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| |-+  JTM 45, 50
| | |-+  12AT7 in Phase Inverter
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Author Topic: 12AT7 in Phase Inverter  (Read 2752 times)
Paul G.
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« on: May 26, 2014, 10:54:30 AM »

Finished up by adding transformers, tubes and a box to Nik's built chassis. Amp sounds quite nice, except it starts breaking up a bit too early for me.

My band is fairly loud and I like to set my amp so it is sorta semi-clean. I like a little Marshall crunch and get into mild distortion when I dig in. More than that and I will use a pedal. I do this for versatility so I can go from cleanish, to dirty to filthy easily in a live situation.

All my voltages were fine, so I did a bit of experimenting with preamp tubes as I have boxes of them.

When I put a 12AT7 (equivalent to ECC81) in the P.I. the amp seemed to come alive. Much less compressed, much cleaner and more powerful sounding. Luckily I had an RCA blackplate pull in my box and I think it will live in V3 of my JTM45 until it dies.

Anyway, anybody looking for more clean headroom, try lower gain tubes in the PI, apparently a lot of distortion and compression is coming from that stage. Low gain 12AX7s are available, as well as tubes such as 12AT7 or 5751 Just might be just the ticket.P.
« Last Edit: May 26, 2014, 10:56:08 AM by Paul G. » Logged
« Reply #1 on: May 26, 2014, 09:36:32 PM »

This isn't a matter of mu factor (gain).

There is a myth that that 12AX7, 12AY7, 12AT7, 12AU7, etc. are all interchangeable but offer different mu factors. They actually are very different tubes, with different internal resistance and different current handling.

A big factor of why a 12AT7 PI stays clean is it can deliver nearly 10 times the current of a 12AX7, meaning it can drive the power tubes better without breaking down and clipping itself.

Also, the 12AT7, because of it's much lower internal resistance, can handle the load from negative feedback loops better without clipping.

A 5751, which was designed to be a direct lower mu substitute for the 12AX7 (same current output and internal resistance), would work more similar to a 12AX7, despite it's mu factor.
« Last Edit: May 26, 2014, 09:38:03 PM by wyatt » Logged
Paul G.
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« Reply #2 on: May 27, 2014, 04:52:49 AM »

That makes a lot of sense.

I had checked the spec sheets but my eyes glazed over a bit. I just used my ears and found what I was looking for.

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