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jrdmedford 10th September 2010 08:02 PM

Recommend a project to fit in a Counterpoint SA100 chassis
Friend has a dead SA-100. For the past week, I've been researching some parts replacements (Exicon 10N/P16) that are alleged to be made workable. I also got the schematics. Redux: To properly repair this amp requires more than simplying coaxing a new set of devices into service - it requires additional fixes to the marginal gate bias references and the overloaded filament supply. For the amount of work (and capital risk) involved - it seems far more useful to throw everything out and reuse the lovely metal case, power supply and heatsinks.

So, I've looking for recommendations for a PassDIY project that can fit within these re-use requirements:

1. Four TO-3 devices per side.
2. 250VA power transformer with 72VCT 7A windings that provides a bipolar supply of +/- 53VDC.
4. Estimates of the heatsink capabilities as follows: The SA100 ran its devices at 100mA and 53V. So thats 53 * .4 = 21W dissipation at idle bias.
Having never made formal steady state temperature measurements, I'm estimating these sinks hit about 50C steady state because they were certainly very hot, and you could only keep your fingers on them for maybe 4 seconds. So, my estimate is that the heatsink capability was (50-25 C)/20 W = 1.25 C/W. Does this sound fair as an estimate ?

So, I'm curious what I can transplant into this chassis that hits on all three
requirements of TO-3 can devices, 53V supply and perhaps a safe estimate of 1.1 C/W heatsink assembly capacity. My guess is that this is going to be a lean Class AB amp, but you tell me.

-- Jim

frags 10th September 2010 08:26 PM

I would try to rewire the transformer to have 2x18VDC or so and build F5 with bias at 0.7A.

jrdmedford 12th September 2010 01:31 AM

I think I've discovered the answer to this question: Can't be done because
the heatsink and PSU capacity of the amp chasis is not really up for more
than a simple 10W Class A application.

This topic will be moved to the Solid State section, where I will discuss
Class AB output stage redesign.


-- Jim (who sometimes can find answers to his own questions given time) :)

Nelson Pass 12th September 2010 07:00 AM

If the transformer has a 240V AC tap, use it to get about 25W
heavily biased class AB.

Or think about a fan...


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