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SPP cap values

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jonwhitear

Member
Joined 2014
I'm also using the 25W Edcors, in my case the CXPP25-MS-7.6K.

I asked George the same question. His response: "I have not used the same Edcor OPT's that you are using, but I would build the board without caps, and listen to it for a few days, then try something about 250 pF and see if it makes an audible difference."

I haven't got as far as doing so yet ;-)
 
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akimmet

Member
Joined 2011
I end up needing 680pF with my CXPP25-MS-7.6K transformers. The correct value depends quite a bit upon the transformer used.

Hopefully you didn't order your transformers already, or you made a typo. CXSE25-8-5K is a SE (single-end) transformer. It won't work with the SPP, you need a PP (push-pull) output transformer with 8K primaries.

These are some approriate and affordable transformers that I know of:
Transcendar TT-124-OT and TT-039-OT
reproduction Dynaco Z565
Edcor CXPP25-MS-8K[FONT=arial, helvetica]
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akimmet

Member
Joined 2011
jonwhitear said:
Is that based on measurement or listening test (or both)?

As I have the same transformers as you, can I assume I need the same caps, or is it a matter of preference?
Click to expand...

I determined the capacitor value using my oscilloscope and 1kHz square waves. Your capacitor value will be very close to mine since the OPT is responsible for almost all of the ringing seen on the scope.

If you have any motor-boating problems after adding the capacitor, add a 100R resistor in series with the feedback terminal on the board and output transformer. I needed to do that until I fixed my wiring layout.
 
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akimmet

Member
Joined 2011
chilihed said:
My transformer only has 3 primary wires, so I can't do ultralinear hookup! So, do I just hook blue and brown to L1 Pri 2, and L2 Pri 1, then black and brown are out to speakers on the secondary side?
Click to expand...

You can still run in triode or pentode mode.

Here are some diagrams showing each common way to wire the screen grids.

SimplePPschemUL.gif gif by birdy81260 | Photobucket
SimplePPschemTriode.gif gif by birdy81260 | Photobucket
SimplePPschemPentode.gif gif by birdy81260 | Photobucket
 
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