The ultimate DIY? A Stax SRM-T2!

[JoaMat]

@@bui501, I’ve no idea if the PBHVs will do or not – someone should try. But how about KSA1156? Max voltage cross pnp in battery is around 370V. As for space it might be a little tight though.

On ‎2018‎-‎04‎-‎18 at 9:54 AM, bui501 said:

Even the more recent sands are starting to be difficult to source.

The squirrels sit on them.

[spritzer]

The KSA1156 should work and then there is the 2SA1627 which is in current production in China.  They are either a TO126 and Dpak which is just annoying... 

[bui501]

15 hours ago, JoaMat said:

@@bui501, I’ve no idea if the PBHVs will do or not – someone should try. But how about KSA1156? Max voltage cross pnp in battery is around 370V. As for space it might be a little tight though.

The squirrels sit on them.

Awe, Nuts! :-D

13 hours ago, spritzer said:

The KSA1156 should work and then there is the 2SA1627 which is in current production in China.  They are either a TO126 and Dpak which is just annoying... 

Thank you, JoaMat & Spritzer. I’ll look them up on mouser. This is the very last sand I need to source from the T2 BOM... assuming none of the older items are not fake (knock on wood).

 

[Whitigir]

19 hours ago, bui501 said:

Awe, Nuts! :-D

Thank you, JoaMat & Spritzer. I’ll look them up on mouser. This is the very last sand I need to source from the T2 BOM... assuming none of the older items are not fake (knock on wood).

 

Test it first Vinh.  Make sure you meticulously test them through and through

[JoaMat]

Reduced voltage on my “modified” T2.

DHT by night.

 

[bui501]

7 hours ago, Whitigir said:

Test it first Vinh.  Make sure you meticulously test them through and through

Thanks.  I'm planning to after my experience with the counterfeit CREE parts from alibaba during my Carbon build.  I've purchased the DY294 transistor tester that was recommended a few years back in this thread.  Is there are better/more accurate equivalent that you're using?  My DY294 sometimes gives me a bad reading for a known good part from mouser.   I would purchase a curve tracer for this project, but they all seem to be prohibitively expensive compared to all other lab equipment. -Vinh

[JoaMat]

Suddenly left channel went down – turned out that one mosfet 2sj79 had transformed itself to a PNP.

[Whitigir]

Ouchie! Can’t wait to see what caused it 

Edited April 23, 2018 by Whitigir

[GeorgeP]

Guessing all the (fun) experimentation has over time totally conflicted that J79...

[Kerry]

Got the offset servo working.  Thanks @joamat

I used a 6.8uF cap that you suggested.

[luvdunhill]

For the feedback cap? The original schematic has this as 10uF?

[Kerry]

Yes - C3 in my schematic above.  Joachim suggested a value of 6.8uF works best here. 

I've had it up for several hours now and and it seems very stable.

I also added a 1K resistor from the -560V rail to the collector (external to this board).

Edited April 30, 2018 by Kerry

[JoaMat]

Kerry, do you hear any bump or whistle in the headphone when power off with the servo?

[Kerry]

There is no whistle when I power off and the power off bump is a little bit softer that it was before (a little more bass and less pop).

[Whitigir]

Just now, Kerry said:

There is no whistle when I power off and the power off bump is a little bit softer that it was before (a little more bass and less pop).

Ah, yes, regarding this power of Bump.  Does it damage anything ? The Grounded Grid doesn’t have it

[Kerry]

Not to my knowledge.  I believe it brings the stators to a more positive voltage (closer to the 580V bias).  AFAIK, this has been in the DIY T2 build from the beginning and has not caused any issues.  From a volume perspective, it's not particularly loud and I can have the headphones on my head when I power down without discomfort.

[mwl168]

@ Kevin and Kerry; 

with all these great work and progress being made, will there be a second generation T2 soon that makes use of these advancements and current production parts and maybe a bit smaller in size? I would be happy to organize a GB for it.

[spritzer]

2 hours ago, Kerry said:

Not to my knowledge.  I believe it brings the stators to a more positive voltage (closer to the 580V bias).  AFAIK, this has been in the DIY T2 build from the beginning and has not caused any issues.  From a volume perspective, it's not particularly loud and I can have the headphones on my head when I power down without discomfort.

The thump shouldn't do any damage but it might be a good idea to follow the original and add some relays to the output.  Stax used a 4PDT Omron relay but it is rated to nowhere near the specs we really need. 

[JoaMat]

If running short of 2sj79 one could rebuild the T2 to something similar to a Grounded Grid. I removed j79s, 750R resistors, a1486s and corresponding LED. Works/ed like charm. Unfortunately right channel started to obstruct after a while. Perhaps this kind of modification isn’t appropriate.

For the time being I run left channel to both sides.........

Edited May 6, 2018 by JoaMat

[GeorgeP]

Joachim, let me know if you need any j79s, as I have a healthy supply.

[spritzer]

Same thing here, I have thousands of them. 

[bui501]

1 hour ago, spritzer said:

Same thing here, I have thousands of them. 

Now I know who the hoarders are -- and why they're so expensive from reputable vendors -- GRRRR  

[JoaMat]

Thanks a lot George, but I actually have a small stock of j79/k216 myself. Living in the middle of nowhere you need to be prepared.

Have been socializing with the faulty channel today and we are now both happy again. Maybe it’s more of a Grounded Grid with dual tube input and active batteries than a T2.

Picture below shows the modded T2. No j79/a1486 at the output stage. Upper right the middle a1486 is missing – replaced with resistor so upper tube grid voltage is set by a voltage divider. If you have a well defined supply voltage you don’t need to send 2.5mA through a 27K resistor – I think.

Edited May 7, 2018 by JoaMat

[Kerry]

Interesting.  I want to play with that a bit in spice 

[JoaMat]

My T2 PSU differs some from the original DIY T2 PSU as I have a -560V section which is as capable as the -500V. So with this modification the only work the -500v is doing is to set the 6CA7 G1 potential.

LTspice simulation I’ve done indicates that -500V at G1 might be desirable. But I don't trust myself doing this kind of work in LTspice.

Below schematic: removed Q24,Q25,Q28,Q29,LED, R48,R49. Added jumpers between drain k216 and cathode 6CA7.