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Differences between 12ax7 vs 12au7 other than gain?


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1/28/2000 4:16 PM
lion
Differences between 12ax7 vs 12au7 other than gain?
In an attempt to lower the gain in a Vox-style preamp Ive found that I really like a 12au7 in the second position instead of a 12ax7.  
(V2a = second preamp stage. V2b = cathode follower driving the tone stack)  
 
I know that 12au7/12ay7s are more commonly used in the first position and I have tried that too - but I like it better in the second position in this amp.  
 
But now Im wondering about the use of a 12au7 as a cathode follower and a few other things:  
 
1) I cant recall Ive ever seen a 12au7 used as a CF - is this a coincidence, or is it just not suitable - or am I missing something here?  
 
2) What are the differences between the two other than the gain (19.5/100)?  
 
3) I seem to remember something about added stress on the power supply with 12au7s is this correct?  
 
4) Of course I could put the 12ax7 back in and drop the gain (say with a split load plate resistor) into the cathode follower - to simulate the lower gain of a 12au7. Would this be a better solution from a tech point of view?  
 
There seems to be a limited selection of 12au7 tubes which certainly speaks in favor of the 12ax7 solution.  
Anything else to consider?  
 
Any advise or comments will be appreciated?  
Lion  
 
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1/28/2000 6:10 PM
Scott Swartz

 
quote:
"1) I cant recall Ive ever seen a 12au7 used as a CF - is this a coincidence, or is it just not suitable - or am I missing something here?"
 
 
Not in guitar amps, fairly common in tube hifi.  
 
quote:
"2) What are the differences between the two other than the gain (19.5/100)?"
 
 
Gain, plate resistance, transconductance - which means correct plate resistors around 30K 2 watt, not 100K. Resistors for CF also lower, maybe 40K, 2 watt.  
 
quote:
"3) I seem to remember something about added stress on the power supply with 12au7s is this correct?"
 
 
Typical current draw 4-5 ma per triode, instead of 1-2 for 12AX7  
 
 
quote:
" 4) Of course I could put the 12ax7 back in and drop the gain (say with a split load plate resistor) into the cathode follower - to simulate the lower gain of a 12au7. Would this be a better solution from a tech point of view?"
 
 
The 12AU7 approach will have less noise and distortion, but maybe not enough to notice in a guitar amp.  
 
quote:
"There seems to be a limited selection of 12au7 tubes which certainly speaks in favor of the 12ax7 solution."
 
 
I think that it is currently out of production, so NOS is only choice  
 
My main Fender like amp uses a 12AU7 as 2nd gain stage/CF in a Bassman preamp, so I am with you on your choice to use it there.  
 
1/29/2000 12:13 AM
Reid Kneeland

There's (sort of) one hiding in the Ampeg  
V-series amps and the SVT - section 2  
(pins 1,2,3) of a 12DW7 is pretty much  
a 12AU7, and is used as a CF in those amps.  
 
Reid  
 
2/1/2000 4:44 AM
Charlie Escher

I use 12 AU7s a lot. I like using them in the PI/driver slot, as well as the other places you've mentioned. I typically use one somewhere in the gain stages (good for driving the tone stack, even in non CF configuration), and one as a cathode follower for a buffered line out. There are plenty of NOS 12AU7s available, they're cheap (~$5US) too.  
 
They work especially well in bass amps when you're going for that Ampeg vibe, in my experience.
 
2/2/2000 6:31 PM
lion

Thanks for the replies.  
 
Im reassured - and I think Ill just keep it there in the 2nd position, even though I havent changed the resistor values - plate 100K - CF res 56K.  
 
The voltage readings are as follows:  
2nd gain stage plate 102V cathode 4.1V  
CF plate 376V cathode 117V  
 
I hope this is OK?  
 
Lion  
 

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