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Patrick Turner Patrick Turner is offline
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Default 12AX7 grid current

On Sep 22, 6:37*am, Ian Bell wrote:
Patrick Turner wrote:
On Sep 20, 5:46 am, Ian *wrote:
Patrick Turner wrote:
On Sep 19, 7:14 pm, Ian * *wrote:
Patrick Turner wrote:
On Sep 17, 6:58 am, Ian * * *wrote:
Patrick Turner wrote:
On Sep 16, 5:34 am, Ian * * * *wrote:


* * snip a bit,


Interesting story ( I was aware of the original MR. Sowter and I think I have an AES paper of his
about audio transformer design. As you say, Brian is always very friendly.. He has always answered my
questions no matter how technical but I admit to not having the skills to TELL him how I think he
should wind a transformer.


I have far less experience than the sum total of the Sowter company
but that didn't make me wrong about where exactly to place CFB
windings in an OPT.

If I investigate the possibility of having someone else wind something
rather than doing it myself, then I'd like it done my way or not at
all. And yes Brian is a gentleman, but he wouldn't do it my way. OK,
so, no problem, I don't expect everyone in the world to be prepared to
work for me as instructed - I try to get someone else to do it.

And last month i found a guy in Sydney who did EXACTLY what Brian
would not, and without any bull****, and the job looked well and
performed quite flawlessly, like all my OPT designs, and the two wound
bobbins cost me aud $280. I have added the laminations after figuring
out the amount of "partial air gapping" and done the terminal boards
and will do the varnishing which is more donkey work. I end up with
two superlative OPTs for Quad-II I have and 50W rated and low loss and
the value of OPTs is at least equal to the best on the planet which
might be at least 3 times the Hammond price of something with a 75m
stack of 32mm tongue GOSS lams, but it has not cost me that much, so
if I sell the amps they will fetch a good price and be exceptional
quality and equal to how the Quad company could/should have done their
Quad-40 amps. I will have a website page on them.

102VP-P is 36Vrms, and if AX7 gain = 60, then you'd need 0.6Vrms grid
input = 1.69VP-P and yes, you'd have a bother with distortion getting
high.


In the last Phone amp I made for a customer last year I used a single
EL84 in triode with an OPT from an old Trio stereo receiver.
I had 12AU7 driving the EL84, and some global NFB and a resistance
divider off the 16 oms sec to drive the phones.
The OPT was about 5K:16, 8&4.
Noise ended up less than could be heard with phones and the drive
signal to the EL84 was below about 6Vrms at clipping which was way
above what was needed for normal listening so the driver tube
contributed negligible Dn.


It is good to know that an SE triode EL84 works OK. Sowter also make an SE phones transformer but it
is a huge expensive monster (type 9351)


It does not need to be huge. The OPT I used were very generic 5W types
meant for EL84 in pentode mode, and have plain GOSS with air gap, and
they saturate at 50Hz at full power which is bloody awful, but then
Trio was just another cheap **** brand and transformer weight size and
cost was never a priority.


The core was a 20mm stack of 20mm wasteless material with Lp maybe
less than 20H.
But they function well with the low levels which are well away from
saturation when using headphones.


That is interesting. Does anyone still make little OP transformers like this?


I am not sure. Maybe Lundahl make something quite nice for 5W and
EL84, or one 2A3.
Maybe Hammond do something suitable and about equivalent to the
generic old OPTs that were used for receivers and radios.
But most old radio OPTs for say 6V6 are a lot worse than what was used
in a Hi-fi receiver.





EL86 is another pentode which makes a good little triode with Ra =
1.4k and gain about 9 and it is best with Ea = 200V approx and maybe
Ia = 40mA. But you could use EL86 with a 300V supply with Ea = 150V
approx to make a White follower and with Ia = 40mA and get +/- 0.64VP-
P into 32 ohms without anything clipping. Its not going to be as
linear as a load of 320ohms or 1k which would be much better.


If you don't know what the primary load is it is easily found with a
series 10R between cathode and OPT primary when loaded by the
secondary load. Primary load = Cathode output signal voltage / current
measured in 10R.


http://www.sowter.co.uk/headphone-transformer.php


I think right now that is my last resort but EL84 based white follower seems an interesting option.


I recall that the White CF does have advantages - it allows PP
operation and thus 2H cancelling, and output load regulation via the
current flow in the R above the top triode. If the load is low, the
load current is high and the signal produced in the top anode R is
then transfered to the botom triode to make it work harder. Its not an
ideal way of doing things but is OK over a range of loads. The Rout of
a White follower using a twin triode is slightly lower than if you
simply paralleled the two triodes ina plain CF which of course can be
made to work optimally using a CCS for the dc load sink. The plain CF
has only marginally higher Rout.


Agreed. As I saif before, I don't think my problem is in the output stage but more in the driver. I
am still experimenting with the 12AX7 but the results are still confusing.. I think I might just
build a little rig with a 12AX7 on its own to get a feel for what is actually going on.#

In the meantime I am considering replacing it with a pentode - perhaps an EF86 - that should give me
plenty of open loop gain and op swing - what do you think?


Not a bad idea, but pentode THD at high Vo becomes worse than triode.

EF86 is best with Ea at 200Vdc and Ia at 3mA.
If you examine the anode curves the variations in Gm either side of
the ia = 3mA is minimal, hence the linearity approaches triode. Most
ppl use EF86 with 0.8mA but 3mA is quite OK, and gm is double, and
gain can be high so maybe an unbypassed Rk can be used for local
current FB to linearize the pentode a bit. But you have a current
source where the Rout is determined by RLdc and following Rg in
parallel, so bandwidth must be considered.

What about a µ-follower gain stage with 12AY7 which is almost the same
as 6SL7?

This should be VERY linear, and maybe not so much input grid current.
And I don't know anyone who doesn't like the 12AY7.

Then of course you could try EF80 or 6BX6 strapped in triode for gain
= 45 approx and lowish Rout about 12k.
EF86, or 6AU6 in triode don't give very high gain.

At the end of all this you'll have SET character using the triode inut
with big Va swing followed by low THD neutral output buffer to put
current into the signal.

Done right, it should sound well.

Patrick Turner.

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