I've been experimenting with a 12AY7 in V1 of my Bluesmaster. I figure if it's gonna be tweedy, go all the way so far I really dig it. Still nice and warm, but a lil brighter.

Gregg

Somewhere I read a long article about which tubes were best for the PI application. This was by the owner of Groove Tubes, so I guess he kind of knows the stuff. His suggestion was that a 12at7 was probably the optimal tube for this. I'm not sure if he had a specific amp in mind though. I don't remember if he mentioned the 12ay7 or whatever else. I have a nice new 12at7 here which I've been meaning to try out, but just haven't gotten around to trying yet.

I guess i should clarify, it's not so much brighter, but less bass heavy. gives the impression of brightness, in the BM, anyway

oh, and here is a snippet from a discussion we had about 12AT7's back in October hope it helps!

Here are some notes from guitarampliferblueprinting.com:

Some basic tube facts
• 12AX7
o Has a published spec gain of 100
o Has a publishes spec current output of 1.2 milliamps
• 12AT7
o Has a published spec gain of 60-70
o Has a published spec current output of 10.0 milliamps (ten times that of the 12AX7 as a side note)
As a third example, a 5751 has a gain which is almost identical to the 12AT7 but it’s standing current is 1.0 milliamps (about the same as the 12AX7). But, there is a third factor, transconductance, to be considered. The 5751 has a transconductance of about 1200. A 12AX7 has a “TC” of about 1600 and a 12AT7 has a TC over 5000. These three tubes act quite differently. A 5751 and 12AX7 are much more closely related than the 5751 and 12AT7.
We will stick to the basic 12AX7 and 12AT7.


When you push your amp hard it is not as much the output tubes distorting as it is the phase inverter breaking down and distorting. We are talking output stage distortion here. We are not talking about how you may have messed up the signal with preamp tube distortion and compression already. The phase inverter may be the hardest worked tube in most amps. I cannot begin to count the times when I have found phase inverters that were long past their service life. When you change your output tubes change that phase inverter. At the least change it every other output tube change.

Many folks think that when they want to have their amp have more clean headroom they can insert a 12AT7 in place of the 12AX7. Very true. (By the way, the 12AT7 in a first gain stage is an awful tone generator in a guitar amp. If you want to drop front end gain use a 5751 (gain of 60-70) or a 12AY7 (gain of 40). The 12AY7 was the first gain stage in the classic Fender Tweed Bassman, Deluxe, etc.
Going from a 12AX7 to a 12AT7 in the PI (phase inverter) will yield a change in output tube distortion, touch, and output dynamics in most amps. Is this because the gain is lower in the 12AT7? 10% yes perhaps. The lower gain is a factor but the larger factors are:
• We have almost 10 times the current available to drive the output tubes before the phase inverter starts to break down.
• We have a transconductance of 5500 vs 1600 of the 12AX7. Keeping this simple, it means it takes a lot less input signal for a given output signal
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Also, in my testing i found there was a pretty large voltage difference between having a 12AX7 vs a 12AT7

V3   12AX7 GE Long    Pin1: 239      Pin 3: 39.2         Pin 6: 232     Pin 8: 39.4



V3   Mullard CV4024  (12AT7)   Pin1: 185.1   Pin3: 55.2       Pin 6: 180.2  Pin 8: 55.1

The 12AT7 sounded pretty good with about a 8 volt swing between Pin 1 and Pin 2, and with the JAN Phillips 6L6WGB's in the output spot, pushed hard and biased hot. With standard biasing, 6 volt swing, and standard 6L6's, not so much.

Gregg