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Old 14th January 2014, 07:57 AM   #1
Courtec is offline Courtec  Netherlands
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Question Sphinx Project Fourteen help wanted

Dear all,

Recently a Sphinx amplifier entered my workshop. It is a model 14 with 2 tubes and Mosfet's in the amplifier stage. These are the 2SJ50 and the 2SK135, very famous in Holland in the 80's when Elektuur came out with the Cresendo amplifier for diy purposes and these mosfet's.
The tubes are ECC88

The reputation of the 14 is not so good. Later, Sphinx made a rebuild which was more reliable. The biggest problem is the design of the PCB and the power supply for the tubes. But the sound is fantastic!

Well, my question, does anybody have a diagram for this amp?

Thanks in advance for the help and i'll keep you folks posted.

Ah, the problem is loud hum and oscillation.
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Old 18th June 2015, 11:44 AM   #2
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A diagram seems to be impossible to find even from employees of the company.

I redesigned the hv power supply to a simple follower with zener diodes and it seems to work.

Had a look for all elcaps on the board, because the unit runs hot.

Oscillation problem could be solved by substituting ECC88 by ECC85 as second tube.
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Old 18th June 2015, 01:27 PM   #3
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Try anti-parasitic resistors (grid stoppers) if HF oscillation.
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Old 18th June 2015, 01:29 PM   #4
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Did this on all 8 grids.
A low frequency oscillation was also apparent.
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