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Re: Unusual Gain Stage Question


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5/3/2004 10:45 PM
Daver
Re: Unusual Gain Stage Question
Dutch,  
Gotcha. I understand the usual Rk/Ck roll off. I guess I'll have to try it out sometime. Everything old is new again or is it there's nothing new under the sun or..... never mind. :D Thanks again!  
Daver
 
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5/5/2004 12:42 AM
Doug B.

Daver -  
 
Hey again!  
 
You can think about it as a way to kinda do "fixed bias" without the extra C- supply. Tremane talks about it briefly in his "Audio Cyclopedia", which, of course, i don't have in front of me right now. I believe the method also aids in noise reduction, as residual B+ hum is injected into the cathode out of phase. (But i need to check....)  
 
If you're interested, i can post his description within a couple of days.  
 
- Doug B.
 
5/5/2004 10:48 PM
Daver

Doug,  
Thanks. Cool info. Please post what you have. I'd really be interested. I'm going to try this out soon. Thanks again!  
Daver
 
5/9/2004 4:10 AM
Doug B.
Audio Cyclopedia excerpt - comments?
Upon reading it, i found the description not all that informative. But here it is anyway, for what it's worth. (Comments in brackets [ ] are mine.) From AUDIO CYCLOPEDIA (2ed), Howard M. Tremaine, Howard W. Sams & Co. Inc. (1969), p 532:  
 
- - - - -  
 
Fig 12-28. Fixed-bias circuit in a low-level, high-gain amplifier stage.  
 
[Diagram is a triode gain stage, tube not identified, B+ = 200v, Rl value not specified, Rk = 390, Rd = 68k connected from the cathode to B+.]  
 
The cathode capacitor [in a self-bias circuit] may be eliminated by use of the circuit shown in Fig. 12-28 in which the cathode element is connected to the junction of a voltage divider circuit formed by a resistor Rd and the cathode resistor Rk.  
 
The circuit places the cathode close to ground and eliminates the need for the usual heavy bypass capacitor. This circuit is quite useful in low-level high-gain amplifier stages in reducing hum. As an example: a given tube requires a bias voltage of 1.2 volts. Connecting a 68,000 ohm resistor from a source of 200 volts dc, to a cathode resistor of 390 ohms places the cathode element only 390 ohms above ground.  
 
Because the cathode resistor is not bypassed, degeneration will take place across the cathode resistor; however, the benefits gained by the reduction in hum in this stage offsets the loss of gain suffered from the degeneration.  
 
- - - - -  
 
It seems to me more like something midway between self bias and fixed bias. ("Quasi-fixed bias"? "Semi-fixed bias"?)  
 
- Doug B.
 
5/9/2004 3:16 PM
Daver

Doug,  
Thanks. So it seems that it's more for noise reduction than anything. Dutch's comments allude to the fact that this arrangement produces more gain than a conventional unbypassed gain stage, but probably less than one with a bypassed Rk. I still need to try it soon to see what it "sounds" like. Any other comments or experiences would be appreciated.  
Daver
 
5/10/2004 10:53 PM
Enzo

The purpose of a bypass cap is to keep the voltage at the cathode steady. You can define this as low impedance at certain frequencies or even as feedback, but the bottom line is keeping the voltage steady. This arrangement does the same thing in a different way. They make a voltage divider off the B+ supply to make a steady voltage at the cathode.
 
5/11/2004 8:39 AM
Dave H

As the bias voltage is 1.2V the cathode resistor could be replaced by a couple of series diodes. That would fix the cathode voltage and give it the same gain as a fully bypassed cathode resistor.  
 
Dave
 

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