The ultimate DIY? A Stax SRM-T2!

[JoaMat]

The Drum Major leads the seven LEDs.

[Kerry]

Are you still using the jfet / trim pot?

[JoaMat]

No, trimmer is replaced by a resistor long time ago. The 10m90s current source is connected to ground and set to 1mA.

Today I modified the input tubes current sources with a trimmer like T8000. Also changed the servo, using single OP tied direct from output via resistor to one of the lower tube cathode. One channel had more than 200 volts unbalance with no servo and both batteries set to 740 V. With the trimmer I managed to get outputs balanced – have been within +/- a couple of volts for half an hour now (servo disabled)….. to be continued….

Edited December 3, 2017 by JoaMat

[JoaMat]

Finally I did notice……  Four Bugle Boys.

Ref. link

Nice!

[joehpj]

On 2017/12/4 at 12:37 AM, JoaMat said:

No, trimmer is replaced by a resistor long time ago. The 10m90s current source is connected to ground and set to 1mA.

Today I modified the input tubes current sources with a trimmer like T8000. Also changed the servo, using single OP tied direct from output via resistor to one of the lower tube cathode. One channel had more than 200 volts unbalance with no servo and both batteries set to 740 V. With the trimmer I managed to get outputs balanced – have been within +/- a couple of volts for half an hour now (servo disabled)….. to be continued….

Was the trimmer in series of two 240R?

[JoaMat]

No, schematically like this: added RV3 and R60.

I got it balanced when current was 5.1 respective 5.8 mA.
When upper and lower tubes changed place I had to reset back to original setting – the unbalanced like of disappeared.

[JoaMat]

This is how the small board with trimmer and resistor looks like.

I do think the T2 amp board by Kevin is a very good one. Easy to build, just put the right component in right position, solder properly and then you are done. If something goes wrong it’s possible to work on the board. And it's also a terrific board for experiments, tests, modifications…...

[joehpj]

What value of pot should be used? Kevin said he always use 10k but I wonder if it should be higher because of tube input?

[JoaMat]

If the pot you are asking about is same as RV3 in schematic a few post earlier then;

• With 240R as in the original DIY T2 the current theoretically is 5.1 mA.
• Adding 10K pot and 1K resistor with pot at mid position give 5.2 mA each CCS.
• Pot at end point gives you 5.1 respectively 6.2 mA.

Just increasing the pot value won’t change the “current range”.

Edited December 30, 2017 by JoaMat

[joehpj]

Oh I was asking about the input attenuator. I used 10K for Carbon, GG which was JFET input. Should I increase to 25K or 50K for tube input? 

And thanks for the information! 

One more thing. Was the R65-R68 on the PS schematic used to keep the filaments grounded? I wanted to adopt the T8000 way of regulated input tubes filament power supply. So I will have to remove the two 47R of 6.3v and keep the rest two tied to 0V, right?

Edited December 30, 2017 by joehpj

[JoaMat]

Oh, sorry. Too much Amarone.

Happy New Year to you all!!!!

[joehpj]

Studied T2 schematic and found filament of EL34 was connected to -500V. Is there any benefit to separate -500V from AC in order to reduce AC noise? I was thinking floating the filaments just like GG and BH.

[spritzer]

The B- is way higher than the cathode to heater level so it is impossible to separate them. 

[GeorgeP]

Just checking if there is any interest in the original Kevin Gilmore boards. The boards fit the original KG DIY-T2 chassis perfectly (group buy for chassis was a little while back). Will be using Custom Circuit Boards. Not looking to do a big group buy - just looking for 2-3 additional interested folks to get pricing to be reasonable (for 6 boardsets it is $204.16/set, for 7 it is $189.30/set).

Will be placing the order in a week or so. So just shoot me a pm if you want a boardset.

Edit: Order placed.

Edited January 23, 2018 by GeorgeP

[luvdunhill]

Still interested in another trafo set, if that were to happen or if there are extras.

[GeorgeP]

Yeah, that has fallen aside a bit.

Would SumR be ok with you, because I was originally thinking of doing one with Paul Houlden, but apart from the high shipping cost (I recalled he wouldn't ship more than two sets in one box from England to Canada), I did a buy with some other folks a couple of years ago and we were about 6 sets or so and the savings was only about $15-20/set. As well, I had him make me a traffo for the Carbon that became the loudest buzz monster I ever heard after a couple weeks of use. That said, I never had any issues with his T2 traffos, so don't know what to make of it.

Edited January 17, 2018 by GeorgeP

[JoaMat]

On ‎2017‎-‎12‎-‎03 at 5:37 PM, JoaMat said:

No, trimmer is replaced by a resistor long time ago. The 10m90s current source is connected to ground and set to 1mA.

Today I modified the input tubes current sources with a trimmer like T8000. Also changed the servo, using single OP tied direct from output via resistor to one of the lower tube cathode. One channel had more than 200 volts unbalance with no servo and both batteries set to 740 V. With the trimmer I managed to get outputs balanced – have been within +/- a couple of volts for half an hour now (servo disabled)….. to be continued….

Two months have elapsed. The modifications work alright. Almost forgot what I’ve done.

Schematic below shows the difference between the servos.

Edited February 3, 2018 by JoaMat

[JoaMat]

I’ve two PSUs for my electrostatic amplifiers. The DIY T2 has 500 volts rails so I’m feeding all my amplifiers with 500 volts. Now I’ve been thinking of building a new PSU with 400 volts. From what I understand the only reason to go higher then 400 volts is if you really want high sound levels.

Today I’ve brought down the T2 PSU to +/- 400 V and -460 V (the -560 V in my PSU is produced by its own regulator connected to ground). The only modification on T2 amplifier needed was to change the battery voltage from 740 V to 640 V – swapped the 4.7K resistors to 6.8K. Since I use offset servos similar to the opto servo Kevin introduced in his designs a couple of years ago I didn’t have to change anything else.

[luvdunhill]

Cool! The servo changes documented in the above post and that’s it?

 

[JoaMat]

Above post show changes to the balance servo I’ve made.

Below schematic shows changes made to get the KGOOS (Opto Offset Servo)  -  the only offset servo you need.

Q33 and R73 replaced by a small daughter board and the resistor strings changed accordingly.

For reference here is t2schemop.pdf, schematic of the STAX T2 output section. Drawn by Kevin Gilmore and redrawn by Linear.

Edited April 5, 2018 by JoaMat

[luvdunhill]

Interesting, I added a simple trimmer here instead. Would these modifications work with the default batterys? I would think so. I am tempted to try these changes out [emoji4]

 

What working voltage is needed for the 10uF cap and the one in the same position for the previous schematic (feedback/integrator cap)?

[Laowei]

/

Edited April 5, 2018 by Laowei

[bui501]

(1) One of Kevin’s later boards (t2shrinkedv10) has stn9360’s shown in the layout as TO220 parts, but when I did a search for stn9360 on mouser (and digikey), the part shows up as an smd component... does the stn9360 still come in non-SMD format? If so where is a reliable source? If not, what is a suitable replacement?  Can I mount the smd version on a smd-to-TO220 adapter board and use the smd parts, or are they not equivalent components?  I can’t find any data sheet for a non-smd version of the stn9360, so I do not know what the specs are.

(2) the board has a number of parts with leads rather close together with no outlines and with just a simple number by each pair of pads.  Are those resistors to be mounted vertically?

thanks for any helpful guidance.

 

 

 

Edited April 5, 2018 by bui501

[GeorgeP]

hmmm, I thought that that boardset was a work in progress and never completed... maybe Kevin will recall.

[bui501]

On 9/11/2015 at 5:12 PM, Kerry said:

I do have a BOM for the original build.

PM me and I can email it to you.

In other news, I've been working on a shrunken version of the T2 inspired by Joamat.  The bigger challenge was the PS which I've just gotten some boards back from manufacturing.  Lot's of SMD parts especially on the bottom of the board.

I've also included some 3D CAD drawings of what I think the chassis will look like.  The PS board is 206mm (W) x 200mm (D).

The chassis will be 12"(W) x 14.25" (D) x 3.33"(H).  I'm using the new GR HV PS for all of the elements including the 60V supply.  12V supplies are still 7812/2912.  I've also included a 5V supply since I wanted to use the new Digital attenuator.  I should have parts next week and I'll show some more pics once I have the board built up.

This is going to be fun

Kerry, I know it’s been a while ago since you posted this, but would you mind posting the Ferber files and BOM for the compact T2 boards you made and referenced back in 2015? Or pm me a link to them? I’n Determined to build a DIY T2, and am trying to go compact if possible-preferably with parts that are still in production.