Extra Gain Stage - Tremolo?

[MarshallPlexi]

Okay so I'm working on an amp that uses a 6SL7 for the first two gain stages and then uses a 12AU7 (mu of 20) as a 3rd gain stage. Since I have an extra triode left over in the 12AU7, I was thinking it would be cool to add a trem to the amp but most trems use the 12AX7 (mu of 100) or at least the 12AT7 (mu of 70).

Here's what I was thinking: I'll use a 12AX7 for both the 3rd gain stage and the trem. The trem will get one half of the 12AX7 so I know that'll work. I can use a 13k plate resistor and a 1.5k/25uF RC on the 12AX7 that is being used as the 3rd gain stage it would have the same gain as the 12AU7. Will this work?

I used the triode calculator on Adam's Amp page to arrive at the 13k plate load for the 12AX7. http://amps.zugster.net/tools/triode-calculator

Will this sound okay to use a 13k plate resistor for the 3rd gain stage? I guess if it comes down to it, I can use a 12AX7 with a split plate load and dial in the amount of gain that way.

[loudthud]

The gain may be the same, but you won't have the ability to make a very big signal. To make the 12AX7 mimic the 12AU7, you need something like 4.7K in the cathode. Make the circuit look like the Concertina phase inverter in a 5E3 except use a 4.7K on the ground side and 100K on the plate side in place of the 56K resistors. Keep the 1.5K cathode bias resistor. You'll need a cap on the input to isolate the DC if you are coming from a volume control or tone stack or other ground referenced signal.

[loudthud]

Something like this Adjust the 4.7K as necessary to get the gain exactly right.

[MarshallPlexi]

Very cool! Thanks!