now playing.....amp can still use some tweaks...
EF86 buffered by 6N1P driving the 829b paralleled sections in triode mode
sad to say, this is a failed design, i can't find tubes that have sections that are equally matched,
and since the cathodes are common to both sections, they do not conduct equally so that
the resulting distortion is audible....i suppose i can tweak this by manipulating filament
voltage to get the transconductance equal on either side, but that will be too much hard work...
so i ditched the septar socket and put in an octal socket instead....
in came the 6AV5 horizontal output tube rated for 11 watts....but in this
amp it is running about 16 watts plate dissipation, 250volts plate at 60mA cathode,
does this ring a bell? yes, it does, it is the operating point for the 2A3....
the power factor of almost 90% and a power draw of about 80watts...
the power traffo is rated for 180VA, so that the power traffo will be
very cool to touch and will last a long long time....
the high power factor is indicative of a very good power supply...
for those interested to design their own SET,
this is how i sized the cathode bias resistor:
for triode mode, grid 1 bias is calculated as follows:
Vg = -(Vpx0.6)/mu
mu equals 4.5 and Vp is 250 volts,
so that Vg is 40volts,
looking for 60mA. Rk, the cathode resistor then becomes,
40/60 = 600ohms, this is achieved with a parallel combination of
1k and 1.5k 5 watt resistors, making for a 600 ohm 10 watt
cathode bias resistor....this resistor dissipates 2.4 watt
so that use of a 10 watt resistor is very much justified...
it would interest you to note the in the actual amplifier,
bias was at 39.5 volts, am i lucky or what?
