12au7 tube help and directions

[Tone Junkie]

I want to experiment with a 12 au7 tube I have a b+ node measuring 205 what would be good drop down resister and cross over caps and also what cathode resister to bias it properly, what would biasing it hot be and what would biasing it cold be just some general knowledge to try and use it in a guitar amp while im waiting on a couple new tubes to come.  and for the cathode cap should use the standard method of fixing frequencies to go with my bias resister or is this tube differant math to get to the same space. Thanks Bill

[bibi]

do you have an RCA tube manual?  In the back there is a table which details everything you need to know for just about all their preamp tubes.   It's called the Resistance Coupled Amplifier Charts here's a link:  http://www.triodeel.com/tubedata.htm  just scroll down near the end of the page.
That said, it's still pretty vague what you are asking so a schematic would be helpful.  Are you using it for the first position?  PI?  etc. 

[Tone Junkie]

Thanks Makes me have to get out my charts and remember the technical terms, Ive been needing to do that anyway, That chart is very helpfull I can get things for the other tubes i use and compare notes between tubes. Bill

[RicharD]

The 12AU7 is only gonna give you a gain of about 10, but 2 of them in series will get you in the AV100 area.  The 12AU7 has an internal plate resistance of 7k7 ohms, so set your Ra (plate resistor) to 2x that.  Call it 15K.  Now that you have that, you can plot a load line (the black line at about 45 degrees).  You want your Rl or volume pot to be at least 5x your Ra plate resistor so call it 100k.  Anything smaller than that and you need to factor it into your load line.  Now you need to establish an operating point.  Ideally you want it to be right smack dab in the center between Vcutoff and Vmax.  As luck would have it, 4.7mA is almost dead center and that gives you an Rk (cathode resistor) of 820 ohms (standard value). That's where I'd start.

A 2k2 Rk puts the tube drawing 3mA with an output more negative than postitve -86v/+39v

at the other extreme

A 220 Rk puts the tube drawing 6.5mA with an output more positive than negative -36v/+85v 

[Tone Junkie]

My friend where do you get that tube cad from it looks like a killer program. Thank you for the information. Bill

[RicharD]

Antique has it for a whopping $40.00.  I play with it like a kid with a TI calculator in 1976.

[Tone Junkie]

Cool   

[PRR]

You "can" run 12AU7 with 15K load, but why? It will be driving 100K-1Meg loads.... why waste all that power?

Drop the AU in a standard "12AX7" 100K/1.5K circuit. It will work. Gain WILL be low (why we don't use 12AU7--- gitar amps need LOTS of gain). It will run "rich" with little down-swing. This may be fun. For "optimum" you want to scale the cathode resistor up inversely as the Amplification factor. 100/20 is 5 so change the 1.5K to 7.5K. You will have lots of available swing but may not be able to distort it without a booster. For Marshal style starved operation, change the 10K cathode resistor Jim used on the AX to maybe a 50K resistor. But gain will be so pathetic that you may as well not bother.

[Tone Junkie]

Thank PRR I have this in an extra gain stage its already over driven i was looking for sweet tone by trying differant gain facters in tubes looking for that ever elusive sweet spot. thanks Bill