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Old 16th Jan 2009, 7:49 pm   #14
Valvepower
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Join Date: Aug 2008
Location: Southend-On-Sea, Essex, UK.
Posts: 630
Default Re: Sound Sales PX4 Amplifier

Hi All,

I’ve just stumbled on this thread.

I repaired and tidied up one of these amplifiers about 8 years ago.

There was a question in a previous post regarding the setting of the balance pot feeding one of the PX4 via the 6SN7 pin 4. The method I used was to adjust this pot by measuring the THD (set to get a minimum) at a few dB below the clip level in to a 15-Ohm load. This was perfomed after roughly matching the positive and negative half’s of the output wave form using an oscilloscope.

The other aspect I liked was the tone control; where the signal via a high and low pass filter network on the o/p of the 6J7, are added together via level controls, and fed to a mixer stage, where anodes of the 6SN7 are coupled together. The 1M2 provides a signal path for the wide band signal, and the two controls adjust the LF and HF levels.

Sadly I sold this amplifier, its one of those sales I regret - ho hum .

Regards.
Terry.
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