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The ultimate DIY? A Stax SRM-T2!

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7 hours ago, Craig Sawyers said:

A question for the few who might have an original fried-egg-temperature-case original Stax T2 - is there a problem with the battery trimmers in that?

Not that I've heard but then again...they have so many issues (and noise) that it might just be lost in the haze. 

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I finished my fourth original DIY T2 a couple of weeks ago. All with Xicon resistors. No elevation above PCB of components except for power resistors. Initial settings of batteries have been straight forward and the three first amplifiers have been silent, no audible hum or noise to my ears. But this last one has some low “static noise” from power-on which disappears after approximately 10 minutes.

 

Now I’ve replaced all four trimmers on left channel with resistors and it didn’t change the noise behavior at all. Than I thought maybe I’m eventually punished for not elevating components - so desoldered all left channel battery resistors and put them on the other side with no ground plane and… no change on noise. But as it vanish within 10 minutes I maybe should accept Birgir's view of the T2 and be satisfied. Just grabbed a bottle of red wine and enjoying

Cliburn-album-cover.thumb.jpg.8746a8742bb66b1a635d5cd2e6c9242c.jpg

this.

Edited by JoaMat

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Try looking at a scope to see if the noise is on the + or - leg of the channel. This helps to identify where to put your attention. 
 

Good luck and above all, enjoy :) 

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Well, by accident I found Head-Case and “The ultimate DIY? A Stax SRM-T2!” on internet in 2011. Read that thread back and forth several times and I became kind of absorbed by the clone Stax T2. So it’s actually you Craig and Kevin, Birgir, Kerry, INU, et. al. that dragged me into this – I’m very grateful for what you all did to get the DIY T2 possible for armatures like me to build.

 

After managing to build a KGSShv successfully I felt ready for the real thing - DIY T2. Bought boards from Tran and I actually succeeded with this monstrous project and had a working amplifier - at least for a while. Then all kind of problem popped up. Somehow I did fix them but eventually I found it meaningless to continue with that amplifier. Reason for the problems was probably myself (Builder Induced Failure) but it gave me a lot practice.

 

My impression was that the quality of the boards Tran sent me was a the lower end. So I order five high quality amplifier boards. Desolder all the expensive sands including c3381 and used them on a new board. Believe it or not but it succeeded despite reused sands - no popping up failures. Then I started to make some modifications (don’t ask me why – just a bad habit) most of them worked. As I thought I needed an unmodified as reference I built the third T2. Now it felt like - just put the right component in the right position, power on, adjust batteries and you are set. But of course I started to fiddle with the new one as well.

 

After a long hike with my wife this summer. 820 km/ 510 miles in 31 days I felt it is most  appropriate to have an original DIY T2 again (I still had three amp boards left).

 

That’s why four.:)

Edited by JoaMat
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The new T2 has behaved in a good manner for a whole month. Now qualified for old good tubes...

BZ8A0289.thumb.jpg.c202112abe5f9a890324937577b94796.jpg

...Amperex Bugle Boy to the left and Brimar to the right.

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23 hours ago, JoaMat said:

The new T2 has behaved in a good manner for a whole month. Now qualified for old good tubes...

BZ8A0289.thumb.jpg.c202112abe5f9a890324937577b94796.jpg

...Amperex Bugle Boy to the left and Brimar to the right.

Is that a custom chassis of your own crafting?

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Yes, heat sinks and front/rear panels are from modushop.biz. Brackets and aluminum plates for top/bottom covers from local shops. Drilling, tapping and other crafting done in kitchen.

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My mind tells me carbon, but my heart tells me grounded grid/blue hawaii. In the end i was thinking to make universal psu for carbon/grounded grid combination so i can have both. I think summed, this is again closer to the T2 budget. Then someone writes that blue hawaii is somewhere in between. Is this more like different shades of grey, or T2 is really that much better?

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tomislavkufrin There is the mini t2, if you dont mind surface mount for the amp boards, you can use exactly the same golden reference lv supply as the blue hawaii - zero changes, the same golden reference hv supply with two resistors changed to go from -400V to the mini t2 -460V, add another golden ref power supply for the +220V or a dropped resistor from the +400V to save an extra power supply board and transformer winding..

if you use two transformers - one to supply the heaters and the plus and minius 15V DC - that transformer can also be used on the blue hawaii 

the second transformer can supply the voltages for the 220V, -460V and +400V.

I think the sound of the mini t2 is better than the blue hawaii (a big enough difference that my mini T2 is now my daily stax amp and the mini t2 runs cooler) and quite a lot of the mini t2 can be re purposed if you do decide to build a blue hawaii some time. The blue hawaii is also a nice amp if you dont want to do smd soldering. 

personally Im waiting for someone to design a reliable full diy t2 using modern components.... Kevin? JoaMat? 

 

Edited by jamesmking
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5 hours ago, tomislavkufrin said:

My mind tells me carbon, but my heart tells me grounded grid/blue hawaii. In the end i was thinking to make universal psu for carbon/grounded grid combination so i can have both. I think summed, this is again closer to the T2 budget. Then someone writes that blue hawaii is somewhere in between. Is this more like different shades of grey, or T2 is really that much better?

I did exactly that - built a universal PSU for KGSSHV, Carbon, GG, BH and the CFA. 

IME, GG and BH are rather similar. Have not heard a full DIY T2 but building one PSU, a BH and a Carbon will not come close to what it costs to build a T2.  

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mini t2 is not alot more than a bh,

a few extra transformer windings - it needs seperate windings for the 6922s and el34s,

an extra 220V supply or dropper resistor,

extra valve sockets and 4 6922 valves

against that smd parts are generally cheaper than the through hole and I found that in many cases - especially the resistors buying 10 of a value in smd was cheaper than buying 6 - the bulk discounts scale much better than through hole.

It is true that its possible to build a blue hawaii in a single case and still fit in golden reference LV and HV boards, its not realy possible to do this with the mini t2 so you need a second case and wiring between the cases. However, it is possible to fit all the pcbs in a case the same size as the blue hawaii and just have the transformers in a small cheap second case.... - which is what I have done.

 

Edited by jamesmking
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On 3/26/2020 at 1:50 PM, jamesmking said:

personally Im waiting for someone to design a reliable full diy t2 using modern components....

Try those modern components and reduce high voltages to +/-400V (I’ve tried all except KSC1008 and J112).

Old               Modern
2SA1468     KSA1156
2SK216       KSC2690A
2SJ79         KSA1220A
2SC3381    KSC1008
2SK246       J112
2SK3675     FJPF2145
    
Mouser wants $51,51.
 

Edited by JoaMat
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So I have been working on the T2 for a couple months. Yesterday I got the PSU up and running on first power up, no issue.

Now moving on to the amp section, I got stuck at the usual place, batteries. Using Kerry’s adjustment, on one of the batteries, I could only get down to 7.43V on the 22K resistor, 784V on battery, 13V offset. Another one got stuck at 806V but with 470V offset! 😕 

Nothing smokes. What should I do first to check?

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So you have 7.43V with the 10K trimmer at endpoint? What is the voltage cross 22K resistor of the other battery?

Edited by JoaMat

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Yes, I couldn’t get them down any lower... Will remeasure the points when I power it up again, for now I’m kissing goodbye the K216s... these rare and expensive sands.. damn :(

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So I swapped out the parts and fooled around with the trimmers, got all the batteries dialed correctly, offset was ~10V. Plugging the headphones and got sound! My Carbon definitely felt threatened. This was at 2am and I was both excited and sleepy, listened to it for a bit and turned off the amp.

Next morning when I got it up again, some resistors in the amp sparked. Looks like I didn’t elevate them high enough 😕 Also the +250v rail went bad and couldn’t regulate, outputting +320v now. Been checking around but still couldn’t find the culprit. 

I wonder what would be the root of this?

Edited by thuytn
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Oof! So something has also gone in the psu *and* some resistors in the amp sparked (to ground or just blew?)? Sounds like you have a bit of work to do, obviously fix the psu first.

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The resistors sparked to the ground plane. I spent a good whole day elevating all of them. Also fixed the PSU, all voltages are ok now. (*wheww)

Now back the amp section, the same thing happened. I couldn’t get the trimmers as low as 6.55V. Same as 740V. 

The LED at +500V didn’t lit.

I wonder if the sparkling to ground plane affected the board itself?

Edited by thuytn
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Here is a T2 on original DIY T2 board with all modern components. High voltages decreased by 100 V to satisfy fjpf2145 and ksa1156. Batteries are on small daughter boards.

IMG_0309.thumb.JPG.ae76e38029f081d4b7c078f080d4231c.JPG

There is annoying hum on left channel. Right channel is silent so hopefully it’s possible to get rid of the hum. 

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On ‎4‎/‎2‎/‎2020 at 7:37 AM, JoaMat said:

...(I’ve tried all except KSC1008 and J112). ...

Now I’ve tried them.

 

KSC1008, put them in like this, one from component side and the other from solder side. Save you a lot of money.
IMG_0311.thumb.JPG.ea154bde51230e695f7629779869c523.JPG     IMG_0310.thumb.JPG.9ba7341a413856108004eab50520fd53.JPG
 

 J112, changed 22K resistor to 13.7K then able to adjust to 6.55V. Doesn’t save you so much money.

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Kevin has mentioned that the 2SC3381 can be replaced by a pair of MPSA06 as well. Has anyone tried that? 

Also, why do you put one of the KSC1008 on the opposite side? 

Edited by mwl168

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I’ve used mpsw06, big brother to mpsa06. Here in my modified T2 (mpsw06 in red circles).
879116480_IMG_0547_redcircles.thumb.jpg.8d583c4caf5f06ae216ef742ded8967b.jpg

 

Here in a DIY T2. Both pieces on same side of board and held together by heat shrink tube. Than you have to twist legs so they look like a Dancing Fool.
497495172_c3381repacment.thumb.jpg.bc5ec152695ace14ce49d75c105df9db.jpg

 

A lot easier just to stick a pair of TO92 from opposite sides.

 

Want surface mount? HN4C51J.

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Thanks JoaMat. 

I am thinking of using 2 MPSA06 mounted on the same side without bounding them together. No twisting and no dancing ^_^ 

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You use a board without 2sc3381 footprint?

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