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Re: Varible 12ax7 cathode resistor in preamp?


 :
2/29/2000 9:07 AM
Steve A.
Re: Varible 12ax7 cathode resistor in preamp?
John:  
 
    I had put one of these in one of my amps figuring it would be a "way cool" gain control... I was disappointed in how little effect it really had. But Bruce's idea of using a small value trim pot in series with a fixed resistor is handy when designing a preamp. You can measure the voltage across the plate resistor and then divide it by its resistance to see how much current the tube stage is drawing.  
 
    Speaking of weird controls, try disconnecting the grid load for an intermediate gain stage. I had screwed up the OD input trim pot on my D'Clone so that the wiper was not seeing any resistance to ground. It did not sound bad at all, at least until I'd hit the Boost switch and it would then develop a touch sensitive VCA effect (like an envelope follower, only it would control the amplitude and not the tone). Depending on the timing of your playing you could get some really neat tremelo or volume pedal effects. That was so much fun that I've considered adding a switch just to kill the grid ground connection, but figured that it would just eat up my EI 12AX7's (which are sure hard to find these days!). So can anybody tell me what sort of damage this could create?  
 
--Thanks!  
 
Steve Ahola
 
3/1/2000 12:04 AM
John
Dan torres says the plate load resistors have to be consistent with the cathode resistors.That is,if you change the cathode resistor,you have to change the plate resistor.What the hell is he talking about?
 
3/1/2000 2:10 AM
Gil Ayan

quote:
"Dan torres says the plate load resistors have to be consistent with the cathode resistors.That is,if you change the cathode resistor,you have to change the plate resistor.What the hell is he talking about?  
"
 
 
I can see a plate load resistor getting damaged if the tube shorts, because there is a significant amount of power being dissipated... say you have 250VDC on the plate, is the tube shorted, say you're using a 100K plate res, you'd be eating V^2/R = 250 * 250/100,000 = .6W, so you would damage a 1/2 watter resistor (so, use a 1W there!).  
 
But, in the same tube (well, short) the current would be approximately V/R = 250/100,000 = 2.5mA... and the 1.5K resistor would dissipate I^2 * R (assuming a 1.5K) = 2.5 * 2.5 * 1.5 *1000/(1000^2) = .01W... THere is no way that resistor would be stressed at all.  
 
In other words, I don't know what Torres is talking about -- some might like to re-phrase that to read, "I don't think Dan knows what he's talking about!" :)  
 
Gil
 
3/1/2000 5:51 AM
Richie
I think if you look at the chart  
he uses.. it goes by voltages used..  
So you have to take this in consideration.  
{~}==:::Richie
 
3/1/2000 7:12 AM
Gil Ayan

quote:
"I think if you look at the chart  
he uses.. it goes by voltages used..  
So you have to take this in consideration.  
{~"
==:::Richie  
}  
 
Richie... cathode voltages? The cathode resistor harldy sees any voltage at all; I have never seen one go bad.  
 
Gil
 
3/1/2000 7:52 AM
Randall Aiken

Gil,  
 
Not having seen the Torres article in question, I suspect what he is talking about is having to change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing.  
If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip very asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs. The two go hand-in-hand, if you change the cathode resistor, the plate resistor should (ideally) be changed, and if you change the plate resistor, the cathode resistor should be changed, unless you are designing for an asymmetric bias point. At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand.  
 
People tend to view preamp tubes and circuitry differently from power tubes, but in reality, they are the same animal, just on a different scale. Preamp tubes need to be biased, just as power amp tubes do. There are ranges that are more optimum than others. This is why it is not a fair test to just swap different tube types and compare tones. A 12AX7 will require different plate and cathode resistors than a 12AT7, for instance, for optimum bias point and tone. A straight swap without regard for circuit operating conditions will lead to incorrect conclusions regarding the tone.  
 
Randall Aiken  
 
3/1/2000 9:14 AM
Gil Ayan

quote:
"Gil,  
 
Not having seen the Torres article in question, I suspect what he is talking about is having to change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, "
 
 
 
Ahhh, that being the case I of course agree. However, I thought that he recommended changing the cathode resistors when routinely replacing the old 1/2W carbon comps with metal film 1 watters.... Sorry Dan! :)  
 
Gil
 

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