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The ultimate DIY? A Stax SRM-T2!


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Tube sockets arrived today. Really crappily packed - wrapped in bubble wrap, but stuffed in an ordinary large brown envelope with a string wrap seal. Envelope was disintegrating, so it could have arrived empty with the sockets somewhere between here and Hong Kong (from DIY HiFi Supply).

The good news is that they thoughtfully put a customs declared value of $50 (and not $152) on, so it slipped though the radar and attracted to tax. So hats off to them for that. Nice to get one over on our tax police. The other good news is that shipping from HK to UK was only $7.

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I've just been flailing around trying to avoid using the 4171G ceramic insulators, since they aren't expected in to the various stockists until the end of July (nominally). I've been talking to Kevin privately thoughout this as I reasoned things through.

The result of that flailing just resulted in my spending a bit of money and stocking up the spares drawers.

The short conclusion is: THERE IS NO OPTION BUT TO USE THE 4171G!

I tried to use the Bergquist elastomer "alumina and beryllia replacement" with 6kV breakdown, type K10. I sourced insulating bushes with 2.54mm bush length so they went into the heatsink bracket and insulated the screw. Then I spotted the fallacy - there is still a breakdown path from the inside edge of the device mounting hole to the hole through the heatsink bracket - which is just the thickness of the washer away (0.15mm). That limits the potential to 0.15 x 3kV = 450V best case (air breaks down at 3kV/mm). Since the device tabs run up to 630V above ground this is a recipe for disaster. The nonsense is of course that Bergquist supply a TO220 bolt mounting stamping out of this material, which makes a joke out of its 6kV material specification.

The alumina washers that Kevin ended up specifying are 1.78mm thick, separating the device tab and heatsink by that amount. That should hold off 1.78 x 3kV = over 5kV!

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I wonder if you could "persuade" Aavid to either send you samples (was it 60 of them :rofl:), or sell them direct to you? I would guess your credentials are good enough that it might work.

Heat sink producer Aavid Thermalloy - World Leader in Thermal Management

Well, that is not a bad idea. So I've filled out the form, to be assessed by a "sales associate". Lets see.... :horsey:

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Hokay, finished the boards. Remaining is:

1. Japanese semiconductors - expected early next week

2. Transformers. Being built by Paul Houlden, ex MD of Avel Lindberg and supplier to Krell and other high end manufacturers. Cores are on order, so finished transformers should be around 3 weeks delivery. Primary specced at 240V 50Hz and his design regulation is 7%, centre potted fixings. Any other T2 builders in 240V territories who are interested, let me know and I'll put you directly in touch.

3. Ceramic insulators.

4. Casework.

Here's a pic of the stuffed boards http://www.tech-enterprise.com/tekstuff/T2stuffedboards.JPG

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I now have 9 sets of heatsinks done,

15 sets of fronts/backs done

8 sets of tops/bottoms done

(in addition to the 3 sets of everything already shipped)

going to do 6 knobs tomorrow

and a bunch of stax jacks (5 and 6 pin)

next week.

2 chassis with knobs should go out for finishing on friday or monday.

Spritzer gets a different color knob. :D

Only thing left to do is measure for and make the mounting brackets for

the DACT.

Unless i decide to do something stupid like making aluminum feet.

Or titanium feet, or some other very silly thing.

Gold/platinum with or without diamonds available special order :D

Would you write or engrave your autograph with production number on the bottom of the knob? :)

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