Neg Bias Quest

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talbany
Posts: 4696
Joined: Tue Mar 28, 2006 5:03 am
Location: Dumbleland

Re: Neg Bias Quest

Post by talbany »

Firestorm wrote:As long as you're dealing with Fenderish values, it is probably not going to make an audible difference. If you move the grid leaks and the grid stops right to the tube, signal attenuation will be less than one percent and the grid stop will still be in series with the signal so it will still work with the tube's capacitance to block high frequency garbage. The differences come if you overdrive the tube to the point of grid conduction. The time it takes to charge the coupling cap (making it a clamp) will be different and the time it takes for the cap to bleed off its voltage (recover) will be different, as Tony said. But if we're talking about 1K5 and a stock Fender, I'll bet it isn't noticeable.

But again, why bother?
Exactly..Although it's already a common problem in Fender amps that use a .1uf coupler 220k to go into grid clamp and suffer from the deraded farting out effect.. things that would help would be to lower the 220k to a 100k bias feed or go down to a .047 coupler or better yet .02..All this helps speed up bias recovery as well as raise the knee..If you wanted to completely eliminate the grid clamp effect all together you could run class AB2 (CF drive) but I am getting way off the topic now so i'll go Bye Bye!!

Why redesign the wheel?

Tony
" The psychics on my bench is the same as Dumble'"
C Moore
Posts: 1266
Joined: Mon Oct 22, 2007 9:28 am
Location: USA, California, 94585

Re: Neg Bias Quest

Post by C Moore »

talbany - I will reread your post...good info...Thank You

Firestorm -
I see what you are saying ...makes very much sense.
I am rebuilding an amp I had trouble with...so I am reloading all the components, and changing a few things around.
The amp has dual pots for the Neg Bias, and dual pots for the Bias Vary Trem.
I was all done (so I thought) stuffing the board, when I noticed I had omitted those 220k resistors.
With the way the board is configured, it would be real easy to add the 220k into the circuit with them on "the other side: of the grid stops.....that is one reason why I was asking.
I am going to reread some of the "stuff" you guys have written, to get a better understanding of this, and then change my parts around so they are in the Correct/Standard position for these 1.5k and 220k parts.
Thanks for all the help.
I REALLY Appreciate It :)
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