Question about replacing 12AX7 with 12AT7 in PI

[Steve A.]

It is my understanding that you need to bias 12AX7's differently than 12AT7's so you don't want to indiscriminately replace one with the other in a LTP PI. I know that some people like to replace the 12AT7 in a BF PI with a 12AX7 to get a sound more suited for hard rock (I prefer the 12AT7 in there myself). On the Boogie Board one of the member posted that he liked to use a 12AT7 for the PI in a Maverick since he thought he could hear the separate strings better with it in there rather than the stock 12AX7. I just tried it out and I think I prefer it to the 12AX7. It isn't just a matter of a lower MU- a 5751 for the PI doesn't sound as good as the 12AT7 (or the 12AX7 for that matter). Here is a link to the schematic (the PI and power amp is in the second picture from the top):

The Free Information Society - Mesa Boogie Maverick Electronic Circuit Schematic

Any suggestions to improve the PI circuit with a 12AT7 replacing the stock 12AX7? (BTW I believe that the NFB is used for the Clean channel but not the Lead channel, and was wondering why the values are so different from an AB763 SR.)

Thanks!

Steve Ahola

[hasserl]

Yes, change the PI plate load resistor and cathode/tail resistor values to those used in some of the later silver face amps.

Here's some discussion on the topic I saved from the old Ampage forum:

From Doc:

"One thing I'd like to mention about "blackfacing" the PI by changing its plate resistors from
47k/47k to 82k/100k: I was displeased with the higher value resistors. (The cathode string was
also changed out.) My ears tell me that the 47k plate resistors and tail resistor network used in
later SF amps results in a louder, cleaner signal than after BF'ing. My friend's unmodified
AA371 bassman (47k plate res)is louder and stronger than my BF'd version. And his amp has
the stock funky feedback arrangement. A few years ago, I rebuilt a SF Bandmaster reverb head.
I left the 47k network and 330k grid resistors in the 12AT7 PI circuit, and basically recapped
the amp without modifying much of anything in the preamp. (I did remove those partial cathode
bias resistors in the 6L6 circuit, and converted to bias level adjust supply.) This amp had a
wonderful Fender tone when the owner played his Telecaster through it. Everyone asked him
about his amp. I guess I did something right without realizing it.
Although the earlier PI circuit utilizing 82k/100k plate loads with 1meg grid leaks has a higher
ultimate voltage gain, it doesn't seem to provide the output stage drive capability for the AB2
power stage as the 47k (1watt) plate load/ 330k grids network. Another benefit with leaving the
47k's in there is you can plug in a 12AU7 directly and have it biased correctly. The 12AU7
lowers the maximum signal power because of its lower voltage gain, but seems to give the amp
clean headroom and an unusual overdriven sound at max. Hard for me to put into words. You
have to try it and listen."

From Ray Ivers:

"Doc,
I agree 100% about the PI load resistor change. IMO the 82K/100K or 100K/100K values,
while appropriate for a 12AX7, have no business being in a 12AT7 or 12AU7 driver stage
unless you're looking for a somewhat squashy, compressed sound. With a 12AU7, plate loads
can be reduced even further for even more headroom and output tube drive, although eventually
you'll end up with over 300V at the plates, of course. I like my stock AB165, but I'm not trying
for anything like a sparkly clean sound out of it, and I bet I would like it even better with 33K -
47K PI plate loads.
Ray"

also from Ray:

"It's quite possible to get the same gain with a 12AT7 as a 12AX7, especially when using lowvalue
plate loads with both (in this case, you'll probably get more gain with the AT). You'll also
be pushing the output tubes a bit harder, which will translate into more overdrive and crunch;
does that sound like what you want? Plugging a 12AT7 into a 12AX7 circuit, especially a
driver circuit, will not give you a true feeling for what a 12AT7 is capable of IMO, although it
will function.
I'm actually a real fan of 150K - 330K plate loads with 12AX7's, provided that the tube is
biased correctly for the new plate resistor; this is not really a drop-in mod. Changing all the
1.5K's to 820's and then putting in 220K plate resistors would be an example of what not to do
IMO; you would end up with a strange bias point and asymmetrical clipping. I would use a
2.7K cathode resistor instead, and the 150K plate/3.3K cathode combination also looks like a
winner to me in TubeCAD (370V B+).
Ray"

From JWK:

"I just got through walking through Aiken's page on designing a LTP using the load line method.
I printed out a couple of plate curve charts from TubeCAD and went through all the steps using
a 12AT7 instead of the 6SL7 he uses. Here is what I got:
33k plate resistors
120 ohm cathode resistor
10k tail resistor
400v B+
Vo is -50v, +56v"

[Jazz P Bass]

Thanks for the valueable post.
I cut & pasted it as a Word document & saved it in my tech folder

[Steve A.]

Thanks for posting that! We hadn't heard from Doc for many years but I did call him on the phone a few years back- he was without regular internet access and would use the computer at the library. I think I had his phone number in my Firefox/Thunderbird address book, but it was wiped clean with one of the upgrades a few months ago (thanks a lot, Mozilla!) I do have it backed up... somewhere!

I need to reread what Randall Aiken has to say about the LTP, just to better understand the function of the components.

Steve

P.S. I just saved that post to Notepad as a TXT file. I will use Word if I need to capture formatting or pictures, but otherwise I like to use a universal format that everybody can read (including me!)

[Steve A.]

Okay- so how does the math for the Fender BF (AB763) PI work out with a 12AX7 (since a 12AT7 really isn't a good match)? How about the 5F3A PI- does that check out okay with a 12AX7?

Thanks!

Steve

P.S. I do think that the sound of the AB763 PI w/ 12AT7 has been etched into our brains, even though the math doesn't work out that well.

[jpfamps]

Richard Kuehnel's site has this calculator on it which takes some of the pain from the maths of the PI.

Long-Tailed-Pair Phase Inverter Gain Calculator

[pdf64]

'the earlier PI circuit utilizing 82k/100k plate loads with 1meg grid leaks has a higher ultimate voltage gain, it doesn't seem to provide the output stage drive capability for the AB2 power stage as the 47k'

I'm not sure that any such LTP arrangement would facilitate AB2 operation? For that I think that a low output impedance driver (to the power tube grids) would be necessary.