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In Reply to: RE: 12AX7 Replacement posted by AudioSoul on December 11, 2015 at 11:23:13
I had a similar question about the 12A?7 series and posted on the Tubes/DIY forum. The responses were very helpful. In the meantime, I have been doing some reading about triode circuits and design. The bottom line is that the 12A?7 series are not really interchangeable without modifying the overall circuit. It's not like swapping 6199 for a 6DJ8.
BTW I had a problem with a 12AX7 and went to a local repair shop who sold me a used RCA (for $5.00!!!)- What a great tube! I wish he would sell me more.
Follow Ups:
a 30% reduction in gain is OK. No circuit mods are necessary.
Opus 33 1/3
Don't confuse the tube's Mu with the circuit's gain. You won't change the circuit gain by 30% since with the 12AX7 in the circuit it never achieved a gain of 100 anyway.Look at Randall Aiken's site (link below) and you can get a handle on this widely confused concept.
Edited to clarify...
Edits: 12/14/15
If we assume a total plate load of 180k TubeCad shows a stage gain for the 12ax7 of 75. The plate voltage moves up 75 volts as the grid voltage moves from -1 to -2 volts showing a stage gain of 75. (because of the non-linearity of the tube, the plate voltage move even farther when the grid voltage is increased from -1 to 0 volts)
With the 5751 the plate voltage moves only 53 volts when the gird voltage moves from -1.5 to -2.5 showing a stage gain of 53.
This is a reduction of 22 volts. 22 is 29.33% of 75.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 12/20/15
My point was that the mu does not equal the stage gain. Your examples clearly show that, thanks for the homework. Also, based on the relatively high circuit gain I'm assuming the circuit model had the cathode cap fully bypassed. Without the bypass the gain would be significantly lower.
And of course the grid resistor of the next stage is in parallel with the plate load of this stage so the effective load would be lower too, which impacts gain as well.
"Without the bypass the gain would be significantly lower."
Published plate curves always assume there is no feedback applied.
"And of course the grid resistor of the next stage is in parallel with the plate load..."
The grid resistor of the next stage, in parallel with the plate resistor, IS the plate load. That is to say, the plate resistor alone is never the total plate load.
"My point was that the mu does not equal the stage gain."
True but consider a 6sn7 with one of Gary Pimm's CCS in the plate circuit and the grid resistor of the following stage at 1 meg ohm.
At 1kHz Pimm's Self Biased CCS has an impedance of about 3 gig ohms, that in parallel with a 1 meg ohm grid resistor of the following stage gives a total plate load of about 999667 ohms.
The published plate curves of the 6sn7 in TubeCad shows a stage gain of 21.
Now that's impossible but the point is with a horizontal, or nearly horizontal load line, one can get very close to a stage gain equal to the mu of the tube.
Just a note, for triodes a horizontal load line produces the most stage gain with the least amount harmonic distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Of course using a CCS will "horizontalize" the load line with the tremendously high impedance. And while plate curves may indeed not reflect presence of a resistance in the cathode that is because the plate curves are not the same thing as tube performance in the circuit with the cathode resistance and next stage influencing it.
Since the original post was regarding a phono stage I'm going on the assumption that it did not use Gary Pimm's circuit, that it did have some cathode resistance, and that there was a stage driven by the output of the 12AX7 based stage. While the other stuff is interesting it is likely not applicable to the specific situation the original poster was asking about.
But thanks again for sharing your knowledge!
I think the 5751 is a good choice as well.
The 12AX7 provides the most gain =100% of the 12A-7 tubes and is generally crucial otherwise. Thus I wouldn't even consider something like a 12AU7 which has very little gain comparatively.
It is better to have blurbed and blown it than to have never blurbed at all.
The temptation is not just the base which all 9 pin dual triodes share ?B9 ?, but the pinout is compatible making it more tempting to fool around these tubes. I was guilty until I realized the circuit was engineered for a specific tube and I was not gaining anything good trying to ham handedly "re-engineer" my gear. With the exception of the 5751 which I like and was developed with 12AX7s in mind.
It is better to have blurbed and blown it than to have never blurbed at all.
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