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Grid leak (bias)resistor question.


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9/4/1999 4:42 AM
Mark Abbott
Grid leak (bias)resistor question.
I've heard some people talk of reducing the value of these resistors to 100K, my first question is why, and my second question is how does it change the sound of the amp? thanks Mark.
 
9/4/1999 3:11 PM
Bruce

One of the things I've noticed is that by changing the grid load resistors to a lower value, the amps don't seem to fart out and smear the lows as bad at higher volume and the preamp can be cranked up a little hotter for a touch more preamp crunch with out caving in the power tubes like that.  
 
I've stopped using 220Ks unless it's for a real clean Fendery type amp, and then I've also stopped using the .1uF coupling cap and went to .033uF to .047uf on those too.  
 
The only guys that seem to think they hear a bass loss difference is with the guys that can't, won't or don't play with a band, others or with a bass player.  
 
The guy who sits in his own little room by himself "might" hear the amp is cleaner and not quite so flubby-bassy-smeary sounding and then think it's not quite as good... because they can hear their technique too well!! :>)  
 
 
 
Bruce
 
9/5/1999 7:08 PM
Joe Movich

Bruce,  
 
I have a Fender amp that I like the sound of at low or medium volume levels, but at higher volumes it gets too "thick" sounding, nice for playing leads but chords are a little too mushy, will changing the bias feeds to 100K and the coupling caps to .047 clean up the sound at high levels too? Thanks.
 
9/6/1999 1:54 AM
Steve A.

Bruce:  
 
 
 
    So what value have you been using instead of 220k for the bias feed? I'd seen that 220k figure in so many schematics that I figured that Congress must have passed a law forbidding the use of anything besides 220k resistors there... < g >  
 
 
 
Thanks!  
 
 
 
Steve Ahola
 
9/6/1999 3:31 AM
Trace

quote:
"One of the things I've noticed is that by changing the grid load resistors to a lower value, the amps don't seem to fart out and smear the lows as bad at higher volume and the preamp can be cranked up a little hotter for a touch more preamp crunch with out caving in the power tubes like that."
 
 
 
 
Bruce, I think that's a fantastic way of describing it verbally. I also notice that the "harsher" highs are trimmed off as well which leaves you you with the creamier highs that are more pleasing to the ear. Do you happen to notice this at all?  
 
 
 
 
 
quote:
"I've stopped using 220Ks unless it's for a real clean Fendery type amp, and then I've also stopped using the .1uF coupling cap and went to .033uF to .047uf on those too."
 
 
 
 
Same here. I've really been diggin' the .022's and .033's a lot lately. The .1's seem to be too muddy on the low-end and it get's really "mushy" when you start driving the amp up a little bit.  
 
 
 
quote:
"The only guys that seem to think they hear a bass loss difference is with the guys that can't, won't or don't play with a band, others or with a bass player.  
 
The guy who sits in his own little room by himself "might" hear the amp is cleaner and not quite so flubby-bassy-smeary sounding and then think it's not quite as good... because they can hear their technique too well!! :>)"
 
 
 
 
What can I say! (ha, ha) Bruce, you have an uncanny ability to hit the nail on the head every time!  
 
 
 
:^)  
 
Trace  
 
 
 
 
 
 
9/6/1999 8:01 PM
Benjamin Fargen
220K to 100K in a cathode biased amp????
Bruce,  
 
What kind of changes are made to the bias setting when replacing the 220K grid leak resistors to 100K in a cathode biased amp? Will the current across the tube go up or down?  
 
 
 
Thanks,  
 
Benjamin  
 
 
9/7/1999 1:51 PM
Doc

There shouldn't be any change in DC cathode current.  
 
 
 
In general, cathode biased power amps can get by with larger grid resistors than fixed bias, due to the increase in effective negative bias under large signal conditions. A fixed bias amp doesn't have this "safety net", and can go into grid blocking (and sustain it) more easily. For instance, the maximum grid resistance (whether it be a grid leak to ground, or a series bias voltage feed) for a 6L6 is 100k under fixed biasing and 500k under cathode biasing. Stretching it to 220k in a fixed bias circuit will allow smaller coupling capacitors for the same R-C constant, also a higher gain of the previous stage, but possible instability under drive conditions which exceed the grid bias voltage.  
 
 
 
Conversely, dropping the grid resistance lower than maximum recommended will lower the gain slightly and improve stability during overload transient signals. Using a lower grid resistance in conjunction with a smaller coupling capacitor will raise the lowest frequency passed, prevent swamping, and result in an overall more well behaved amp.
 

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