Hi group,

I bought some EL86 recently, and want to build an OTL amplifier for my
800 ohm speakers. Does anyone have some cheap, quality 60Hy double
chokes to spare for this application, or can anyone tell me where to
have them made? I discussed this matter with Sowter Transformers but
they don't know what a double choke is. The Philips book says to use
60H double chokes. There is also a EL86 SPP article in the May 2003
issue of Audioxpress which tells me to use 30H (I think there will be
less bass if a 30H DC is used). Both authors never specify the
details about this double choke. I need more details about them, ie
winding details.

Thanks a lot!

(Lemoncha) wrote:

: Hi group,
:
: I bought some EL86 recently, and want to build an OTL amplifier for my
: 800 ohm speakers. Does anyone have some cheap, quality 60Hy double
: chokes to spare for this application, or can anyone tell me where to
: have them made? I discussed this matter with Sowter Transformers but
: they don't know what a double choke is. The Philips book says to use
: 60H double chokes. There is also a EL86 SPP article in the May 2003
: issue of Audioxpress which tells me to use 30H (I think there will be
: less bass if a 30H DC is used). Both authors never specify the
: details about this double choke. I need more details about them, ie
: winding details.
:
: Thanks a lot!

These are the double chokes to feed the SEPP screen supplies?
I only photocopied the inroduction and a few of the PP amps in the Phillips
(or Mullard - by Rodenhuis?) book when I had it on loan, and didn't get the
SEPP circuit values - although there is a negative image of the bare circuit
on the cover of the one I am thinking of. I didn't know they were specified at
60H, for example. If your book is the one that has an analysis of the circuit
and the problems feeding the screen (for which the double choke is the best
solution) why not show Sowter that page, or pages?

My *guess* is that a bifilar choke might do the trick - they could do that.
If they understand the reasoning given in those pages they would get it, IMO.
Essentially the two windings feeding each screen have DC current in opposing
directions, so the magnetic field cancels, and no gap is required, IIRC.

If your book is different and doesn't have that analysis I can scan and post.
RdM

"Lemoncha" schreef in bericht
om...
Hi group,

I bought some EL86 recently, and want to build an OTL amplifier for my
800 ohm speakers. Does anyone have some cheap, quality 60Hy double
chokes to spare for this application, or can anyone tell me where to
have them made? I discussed this matter with Sowter Transformers but
they don't know what a double choke is. The Philips book says to use
60H double chokes. There is also a EL86 SPP article in the May 2003
issue of Audioxpress which tells me to use 30H (I think there will be
less bass if a 30H DC is used). Both authors never specify the
details about this double choke. I need more details about them, ie
winding details.

Thanks a lot!

I had a look at the file Ross was talking about and there it says 2 x 5H is
enough
for 30Hz .
So the first thing that came to mind was using 2 power trannies and try to
get both
primaries on one bobine/core .
That wouldn't be to hard if the trannies have 2 chambers and a core that can
be taken
appart (not welded or something) .

Ronald .

Although I don't have the circuit, I assume bandwidth is important?
If so, then the capacitance and leakage of the tranny/choke must be
considered. Maybe leakage isn't crucial...depends.

Simulation would be dead easy for anyone with a circuit and the
right valve models. Unfortunately no EL86 in my library.

As Ross says, Sowter could undoubtedly help if they knew what it is
for. They may well have a stock interstage transformer that would
do the job, or they can wind an OPT without the secondaries,
presumably. They don't charge extra for one-offs, or for advice.
They can take a long time to deliver though...

cheers, Ian


"Ronald" wrote in message
...

"Lemoncha" schreef in bericht
om...
Hi group,

I bought some EL86 recently, and want to build an OTL amplifier
for my
800 ohm speakers. Does anyone have some cheap, quality 60Hy
double
chokes to spare for this application, or can anyone tell me
where to
have them made? I discussed this matter with Sowter
Transformers but
they don't know what a double choke is. The Philips book says
to use
60H double chokes. There is also a EL86 SPP article in the May
2003
issue of Audioxpress which tells me to use 30H (I think there
will be
less bass if a 30H DC is used). Both authors never specify the
details about this double choke. I need more details about
them, ie
winding details.

Thanks a lot!

I had a look at the file Ross was talking about and there it says
2 x 5H is
enough
for 30Hz .
So the first thing that came to mind was using 2 power trannies
and try to
get both
primaries on one bobine/core .
That wouldn't be to hard if the trannies have 2 chambers and a
core that can
be taken
appart (not welded or something) .

Ronald .

I just took a look at that article in aX. No need for a hard to get
double choke. A pair of single 20H PS chokes will work OK.
One end of each choke is at AC ground while the other
is at the output AC signal potential. The two chokes are already
tightly coupled through those connexions. The original inventor
of the subject circuits objective was to avoid iron cored
devices completely.

Stray winding C is a consideration, but for an experimental
try that should not stop you. I built something similar to this
around 1960 using 20 of 6AQ5. It made an easy 100 Watts
into a 100 ohm load. I used ordinary PS chokes. Author of
this article even suggests the use of line matching transformers
as an output impedance match to regular 8 ohm speakers.
In general they are no better than a PS choke for audio
applications.

I've posted yet another version of this circuit at ABSE
for your interest.

Good Luck, John Stewart

Lemoncha wrote:

Hi group,

I bought some EL86 recently, and want to build an OTL amplifier for my
800 ohm speakers. Does anyone have some cheap, quality 60Hy double
chokes to spare for this application, or can anyone tell me where to
have them made? I discussed this matter with Sowter Transformers but
they don't know what a double choke is. The Philips book says to use
60H double chokes. There is also a EL86 SPP article in the May 2003
issue of Audioxpress which tells me to use 30H (I think there will be
less bass if a 30H DC is used). Both authors never specify the
details about this double choke. I need more details about them, ie
winding details.

Thanks a lot!

John Stewart wrote:
in ,
on Sat, 26 Jul 2003 07:26:24 -0400,
: I just took a look at that article in aX. No need for a hard to get
: double choke. A pair of single 20H PS chokes will work OK.
: One end of each choke is at AC ground while the other
: is at the output AC signal potential. The two chokes are already
: tightly coupled through those connexions. The original inventor
: of the subject circuits objective was to avoid iron cored
: devices completely.

Are you sure? Completely? I obtained the impression it was ungapped and of
small size, but I assumed it still had a core! (Smaller than OPT's, natch
It'd be good to get an impression of what this dual choke looked like ...

: Stray winding C is a consideration, but for an experimental
: try that should not stop you. I built something similar to this
: around 1960 using 20 of 6AQ5. It made an easy 100 Watts
: into a 100 ohm load. I used ordinary PS chokes. Author of
: this article even suggests the use of line matching transformers
: as an output impedance match to regular 8 ohm speakers.
: In general they are no better than a PS choke for audio
: applications.
:
: I've posted yet another version of this circuit at ABSE
: for your interest.

Coincidentally, I had just gone through the usenet-replayer archive with
the longest timebase option and found that (previously posted) jpeg and
also the WW May 1952 article (a few months younger than I am:-) you also
posted earlier, *still there*! (just), which might give some impression of
it's archive life. (it doesn't appear to date them, as far as I can see...)

Also, there's a 5-part dualSE Glass Audio extract that I'd missed earlier.
A bit odd, that one. I have yet to assemble the collected Matrix Amp parts!
Regards and Cheers! - RdM.

: Good Luck, John Stewart
:
: Lemoncha wrote:
:
: Hi group,
:
: I bought some EL86 recently, and want to build an OTL amplifier for my
: 800 ohm speakers. Does anyone have some cheap, quality 60Hy double
: chokes to spare for this application, or can anyone tell me where to
: have them made? I discussed this matter with Sowter Transformers but
: they don't know what a double choke is. The Philips book says to use
: 60H double chokes. There is also a EL86 SPP article in the May 2003
: issue of Audioxpress which tells me to use 30H (I think there will be
: less bass if a 30H DC is used). Both authors never specify the
: details about this double choke. I need more details about them, ie
: winding details.
:
: Thanks a lot!

In article , Ross Matheson
wrote:

John Stewart wrote:
in ,
on Sat, 26 Jul 2003 07:26:24 -0400,
: I just took a look at that article in aX. No need for a hard to get
: double choke. A pair of single 20H PS chokes will work OK.
: One end of each choke is at AC ground while the other
: is at the output AC signal potential. The two chokes are already
: tightly coupled through those connexions. The original inventor
: of the subject circuits objective was to avoid iron cored
: devices completely.

Are you sure? Completely? I obtained the impression it was ungapped and of
small size, but I assumed it still had a core! (Smaller than OPT's, natch
It'd be good to get an impression of what this dual choke looked like ...

Of course he is sure, completely! The two chokes should work just fine,
you might even get away with just one if you have a convenient source at
the correct voltage for feeding the screen of the lower tube. The use of
the double choke cuts down on the amount of iron necessary, since the DC
current for the two screens flows in opposite directions, the effect of
the DC tends to cancel as in a push pull output transformer, eliminating
the need for an air gap, or at least a large air gap. Eliminating the air
gap means less iron is needed to maintain the required inductance.

I suspect that the "dual choke" looked pretty much like a very small push
pull output transformer, with four lead wires exiting the windings.

Someone posted a link to a neat design for an amplifier like the Philips
about half a year ago. Unfortunately I don't remember who the designer
was, or who originally posted the circuit. You can find a copy on my web
pages at this URL:

http://users.rcn.com/jbyrns/stuff/scan0022.png

Notice that the plate current for the tubes doesn't pass through the "dual
choke", and a secondary winding has been added to the "dual choke" to
allow the connection of an ordinary loudspeaker.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/