[Roger Jones]

On May 18, 5:26*pm, Roger Jones wrote:
On May 16, 7:18*pm, TerryG wrote:









Hi all,

I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.

Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.

I have designed and built several pentode single ended amplifiers at
this point. *And the one point that still alludes me is: *How from what
the datasheets say can you figure out what resistor to start with on g2?

I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.

I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. *Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point. *It seems most g2's have a certain impedance, EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. *This also lowers distortion it appears, but there is
always a point of diminished returns for each tube.

It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.

I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.

Terry

--
TerryG

Terry, I've played with quite a few Single-Ended Pentode (SEP) amps
over the years...in my student days it was for economic reasons! *I
still do them now in vintage radio restoration. I sometimes use a g2
resistor to ensure the g2 voltage does not exceed the plate voltage
(OPT primary resistance drops the latter a bit.) *I do the same in
pentode/beam tetrode P-P amps for the same reason (if they're not UL)
- just done it with an Eico HF12 (EL84's); actually I just moved the
g2 tap further down the B+ filter and adjusted the R values to get the
right voltages back. My reasoning is to reduce screen power and
heating. However, in both, I always decouple the g2 resistor (above B+
filter does it anyway.) *I would expect this to change the dynamic
operating point, tube impedance and max. power wrt a naked R...
obviously, it can't change the DC operation. But I've never studied
the effect... mea culpa! *I trust the tube modellers will help us.
Cheers,
Roger

Good replies... many thanks. I conclude that a lower g2 voltage (than
plate), fully decoupled, is advantageous. FWIW, in vintage radio
restoration (keeping same 6K6, 41, etc), I invariably add NFB from OPT
secondary to the cathode of the triode AF tube and mostly replace the
detector diode with a 1N34A point contact s/s diode. Audio gain goes
down, of course, but since I just listen to local AM radio on these
sets, no problem. Actually, it makes the range on the volume control
better... it's further "off the end".
Cheers,
Roger