Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on December 05, 2018, 05:39:29 pm

Title: Fixed and self bias
Post by: shooter on December 05, 2018, 05:39:29 pm

I should know this but don’t!  :think1:
You start with fixed biased amp, and 1ohm Rk for current calcs.

now I up Rk to 10 ohms cuz I want to made things hard, and re-bias the fixed side – if needed.

If I keep uppin Rk n tweaking fixed, can I get to a point where even and odd harmonics “balance”, unintended consequences, or am I just wasting gray matter n cloud space  :dontknow:

 

Title: Re: Fixed and self bias
Post by: PRR on December 06, 2018, 02:26:17 pm

It's complicated. And maybe not worth much.

Quit picking your brain and apply the soldering iron.

Title: Re: Fixed and self bias
Post by: shooter on December 06, 2018, 03:05:09 pm

Quote

It's complicated.

 :laugh:
ya, it's stuck in an endless loop!  I'm doing woodworking outdoors so my brain just shuts down  :icon_biggrin:

Title: Re: Fixed and self bias
Post by: jjasilli on December 06, 2018, 06:50:44 pm

Maybe that's what Fender hoped when they went from BF to SF.


Anyway 10R can act as fuse & bias sense.

Title: Re: Fixed and self bias
Post by: shooter on December 06, 2018, 07:25:00 pm

Quote

10R can act

That's what got the whole mess in my brain started, Willabe's KT88 conversion.  The PA tubes use 10r guessing as the "correct" value for the meter's, Then...........  :think1:
I have a small understanding of amp history, and a quick search only yielded spy vs spy instead of working together.

I'll be getting the soldering gun out soon, gotta wire up lights for my ghetto camper II  :icon_biggrin:

Title: Re: Fixed and self bias
Post by: PRR on December 06, 2018, 07:55:53 pm

Few power tubes have Gm as low as 1/50r, so a "significant" resistor will be 100 Ohms up.

10 Ohms was standard when good meters read 1.5V on the lowest range. 50mA was 0.5V or 1/3rd-scale, you can read that. With 1 Ohm it is 0.050V, hard to squint on a 1.5V scale. But sweet on DVMs where the natural input is 199mV.