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Unusual Gain Stage Question


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4/29/2004 2:36 AM
Daver
Unusual Gain Stage Question
Well, the question isn't too unusual but the gain stage is. I've seen this stage in Magnatone schematics; 12AX7, 270K plate R, 2.7K cathode R unbypassed by a cap and then a 220K resistor from B+ to the cathode connection. What is this resistor from B+ to the cathode doing? Negative feedback? Some type of biasing arrangement? Thanks for any insight you can give.  
Daver
 
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4/29/2004 11:45 PM
Daver

Bump! Is it really that weird? :)
 
4/30/2004 4:14 AM
woo

Its just another way of setting the bias. I'm not sure why they did it, as you could get the same bias point, a little more gain, and a little less hiss by just using a bigger cathode resistor and a bypass cap. I'd be inclined to give it a weirdness factor of 7.
 
5/1/2004 4:44 PM
Daver

Bias, huh? It has a grid to ground resistor and a cathode resistor. Why would they need to use an additional resistor from B+ to the cathode to set the bias? Could you give me a technical explaination on how this works? Thanks!  
Daver
 
5/2/2004 7:15 PM
Dutch

quote:
"Bias, huh? It has a grid to ground resistor and a cathode resistor. Why would they need to use an additional resistor from B+ to the cathode to set the bias? Could you give me a technical explaination on how this works? Thanks!"
 
 
Daver--  
Basically, the extra current flowing through the resistor from the B+ is added to the current flowing through the tube, to provide extra voltage drop in the cathode resistor over what the cathode resistor would normally develop with just the current flowing through the tube.  
 
The other times I've seen this arrangement (mainly in some of the Magnatone amps), it was used because a very small cathode resistance (470 ohms, IIRC) was used, and extra DC current was needed to bias the stage out of the "grid leak" region.  
 
So what's the reason for doing the small cathode resistor? To reduce the amount of degenerative feedback occuring in the cathode resistor for more gain without introducing the phase shift and imperfect low frequency response characteristic of a capacitor-bypassed cathode resistor. For guitar amps, it's not that critical, but for hi-fi amps, phase shift is a big deal, as is the low frequency rolloff that can occur with cap-bypassed stages.  
 
HTH!  
 
C ya,  
Dutch
 
5/2/2004 7:08 PM
Daver

Dutch,  
Perfect! That's exactly what I was looking for. You're right, I believe the cathode resistor was 470 ohms in the schematic I saw. Now for the big question; does biasing the stage in this way sound different than the "usual" way with a bypass cap? You knew I'd ask! ;) Thanks very much for a great explaination.  
Daver
 
5/2/2004 7:35 PM
Dutch

quote:
"Perfect! That's exactly what I was looking for. You're right, I believe the cathode resistor was 470 ohms in the schematic I saw. Now for the big question; does biasing the stage in this way sound different than the "usual" way with a bypass cap? You knew I'd ask! ;) Thanks very much for a great explaination."
 
 
Daver--  
You're welcome, and thanks!  
 
There is a difference in sound. The frequency response with no cathode bypass cap is essentially as flat as the tube itself will let it be, which is usually pretty flat. With a cap-bypassed cathode resistor, you get more gain, but it's got a certain frequency below which the phase starts to shift and the gain starts to roll off. With a typical 1.5k/25uF Rk/Ck combo, this frequency is way below the guitar's fundamentals, but can affect the transient response some.  
 
 
HTH!  
 
C ya,  
Dutch
 

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