PT choice

[memphis032]

Howdy,

Working through the checklist for this build I've come to the choosing iron stage... I've pretty much decided on the output tx, but I'm not sure on how much breathing room I need for the power

The project is a low-plate classic, but with a parallel SE output.

So far - 2 12ax7s (~ 6mA), 1 12at7 (~ 20mA) and 2 6L6s (~ 150mA)
So, roughly 176mA, plus whatever disappears through all the pots etc... Any suggestions of ballpark figures for how much that extra is?

[martin manning]

What plate voltage are you planning for the 6L6's? What load impedance? The other losses will be small compared to the output tubes.

[memphis032]

I'm looking at PTs between 275 and 325V, so HT of ~380 to 450V, probably leaning towards the higher end, and OT primary of 5k

[martin manning]

You might want to rethink this. My put for the parallel SE output would be 275V Va-k (HT minus cathode voltage) and 1k1 load impedance (for two tubes), idling at 100 mA each.

[Firestorm]

Martin,
Why do you prefer only 1K1 load with 6L6s? Seems low even at 275Va.

[martin manning]

That Va-k and Z would put the load line more-or-less through the knee with the screens at or about the same voltage and the idle point centered at ~90% Pa max. If you look at The GE data sheet for 6L6 there is a class-A pentode case at 250V and 2k5 for one tube, so 1k2-ish for two, and 72 mA Ia0. That's not far from the spot I picked...

[memphis032]

Why such a low HT? A 275V PT is usually the lower end of the scale, and that will give almost 100V more when rectified.

[martin manning]

I suggest you do some reading here: www.freewebs.com/valvewizard1/se.html

[Firestorm]

That Va-k and Z would put the load line more-or-less through the knee with the screens at or about the same voltage and the idle point centered at ~90% Pa max. If you look at The GE data sheet for 6L6 there is a class-A pentode case at 250V and 2k5 for one tube, so 1k2-ish for two, and 72 mA Ia0. That's not far from the spot I picked...

I see it. Personally, I'd be inclined to design around a higher Po. This'll get you 13W from a pair; you should be able to get close to that with a single -- higher Va, lower Ia, higher Z. Actually, I'd probably go P-P Class A. Wouldn't sound quite the same but the post-rectifier ripple would be way easier to handle.