Setting up a 12at7 for 12ay7 like performance ?

[chopstuck]

Let me just say I have a carton of good 12at7s and I would like to change out some of my triodes in my tweed style builds.
I have been experimenting, replacing the 12ax7s in the first and second position in my 5f4 Super clone with different noval triodes from 12ay7 at7 and au7 equivalents. I tried the 5751 and even a 7249.
Thing is, I like the change in the second socket best where both the second gain stage and CF live.

With it's lower plate resistance I have to get the AT7 to the "happy place" in it's plate curve.
Currently I can hear when the tube draws too much current and the bass farts out.

On the second stage the Rk is 1k5 with a 20mfd bypass cap. 2.2v
I have to rebias and change the Kr. I was thinking maybe 2k2 ?
On the CF I see just a 100k Rk with 140v.
I hope to get better drive through the tonestack and get better response from the controls here.

What values might I try to get the AT7 into it's operating range ?

Or should I just reduce gain by leaving in the 12ax7s and change their Rk and bypass caps ?

[martin manning]

For the 12AT7, try 37k and 0.68k for the gain stage, and 37k for the CF Rk. Or, maybe 42k and 0.82k, and 42k. You can leave the 20u Ck for now, I think. Current draw will increase, of course, so you may have to adjust your dropping string, and use a 2W resistor for the CF Rk, and 1W for the gain stage Ra.

[martin manning]

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