[sdiy] tube preamp gain calculation: how to?

[anthony]

I once found a page that showed how to figure out how to get whatever gain 
you wanted from whatever tube.

I'm making a couple of different amp clones with variations: one is a 
Kalamazoo Model 2 and the other is a Champ. Both will be adding extra preamp 
stages at the beginning of the chain using 12AU7's or maybe 12BH7's with no 
front panel adjustment for gain or tone - there'll just be a couple of 
switches to switch them in and out and maybe some chassis mounted trimmers 
(I have a number of brackets and twist-lock full sized trim pots for this).

Actually in the case of the Kalamazoo clone I'm just going to use the unused 
12AX7 section (well they'll get shuffled around...)

The Kalamazoo name struck me as a cool name and it's fitting that this clone 
is going into the tweed cabinet from a dead solid state Epiphone EP-800R 
(since Kalamazoo was a Gibson budget line).

I think I've decided to name my new line of super hi-gain with tremolo, 5 
watt (1xE84), 1x12" combo amps "Oconomowoc MkII" and the "MkIV" big brother 
2x12" combo that will actually have the MkII circuit in it as a preamp and a 
2xEL34 power amp with a reverb that will be fed from the EL84 so the tremolo 
will bang on the reverb. The 2X12" can either have each amp going to its own 
speaker or can be switched to just the big power amp running through both 
12" drivers. I think two models are enough. Well I think they can both be 
offered in a head format. I hope to be able to sell the MkII for $200 and 
the MkIV for $475.

I hope nobody else is using the name Oconomowoc as a brand name for amps or 
audio gear... I could always go with Sheboygan or some other cool Upper 
Midwest town name

I know with the 12AU7 you can get more actual gain than you woud expect if 
you were running it the same was as a 12AX7 because the 12AU7 has a much 
lower plate resistance.

So could anyone tell me the formula for figuring out gain from tube data, 
supply voltage, plate resistor and cathode bias resistor and cathode 
resistor bypass cap? Or at least lead me to the web page. I tried googling 
and looking in my favorites (I know it's in there somewhere...)?

I have a good idea how I'm going to do it from other projects adding preamp 
stages to Fender Champs and the AX84 page. But I don't get all of the 
details I need from these sources.

thanks,
Anthony A