On May 21, 11:02*pm, Roger Jones wrote:
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".
Yikes, WTF are you discussing. Please post a schematic somewhere so a
picture says a thousands words you can't seem to type.
As one reduces Eg2, the G2 biasing Vdc, then the set of Ra curves for
Ia vs Ea for given Eg1 values changes. Most ppl won't use AB2 and the
limit of operation and maximum PO is determined by the Ra curve for
Eg1 = 0.0V. This curve rises steeply at first from 0V, 0A, often on a
slope = 200 ohms, then with Ea at about 50V to 100V the line rolls
over to the right and assumes a slope of perhaps 38k, for an EL84. The
KNEE of the curve and the height of the flat part of the curve above
Ea = 100V appear lower on the graph as Eg2 is reduced.
For class AB and highest possible PO, Eg2 is kept as high as possible
to get maximal Ia swing into minimal RLa which is 1/4 x RLa-a for AB
operation. But for class A operation the RLa = 1/2 RLa-a and Ia change
for this higher RLa value is LESS than for class AB, so you **don't
need** to have Eg2 as close to Ea as possible. This is especially true
for SE pentodes.
Tube data from old books etc often show 3 sets of Ra curves for
differing values of fixed Eg2, and for an EL34 in SE mode, Ea might be
420V, but Eg2 at only 300V. This means Ig2 is much lower than if Eg2 =
Ea = +420V. One will find that the class A performance is no worse for
the lower Eg2, ie, Ra and THD is not significantly higher, see my
pages on SE35 to find out more.
http://www.turneraudio.com.au/se35cfb-monobloc.htm
I use CFB windings wherever I can for maximum linearity, and to get Ra
effectively lower than triode before adding a little GNFB.
The lower Eg2 means that Eg1 can be lower, so that for cathode biasing
there is less power wasted in Rk because Ek is low.
For high PP AB PO, Eg2 is high as possible, and this causes Ia dc to
increase, so Eg1 must be increased to counter the effect of high Eg2,
but of course this is OK because one wants a high Vg1 swing without
getting any grid current.
In guitar amps, you often see 6L6GC with Ea = +450V, and Eg2 a fixed
+440V, and grid stopper series R to G2 are often 470 ohms, 1 Watt.
Some Rg2 are special fusable resistors and they look like they have a
high Watt rating but actually don't. When they fuse, there are no
flames and smoke. Normal G2 power input at idle might be 4mA x 440V =
1.76 Watts, and so Pd in 470 ohms is 0.0075 Watt only, so the 470 ohms
could have a rating of 0.25 Watts, just a small metal film R. If the R
heat went to 1W, R would fail, and to cause this the Ig2 would be
46mA,
and I have seen such G2 resistors go open circuit often where an OP
tube has gone into thermal runaway, red hot anode with Ia dc = 300mA
perhaps, and hence Ig2 increases hugely.
Once Rg2 fuses open, Eg2 collapses to 0V, and Ia is cut off. I've used
low power rated Rg2 as an extra layer of protection against bias
failure. It isn't a good protection measure because after Rg2 fuses
open someone has to solder in a new Rg2, and the best form of
protection is via active monitoring of idle bias current and a circuit
which automatically turns the amp off when Ia dc exceeds 1.5 x idle
value for longer than 4 seconds. My website has details.
Patrick Turner.