The ultimate DIY? A Stax SRM-T2!

[GeorgeP]

Powered up the power supply and all voltages are pretty much bang on. Amp board is nearly done - just waiting for the tube sockets so will complete the wiring and umbilicals while I wait.

 

One thing though that I wanted to check with others is with the Panasonic AHN22112 relay - it takes anywhere between 40 to 60 seconds to click on. Is this normal or should I be replacing the relay?

[spritzer]

Should take a set amount of time based on the 555 timer.  Could be a bad chip?

[GeorgeP]

Do have an extra 555, so I will swap it out and see. How long does your take?

[kevin gilmore]

That is about the right amount of time.

You want the output filaments warm before the high voltage

Kicks in. The timing cap is a lyric so the timing is not exact

[GeorgeP]

Ok - will leave as is then. Makes sense.

 

By the way, your knobs cleared customs yesterday and I should have them on Monday.

[GeorgeP]

Just as a clarification to the above, the relay clicks on at exactly the 60 second mark whenever the power supply is turned on after a period of being turned off. But if turned off and switched back on after only a few seconds the relay can click on a little before the 60 second mark.

 

 

Also, two quick questions I couldn't find answers to in reading through the thread:

 

1) what are the 18 #4 nylon washers (1 each for power supply 2 each for amplifier) in the BOM for? Are these to be added to the stand-offs to gain additional height?

 

2) do the .25-20 traffo bolts need to be nylon or can they be 18-8 ss? I had read in the thread that on at least one T2 the nylon bolts did not survive shipping (though I don't plan on ever shipping mine).

 

As an aside, wiring the amphenol connectors is very painful - would prefer to redo the entire amp board instead (from scratch that is). Though have got it pretty automated now. Definitely pre-soldering the ends of the wires and using additional flux on the wire and connectors made things a whole lot easier.

[kevin gilmore]

1) I think so, but I don't remember at this point.

 

2) just make sure the bolt does not touch the top and the bottom of the chassis at the same time. otherwise you get

a shorted turn and that is real bad.

 

if you use the Teflon wire and pre-tin, makes it much easier to do the amphenol connectors.

at least 1 hour each.

[GeorgeP]

Thanks, Kevin.

 

And yes, pre-tinning made it easier, which is all I did initially. But then I found a swab with the flux pen made it even quicker. I would still hesitate to do another two-box build - though this one has 28 wire connections (with all 30 connectors populated in the umbilicals in case).

[GeorgeP]

So nearly a month after completing the power supply and waiting for the 4 teflon octals to come in, the T2 is finally done. With two small kids and both my wife and I being practising lawyers, this project was relegated to the late eveing hours and so took a loooong time to complete, with sourcing of parts and the case having started in February.

 

I initially fired up each channel individually just to check the LEDs with no Stax attached, but quickly found that firing it up was certainly not that straightforward (having only previously built the KGSSHV).

 

The right channel: LEDs in the batteries fully lit, but everywhere else they were either unlit or extremely dim. Not very encouraging.

 

The left channel: LEDs in the batteries fully lit, but everywhere else they were either unlit or extremely dim, with the LEDs in output stage flashing and a light buzzing sound rising and falling with the flashing of the LEDs. Again, not very encouraging.

 

My first thought was how would I be explaining all the additional hours troubleshooting this thing to my wife - she was not exactly enthuastic about the project to begin with and our home office has been overtaken by it. Now I tested each and every part that went into the amp including matching the resistors and transistors, and everything checked out and was sourced from known parts suppliers. There were no pops or zaps, no smoke or strange odours, and there was no way all those LEDs became defective after testing. So what else to do except fire up both channels and adjust the pots to see what happens.

 

Interestingly, as I went through adjusting the pots, the LEDs did all sorts of weird things - some unlit ones lit up while others turned off, adjusting one channel seemed to affect some of the LEDs on the other channel. While adjusting the first battery, the voltages got stuck and would not adjust any further - but at least the flashing LEDs and buzzing was gone (but those previously flashing LEDs were off by this point). But I didn't let that get me down (well actually, I was as down as could be by this point and it was simply desperation - like fear substituting for courage in a fight) so I started adjusting the pots on the next battery. But it too stopped by a certain point - if I recall they were in the neighbourhood of something like -560 and +20.

 

I then went to the batteries on the other channel and surprisingly they could adjust below -560 and above +20 (got it to something like +140 and -550). Then I went back to the first channel. Miraculously, the voltages were no longer stuck and could be adjusted within striking distance of the proper voltages: landing at +200 and -544. Somehow the two sides are dependent on one another. I was able to do this for each of the batteries, and by this point all the LEDs were brightly lit! The balance checked out at less than 0.0 volts, with the + and - for each channel around 10 volts. Quickly hooked up a pair of 307s without being connected to a source and started adjusting the volume - dead silence. Tried adjusting RV1 and RV2 to try and get things even closer and to get the offset down a bit, but it seems that once the T2 finds a sweet spot it likes to stay there - you know when you are off if you have earspeakers on while adjusting because you will hear what sounds like the output from a crappy microphone in a wind tunnel (and yes, you can also hear the wipers on RV2 as you adjust, but not RV1). Then it was 3am so went to bed.

 

The next evening (yesterday; it was hard to focus at work that day) I connected a (crappy) DAP to give a listen (has yet to be relocated to be near a semi-proper music source) and crisp clear music was coming out! And it sounded really, really good (the 307 also sounded really really good - very underrated)! I ran it for 6 hours with music running through and everything continued to work perfectly. Will try it out with my 007MK1 in the coming days, and my 009 as well (Birgir will shudder at the thought). But the heat, my god this thing was burning hot by the end of six hours (will measure temp when I get a chance). The amp was clearly hotter than the PS, but the metal caps for the HV traffos were the hottest and I could not keep my finger on them for more than a second - while the body of the traffo, though hot, was still bearable to touch - not sure if others have experienced this with their T2 traffos.

 

The reissue Mullards (xf2s are waiting in the basement) and the 6922 JAN Philips tubes (believe NOS from the 1980s - not sure if I should be looking into betters ones or not) all worked perfectly and their heaters had a nice glow about them in the dark.

 

So that makes at least one DIY T2 for Team Canada. Will try to post a few pics over the next couple days. Now what to do with the extra KGSSHV? Though if you ask my wife (which she has asked me already a number of times), the bigger question is where am I going to put this 60 pound beast? I think my nightstand should do the trick.

 

George

[PICaudio]

Congratulations George!!

 

-- o --

Rodrigo

[luvdunhill]

Just curious, what position did you start with all of the pots? I believe there a few posts in this thread with pretty detailed startup sequences, curious if you read those or just tried to wing it.

[GeorgeP]

luvdunhill

Posted Today, 02:13 PM

Just curious, what position did you start with all of the pots? I believe there a few posts in this thread with pretty detailed startup sequences, curious if you read those or just tried to wing it.

 

 

Read through this entire thread multiple times (too many). Saved and printed out many pages. Put the pots in all centred as Kevin recommended, though I believe Inu commented at one point to have RV2 centred and RV1 at the highest resistance (cannot now recall), having previously tried them all centred. Also considered Craig Sawyers' recommendations for order of adjustment based on his spice model, but did not first adjust the voltage across the 22K resistors in the batteries to 6.55V as recommended by Kerry. But despite these careful considerations there was still an element of winging it. Did not get the sense that anyone actually was able to get one of these fully operational with a "step 1, step 2, step 3 etc." approach. I think facing Mecca (or equivalent) helped the most. As Kevin has commented, the battery adjustment thing is certainly weird.

Edited July 10, 2013 by GeorgeP

[spritzer]

Fake it till you make it is my line...

[GeorgeP]

Yes, that is definitely half the battle for most, and more so for me on this as my knowledge level could fill a shot glass compared to the knowledge levels of most of the folks in this thread.

 

Just as a note, what seemed the most useful to me that I didn't see anywhere else was to focus on getting the +200v in range then everything else just seemed to fall in place.

[purk]

Congrats GeorgeP!!

[GeorgeP]

Thanks, Purk! And thanks again for helping me source the case - I am forever in your debt.

 

So I managed to take some pics with my very crappy camera (used to be an SLR guy and still have my 501.4, 85L 1.2, 135L 2 and 70-200L 2.8 IS - just waiting for the right time), so please excuse the harsh flash (I know, I could have adjusted the exposure). Have not yet fully cased it and still need to find some proper feet to go with the chassis.

 

Also took some temp measurements which could in part be affected by the fact that neither of the chassis are getting any real airflow on the bottom due to the absence of feet.

 

The heatsinks near the front of the amp measured 52C/125F with my Fluke 54II and contact thermocouple, while the rear section measured 46C/114F. The traffos as I said before were stinking hot with the hottest one having a temp of 54C/129F at the mylar surface and the metal cap plate measured in at 67C/153F (though the ss bolt securing the cap measured 55C/131F). The Traffos in particular seem really high - not sure if others have measured theirs to see, but should probably try to order a slightly beefier set to have on hand.

 

And if you notice in the pics you will see that the 500v wire just makes it to the soldering point - this is what happens when you are cutting wires to length while being regaled with the day's events by your 5-year old son. There is also some ugly kapton tape under the caps in the power supply - I am sure it was totally unnecessary but have been extremely paranoid ever since reading about the arcing some people experied in the KGSSHV thread.

 

Also couldn't seem to locate any black delrin rod in Canada (though am sure I was just not looking in the right place), but managed to find the white delrin rod from an aviation supply company - $1.25 for 3ft (I always buy extra) plus $12 shipping. Seems to have a bit of flex to it when the pot is bottomed or topped out - not sure if the black delrin is any stiffer.

 

Balance on the two channels are each below 0.1v and the offset on the + and - of each channel is between 4-5v - very pleased with this result. Oh, and it sounds good too.

Edited July 11, 2013 by GeorgeP

[tkam]

Congrats on finishing the build and nice pics!  Those heatsink temps don't seem bad, not sure about the transformer temps though - hopefully someone who knows more will chime in soon.

[s_r]

Congrats on the T2, looks beautiful. Before I got a fully encapsulated replacement from sumr (old transformer was buzzing) I measured about 50-60C on the trafo in my KGSSHV. You can shoot sumr an email if you're worried, but it's not too much hotter than what I measured for mine.

[JoaMat]

Congrats, well done!!

[Lil Knight]

Oh, and it sounds good too.

Something must be wrong with your amp here. It should sound VERY good.

Congrats on finishing the amp, very nice work, George.

Edited July 11, 2013 by Lil Knight

[RudeWolf]

When I first saw this thread two years ago I was completely baffled and thought that this is insane.

 

Now I look through the schematics and PCB's silkscreens and think to myself- "well, this is clever and for only 2,5K; I might pull it off in two years or so..."

[Victor Chew]

Nice work GeorgeP.

[chinsettawong]

Nicely done GeorgeP.

[palchiu]

GeorgeP, Congrats!  It's great amp!

[GeorgeP]

Yes, I am not worried about the heatsink temps, but those transformer temps took me a bit by surprise - though just don't know if those temps are normal.

 

 

Congrats on the T2, looks beautiful. Before I got a fully encapsulated replacement from sumr (old transformer was buzzing) I measured about 50-60C on the trafo in my KGSSHV. You can shoot sumr an email if you're worried, but it's not too much hotter than what I measured for mine.

 

Did Peter consider buzzing to be a need for replacement or did you simply opt for getting the encapsulated version? There is a very light buzzing from one of the transformers but only audible if my head is within 3 feet with the cover off - so not really concerned.

 

 

When I first saw this thread two years ago I was completely baffled and thought that this is insane.

 

Now I look through the schematics and PCB's silkscreens and think to myself- "well, this is clever and for only 2,5K; I might pull it off in two years or so..."

 

I wish it was only 2.5k - maybe if you don't count the case. I still have to go back and check (though part of me says not to), but I think it was closer to 4k all said and done.