In my last post I made a slight mistake in the calculations of the
power supply impedance for Chris Parkin's 300B amp.

I will place the corrections and notes into my text as follows....

The power supply he wants to use is on the circuit shown at
http://www.drtube.com/schematics/an/conquest.gif

This has a 425-0-425 tranny winding feeding a 5U4 with 47 ohms in series
with the C1, then a choke, then C2 to the OPTs.
With no load, the B+ would soar to +600v.

This is because without load, the B+ will be 1.414 x the vrms of the
HT winding.
With a tube rectifier, one never sees an AV to DV conversion rate of
1.414 at the output of the CLC filter when the circuit is loaded.
The AV to DV factor is usually 1.1, or 1.2,
and its only with silicon rectifiers that you start seeing
B+ voltages = 1.35 x the AV winding voltages.
So 425vrms with silicon would give +573v but with a tube rectifier
its more like 510v, but then we have that 47 ohms in series with C1
and this limits the peak charge currents, and reduces the B+.



Our friend used a pair of 10k resistors to replace the tubes, and measured B+ of 500v,
so we assume a current total of around 100 mA.
This indicates a series impedance of the power supply
of at least 100 ohms or more.

The drop in voltage level with 100mA load is about 100v,
and thus the effective series R is 100 / 0.1 = 1,000 ohms, not 100 ohms.
The 100 ohms seemed a bit wrong.


But with the tubes in place with a draw of say 180 mA, the drop across
the supply would be 180v, and so B+ would be 420v,
much less than the +475v I estimated at first,
and in fact the drop across the tubes would be a lot less than the
400v that I initially predicted, based on a overly fast guestimate.

1,000 ohms x 180 mA = 180v.
I must have had that 1k PS impedance in mind.....
Just exactly what the impedance of the PS works out to be
is not within my time constraints to calculate.


Therefore, it seems the advice I gave him is very unlikely
to cause the man any grief, which you seem to want to point out to all.

With luck, he will get back to us to tell us how it all went when he fired up his amp,
and if in fact his 300B tubes settle down with 33 watts dissipation instead of
36 watts, he should be well pleased.

Patrick Turner.

Let's here from Chris how he got on.

Patrick Turner.