Re: Need EL34 grounded-grid curves



Prune wrote:

I did the XY thing with the scope, feeding a triangular wave from my
soundcard into the amp. I reduced voltage and bias current about 7%
each, and the display remains a straight line, I can't see any
curvature, unless I push it close to clipping. I don't know to what
extent it's the feedback correcting this, since the amp doesn't work
with feedback disconnected, the DC offset gets huge (even though it's
pretty low feedback since it goes back to the JFET sources, not gates).
I also can't hear any difference. So I guess I didn't have anything to
worry about...


There are 2 x EL34 and 28 SS devices in that circuit of yours.
Worked out how each and everyone of them work?

There is also a lot of NFB in action, and slight changes of bias may not
make a visible
difference to X-Y lines on a CRO.

Harmonic Distortion measurements with something capable of measuring
down to 0.005% might tell you more.



As for the circuit being PP, I don't see it. Each side drives a
separate electrode on the headphones, but each electrode is driven SE.
A differential output amplifier is not the same as PP since the side
pushing is on the other side of the load from the one pulling at that
time.

All differential amps are PP amps, because one side turns on while the
other side turns off,
and the even number harmonic currents cancel at the cathodes, emitters,
or sources which are commoned
together to allow their combined current flow in one R or a CCS.
This occurs regardless of whether
there is a balanced input, or only one side is driven, and there does
not need to be a
mutually coupled load from anode, collector or drain as you have with an
OPT
for the circuit to be a PP circuit.
Each 1/2 of any differential circuit is an SE device, but the whole
circuit is PP,
and with all the tell tale signs of reduced 2H,4H etc.


The one problem I still have is some drift of the DC offset as the amp
warms up, so I'm hoping that several DC volts offset wouldn't be a
problem since the load is a capacitor and shouldn't draw DC.


The load is indeed a capacitor and dc drift won't worry the signal,
but you may find dc drift occurs during low level op and high level op,
and you have to analyse why this occurs if it does.

ESL usually only have a significantly pure C load which dominates the
loading at HF above 7kHz
because a treble panel is typically only 200pF measured stator to
stator.
Bass panels usually have a series resistance to feed them lest treble
production become too high.
Bass panels are typically 400 to 1,000 pF.
Some ESL have midrange panels also with series R so high pure C loads
are never seen by the amplifier
which thinks the load at 100Hz is purely resistive, and a high
resistance,
and without the shunting effect of an inductive primary
of a step up tranny. EL34 in pentode mode will have gain approaching
pentode µ at high RL values,
and so the NFB is at a maximum at LF, and declines as F rises and Z
falls for the ESL.

You need to know exactly what the L, C and R equivalent circuit is for
your arrangement
before you can say you understand your loadings.

If you were to understand the working, you could work out
at what dc anode current and at what ac signal level where there was
distortion
caused by cap charge and discharge.
200pF at 20kHz is 39k ohms and would seem to me to be an easy load
for a pair of class A1 EL34 with even a fairly low bias current.
As the C load is increased, the C load current increases.

The beauty of direct drive to ESL is that you avoid the usual step up
transformer self capacitance
which is more than the treble panel for many designs.
But otherwise, there are few if any technical benefits, so sonic
benefits are minimal.

A step up tranny may have a 1:100 ratio, so ZR = 10,000, so 200pF
connected at its output secondary without any series R becomes 2uF at
the input.
If the self C of the tranny is 200pF, you will have 4 uF at the primary,
and some amps will not cope well.

Patrick Turner.









Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wrote in
news:4608F2C3.953798F0@xxxxxxxxxxxxxxxxxx:



Prune wrote:

Thanks, I guess that answers that question. Now if I could only find
curves for the MOSFET. The datasheet only has current vs voltage
ones.

But what current and what voltage at what electrodes?

Mosfet data should appear like pentode data, ie with Id plotted on a
vertical
scale and Ed horizontally, and the curves you see are Rd curves at
various values
of Egate.

But you have a PP circuit, and wishing for complementary oddorder
distortion cancelling is rather like like
wishing santa make a special trip in July.

Just remember that a +1V applied at the cathode gives +20V at the
anode; the tube becomes a non-inverting amplifier.

One way is to fire it up and start fiddling with bias currents,
but without tempting smoke, and see what your THD meter tells you.

If I attempted direct drive of ESL panels I'd use a far simpler
amp circuit without any SS within, except perhaps for a CCS load in an
input LTP stage.

Good Luck,

Patrick Turner.


Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wrote in
news:4608C3FE.2DCAD8A1 @turneraudio.com.au:



Prune wrote:

Hello gentlemen,

I've not been able to find EL34 curves for cathode-driven
operation anywhere. Help would be greatly appreciated!


The grounded grid curves are the same as for grounded cathode in
either pentode, UL or triode.

Voltage gains are indentical.

All that changes is the input impedance which is high for grounded
cathode, and very low for
grounded grid, and Rk in = RL / voltage gain.

There are no separtae groups of curves for grounded grid.

Musicman guitar amps used transistors to drive the cathodes of
output tubes,
and this could have been a reason why they went out of business.

Patrick Turner.

.

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