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Varible 12ax7 cathode resistor in preamp?

2/29/2000 1:35 AM
John Varible 12ax7 cathode resistor in preamp?
Can it be done?Thanks.
2/29/2000 2:12 AM

Hi again John, I did do that in a V-twin to adjust the clean level of that channel to match the lead channels vol. but there is a mod other than that out now. I believe you have to watch the voltage drop at the plate or the cathode as to not get away from a properly biased 12AX7, but I believe there is a tolerance for that. Perhaps Randall Aiken could give much more of a detailed desciption on that.
2/29/2000 5:00 AM

Until I decided on a particular design and quit wasting my time, I used a few 5K trimmer pots in series with an 820 ohm cathode resistor.  
Bypass the whole thing with the cathode bypass cap and vary the trim pot until you get the tone you like.  
Don't be dissapointed when you find out how close the original 1500 ohm resistor is with a 100K plate load resistor.  
I very seldom find it necessary to change from 1300 to 1600 ohms in most all applications.  
You might find in a particular amp, a higher value works better because the stage has too much gain.  
2/29/2000 5:09 AM
BRUCE, 5K? How about a 50K?
2/29/2000 7:51 AM

Well once you get up that high in the cathode resistance you just about can shut the tube off.  
It will have a gain approaching 1-2 (or + unity) when the cathode is only half the plate load.  
I've used 56K cathode resistors on cathode follower tone stack drivers just for that reason.  
I think you'll be pretty suprised how little gain there would be even with a 25K pot.  
No harm in experimenting using 50K trimmer, but buy a 10K to try out too.  
2/29/2000 8:32 AM
I think sound city amps have a pot  
in some amps to adjust this.  
2/29/2000 12:52 PM

I've considered replacing the cathode follower one stack driver resistor on my mini Plexi with a resistor and pot in series, like Bruce suggested. On this particular amp its preamp had a little too much gain, and increasing this resistor from 1K to 2.2K helped a lot. The gain of the preamp was reduced slightly and it really helped the tone with humbuckers. But sometimes I would like the gain back for single coils, hence the pot.

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