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The ultimate DIY? A Stax SRM-T2!


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Guest sawyers
Filament transformer is a slight bit high... The output tubes are 6.5 vac

the input tubes are at 6.7 vac

That is alas part and parcel of using AC heater supplies - they are locked to the mains voltage, which is only known nominally and dependent on territory.

Here in the UK, we are supposedly harmonised to the the EU standard of 230V, +/10%. So it can be anywhere in the range 216V to 253V and still be in tolerance. So 6.3V to the heaters could be anywhere in the range 5.7V to 6.9V in the UK. In the US the utility companies "try" to keep the voltage in the range 114V to 126V "most of the time", whatever that means. If serious, that means half the percentage change allowed in the EU - so in the US the heaters could be in the range 6V to 6.6V. So your measurements could just be a reflection on the mains power voltage at that time.

Alas the only way to get accurate heater voltages is to use regulated DC. For that read less efficient, larger transformers and more heat, because a capacitor input filter is less efficient than raw ac, added to power loss in the regulator.

FWIW I've usually just use ac - but I tend to use Sovtek tubes and the like. But if I was using NOS Mullard EL34's I might get a whole load more fussy.

Craig

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Guest sawyers
The UK is mostly to blame for the wide tolerance of voltage (at least on the high side) in EU because they were originally high at 240V. Instead of changing the system, EU just increased the tolerances.

It is unfortunately more complicated historically. Once upon a time, (ie back in the 1990's) the UK used 240V and the rest of the EU used 220V. So a harmonised voltage of 230V was introduced in 1995, with transitional asymmetric tolerances - so the UK had +10%/-6% and the rest of the EU +6%/-10% from 1995 to 2008. Then the tolerance was harmonised to 230V +/-10%.

The "old" 240V UK standard is defunct, but the nominal range was from 228V to 253V (+/-5.5%).

That is theoretical, because each country's power generation system is optimised for a particular voltage, the UK continues to generate 240V and the rest of Europe 220V, 50Hz.

The practicalities of multi-standard transformers is horrid, particularly when a transformer is designed for 60Hz that is then used on a 50Hz power system. The core and winding design depends on the power line frequency, and results in mechanical buzzing or humming from the transformer of either EI or toroidal construction. A 60Hz transformer that is going to be used on a 50Hz system needs to have a seemingly overrated *core* - it needs 60/50 times as much material (20% more) to keep the core flux density the same, assuming the same number of turns. Not overrating the core gives a buzz as a result of pushing the core towards the non-linear part of the hysteresis loop - so you get more core loss, and output wave form distortion. The non-linear core flux density adds harmonics to the acoustic output - you can't easily hear a faint 50Hz hum - but add harmonics and you get a loud sounding buzz. It is a point that is missed time and again by transformer designers. I have products as diverse as a Fluke 8506A (true thermal rms digital multimeter) and an Audio Research D125 power amplifier that exhibit precisely this intensely irritating phenomenon.

<soap box mode off>

Craig

Craig

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Guest sawyers
And after a few hours this is what happened. Due to the current limited power supplies, absolutely

NO components burned up.

http://gilmore.chem.northwestern.edu/arcscrew.jpg

I'm switching to acetate screws once i find some.

Wow. Was that on the power supply or the amp board?

Regarding screws, PEEK is a good option - and engineering grade plastic and available quite commonly. Just google Peek screw.

Craig

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yep it was in the power supply... currently the screws are all stainless. Because i have a bunch of them, and they

look cool. Got nylon screws coming, but if i can find peek screws that is even better. I use peek tubing for all the

LC's...

on second thought, at about $7 per screw, and 64 screws needed... maybe spritzer will do it.

I know there is no reason not to be excessive, but that is a bit nutty. Need to verify that they are

really stronger first.

Edited by kevin gilmore
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on second thought, at about $7 per screw, and 64 screws needed... maybe spritzer will do it.

I know there is no reason not to be excessive, but that is a bit nutty. Need to verify that they are

really stronger first.

I've been meaning to cut back on the insanity... :D

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Guest sawyers
yep it was in the power supply... currently the screws are all stainless. Because i have a bunch of them, and they

look cool. Got nylon screws coming, but if i can find peek screws that is even better. I use peek tubing for all the

LC's...

on second thought, at about $7 per screw, and 64 screws needed... maybe spritzer will do it.

I know there is no reason not to be excessive, but that is a bit nutty. Need to verify that they are

really stronger first.

The other possibility is Schurter Transipillars. These are nylon pillars with a selection of captive threaded insert or threaded rod at the ends. The advantage is that the nylon is always used in compression. If you use nylon screws the material is used in tension - and it is very easy to strip the threads as the screw stretches.

Have a look for instance at Farnell part number 627185. Datasheet with dimensions is here http://www.farnell.com/datasheets/85817.pdf . These have got to be available statside, at around a dollar each.

Craig

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Guest sawyers
I like inu's parts, and just ordered a bunch of them.

I wonder what the spec on the voltage of the washer is?

Ah - the offending screw was securing a power device to the heatsink! My suggestion of transipillars is entirely redundant. I too like the ceramic insulator and PPS bush that Inu put up too.

Craig

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I have a bunch of 4169G and 4180G pads. They are really nice, but the widths are more than .5", which is the mounting width of the devices. It's nice to know that the 4171G will work. I cracked one by over tightening, so you have to be careful. I think I will order some of these as well.

Not sure on the voltage rating of the glass filled PPS screws. I do worry a bit about the temperature limits (around 185 deg F for the ones I was looking at). Shouldn't normally be an issue but in a hot room it starts to get close.

Great idea on the current limiting supply!

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Does anyone have any specs that make sense on the 4171G. Like what the real max voltage is?

Errors in the spec sheet have the thinner one being able to stand more voltage.

They only need to withstand 600 volts max.

I assume that thermal grease is required when using the ceramics...

Too bad no one makes berrylium oxide pads.

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