The ultimate DIY? A Stax SRM-T2!

[JoaMat]

R11 determines the current through LT1021-10 which needs 1.1 mA at 25 degree C. Choose R11 value accordingly.

For +/-500V sections; R11 = 400K gives 1.4mA and 0.8W.

Edited February 6, 2023 by JoaMat

[jamesmking]

7 hours ago, JoaMat said:

R11 determines the current through LT1021-10 which needs 1.1 mA at 25 degree C. Choose R11 value accordingly.

For +/-500V sections; R11 = 400K gives 1.4mA and 0.8W.

The original published gerbers has All the rails running at around 1.8mA through the R11 resistor except the +-500V rails at almost 2.3mA!  The original published gerbers has:

+250V rail (300V zenner string) using 160K so 1.875mA, 0.56W

-260V rail (actually a +300V supply (with 350V zenner string) ) using 200K so 1.75mA, 0.61W

-560V rail (600V zenner string) using 330K at 1.82mA, 1,09W 

+-500V rails (550V zenner string) using 240K almost double the required current at 2.29mA... and around 90C+ temperature with the lid off dissipating 1.26W...

so if we set a target of around 1.4mA to provide a little headroom and go for values in the vishay PR03 resistor range available at mouser or radio spares:

+250V rail 220K (available mouser and radio spares) for 1.36mA 0.41W 

-260V rail 240K (available mouser) for 1.46mA 0.51W   or 270K (available radio spares) for 1.3mA 0.45W

 -560V rail 430K (available mouser) for 1.395mA 0.84W or 470K (available radio spares) for 1.28mA 0.77W

+-500V rails 390K (available mouser and radio spares) for 1.41mA 0.78W... that's half a watt reduction in dissipation 🙂 compared to the original published gerbers.

These lower dissipations might even allow the use of Vishay PR02 2W resistors instead of the 3W... time to update the gerbers and schematic again. I will update my gerbers and schematic post.

 

 

Edited February 6, 2023 by jamesmking

[JoaMat]

Well, it’s almost 10 years ago I made the “original layouts”. Just found another version of a layout on my computer from that time. A part of it shows (in ugly yellow circle) a RES90 resistor with text "Resistor I=1,3mA". There is also room for three standing resistors instead of one power resistor.

[bbest]

On 2/6/2023 at 4:22 AM, jamesmking said:

Here the latest T2 psus, I have modified the layout to provide more space around the rather hot R11 3W resistor and added vent holes to the pcb. The pcbs, transformers etc still fit inside a 400mm deep case without issue.

...

This pcb is 220mm wide by 137mm deep and contains the -260V, -560V and +250V rails

 

 

tiny mistake: FQPF8n80s -> FQPF8n80c. But anyway, FQPF8n80c completely obsolete. May be it make sence to change it on FQP8n80c.

PS: compared to previous gerber version

fixed wrong silk for IXCP10M90S: KAG -> GAK, good 👍

Edited February 11, 2023 by bbest

[bbest]

some intermediate results of PSU (followed by jamesmking assembly)

Used 0.5mm transformer wires for AC HV (red ones): compact and reliable.

[jokerman777]

Hi, I'm slowly sourcing parts for a T2 build and wondering if 2SA1627 will be a fine replacement for the 2SA1486s that are not mounting to the heatsinks (those I will just use KSA1156)? Seems to have mostly the same ratings other than only 1W of maximum dissipation, would make my life easier if it works comparing to using the SMD STN9360...

Thanks! 🙂

[JoaMat]

I think 2sa1627 will work.

Someone have to test it. Let us know how it turns out. 

[jokerman777]

2 hours ago, JoaMat said:

I think 2sa1627 will work.

Someone have to test it. Let us know how it turns out. 

Thanks! I will give them a try and report back, tho won't be able to find out till after a good while.  😀

[JoaMat]

DIY T2 amplifier with 300B tubes.

Amplifier is built on Kevin’s board. Only modification is decreased high voltages, +/-400V instead of +/-500V. 

Homemade 300B to EL34 adapters connected to four Traco 5V switched PS for filament power. 

I only replaced the EL34 with 300B along with the filament power supply. No other adjustments. It works – I’m in no harry to change back to EL34.

[JoaMat]

I 3D printed a cover to my filament power supply today.



Just four Traco TPP 15-105-D and some tblocks. I don’t think my tubes need slow start – and if they do – they are not my tubes.

[JoaMat]

Soft start would be nice...

…if it works. Not sure it will, but I know a way to find out.

Above is based and inspired by this:

 

 

 

Edited April 23, 2023 by JoaMat

[Rinat]

On 3/29/2023 at 12:09 AM, jokerman777 said:

Hi, I'm slowly sourcing parts for a T2 build and wondering if 2SA1627 will be a fine replacement for the 2SA1486s that are not mounting to the heatsinks (those I will just use KSA1156)? Seems to have mostly the same ratings other than only 1W of maximum dissipation, would make my life easier if it works comparing to using the SMD STN9360...

Thanks! 🙂

I use 2SA1413 (It is the same transistor in a different package). 

Edited April 27, 2023 by Rinat

[simmconn]

Some bench notes on the DIY T2 - not a builder, just a tinkerer...

T2_AB.pdf

[JoaMat]

On 4/23/2023 at 2:01 AM, JoaMat said:

…if it works. Not sure it will, but I know a way to find out.

Yep, it seems to work.

So far so good. No heat problems. Current set to 1.5A and when filament voltage reaches 3.7V solid state relay closes.

[JoaMat]

Now I think I’ve a 300B filament power unit I like.

All smd on bottom side and common (through hole) trimmers and LEDs, and the Tracos of course on top side. With trimmers voltage at tube pins is easily set to 5.00 +/-0,05V. There is voltage drop cross wires and solid-state relays.

Cover with ventilation slots. The Tracos produce some heat.

[simmconn]

What’s the inter-winding capacitance look like on the DC-DC modules? That together with 300B’s mu of only 3.9, the circuit would have to work a lot harder to get the output swings the EL34 is capable of.

[demonkuro]

Hi guys, I spent 2 months building a T2 and today was expected to be the day of completion, but it exploded. When the power supply is not loaded, everything is fine and the correct voltage can be output. However, I connected the amplifier and turned on the power, and after 6 seconds of high voltage start-up, the power supply exploded. 10M90S I get it from MOUSER, it should not be a fake. I set both RV1 and RV2 in the center. The explosion killed the  10M90S，fqp8n80c，1N4007 and 1n4751a at +500V, and the same situation over at -500V. Did I do something wrong?Why did it explode？Expect someone to answer,THX

 

 

[Rinat]

Have you checked the output voltages without a load? 

[simmconn]

Please check the wiring from the PSU board, through the connectors and umbilical cord all the way to the amp board. If a PSU regulator output is shorted to GND or to another power rail due to a wiring mistake, you’ll see the explosion. Check every component in the PSU as some may look okay from the outside but may have been damaged already. I know it could be heart sunken but that’s what you need to do to bring it back to life. Btw I see you didn’t use the ceramic insulators on the PSU transistors. Although it may not be the cause of the failure this time, the ceramic insulators are recommended over the silicone ones.

[jamesmking]

I am sorry for your loss. Its difficult to evaluate what could have gone wrong, if the psu worked without being connected to the amp then the possibilities are bad wiring between psu and amp board(s) or an issue with the amp board(s)...

 

generally it is a high risk strategy to power up the entre amp to full voltages immediately...

during building I hand measure *every* component to make sure it is what I think it is. I have once received different transistors from what I ordered (from a reputable supplier) in bags marked with the components I thought I was getting... turned out to be a picking error... Putting them through a component tester alerted me to the fact they were not what I was expecting. 

I use an insulation tester to verify there is no leakage between wires within the cable(s) that go from psu to amp. I use a pat tester to verify no leakage from transformer(s) to the psu case no leakage primary to secondary within the transformer(s). 

before power up I check there is no continuity between any metal tab on the transistors and the L brackets that connect to the heatsinks. I also check the ground wiring very carefully to make sure the amp pcb(s) amp pcbs share a common ground point with all the psu pcb(s)

after that I connect the psu to one amp channel only (if possible) and check for continuity between each voltage rail on the psu and the appropriate voltage input on the amp board. This makes sure the wiring to the psu socket, the cable from psu to amp and amp socket to amp pcb is correct. I then do the same for the other amp channel. If the amp connects a voltage rail to the heaters of a valve, I also check for continuity between that voltage rail and the appropriate pins of the valve socket. 

I prefer to test the psu by itself and run it for several hours not connected to an amp. I bring the psu up slowly on a variac and monitor output voltages and their drift over time. I also use a thermal probe and (carefully) check for overheating components. 

When possible I power test up one amp channel at a time (again with the psu on a variac) and monitoring voltages on the amp board as I slowly increase the voltage to the transformers. If I don't see all the voltages increasing at about the same rate I abort because a voltage rail is being too heavily loaded - probably by a short.. If I hear the variac buzzing loudly I have a short and abort. I also monitor DC offset and DC ballance on the amp audio output.

If this goes well I leave the channel for an hour or so again periodically monitoring voltages, temperatures and DC offset, ballance etc. I then power down, disconnect that channel and test the other amp channel (if possible) before testing both amp channels at the same time.

After a few hours powered up I proceed to sine wave testing using a signal generator and monitoring using an oscilloscope. Connecting headphones does not happen until I am very sure the am is stable and reliable.

Its much slower this way but provides more opportunity for catching a fault early and less damage if a fault does occur.

 

 

 

[demonkuro]

7 hours ago, Rinat said:

Have you checked the output voltages without a load? 

Checked,all output voltages without a load work right

[demonkuro]

7 hours ago, simmconn said:

Please check the wiring from the PSU board, through the connectors and umbilical cord all the way to the amp board. If a PSU regulator output is shorted to GND or to another power rail due to a wiring mistake, you’ll see the explosion. Check every component in the PSU as some may look okay from the outside but may have been damaged already. I know it could be heart sunken but that’s what you need to do to bring it back to life. Btw I see you didn’t use the ceramic insulators on the PSU transistors. Although it may not be the cause of the failure this time, the ceramic insulators are recommended over the silicone ones.

Thanks to your advice, I will replace the connectors and cables, and replace the parts that have been damaged. The seller told me that this silicone insulator can withstand 2KV, I agree with you, it should not be the cause of this explosion.

[demonkuro]

7 hours ago, jamesmking said:

I am sorry for your loss. Its difficult to evaluate what could have gone wrong, if the psu worked without being connected to the amp then the possibilities are bad wiring between psu and amp board(s) or an issue with the amp board(s)...

 

generally it is a high risk strategy to power up the entre amp to full voltages immediately...

during building I hand measure *every* component to make sure it is what I think it is. I have once received different transistors from what I ordered (from a reputable supplier) in bags marked with the components I thought I was getting... turned out to be a picking error... Putting them through a component tester alerted me to the fact they were not what I was expecting. 

I use an insulation tester to verify there is no leakage between wires within the cable(s) that go from psu to amp. I use a pat tester to verify no leakage from transformer(s) to the psu case no leakage primary to secondary within the transformer(s). 

before power up I check there is no continuity between any metal tab on the transistors and the L brackets that connect to the heatsinks. I also check the ground wiring very carefully to make sure the amp pcb(s) amp pcbs share a common ground point with all the psu pcb(s)

after that I connect the psu to one amp channel only (if possible) and check for continuity between each voltage rail on the psu and the appropriate voltage input on the amp board. This makes sure the wiring to the psu socket, the cable from psu to amp and amp socket to amp pcb is correct. I then do the same for the other amp channel. If the amp connects a voltage rail to the heaters of a valve, I also check for continuity between that voltage rail and the appropriate pins of the valve socket. 

I prefer to test the psu by itself and run it for several hours not connected to an amp. I bring the psu up slowly on a variac and monitor output voltages and their drift over time. I also use a thermal probe and (carefully) check for overheating components. 

When possible I power test up one amp channel at a time (again with the psu on a variac) and monitoring voltages on the amp board as I slowly increase the voltage to the transformers. If I don't see all the voltages increasing at about the same rate I abort because a voltage rail is being too heavily loaded - probably by a short.. If I hear the variac buzzing loudly I have a short and abort. I also monitor DC offset and DC ballance on the amp audio output.

If this goes well I leave the channel for an hour or so again periodically monitoring voltages, temperatures and DC offset, ballance etc. I then power down, disconnect that channel and test the other amp channel (if possible) before testing both amp channels at the same time.

After a few hours powered up I proceed to sine wave testing using a signal generator and monitoring using an oscilloscope. Connecting headphones does not happen until I am very sure the am is stable and reliable.

Its much slower this way but provides more opportunity for catching a fault early and less damage if a fault does occur.

 

 

 

Thank you for your concern, before connecting the PSU to AMP, I let it work independently for several hours, all the output voltages are right, the drift is very small, and it looks very stable. So I confidently linked it to AMP, but nothing good happened... Just like the carbon I made 8 years ago, the PSU also exploded, and that time I replaced the damaged part and it worked fine. Thanks to your advice, I will go find an insulation tester to test each power supply. I should be more cautious.The PSU was tested today and luckily all worked fine except for -500V and +500V.Waiting for new parts and replace the wire with a higher withstand voltage rating.

[Helium]

8 hours ago, demonkuro said:

Thank you for your concern, before connecting the PSU to AMP, I let it work independently for several hours, all the output voltages are right, the drift is very small, and it looks very stable. So I confidently linked it to AMP, but nothing good happened... Just like the carbon I made 8 years ago, the PSU also exploded, and that time I replaced the damaged part and it worked fine. Thanks to your advice, I will go find an insulation tester to test each power supply. I should be more cautious.The PSU was tested today and luckily all worked fine except for -500V and +500V.Waiting for new parts and replace the wire with a higher withstand voltage rating.

Did you trim snap-in capacitors leads underneath PCB? Seems that PCB is quite low and may be touching bottom cover. Once I made same stupid mistake. Without load PSU was OK, but when I connected amp section, it exploded quite spectacularly.

Edited May 22, 2023 by Helium

[demonkuro]

1 hour ago, Helium said:

Did you trim snap-in capacitors leads underneath PCB? Seems that PCB is quite low and may be touching bottom cover. Once I made same stupid mistake. Without load PSU was OK, but when I connected amp section, it exploded quite spectacularly.

The PCB is 15mm from the bottom cover, they are not accessible, thanks for the reminder, that should not be the cause of the explosion