Chris Parkin wrote:

Thanks for the advice guys, certainly given me food for thought. I'll
probably increase the cathode resistors a little, and maybe put a 500 ohm
after the choke to drop the ht a little (also a little worried about the
6sn7's). And I have a mate with a variac, so i can crank it up slowly.
I have built a couple of kits in the past, a 5881 pp and the audio note
pre amp kit, just soldering stuff together as per the instructions didn't
give me much of a feeling of input though, so this is more like it!
I wasn't expecting a walk in the park, though maybe i should have gone for
something a little more run of the mill. I have been picking at Morgan
Jones' book for the last 6 months, though he doesn't dwell much on either
single ended or power triodes, yet alone zero nfb. Ploughed through a lot
of theory on the net which has been useful.
I would spend more time experimenting, but i haven't really got the space
to leave things set up, got the guts of a valve electronic organ in the
cupboard, ht is about 750v unloaded which scares the crap out of me!
though looking at the size of the choke I think it's probably designed as
a choke input, which should bring it down a little. Might dig it out
again soon.
Anyway thanks again Patrick, bear and Andre.

Cheers, Chris

In addition to my last advice, if you test the drive amp without the
output tubes plugged in, the B+ will rise to quite a higher value
than the normal operating value.
With only 20 mA of current supplied to the drive amp,
its supply voltage will be up around +550v perhaps.
I assume the PS seriesed electros are each rated for +350vwkg
The 6SN7 should cope with that B+, but I would
place in some high powered resistor to replace the current draw of
the output tubes.
I sometimes use a couple of 25,40, or 60 watt 240v lightbulbs in series.
For the US, 4 x 110v bulbs are used.
If two 40 watt bulbs result in a voltage between the OPT and cathode pin
equal to say 380v, and the cathode R indicates say 170 mA,
this means that the power in the resistor is 64.6 watts, and the value of the
R in the lamps is 2.235 k. An R of 2.2 k could be used as a check,
but ithas to be a rugged R for long period testing, which is why I like lamps,

and after trying a couple of different values one can guess about what is
possible
to get with the tubes plugged in.
If the tubes are inserted, the cathode bias R value could be adjusted to
produce the same
current, or near it.

BTW, a 500 ohm series R after the choke will cause a B+ drop of 75 volts
at 150 mA.
That value could be fiddled with, to trim the final working point of the tube.

One can always place 500 ohms in, and shunt it with other R later,
or remove it.

I don't think you should have too much trouble with the circuit
as it is, although I think the driver stage with half a 6SN7
having to produce around 50vrms is driving the tube too hard.
Its total signal load is 19.5k plus the 100k in parallel,
which is 16.6 k, and 70 peak volts makes a 4.2 mA peak current change,
and thd will be quite high, maybe 6 %.
The supply plate voltage for the second 1/2 of the 6SN7 driver tube
is not from the HT of the PS to 0V, because there is 140v across the
19 k cathode R of the driver. The anode current is around 7.2 mA,
so the effective B+ of the 2nd triode is the HT-140v, approx, which
is probably around 310v working.
My load line analysis reveals the driver barely makes
65 peak volts swing before driver grid current flows.
But the designer may have deliberately chosen the working
condition to cause some 2H cancelation when the 2H of the driver stage is fed
into
the 300B, which are not without their thd at full output,
perhaps about 7%.
Theoretically, if both stages had the same % of 2H,
then the output should be free of 2H, but rarely
does the 2H increase of each stage occur to make perfect 2H cancelation
from low power to high power.
Sometimes one gets more 3H and 4H after trying to achieve
distortion cancelling.
In my SEUL amps, I designed the driver stage for reasonable
linearity, and a large v swing ability, with little regard for 2H cancelation.

But in my latest SE amp with CFB windings, the 2H in the output stage is
only 2% at clip, so the 2H from the driver gives significant
2H cancelation, without having to set up a driver triode
with a high Ia and low value RL.
I found it was difficult to make the 2H track each other
exactly over a wide power range, or with different value speaker loads.

The bottom line is how it all sounds, and 12 watts max from two 300B
should be possible, and at 2 watts, with sensitive speakers,
the music should be excellent.
Its not too hard to change the circuit around, and I have a friend with a GM70
SET,
with 2A3 driver, 417A input, and he changes tubes, caps, loads, chokes et all
almost daily, with a breadboard arrangement spread all over the lounge room
floor.
SE is so simple, it is all doable with aligator clipped leads.
I get a frown when I say,
"Geez Doug, those alligator clips sound a bit leathery and swampy."

Some say the 300B is bass shy, and top end lazy,
but one dies for the midrange.
In a comparison recently with a renowned SS amp
of 100w per/ch which was made locally, a chinese tubed ValveMark
amp with a single 300B/ch was superior in its bass response,
and had no trouble with the highs.
300B amps are not for the head bangers, the SS is right for that,
but for easy listening, 300B is OK.

Patrick Turner.