Tube Amps / Music Electronics
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|1/28/2000 4:16 PM|
||Differences between 12ax7 vs 12au7 – other than gain?|
In an attempt to lower the gain in a Vox-style preamp I’ve 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 12au7/12ay7’s 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 I’m wondering about the use of a 12au7 as a cathode follower and a few other things:
1) I can’t recall I’ve 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 12au7’s – 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?
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|1/28/2000 6:10 PM|
Not in guitar amps, fairly common in tube hifi.
Gain, plate resistance, transconductance - which means correct plate resistors around 30K 2 watt, not 100K. Resistors for CF also lower, maybe 40K, 2 watt.
Typical current draw 4-5 ma per triode, instead of 1-2 for 12AX7
The 12AU7 approach will have less noise and distortion, but maybe not enough to notice in a guitar amp.
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|
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.
|2/1/2000 4:44 AM|
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|
Thanks for the replies.
I’m reassured - and I think I’ll just keep it there in the 2nd position, even though I haven’t 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?
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