29 Aug
2008
29 Aug
'08
10:13 a.m.
On 29 Aug 2008 at 9:13, William Blair wrote:
Yes, I found that excellent site back when I was
playing with
low-voltage tube headphone amps and I have a copy of the Tube
Collector Magazine "Car Tubes" article cited there. However, the tube
amps I built from modern plans were ones that used common dual triodes
like the 12AU7 but with _very_ low plate voltages.
My first exposure to very-low plate voltage applications was with a
code-practice oscillator built from a circuit in the ARRL Radio
Amateur's Handbook. ISTR it used a 6SN7 that used the 6.3v from the
heater winding as plate voltage. At any rate, it would drive a pair
of earphones. Perhaps someone with a RAH collection from the 1950's
might locate the circuit.
While dual (identical) pentodes aren't too common for you to use the
first grid as a space-charge element, I wonder if one of the sync-
separator tubes, such as the 6BU8 might not do the trick. Single
cathode and first grid, independent 3rd grids and plates.
In the 20's and 30's before small-geometry power tubes became
available, "microwave" experimenters would take a hefty triode and
put a high positive voltage on the grid and a slightly negative
voltage on the plate to induce Barkhausen oscillation. But that has
nothing to do with your problem, so forget that I mentioned it.
Cheers,
Chuck