S.E.X.3 Plate choke replacement

craigius · 503

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Offline craigius

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on: December 27, 2023, 05:15:20 AM
I finally decided to build my S.E.X 3.0 amplifier over Xmas holiday, but when checking my parts list I now only have one of the two PC-3 plate chokes.
The manual schematic has the PC-3 down as 20/40H is this a 40 Henry 20mA choke or is the 20 referring to a 20v drop from the 400v B+ power line to 380v on pin 5 of the 6FJ7 ?.
Can anyone recommend an alternative to the PC-3 that I can use. I am based in the UK so it may be easier to obtain parts from here such as from Mouser UK etc. As I have to replace the chokes I would prefer to upgrade the original PC-3 part would the following Hammond 194C (40H 50mA) chassis mount be a good replacement. I would replace both left and right channels with Hammond 194c and discard my single PC-3; or could I use the larger Hammond 193a (200 henry 30mA) as an upgrade I imagine there maybe a slight loss in overall output power from 2W but can live with this if it provides  better (ripple rejection) hence sound quality with the larger choke.
I plan to use a larger case anyway for further upgrades so can fit larger parts.



Offline Paul Joppa

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Reply #1 on: December 27, 2023, 05:59:24 AM
PC-3 is rated 40 henries at 35mA, for use with the S.E.X. amps. It has a tap rated 20 henries at 50mA, which is used with the Stereomour II amp. You can obtain one from Bottlehead replacement parts.

Hammond does not provide enough information (specifically, identify the start and end of the winding, and the winding capacitances) needed to confirm whether it will work well as a plate choke at 8 Kohms.

Paul Joppa


Offline Paul Birkeland

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Reply #2 on: December 30, 2023, 06:07:27 AM
You also have to consider that there's a specific amount of space and a specific set of mounting holes for the PC-3.   I'm sure you could try the 193A, but that would involve making a bigger chassis plate, getting a bigger wood base, and redesigning the power supply to account for the substantial voltage drop across that part.  As PJ mentions, you'd have to evaluate the part for anode choke duty to figure out how well it performs too. 

There's a small possibility that these would fit:
https://www.thermioniclab.com/en/anode-chokes/anode-choke-40h-35ma-for-bridge-of-zen-preamp.html
I kind of doubt it though because I think he uses metric lams/bobbins and our winder uses US spec parts. 

The least expensive and most convenient option would be to just order up a replacement choke from us. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline craigius

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Reply #3 on: January 01, 2024, 06:01:30 AM
Thank you both for your suggestions it is appreciated as not come Thermionic labs before and still considering all options. I know the Hammond 193a is a bit of an unknown that may work but with modifications required so I looked also into the Lundahl plate chokes as they are very well regarded for high end audio use, the off the shelf LL1668 (100H 40mA rDC 680ohm choke series connected for single ended usage) stood out as an option that could work also any thoughts on the Lundahl. I plan to use a case I already have which is 380x280x70mm solid wood with aluminum top/bottom plates similar look to your supplied enclosure, I don't intend to use the amp to power speakers only headphones so will leave those connections off (4ohm output transformer to banana plugs) so space is not an issue so not a limiting factor to any anode choke options.



Offline Paul Birkeland

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Reply #4 on: January 01, 2024, 06:21:52 AM
If you have a lot of builds under your belt, it might be feasible to do what you're describing.  If this is your first kit build (especially from us), you should keep in mind that these kinds of changes will make very difficult for us to help you with the kit if anything ends up not working properly, and especially if the Lundahl chokes end up picking up magnetic hum from the power transformer.   It would still be most cost and time effect to just buy a replacement plate choke from us. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man