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!