6AK6 OT Recomendations and math help

[stevehoover]

Hi Guys,
I'm confused (as usual)...

What would be the ball park OT impedance for a pair of 6AK6's in Push/ Pull at a B+ of approximately 250V.
The data sheets say optimum load is 10K.   
Is that single ended? If so...What is the correct formula for calculating this in push / pull?

The other mystery is the cathode resistor....it says 520 ohms. Again is this single ended?
I cut that value in half for push / pull?? right?
I want to use separate resistors (cathode biased) in push/ pull, so that I can use unmatched tubes if I have to.

Thanks ahead of time for your replies....I've always had a bit of a challenge when it comes to higher math,
and the data sheets are a mystery as well at times. 
It would rock to find a tutorial on interpreting data sheets and plugging in those numbers for common values needed.

I will cut and paste your replies into a word doc and save it in my "Amp Build Notes" file so I wont have to ask again.
THXS
Steve

[PRR]

> What would be the ball park OT impedance for...

"Stupid high". 20K for being push-pull class A, 1.93X ((250/180)^2) for higher voltage and lower current: 38K.

38K on a 2.5W core leads to SUPER-thin wire, excess winding cost. Compromise leads to bass loss. Also loads of stray inductance causing a ring-and-drop in the top of the audio band.

Hammond 125B is rated 15H inductance, 7K at 82Hz, bottom gitar note. If you wired it ratio-wise to give "38K", the bass roll-off would be at 400Hz, WAY bass-shy.

10K is a much more reasonable upper limit on end-to-end OT impedance.

If you are a simple engineer tasked with 2.5W output, and heater current is not costly, a single (no phase-splitter) 6Y6 6F6 or 6v6 at 200V-250V and 3K-7K load is a heck of a lot more sensible.

Work two 6AK6/6G6G at 20KCT load, 180V plate and G2 supply (use resistance to drop from higher supply), and 300 ohms common cathode resistor.