The ultimate DIY? A Stax SRM-T2!

[jamesmking]

The Gerbers for my T2 build.

My T2 has been working without issues for some months now with heavy usage almost every day so I feel reasonably confident about posting the gerbers I used. (see my previous build posts for more details and photos.). full credit goes to Kevin for the original design and joamat et al. for the modifications to modern components. I simply hacked up the gerbers to fit my build style.

The amp gerbers are slight modifications of Joamats mostly modern T2 (his build uses all available components except for 4 of 2sj79 and 8 of 2sk216). My modifications include the addition of screw terminals for all voltages, shortening to fit into a hifi 2000 disapante 2U 400mm deep case, split into separate channels, removal of the pads for the pot, wider clearance between components and the ground plane, instructions on the top side for the adjustment pots.

T2 left.zip   T2 right.zip

(note left and right are swappable I just like the 6922 valves at the front)

The power supply is also based on Joamats T2 psu, with a couple of error corrections, there was a missing track and a couple of incorrectly labelled components. also the solid state relay in the HV delay section is obsolete and I replaced the footprint with a standard relay.

minus and 250v cleaned .zip  cleaned t2 hv and lv bias and delay.zip

Feel free to use/modify/ignore.

I do find that in the power supplies the 3W resistor close to the film cap and zener ladder gets very hot ~90C on the high voltage tails. I have therefore modified the power supply boards to make them slightly longer and provide more space around this resistor and put some holes in the pcb under it for better air flow.

minus and 250v cleaned COMP -560v lengthened.zip  cleaned t2 kgsshv hv and lv bias and delay comp lengthened.zip

I have ordered a set of these lengthened boards but I have not received them yet or made a build using them so if you are going to use them please check them carefully in case I made a mistake.

regards and good building

James

P.S.

looks like next on the list will be a megatron build. 

 

Edited January 9, 2021 by jamesmking

[starcat]

Hey James, which one do you like more the T2 or the BH, you have build both, what is your experience during building and then listening to them? Cheers

[jamesmking]

On 1/16/2021 at 12:59 AM, starcat said:

Hey James, which one do you like more the T2 or the BH, you have build both, what is your experience during building and then listening to them? Cheers

I built a blue hawaii with golden reference HV supply and the simple voltage regulator based lv supply built into the golden reference hv board. Later I  added a golden reference lv supply which improved the sound a bit. I also replaced the diode bridge on the LV with a synchronous rectifier for even lower noise. The blue hawaii is a very good amp. Far better than anything I had listened to previously and was a quite straightforward build I did not modify any of the gerber files. I did solder the leds, the resistors you need to measure the voltage across to set the constant anode current and the adjustment pots onto the top side of the board for easy checking and adjustment. I managed to fit everything into one 400mm deep disapante case

The T2 build is by its nature almost twice the size of the blue hawaii and of course more expensive. The pcbs are bigger, there's more values and components and more power supply rails and multiple transformers etc. I also had to modify the gerbers to fit my build style, selected volume pot and the case I wanted to fit the amp into. The T2 is just magical and is my daily listening device. If you only have the time and resources to build one build the T2 and get the best to begin with. I don't regret building the blue hawaii but I have not listened to it since I built the T2. I found the T2 build to be big but not difficult. I read the forum posts carefully followed their advice and used xicon resistors and ultrasonically cleaned the pcbs. I also measured every component with a LCR meter and transistor tester etc before soldering them in and checked everything multiple times... I even tested the zeners and diodes and matched all components between channels and between + and - halves of a channel. I actually finished the pcbs about three weeks before I switched the amp on.. I got locked into a cycle of checking and rechecking everything and was convinced I would get sparking or noise batteries. So far I have had no issues at all with the amp and the initial fear I felt from reading the forum posts on failed T2 builds has dissipated. 

most importantly the T2 has 4 more glowing valves than the blue hawaii and so looks better in the dark... but then the megatron has even more valves

Edited January 24, 2021 by jamesmking

[starcat]

Thanks, James. So, next for you would be the Megatron? 

I am just getting PCBs and a case from GeorgeP for the T2. 

 

Anyone have some 34-40x  2sc3675 available for sale? 

[jamesmking]

7 hours ago, starcat said:

Thanks, James. So, next for you would be the Megatron? 

I am just getting PCBs and a case from GeorgeP for the T2. 

 

Anyone have some 34-40x  cavailable for sale? 

Glad to help starcat.

The Megatron build is in the early stages. Im taking this one slowly because I have been suffering from work related stress for a while now... and don't have the energy to work at my usual build rate. I have reverse engineered the gerbers, built my own circuit diagram and edited the gerber files to my build style. But have not ordered any pcbs yet. Worse im going to use golden reference HV supplies and the world seems to be out of stock of C2M1000170D pass transistors and I don't have any spares.

30 to 40 non fake 2sc3675 ..... wow.... I have a total of 6 in my parts inventory and they came from ebay, I tested them with a breakdown tester and they look ok but that's still no guarantee they are originals.

Spritzer on the forum helped me source the transistors I needed for the mostly modern T2 build (he runs a company building stax amplifiers and has a large stock of original genuine parts) but some of the transistors for an original t2 build are getting very very rare. You might want to private message him and see if he is willing to sell you any nos transistors you need. I only asked for very small numbers - just enough to build the T2 plus a spare set in case I messed up.

I have also replied to you diy t2 post with a breakdown of what the different versions of the T2 are and the gerbers for them. I assume you are going original T2 rather than the mostly modern version...

 

[JoaMat]

7 hours ago, starcat said:

Anyone have some 34-40x  2sc3675 available for sale? 

bdent.com has 83 pieces of 2sc3675. Grab them before they are gone.

Edited January 24, 2021 by JoaMat

[starcat]

On 9/28/2017 at 3:43 AM, Skooby said:

Are these valid DIY T2 tranny specs?

30-0-30/500mA, 2x6.3V/1.5A, 2x6.3V/4.5A

Two of 485V/150mA, 285V/60mA, 75V/60mA

Kevin states here that 475V are needed instead of 485V. 

Or is 485V a new update? 

[jamesmking]

59 minutes ago, starcat said:

Kevin states here that 475V are needed instead of 485V. 

Or is 485V a new update? 

mains line voltage varies by +-10% or more, transformer output varies with load 10Vac on 475Vac is not going to make much difference....

[starcat]

Anyone has used other .5 watt resistors instead of Xicon 273 or know which Vishay series is a direct replacement for Xicon 273?

Pretty much sourced all the transistors, except for the 8x 2sk216

[jamesmking]

5 hours ago, starcat said:

Anyone has used other .5 watt resistors instead of Xicon 273 or know which Vishay series is a direct replacement for Xicon 273?

using mousers parametric search I can't find any vishay resistors !% 50ppm 1/2w 350v working.

koa mf1/2 series are same ppm, working voltage and wattage and similar size.

https://www.mouser.co.uk/KOA-Speer/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/MF-Series/_/N-7gz41Z1yzvvqx?P=1yzbppyZ1z0zlrrZ1z0ynpy

I use them interchangeably with the Xicon. Xicon is far cheaper if you don't mind buying 100. koa is an option if you want to buy just a few. Per resistor koa is more expensive than the xicon.

 

vishay MBB02070C are close to the xicon 1% 50ppm 350V working but 0.6W

https://www.mouser.co.uk/Vishay/Passive-Components/Resistors/Through-Hole-Resistors/Metal-Film-Resistors/MBx-SMA-Professional-Series/_/N-7h7zb?P=1y97ob9Z1z0jjxqZ1yzrof0Z1z0x6pwZ1z0zls5Z1z0wljo

(I have not tried these resistors)

 

Edited January 26, 2021 by jamesmking

[starcat]

Piling up lately all parts for the absolute original DYI T2. When I say original I mean even in the Gilmore case. Last package come in today as well as the transformers.

 

Edited March 9, 2021 by starcat

[starcat]

Started with the PSU board, half through, made first test fit into PSU chassis in order to properly adjust and fix the brackets to the board and make sure everything fits perfectly. Up to the transistors for now...  

Edited March 9, 2021 by starcat

[JoaMat]

Proxxon microclicker – nice. What torque setting will you use for transistors at brackets?

[starcat]

Hey JoaMat,

this note from ONsemi with mounting considerations Craig posted earlier states 8 inch-Lb for TO-220 package which is about 0.9 Nm.

The only thing I fear is the metal ring on the holes of the board potentially touching the inox screw and also the bracket. Need to investigate as I do not want to use plastic screws. I am using these PPS insulation washers on top, might throw in one from below for peace of mind.

I am also using belleville disc spring washers from below to hold it safely in place.

Edited March 10, 2021 by starcat

[Pars]

I noticed that you didn't tap the holes in the bracket for the screws? Any reason? Seems better to me than using nuts on the bottom. Also, I don't believe the metal rings on most of Kevin's boards are connected to anything.

[starcat]

Fallacy - the bolt might be short to the bracket, but not the transistor! All good. 

No, I didn't tap the holes, feel like using a screw was easier and followed the ONsemi note. I can use a disc spring washer with the nut below as well. 

Edited March 11, 2021 by starcat

[Craig Sawyers]

Plastic screws cannot take the torque, even at the correct an surprisingly low value from the On-Semi apps note. Found that out having bought a shed load of PEEK ones, only to resort to stainless.

Don't forget a smear of heat sink grease between the package and ceramic washer, and between the washer and and bracket.

Where did you get the rectangular bush from? That is very neat.

[starcat]

Hi Craig, thanks. 

The rectangular bush is Aavid 7721-10PPSG (Mouser p/n 532-7721-10PPSG). It protrudes into the TO-220 package as well insulating the steel bolt from the hole inside. 

PS: Round for the downside (if used at all), same PPS material would be the Aavid 7721-3PPSG. 

Edited March 10, 2021 by starcat

[Craig Sawyers]

I like that washer a lot.

I used the 7721-3PPSG. That has a longer bush length than the 7721-10PPSG (3.18mm compared with 2.41mm). But 2.41mm should be just fine.

The downside of both our choices is the bush diameter of 3.56mm. The tolerance for a TO220 hole is 3.53mm to 4.09mm. Which meant that for some of my devices it was a really tight fit and needed to be pressed home. On other devices it just dropped in.

But they fit into the hole in the ceramic washer just fine.

[Craig Sawyers]

The only Aavid shoulder washer that is formally intended for the TO220 package is the 7721-7PPSG, but that has a really short bush length of 0.81mm, and will absolutely guarantee flash over. ISTR Kevin used those in development of the original T2 clone, and indeed got flash over. That must be buried somewhere in the early pages of this thread.

[JoaMat]

Be aware. Recommended torque for 10PPSG is 0.565Nm to  0.678Nm. See attached Page from Aavid-Board-Level-Heatsinks-Catalog-2018-1507171.pdf for reference.

I use this setting

...and I don’t know how to convert Ncm from Nm 😢

[starcat]

Thanks, Joamat. I have missed that Aavid note. Didn't start tightening the transistors yet, so that's right on the moment.

Nm in Ncm should be divided by 100, meter by centimeters. So 100 Ncm is 1 Nm and 82 Ncm is 0.82 Nm. Looks like the 10PPSG can withstand more pressure than stated. 

Btw, the Wiha is very nice too 🙂

Edited March 11, 2021 by starcat

[JoaMat]

To be a DIYer gives you the privilege to do what ever you like (maybe not to break federal laws). I think a proper torque considering the thermal aspect is more important than the recommendation by Aavid in this case. And the shoulder washer will stand a higher torque than recommended. 

Thank you, I like the Wiha a lot. It even counts the clicks.

[starcat]

Looking good. Thermal grease applied evenly on both sides of isolation pad (to transistor and bracket). Tightened with 0.82 Nm. M3 stainless steel mounting hardware used with disc spring washers underneath. 

[starcat]

I've got the BC556ABU for the PSU as this is what they had at Mouser, but understand the BC556C has a much higher hFE. Is the C version really needed (as it is used here only for the timer circuit)?

Found some BC556C at TME and could swap it out of course.

Edited March 14, 2021 by starcat