Fourier Sans Pareil OTL

[rhthatcher]

I have a friend's pair of Fourier Sans Pareil OTL monoblocks for diagnosis and repair. I understand he and a friend have swapped them back and forth over the decades and they have low hours. Apparently, they were glorious, and then they weren't. And one isn't currently working. The other one is ???? I haven't put power on them yet. My bench has a preamp project in progress, so I'll get to the OTL's when that's done. I'll start with information gathering.

Attached is a schematic, but I don't have any other documentation.

I'm already planning to replace 360V caps with 450 or 500V caps. I read that these are underspec'ed, and with 340V on secondaries, that's obvious.

Some of my initial questions are:
•Where can I get a manual? Operator manual or service manual?
•How to bias? Procedure?
•What are the target voltages at the Test Points?

 

[poseidonsvoice]

Randy,

Have you tried contacting John Atwood?

I'll be curious to see how your repair goes, it definitely looks interesting!😀

Best,
Anand.

 

[rhthatcher]

Thanks Anand - I just shot him an email. Good call!!

 

[decramer]

The schematic looks like Alan Kimmel OTL ??

 

[tubelectron]

This design reminds me some late Futterman OTL circuits... Maybe the H-3aa or the OTL-3 :

https://www.solaris.no/electronics/power_amp/tube-amp/futterman/h3aa_sch_orig.jpg

https://ldsound.info/futterman-h3-otl/futterman-h3-otl-ldsound-ru-3/

http://www.ys-audio.com/wce/futtermanOTL3.JPG

I note that it is not a direct to speaker coupled OTL, since the output signal passes through a 7x 1200µF/360V + 3.9µF output capacitor assembly.

Not easy at all to find the key informations you need for a repair and setup of such an amp...

https://www.one-electron.com/Projects/Fourier/FC_SP_ad_2.png

T

 

[rhthatcher]

I finally got my hands on the bias procedure and want to share this information for posterity.

I have updated 1 of the 2 monoblocks so far:

• All new electrolytic caps with increased voltage rating.
• Updated film caps
• Replaced a few resistors that were out of spec
• Checked all solder points after finding a fuse clip that was loose
• replaced trimpots
• added test points so j-hook type DMM probes can be hooked up for biasing

You can see pix of the bias test setup, and a picture of first / rough pass at biasing. It will get some more fine tuning and then tested on bench speakers. Then I'll do the same updates to the other amp.