Check over my thinking please - adding gain stage

[plumcrazyfx]

So you all helped me with the dual channel 5C1 build I did a couple months back - it sounds good but never quite crunched the way I wanted it to.  So I decided to add a gain stage - I figured with a 12AX? after the pentodes I could use each triode as a gain stage for each of the pentode inputs.  This would still allow me to parallel the inputs with a jumper.  I had a failure at first - I think because of lacking grid leak resistors.  So I added them and the thing sounds killer - everything I wanted and more.  I was doing some reading afterward and forum the Aiken site talking about max grid resistance where it suggested no more than 300K.  My question is I have 180K mixing resistors and 240K grid leak resistors because they were closest to my estimation of what I should start with that I had handy.  Am I thinking right that this is not 420K on the grid because of the parallel channels - the grid is seeing 210K?

I am also wondering about my decision to drive the grid directly from the middle lug of the volume pot.

Like I said the thing sounds awesome and I'd rather not change something potentially changing the sound unless I've done something dangerous or something that with impact reliability.  Any help will be appreciated.

[66Strat]

I have a couple of suggestions. The maximum grid resistance for an EL34 is 500K per the datasheet. With that in mind, you could safely increase each grid leak resistor to 500K. As the resistors are parallel to ground, the EL34 grid would only see half of the rated value.

You also might consider using a single grid leak resistor and implement the mixing resistors prior to the grid leak resistor (see attached). Also, a 12AU7 may be a better option than a 12AX7. I would implement the circuit as drawn with 47K plate resistors and 820R to 1K5 cathode resistors. These values would work well with either tube.

[sluckey]

Am I thinking right that this is not 420K on the grid because of the parallel channels - the grid is seeing 210K?

That's correct.

I am also wondering about my decision to drive the grid directly from the middle lug of the volume pot.

That's fine. It's quite common.

Sounds like you have found your sound.

[plumcrazyfx]

Thanks.  It really does sound good.  I'm planning on building the final with octals so I can use 6SJ7, or with adaptors EF86, 5879, or 6AU6.  All sound good and parallel it's super fat.  I do think an AT7 or AU7 might be the thing.