of thumb for a Class AB push pull amps is to bias the tubes to 70% of the maximum dissipation, which is 30w for 6L6GC tubes (6L6GB, 6L6WGB and real Tungsol 5881 tubes have a maximum dissipation of 23w, IIRC).

So 430 x .038A = 16.34w or about 54% of 30 watts.

Try increasing the bias to 48mA. 430 x .048A = 20.64W or 68%.

Try this and see if it improves your OD sound.

FWIW, here is a convenient tool:

http://www.webervst.com/tubes/calcbias.htm

However, if you play with a lot of preamp distortion, it might be the voicing of the OD channel that is causing the harshness. To get power amp distortion, you'd have to be playing the amp very loud. So I'm thinking that the bias may not be the issue.

tung

Question about plate voltage and bias adjustment for the Rivera Quiana 112 combo.
The specs indicate Tube voltage at 430 DC. When it’s time to retube with new 6L6s, where do I want the bias (mA) set so it maintains the gain and tone structure??
I tried a setting of 38mA, and I'm thinking it's a little hot for the amp. The cleans are still wonderful, but the overdrive channel got a tad harsh.
Where's the nice happy middle ground for average breakup...not too hot/not too cold?

 

[Radioboy950]

Hmmm...
Good stuff to chew on.
Cool table/charts too.
Thanks much.