The ultimate DIY? A Stax SRM-T2!

[ibuski]

1 hour ago, kevin gilmore said:

ciss of 1350pf

work, yes. sound good, unlikely.

Thanks for quick confirmation.  Is 1350 good enough?  Now I find K216 has Ciss only 90 pF, more than 10 times small.  I don't know it's good or not though.

[Rinat]

3 hours ago, ibuski said:

Hi, I read Kevin mentioned in the past that KSA2690AYS (160V capacity) works fine as substitute for K216 except pin layout.  They are cheap on Mouser.  BTW KSA1220A can be used as J79 replacement, but it's not available.    Then I learned that J78/K215 (180 V capacity) or J77/K214 (160 V capacity) should work as well, and they are pin compatible with J79/K216.   Do I understand anything wrong?   They also aren't popular, but maybe easier and slightly cheaper than J79/K216.

I have a bunch of 2sk214 from bdent and their Vbr is measured as 220-240v by DY294. This is sudden for me, because I have the same result with 2sk216s. 

But I still afraid to use them as a replacenment because in other conditions, like temperature, Vbr could be lower than it was measured by DY294. 

But maybe I'm just too scared to burn something expensive 🙂 

[Rinat]

49 minutes ago, ibuski said:

Thanks for quick confirmation.  Is 1350 good enough?  Now I find K216 has Ciss only 90 pF, more than 10 times small.  I don't know it's good or not though.

Lower capacitance is better. 

And in this context I want to ask another question to all: I want to use AlN thermal pads for the transistors with 1mm thikness instead 1.6mm AlO.

It adds a few picofarads to a capacitance between a metal tab and the ground. Is it ok ?

Edited December 25, 2021 by Rinat

[JoaMat]

On 12/25/2021 at 2:25 PM, ibuski said:

...
JoaMat also confirmed that Ksa1220A worked for his BH and DIY T2, but probably with lower voltage setting.
...

A few years ago I replaced 2sj79 and 2sk216 with ksa1220 respective ksc2690 on both Blue Hawaii and T2 and that without changing any voltages.

Below chart show voltage cross source and drain on T2´s Q26,Q27 during power on, 30 second wait and power off.

So far I haven’t had any ksa1220 or ksc2690 blowing up.

Today I reduced voltage further on my T2 PSU, 300V section now 200V. Seems to work as good as with 300V. High voltages are now +/-400V, +220V, -260V and -460V. The reduce voltage also lowered the maximum voltage on graph above to 200V

[JoaMat]

Reduced “300V section” a bit more, to 150V. Now the high voltages are +/-400V, +220V, -310V and -460V.

Graph below shows voltage cross 2sj79 (Q24/25) in blue and cross 2sk216 (Q26/27) in red during power on, wait 30 seconds and power off.

 

I can’t hear sonically differences between original voltages and reduced voltages.

[jamesmking]

A tung-sol el34 in my mostly modern DIY T2 goes SPARK SPARK white lightshow! 😬

hi everyone, until Thrsday my diy T2 has been working fine... I put in a matched quad of tung-sol el34 in September 2021 and on Thursday as soon as i switched on I got a loud sparking noise from the right headphone. I immediately cut the power. I installed a blue hawaii and checked my sr007s where not damaged and they seem fine...

I powered up just the T2 psu - all voltages good, stable etc. I powered up the amp with heaters and low voltages only - all looked good. I powered up the entire amp on a variac and at 50% of normal mains voltage and one of the el34s had bright white sparking coming from deep inside, visible through the rectangular cutouts in the plate.. I cut the power immediately and tested all the valves on my avo mk4 valve tester. All measured good and fine expect for the valve with the sparking which showed no leakage when cold but when heated the insulation test meter started varying all over the place i.e. not constant leakage and swing to very low insulation resistances <1Mohm and back to about a few Mohm. I replaced just the failed el34 with another known good el34 and powered the amp up again on the variac. I got to full mains voltage without any sparking but only tested for a minute or so. I checked the + and - outputs and Dc balance and offset was fine given the short warm up period.

So I think the valve failed rather than the T2 has an issue and damaged the value. Obviously I need to do more testing but my questions are:

1. is anyone else using sung-sol el34s on a t2 at full voltages?

2. anyone had a el34 fail hot heater insulation tests with the insulation meter jumping all over the place, rather than a constant leakage?

3. does anyone know if there is an issue using tung-sol el34s in the T2

4. any idea if there are any mostly modern T2 components I should pay special attention to that could have been damaged by the sparking?

regards and happy T2ing

 

 

 

 

Edited January 15, 2022 by jamesmking

[Kerry]

Not on a T2, but about 10 years ago I had a similar situation with an EL34 on a Blue Hawaii.  Spectacular lightshow from one of the tubes.  No harm done to the amp, so after replacing the offending tube, everything was fine.  I'd don't recall what brand tube it was, but either JJ or Mullard new issue.

I haven't had any issues since then.

Hoping all is fine, though it sounds like it is.

[Rinat]

3 hours ago, jamesmking said:

A tung-sol el34 in my mostly modern DIY T2 goes SPARK SPARK white lightshow! 😬

hi everyone, until Thrsday my diy T2 has been working fine... I put in a matched quad of tung-sol el34 in September 2021 and on Thursday as soon as i switched on I got a loud sparking noise from the right headphone. I immediately cut the power. I installed a blue hawaii and checked my sr007s where not damaged and they seem fine...

I powered up just the T2 psu - all voltages good, stable etc. I powered up the amp with heaters and low voltages only - all looked good. I powered up the entire amp on a variac and at 50% of normal mains voltage and one of the el34s had bright white sparking coming from deep inside, visible through the rectangular cutouts in the plate.. I cut the power immediately and tested all the valves on my avo mk4 valve tester. All measured good and fine expect for the valve with the sparking which showed no leakage when cold but when heated the insulation test meter started varying all over the place i.e. not constant leakage and swing to very low insulation resistances <1Mohm and back to about a few Mohm. I replaced just the failed el34 with another known good el34 and powered the amp up again on the variac. I got to full mains voltage without any sparking but only tested for a minute or so. I checked the + and - outputs and Dc balance and offset was fine given the short warm up period.

So I think the valve failed rather than the T2 has an issue and damaged the value. Obviously I need to do more testing but my questions are:

1. is anyone else using sung-sol el34s on a t2 at full voltages?

2. anyone had a el34 fail hot heater insulation tests with the insulation meter jumping all over the place, rather than a constant leakage?

3. does anyone know if there is an issue using tung-sol el34s in the T2

4. any idea if there are any mostly modern T2 components I should pay special attention to that could have been damaged by the sparking?

regards and happy T2ing

 

 

 

 

I feel for you and could only imagine how it feels to see such a lightshow (

4. I would check if everything is ok with Q24/Q25, just in case. 

 

And a question about your version of PSU: could I replace KSC5026 with 2SC3675 ?  I have hundreds of the latter. And there is nowhere to buy the first ones(. I did not make calculations yet. 

Edited January 15, 2022 by Rinat

[JoaMat]

8 hours ago, jamesmking said:

... I powered up the entire amp on a variac and at 50% of normal mains voltage ...

Is the T2 working at 50% of the mains?

Edited January 15, 2022 by JoaMat

[jamesmking]

1 hour ago, JoaMat said:

Is the T2 working at 50% of the mains?

50% of 240Vac was as far as I got turning up the variac before the valve sparked.... I suspect because it took that much voltage to fire the HV delay relay and as soon as the hv was applied spark city...

I replaced the valve I got to 100% mains before testing DC offset and DC balance but only ran the amp for a minute or so at full hv, It was already 1am so I did not do any further testing.

Sorry rinat, my psu has KSC5026 in it. I have not tried the 2SC3675. 

Edited January 15, 2022 by jamesmking

[JoaMat]

Thanks James, my clock always tends to speed up when an amplifier needs attention.

I have never used a variac when testing an amplifier. That’s way I asked.

[Rinat]

33 minutes ago, jamesmking said:

Sorry rinat, my psu has KSC5026 in it. I have not tried the 2SC3675. 

Ok, thanks. I'll calculate if the replacement could be done. 

[audiostar]

27 minutes ago, JoaMat said:

I have never used a variac when testing an amplifier. That’s way I asked.

Me neither (using the incandescent light bulb tester) but James is a Variac specialist 🙂

[jamesmking]

6 hours ago, JoaMat said:

Thanks James, my clock always tends to speed up when an amplifier needs attention.

I have never used a variac when testing an amplifier. That’s way I asked.

I find a variac really useful.

with one you can slowly bring up the psu not connected to the amp and see if there is strange behaviour and note

what minimum input voltage is required to maintain no load regulation. You can then compare this value when under load. You can look for rails that come up more slowly than the others etc. I also find that if there is a short the variac will make a loud hummmm/buzz even at low voltages e.g. 1/4 or less full mains AC and this can act as a warning.

testing update. So far so good. been at full power for about 1 hour and no issues yet. All psu and virtual battery voltages, DC offsets and DC balance look good. So this looks like I had the same experience as Kerry.... I am currently measuring the DC operating points of all values and comparing. So far looking within 1-2% between channels and + and - halves of a channel.

update All DC operating points of all values look sane and similar. *massive sigh of relief*

update listening to music through the amp all seems good. I am almost certain it was a isolated valve failure and not caused by the amp. Its so nice to have the T2 working again. there is just something magic about the way it sounds.

 

Here are the measured operating points all measured against ground.

input 6922 (furthest from the el34s)

Heater ~0V to ground

Anode 71V

Cathode approximately 1.6 to 1.7V

Screen 0V

Grid 0V

same measurements for second triode in the envelope

 

6922 closest to the el34s

heater 0V to ground

anode 197V

cathode 71V

screen 0V

grid 67V

same measurements for second triode in the envelope

 

EL34s all 4 should measure approximately the same

Heater -500V to ground

anode/plate   nominally 0v but varies under measurement and matches with any dc offset in the amp

G2, G3 same voltage as anode/plate

cathode -452V

G1 -499.5V

 

 

 

Edited January 15, 2022 by jamesmking

[audiostar]

The light bulb protects from any shorts, limits the current as well. The Variac would not really protect from shorts, but it allows for testing 115 volts nicely (I have a step down transformer for US mains).

[jamesmking]

17 minutes ago, audiostar said:

The light bulb protects from any shorts, limits the current as well. The Variac would not really protect from shorts, but it allows for testing 115 volts nicely (I have a step down transformer for US mains).

I agree a variac is not a substitute for a dim blub tester. I use a variac because if something is going to go wrong I would rather it go wrong at lower voltages than full to limit the damage.

[JoaMat]

Man or mouse? I always start with the Driver.

https://www.youtube.com/watch?v=J2RoYgRhXjw

 

[ibuski]

12 hours ago, Rinat said:

And a question about your version of PSU: could I replace KSC5026 with 2SC3675 ?  I have hundreds of the latter. And there is nowhere to buy the first ones(. I did not make calculations yet. 

Hundreds of 2SC3675?  I'm drooling!   Have never found such huge stock of "real" C3675 even in Japan.   We know one shop shows hundreds of stock, but there is rumor they are not really....healthy.   Original T2 requires 28 pcs. of C3675, right?   How many T2s are you building?

I understand that the KSC5026 takes current through KSA1156.  When comparing three Tr, C3675's Ic looks low.

KSA1156: Ic=0.5A, KSC5026: Vceo=800V, Ic=1.5A, 2SC3675: Vceo=900V, Ic=0.1A

However there is 50k Ohm resister, thus the current is limited to 500 V/ 50k Ohm = 10mA for two Transistors, isn't it?   I see no issue to use C3675.   On initial GRHV design, there were STN0214.

[Rinat]

4 hours ago, ibuski said:

We know one shop shows hundreds of stock, but there is rumor they are not really....healthy.

I've asked @ibuskiin PM and no - I've bought these transistors from another and more reliable source, the current official dealer of Onsemi. 

I've measured them with DY294 and tried to build curves up to 750v. Everything is ok. I've got them in a factory package with stamps, date codes, etc. 

Before I've made a payment they said that these transistors have some discoloration due to long storage. But actually, they look better than the ones I got from bdent.com

🙃

Edited January 15, 2022 by Rinat

[Rinat]

On 1/15/2022 at 7:48 PM, ibuski said:

However there is 50k Ohm resister, thus the current is limited to 500 V/ 50k Ohm = 10mA for two Transistors, isn't it?

It seems that current is limited to 0.20 ma. Look at the voltage across the 50k R12 resistor. It is 10v - Vbe(0v min). 10/50000 = 0.00020. 

So, it looks like 2sc3675 will work perfectly here.  

Edited January 17, 2022 by Rinat

[JoaMat]

Well, today I swinged my driver really hard and hit and killed +250v and -500V sections. Here are the casualties…
 
  …RIP.

While repairing I also replaced ksc5026 in 250v section with 2sc3675 and that seems to work all right. Can you find them?

By the way – the first version of KGSShv power supply used 2sc3380 along with 2sa1486, later replaced by ksc5026 and ksa1156 plus a Zener diode.

[JoaMat]

The above misfortune was due to a battery problem in T2 amplifier. I replaced a mosfet j112(2sk246) and left the 10K trimmer in max position. My guess (I’m just an amateur but this is my theory) is the battery with the new mosfet tried to set voltage to more then 100V lower than expected. Resulting in damages to Q5, Q23, Q26 and D23 along with damages to the power supply.

In order to find out what voltage cross 22R resistor R42 I put 27V from lab PSU from R42 to RV2 with 10K resistor in series.

When measuring voltage cross 22K resistor of each battery I got 1V lower value at the battery with the new mosfet then the other three. That’s why I think there was more than 100V lower voltage at that battery.

Now I have gotten really tiered of led batteries so I removed them and replaced them with those daughter boards.

The amplifier has only modern components. To my ears it now works as good as the original DIY T2.

[Kerry]

A nice benefit of the modules is that you can set them to 740v before you install them in the amp. 

I’m glad everything is up and running again. 

[audiostar]

8 hours ago, JoaMat said:

 

If I were to build the T2 again, I would put as much (lower hight) components as possible on the top side of the board for better cooling, at least all the 0.5w Xicon resistors, in addition to the LEDs.

Very nice work on the modular batteries. Surely the way to go nowadays. 

 

Edited January 17, 2022 by audiostar

[JoaMat]

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Edited January 18, 2022 by JoaMat