reyer wrote:
"Patrick Turner" schreef in bericht
...
West wrote:
"Patrick Turner" wrote in message
...
The 845 amps I have been strugling to find time to build
are approaching a stage where I must decide
on the actual working points that would be most suitable for
both 211 and 845 including class A2 operation,
by means of cathode follower buffer choke loaded to the
fixed bias supply.
So far having Ea at 1,000V and Ea at 80mA with RL = 12.5k
could seem OK but it means Pda = 80 watts,
and only KR845 or KR211 look able to take this idling condition,
one reason being that the KR heaters are 10V at 1A instead
of the RCA GE use of 10V at 3.3A.
RCA data gives Pda = 67 watts, and if Pd-filament = 33 watts,
the total tube Pd = 100 watts
Anyone here with actual experience of using either tube
in an SE amp???
ALSO, anyone here have longterm experience of use of Shuguang
845?
Patrick Turner.
I remember reading, not long, ago that the Boss (AJ) found 2 sweet spots
with his 845's, 970v & 1046v. The voltages are rough approximates that
are
the best my memory can do at the moment. Good luck and keep us posted on
this venture and why not ping him?
Well yes, between 970V and 1,050V seems about right...
211 has Ra = about 3.3k and 845 has Ra about 1.9k.
So the damping factor of the 211 is poorer and NFB would seem to me
to be almost unavoidable.
The A2 operation means THD tends to rise when grid current happens...
A SRPP with a pair of EL84 in triode looks promising to make the needed
150Vrms drive at low impedance for the 845, and with a choke to bias the
845 grids so the
bias can't change easily when in A2.
AJ may not be reading the group at the moment.
Patrick Turner.
west
Has anyone checked this?
http://www.tubelab.com/powerdrive.htm
It seems very promising, being able to go into A2 without the grid current
problems. I know the powerdrive involves mosfets but, since I really like
the sound of a transistorized CCS in the tail of my LTP phase splitter, I am
very tempted to try a 845 amp with this powerdrive circuit.
Using some kind of voltage follower directly connected to the
output 845 or 211 grid is the normal way to over come the
sudden low impedance input when the grid of the output suddenly
begins to draw grid current when it is forced above 0V potential
The A2 distortion in the output signal is due to the sag in the input
signal drive, because the output grid appears as a low resistance in
series with a diode to 0V,
so it conducts above 0V.
The mosfet follower isn't too bad a choice because of its
very low source resistance of 1/gm and for the mosfet concerned,
its probably less than 10 ohms.
So there is almost no distortion created at the power grids due to
normal
plate drive stalling when the load changed to low resistance with grid
current,
maybe a couple of thousand ohms.
I am not sure how much grid current is drawn by either 845 or 211 when
the grid is at +50V
and the anode has swung down to a low voltage.
So I don't know what the equivalent diode + R circuit is that any
follower has to deal whith
when grid current is drawn.
My plan so far for this amp is as follows :-
Anode supply, B+, +800V approx, CLC filtering.
Cathode supply, B, -400V approx, CLC filtering.
211 or 845 is set up with OPT and cathode bias resistor between +800
and -400V and grid is biased via a 100H choke from the -400.
The cathode with 845 will be at about 160V above the -400V at -240V.
Rk will carry the tube Ia of approx 70mA so Rk = 1k2 approx.
The cathode has 10Vdc applied, CLC filtered, and with
negative end as the connection for the Rk.
Also from this point 1,000uF is used to bypass the cathode to the 0V
point in the power supply and rest of the input circuitry.
The input tube V1 is one x paralleled 6CG7 with CCS load to +350V
to produce a low distortion signal up to about 10Vrms for the driver
stage.
Driver gain stage stage is EL84/6BQ5 in triode with Ea at 350V approx.
Its B+ of +750V is derived from output B+.
Ia is about 12mA, RL about 40k, so 200Vrms is easy from this set up
and distortion is mainly 2H and has a phase which cancels the 2H of the
output tubes.
Between this voltage gain stage and the 845/211 grids, there is a buffer
cathode follower stage.
It does the job of the mosfet used in the "power drive" scheme mentioned
by our learned
experimenter at the website quoted above.
I see little reaon at all to use a solid state follower in series in the
signal path.
The buffer cathode follower is set up with its anode grounded to 0V.
Its cathode is connected to the output tube grids directly.
Then there is a small R of perhaps 560 ohms towards -400V,
and then choke to -400V.
So there is a very low impedance biasing the output grids to -400V,
while the cathodes of output tubes adjust themselves ordinarily as with
cathode bias to
a point of eqilibrium.
There is no fixed bias to adjust, and no need.
The EL84 cathode follower grid is biased via 220k also from the -400V,
and the cathode
voltage will rise with the Ia flow of about 15mA to about 15V, depending
on the
choke winding resistance and the small series resistor.
The grid of the CF is cap coupled with two 0.47uF to the
voltage amp EL84 whose anode is at +350V.
The centre join of the two caps is biased via 1M to 0V.
Thus 630V rated coupling caps can be used.
This is a simple arrangement, and for changing from
845 to 211, some reduced Rk is needed, and some added R in the cathode
power supply
so the headroom taken up with cathode bias voltage is used better.
Cathode biasing in class AB and B PP amps isn't used because the
Ia is non-linear, and positive going currents are much higher than
negative going currents, so Ck charge up and biase drifts.
This does not occur in class A2 amps. The +Ik and -Ik swings are
very similar, changeing only 5% if that is what the THD is at clipping
even when into A2 operation.
So all we need to guard against is the distortion to the drive signal
caused by the grid current stalling the voltage amp.
In my scheme, the output tube grid bias voltage is kept constant
because of the low DC R of the choke and trimming R for the buffer CF.
The output resistance of the EL84 in triode in CF = 1 / gm = approx 100
ohms.
AFAIK, this low resiostance is very much lower than the load offered by
the
output tubes when drawing grid current.
The EL84 will itself saturate at about 120mA and I
think that current ability would be sufficient when 845 or 211 have
their grids
forced to being about +50v at the Ea minimum value on negative going
anode swings.
A 120mA change in Ig for a 50V swing indicates Rg input resistance when
positive to be
50 / 0.12 = 416 ohms.
So the question is, what is the gid current when output
grid is at say +55V above the cathode voltage???
Does one need to use a mosfet??
In the website above or at
http://www.tubelab.com/powerdrive.htm
the guy has a photo of a 211 which has turned bright blue
while in the process of dying, and with 300mA of anode current,
and if he had a simple active protection scheem which reacted to
a voltage across a cathode resistance, there would be no panic
about turning off the amp when ****e like this happens.
If you build tube amps, especially high voltage types,
you should always take the time to add in the protection
and auto shut down circuits.
Patrick Turner.
Cheers!
Reyer