by EWBrown » Sat Dec 20, 2008 6:11 pm
I should have read the original post closer (but then I was still in a power outage "haze" brought on by sleep deprivation)... I'm slowly recovering back to "normal" but that will take a few more days and some more decent uninterrupted nights' sleep.
My main 12AT7 experience is with ST70 drivers, and as the VA in various 2A3, 6B4G abd 300B SET designs, both Bottlehead, and my own "mutants" on BH's theme.
The 10 ohms resistor for the LM334 would set the total current to around 7 mA, so half that or 3.5 per 12AT7 section. THat should be fine, I've run 12AT7s in VA duty at 3.4 to 4 mA, and the ST70 LTPIs aer 4.5 mA or 2.25 mA per trioode sectioin, which seems low, but still works fine.
100K per plate resistor would give you around 350V drop across each plate resistor, so that value may be high, 62K may be low, I'd ain for something in between, perhaps 82K, 2W, for a good starting poing (that gives around 280 to 290V drop across each resistor)
Usually the second plate rsistor shopuld be around 5% to 10% higher, but it isn't absolutely necessary.
12AT7s are pretty rugged,, somewhat more so than 12AU7s. Even though the plate structure seems small, they still have plenty of "ballz" Yellow_Light_Colorz_PDT_06
I recently re-calculated the component values to run the "classic" VTL VA / LTPI circuit on 365V B+ in order to run PPP EL84s, in a yet to be resuscitaqted old project that I furtively started a few years ago.
Basically a "ripoff" of the VTL "Tiny Triode" Monoblocks, 45W in PPP pentode mode, and 26W in PPP triode mode. Tube CAD showed that I only had to change the LTPI plate resistors to 43 or 47K. The VA remains unchanged and the NFB loop R and C values are YTBD.
VTL runs their EL84s at almost 400V B+ and the plate current at 30 mA, fixed bias. I'll stick with the tried and proven ST35 standards of 365V and 35 mA, as I can use standard ST35 PSU design and parts.
/ed B
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