12AT7 Express PI

[Smokebreak]

I was rollin tubes today in a stable express and while I only own 3 12AT7s, they ALL exhibited really nasty fuzzy distortion, that cuts in and out, when either treble or volume are dimed, or in different positions on the bright switch. I can put any 12AX7 in there and it's fine.
Anyone else run across this?

[Firestorm]

A lot of people consider 12AT7s to be "function" tubes, rather than audio. They draw a lot more current than AXs, so a lot of voltages and swings will be different. There are some good ones, though: Mullard comes to mind. Maybe Brimar.

[gingertube]

Smokebreak,
Just a heads up - I had a similar problem on the Express I built (see "Junkbox Trainwreck" thread). The Phase Inverter was actually breaking into ultrasonic oscillation at high treble and volume settings.

If this is your problem then fit 22K or even 47K grid stops to both triode sections of the PI for a cure (I used 47K but never checked if a smaller value would do).

Cheers,
Ian

[Bob S]

A good use for 12AT7's is reverb drivers.
They can definitely give the springs plenty of boing.
Maybe use an AX7 in the PI & knock down the preceding gain stage a little.

[Smokebreak]

Ok thanks guys.

Gingertube, I'll try that for sure, as I really dug the at7 tone in there.

I've tried just about everything I've read to knock down the gain, without straying too much for the topography, and can tell this is one of those amps I break open every couple of months, mod the crap out of it, and wind back up at stock, as that's as good as I can get it to sound(for my ears). I've got another junk express build 6V6 that I love after knocking the gain down and pulling some caps, but it never goes above 9 oclock

[Firestorm]

Ok thanks guys.

Gingertube, I'll try that for sure, as I really dug the at7 tone in there.

I've tried just about everything I've read to knock down the gain, without straying too much for the topography, and can tell this is one of those amps I break open every couple of months, mod the crap out of it, and wind back up at stock, as that's as good as I can get it to sound(for my ears). I've got another junk express build 6V6 that I love after knocking the gain down and pulling some caps, but it never goes above 9 oclock

It's very difficult to reduce the gain in an Express and have it sound right. Because there's no choke, the output pulls the screens down and in turn the PI, reducing its gain a little, so you get that singing sustain. So no matter what you do, it won't be Express-like until you turn it up enough to overdrive the output. Back to square one.

It's a great candidate for scaling, but that has a set of challenges all its own.