Mark IIB: 12AT7 instead of a 12AX7 in the phase inverter

[Tommy]

Can I use a 12AT7 instead of a 12AX7 in the phase inverter slot? Does it real change the tone?

 

[sundaypunch]

Sure, if you do a search you will see that many people do this. I do it in all of my Mark series amps. IMHO, the cleans improve and you lose a little bit of gain. I find there is still more gain available than I need so I like using the 12AT7 for the improved cleans.

 

[gonzo]

on the same thought...

does this reduce the hiss in the lead channel, when you're driving it realy hard?

i've always thought this might be an 'easy' experiment.....
but never did the change.

i find that i'm backing off the vol1 gain anyway, so, where's the harm in driving it a bit harder, with a cleaner tube in V1?

couldn't you also end up driving the lead channel gain harder too?
to get to the same spot you were at with the 12ax7?

 

[lyman]

i just recently switched my phase inverter from a JJ 12ax7 to an electroharmonix 12at7. I don't have a mark II, it's a mesa maverick (el84) by the way. I found that it tightened up the sound a bit, maybe gave it a little more headroom too. The output was a little less than with the 12ax7, I had the output knob a little higher than before to get the same volume. To me the amp sounds better and I'm quite happy with it. I didn't bother getting a balanced tube either (which doesn't mean it's UNbalanced though).

My logic was that the 12at7 is what most fender amps call for, and the maverick clean channel (and mark series too?) after all is designed around a modded fender circuit.

Maybe it sounds better due to the lower gain of the 12at7, which explains the increased headroom, tighter bass, lower output etc. Maybe the 12ax7 was driving the power tubes on the hard side. The maverick does run the el84's hot.... Anyway try it out and let us know. It's a cheap experiment!

 

[edward]

I've put in a NOS RCA 12AT7 in the PI spot of both my DC5 and MKIII, and I swear by it. That single swap made the most tonal difference of all the tube changes I made, and it was a significant change for the better, IMHO. Sweeter cleans, smoother OD with less grain, and zero drawbacks that I can hear. I coupled that tube swap with a few other preamp tube changes and I am in tone heaven. I can't say what it'll do for a II, though ...gotta try it and decide for yourself. But it made my "nice" boogies that I liked into real keepers that I absolutely love. I can't say that about other amps I've tried and owned

Edward

 

[Dutchman]

I thought I would give it a try in my Mark 2B, so I replaced a GE 12AX7 with a Sylvania 12At7.
Seems to make it a little more focused, not as loose on the lead channel,
My V1 acts like a tone control when you hit 6.5 on up to 8 really fattens the clean channel up.

It's different.....I like it ...... for now......

Mark 2B = the poormans Two Rock........