The ultimate DIY? A Stax SRM-T2!

[Flyingsparks]

Add me to the list, I'm definitely interested

[Hens]

For those interested here are the prices. It really only makes sense if i do another 20 piece board run.

power supply board quantity 5 $157 each

quantity 10 $97 each

quantity 20 $61 each

amp board quantity 5 $242 each

quantity 10 $166 each

quantity 20 $102 each

so prices did go up ever so slightly due to increase in sales tax.

which makes a set of boards quantity 5 $399

quantity 10 $263

quantity 20 $163

Are you still offering chassis and parts kits?

[kevin gilmore]

I never did the parts kits. Never will either, not enough time.

All current chassis are spoken for. There is actually one extra chassis, but no board set to go with it.

I just don't see me doing another chassis run.

Some other fool would have to take that on.

[Hens]

Thanks for the reply Kevin, I thought I remember reading towards the beginning of the thread that you were offering part kits of some sort, it may have just been the sockets. Oh, and lets not forget the personally laser engraved titanium knobs.

As I am horribly lazy, could you point me to the BOM so I can figure out how hard it will be to find all the bits and pieces needed to populate the board?

[luvdunhill]

For those interested here are the prices. It really only makes sense if i do another 20 piece board run.

power supply board quantity 5 $157 each

quantity 10 $97 each

quantity 20 $61 each

amp board quantity 5 $242 each

quantity 10 $166 each

quantity 20 $102 each

so prices did go up ever so slightly due to increase in sales tax.

which makes a set of boards quantity 5 $399

quantity 10 $263

quantity 20 $163

wow, I'd have to agree 20 is pretty much a minimum. I'd recommend just starting a thread where people can post their interest and holding tight until that number is reached. That's some serious outlay for something that chances are very large will never meet 20 to begin with.

[jgazal]

As I am horribly lazy, could you point me to the BOM so I can figure out how hard it will be to find all the bits and pieces needed to populate the board?

Directly from the source:

uploaded struts spreadsheet.

http://gilmore.chem.northwestern.edu/t2parts3as.xlsx

[Craig Sawyers]

Well, I tried and failed to get the beast finished before wife and daughter got back from Rhodes. What I do have now is all four Amphenols pigtailed and installed in the now completed chassis.

With that in mind, you have to leave the Amphenol fixing screws loose, plug in the umbilical to centre the chassis connector in the hole and then fully tighten the nuts.

So only the internal wiring to complete now.

[luvdunhill]

would it be possible to get the "official" PCB width?

[kevin gilmore]

The amp board is 11.950 x 16.000

The power supply board is 11.950 x 10.000

but the inside of the chassis should be exactly 12.00 inches wide.

allows for .025 per side for irregularities due to warping of the heatsinks.

[luvdunhill]

Thanks Kevin!

One more question. How stable is the chassis without the four screws that go through the front and back plate? I was thinking it would look nice without these, and since there are screws that secure the front and rear extrusions to the top and bottom, I figured it might be possible to omit these. I just don't have the ability to accurately tap holes in the end of my heat sinks..

[Craig Sawyers]

While investigating' date=' sparked somewhere in the Active battery area and all of 2SK216 and 15V zener diodes in R-channel went to heaven.[/font']

You know - that just happened to me. Uncertain what caused it in my case, but was merrily adjusting the active battery, both test points at very close to 740V, then a sound of oscillation (whistling noise). Never a good sign - then crack - certainly 2SK216 are fried.

The other channel refused to adjust. Stuck at about 560V no matter what I do (-540, +20 WRT 0V).

[Craig Sawyers]

Well this is interesting.

The channel that self oscillated itself to death has for certain dead:

Two dead 2SK216 (the ones in the cathode circuit)

One 2SJ19

Two 100V zeners

One 2SC3675 - the one in one of the 740V batteries

Several dead LED's associated with the current sources.

The last is interesting, because in one case only *one* of two series connected diodes is dead. This was established by using a 9V battery and 5.6k series resistor.

In the channel with stuck battery voltage there is also at least a pair of dead LED's in one of the anode current sources (the pair associated with the current triple). Which would explain completely why I could not adjust the battery voltages. There are no other dead semiconductors in that channel (at least that I have thus found).

All the power supply voltages are correct.

[luvdunhill]

I'd guess those LEDs were dead to start with then (in the stuck battery voltage case).

[Craig Sawyers]

I'd guess those LEDs were dead to start with then (in the stuck battery voltage case).

I'd like to think so. But I don't think that is the case - the only dead LED's that I have found are the ones in the current triples.

I'm wondering whether it is something to do with the EL34's I'm using. Not minded to put in new tubes for the first run, I used a serviceable old pair - just running one channel at a time. Now those tubes measure pretty much OK - but they could be just off enough, or have some odd electrode short that doesn't show up on test, to trigger a problem.

I'll fix the unit up, replacing the LED's as a matter of policy, and use new EL34's. It'll be a few days since I don't have spares for the FETs.

Not the best of days....

[luvdunhill]

wierd. very wierd. you still have one channel to play with though... double down and try the other channel the good pair of tubes? I'm assuming you have high enough Idss on the JFET and aren't running into the issue Inu did?

[Craig Sawyers]

wierd. very wierd. you still have one channel to play with though... double down and try the other channel the good pair of tubes? I'm assuming you have high enough Idss on the JFET and aren't running into the issue Inu did?

Wierd is right. The channel that is nominally working still has a pair of dead LED's in one of the current triples. The more I think about it, it just has to be a tube issue. Everything that died on the deceased channel was associated with one of the EL34's. I'll do some more serious checks on those tubes and see if I can find out if that is the problem, although I can only get to 400V anode-cathode voltage on my tester (AVO CT160).

JFET's are GR Idss band, which is what was called for. But even if the Idss band was an issue, it wouldn't explain mass silicon suicide.

Anyway, I've just ordered a bucket full of spare semiconductors. Enough to fix the sucker, and plenty left over (which of course I'll now never need). After all, what is another hundred quid on this beast?

[Inu]

Wierd is right. The channel that is nominally working still has a pair of dead LED's in one of the current triples. The more I think about it, it just has to be a tube issue. Everything that died on the deceased channel was associated with one of the EL34's. I'll do some more serious checks on those tubes and see if I can find out if that is the problem, although I can only get to 400V anode-cathode voltage on my tester (AVO CT160).

JFET's are GR Idss band, which is what was called for. But even if the Idss band was an issue, it wouldn't explain mass silicon suicide.

Anyway, I've just ordered a bucket full of spare semiconductors. Enough to fix the sucker, and plenty left over (which of course I'll now never need). After all, what is another hundred quid on this beast?

In my experience,

You will be better to check the 6DJ8 if you using a vintage Ampelex.

My old Bugle boy killed 2SK216s twice so.

If you use the unbalanced or low-Ip 6DJ8, you will be into the mud.

I am stop using the vintage tubes for the T2, happy with the Silvania tubes now.

As Kevin mentioned before, adjust Active Battery is tricky.

While adjusting the active batteries, you will better to monitoring voltage between +/-Out to GND or +Out / -Out at same time.

Not necessary to adjust it to exact 740V. My unit is balanced at 749

[Craig Sawyers]

In my experience,

You will be better to check the 6DJ8 if you using a vintage Ampelex.

I am stop using the vintage tubes for the T2, happy with the Silvania tubes now.

That is very useful information Inu. I am using old stock Mullard ECC88/6DJ8, probably 1960's vintage. They measure fine on a tube tester, but maybe the T2 is sensitive regarding input tubes.

As Kevin mentioned before, adjust Active Battery is tricky.

While adjusting the active batteries, you will better to monitoring voltage between +/-Out to GND or +Out / -Out at same time.

Not necessary to adjust it to exact 740V. My unit is balanced at 749 – 750V.

If adjusted point of the RV1(2K) is out of the safe area, loud buzzing noise comes from the ear speakers. It sounds like oscillating and very loud.

Turn both RV1 quickly to find the point that stop the loud noise.

[Inu]

Aha - that sounds like the oscillatory process that destroyed one channel. New semis in a few days, and it sounds like some new 6DJ8's are in order too.

Avoid 6N1P and 6N11. These are different animal. These are bad linearity with the low voltage (<100V) operation.

[luvdunhill]

I'm not liking the fact that a non-linearity in an input tube causes things to melt. How sensitive was the original T2 to this? I'd be surprised if Stax let something leave their factory that did this...

[Craig Sawyers]

As Kevin mentioned before, adjust Active Battery is tricky.

While adjusting the active batteries, you will better to monitoring voltage between +/-Out to GND or +Out / -Out at same time.

Not necessary to adjust it to exact 740V. My unit is balanced at 749 – 750V.

If adjusted point of the RV1(2K) is out of the safe area, loud buzzing noise comes from the ear speakers. It sounds like oscillating and very loud.

Turn both RV1 quickly to find the point that stop the loud noise.

Inu, what adjustment protocol did you use? ie where to preset RV1 and RV2, and which order in which to adjust, whether they need to be adjusted simultaneously or not, etc.

And how many meters, and connected where?

Apart from KG, I seem to be number 2 to get to this stage (you were first), and we need to work out a foolproof method of setting this beast up to prevent auto-destruct setting of the battery pots, or there will be a further 17 learning curves down the line, and buckets of dead silicon.

[justin]

I'm not liking the fact that a non-linearity in an input tube causes things to melt. How sensitive was the original T2 to this? I'd be surprised if Stax let something leave their factory that did this...

some kind of protection diode missing somewhere?

[Craig Sawyers]

some kind of protection diode missing somewhere?

I've been peering at the photos of the original T2, and the only difference I can find after a couple of hours is that pin 9 (the shield between the two halves of the triode) is connected to ground in the original, but not in the clone. Also Stax use a grounded shield tube around each 6DJ8.

I've also been searching the web for the significance of connecting pin 9 to ground, or not. Most audio applications ground it, but it is uncertain whether this has a grounding in theory or not. The changes in interelectrode capacitance seem trivial from the datasheet.

But I also read that the 6DJ8 was originally introduced as a TV RF stage, and that with its high mu if it can burst into self oscillation it will. Most folks seem to add a 1k grid stopper to quite the things down.

So maybe pin 9 to ground and a shield are important in taming the 6DJ8, particularly in an ultrahigh bandwidth cascode/constant current arrangement. Plus perhaps a grid stopper tight up to the pin, although that would need a track cut, and wasn't necessary in the orignal.

[kevin gilmore]

It is possible that current production 2sk216 parts don't have the protection zener built in...

That could explain it.

I've been peering at the photos of the original T2, and the only difference I can find after a couple of hours is that pin 9 (the shield between the two halves of the triode) is connected to ground in the original, but not in the clone.

Not true, pin 9 is connected to ground on the component side of the board. Measure with an ohmmeter and you will see.

Also the center shield pin on the socket is connected to ground.

Edited September 10, 2010 by kevin gilmore

[Craig Sawyers]

Not true, pin 9 is connected to ground on the component side of the board. Measure with an ohmmeter and you will see.

Also the center shield pin on the socket is connected to ground.

I do beg your pardon - you are quite right. I hadn't spotted the thermals that make the connection.