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Fischer master volume type 3


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8/5/2000 1:26 PM
Carlos
Fischer master volume type 3
Hi everybody!  
 
There's something I don't understand about Ken Fischer's master volume type 3  
 
http://www.lynx.bc.ca/~jc/KenFischerMaster.gif  
 
Doesn't the pot affect the negative grid voltage so that the bias changes???  
 
BTW, how does the circuit sound compared to the "Matchless" type master?  
 
Regards  
 
Carlos
 
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8/5/2000 2:09 PM
Randy Jamz

No, the bias won't be effected. The impedance of the bias circuit is high enough that the 100K potentiometer won't offer much of a divider circuit. I've used this Master setup for over 10 years and had no trouble so far. It does require the added expense of a dual ganged potentiometer, as opposed to the single 1Meg pot Matchless use. The Matchless one works, I suppose, but the frequency response over the whole rotation is not very nice.
 
8/5/2000 3:22 PM
Trace

I agree with Randy. I've never had a problem at all with these so far. I realize that something could eventually happen but so far so good! (ha, ha) I think it sounds pretty "natural" and there's no loss of low-end at lower volumes in comparison to many of the other master volumes.  
 
As far as the Matchless Master Volume I think it's a cool Master Volume and it works great. In my opinion I think the dual ganged pot sounds better and more natural but that's just my opinion. Try both and see what you like and let me know!  
 
Trace
 
8/5/2000 3:57 PM
SteveG
I wonder why he chose 100k instead of 220k?
 
8/5/2000 9:58 PM
Trace

quote:
"I wonder why he chose 100k instead of 220k? "
 
 
As it was explained to me (hopefully I'm not way off base here--ha, ha) the 100K's are easier on the power tubes therefore extending the life of the power tubes. 100K's on place of the 220K's also loads down the preamp a touch more or make it a bit more "crunchy" (for lack of a better word). I'm sure someone will jump in with a mre detailed explaination (ha, ha).  
 
Trace
 
8/6/2000 5:04 AM
PaulC

It's all to do with grid blocking and the build up of a charge on the grid of the pwr tubes. The 100k just bleeds the charge off faster. there's been alot of talk about these resistors on here, so a little diggin' will find some great info that really explains it. One thing I'd think about though is to go with the 250k pots and then adding a 220k resistor from the wiper to the leg going to the bias supply. If that wiper fails your tubes will die in a blaze of glory! The resistor will keep things hooked up. I'm just guessing at the values. The hard part will be in finding a pot and resistor value that gives a good taper. Later - PaulC
 
8/6/2000 2:29 PM
Trace

quote:
"The hard part will be in finding a pot and resistor value that gives a good taper. "
 
 
You'll laugh your butt off but to be completely honest the Radio Shack dual ganged Audio Tapr pot works REALLY well as far as the taper. It's smooth and doesn't "come on" all at one which is really nice for when you are playing at home and don't want to blow the roof off! (ha,ha)  
 
I ran out of what I had and grabbed one just to try to see how it would would work and I was really surprised at the taper. It's around $2 I think for a pot.  
 
Trace
 

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