SRX-Plus Build Thread

[JoaMat]

Nice! I like them chassis.

[mwl168]

The chimp looks really immersed in the music...

[Blueman2]

I like the top transformer design as well.  Reminds me of my first SRX build (on the right).  Left one was my second where put trafo in the chassis.  

 

 

[tomislavkufrin]

On 3/19/2020 at 3:34 PM, mwl168 said:

The chimp looks really immersed in the music...

the chimp is re-listening his music catalog.

this forum is a bit like AA meetings. except, instead to stop, people pull you in to do more alcohol. 

[mwl168]

@Blueman2, what wattage rating resistors do you use for the voltage divider to elevate the front tube filament supplies?

thanks!

[Blueman2]

12 minutes ago, mwl168 said:

@Blueman2, what wattage rating resistors do you use for the voltage divider to elevate the front tube filament supplies?

thanks!

The current draw is minimal.  1/4W will do fine, though I recall using 1/2W.  

[mwl168]

Great! Thanks!

I thought 1/2W resistors should do too but looking at the SRX Plus shunt regulator PCB, the resistors look to be physically much bigger. That’s why I ask.

[tomislavkufrin]

What is the maximum voltage difference you can have between output + and - of a channel? 22-25 volts is still ok, or on the high side?

Whatever i do with the pots it looks like this voltage difference is just shifting, not even changing. minimum i can get is 21V. I do not have this problem on the other channel. When i tried to swap them, i could again zero the offset and +/-  on the "good" tube in opposite place. 

There are no sound issues but i would not like to damage the headphones.

[Blueman2]

Right now, my SRX is the most stable amp I own in terms of L- to L+, R- to R+, L-L+R-R+ to ground. All rock steady less than 1V.  But before I did the better tubes and heater elevation, I had your issues.  Sounds like the offset issue follows your tubes?  So I would definitely try another set of tubes.

As far as hurting your earspeakers, 20V between L- to L+ or R- to R+ does seem a bit high but I doubt it will cause any permanent damage.  But others can chime in on how dangerous this is.  I know I used my first SRX build at 20-30V offsets between -/+ and my SR-007 survived that.  It took about 2 weeks before a new set of tubes arrived, which solved the offset issue for me.  

What is your L- to GND and R- to GND voltage?  Are they OK? 

Edited March 23, 2020 by Blueman2

[tomislavkufrin]

22 minutes ago, Blueman2 said:

Right now, my SRX is the most stable amp I own in terms of L- to L+, R- to R+, L-L+R-R+ to ground. All rock steady less than 1V.  But before I did the better tubes and heater elevation, I had your issues.  Sounds like the offset issue follows your tubes?  So I would definitely try another set of tubes.

As far as hurting your earspeakers, 20V between L- to L+ or R- to R+ does seem a bit high but I doubt it will cause any permanent damage.  But others can chime in on how dangerous this is.  I know I used my first SRX build at 20-30V offsets between -/+ and my SR-007 survived that.  It took about 2 weeks before a new set of tubes arrived, which solved the offset issue for me.  

What is your L- to GND and R- to GND voltage?  Are they OK? 

right now R+ to R- is <1V and R+ or R- to GND is <1V. And tends to stay like this. When i measure during the day R+ or R- to GND is around 0 +-3V. 

L+ to L- is 25, 22V, which i try to split in reference GND. Lets say L+ to GND ~= 11V and L- to GND ~= -11V. That is like a best case. 

 

[mwl168]

A few things:

Offset and balance voltage will settle as the amp warms up. Are these voltage measure when the amp was cold or warm?

IME, high DC offset usually caused by poorly matched tubes. Swap the 6SN7 tubes between channels. If the high offset follows the tube, replace with better matched tube(s).

[tomislavkufrin]

16 minutes ago, mwl168 said:

A few things:

Offset and balance voltage will settle as the amp warms up. Are these voltage measure when the amp was cold or warm?

IME, high DC offset usually caused by poorly matched tubes. Swap the 6SN7 tubes between channels. If the high offset follows the tube, replace with better matched tube(s).

Warm, i measured 8 hours after start. Value looks stable, i did not notice it change too much. I just do not like the numbers.

I did exactly that. Same offset value, it just moved to the other channel. Next time i'll just buy everything matched.

 

 

[mwl168]

Also, check to see if your front end tubes are reasonably matched. If the 2 5K pots are centered (ones you use to balance the driver tube plate voltages) and the driver tube plate voltages of the two sections are more than 10V off then I would consider the tube does not have well matched triode sections.

You can center the 5K pots without removing them from the PCB.  

 

Edited March 23, 2020 by mwl168

[chinsettawong]

Don't worry too much about the offset 20-30V voltage.  It will not hurt your headphones at all.

[tomislavkufrin]

10 hours ago, mwl168 said:

Also, check to see if your front end tubes are reasonably matched. If the 2 5K pots are centered (ones you use to balance the driver tube plate voltages) and the driver tube plate voltages of the two sections are more than 10V off then I would consider the tube does not have well matched triode sections.

You can center the 5K pots without removing them from the PCB.  

 

I did not have problems balancing the driver tube plate voltages. Resistances are in 2500 +-200 ohm range. So, not to far off center. Looks like only that output tube is not matched well.

 

4 hours ago, chinsettawong said:

Don't worry too much about the offset 20-30V voltage.  It will not hurt your headphones at all.

Excellent, because i really like the sound

[jose]

The SRX-P's design is great for its simplicity. (In my experience with this amp) Most of the problems are caused by the tubes. 
We shouldn't be as reluctant to something as simple as tubes replacement. It is a simple and inexpensive test (a set of current production tubes is cheap)

[tomislavkufrin]

New tubes are in, 25V difference between + and - is gone. Works like a charm. 

Just for info, are there any other tube options that are not Tung Sol GTB, and not NOS?

[mwl168]

5 hours ago, tomislavkufrin said:

New tubes are in, 25V difference between + and - is gone. Works like a charm. 

Just for info, are there any other tube options that are not Tung Sol GTB, and not NOS?

That's great news! Are you able to get rid of the hum too?

Don't understand your question but all major tube manufacturers back in the days, RCA, GE, Sylvania, etc.,made 6SN7 GTA\GTB tubes. Looks to me most, if not all, current production 6SN7 tubes are of the GTA\GTB variant. Just check the plate voltage and dissipation ratings in the datasheet. 

JimL picked fairly conservative operating points for the output tubes of the SRX Plus but I would personally want the plate voltage rating of the tube to be 20% above the B+.   

Edited April 1, 2020 by mwl168

[tomislavkufrin]

13 hours ago, mwl168 said:

That's great news! Are you able to get rid of the hum too?

Don't understand your question but all major tube manufacturers back in the days, RCA, GE, Sylvania, etc.,made 6SN7 GTA\GTB tubes. Looks to me most, if not all, current production 6SN7 tubes are of the GTA\GTB variant. Just check the plate voltage and dissipation ratings in the datasheet. 

JimL picked fairly conservative operating points for the output tubes of the SRX Plus but I would personally want the plate voltage rating of the tube to be 20% above the B+.   

My father told me that he cannot hear anything, but i can hear a very faint hum. There is room for improvement in my heater wiring. And i need to check if my soldering of socket pins is good. 

Better to ignore that question, i got confused about nothing..

[tomislavkufrin]

Hm, it looks there was something else also causing the noise issues. Trafo itself was buzzing. I have removed the cover, now there is no noise on the trafo and seems that i cannot hear any noise on the headphones.. I will try again in the morning when there is no noise in the house but i think now it is silent. As i saw some scratch marks on the inside of the cover, it looks like the bolt was touching the cover and possibly making a shorted turn. Or i moved some wires and now it is magically better. Or both.

[mwl168]

Cased my SRX Plus last weekend at long last. Finally got down to adjust and test it last night. 

The PSU inside is the Kerry design from the recent GB with a high tension supply timed delay. 

 

[Blueman2]

Nice!!!  Did you mill the stops yourself or job that out to someone else?  

[mwl168]

1 hour ago, Blueman2 said:

Nice!!!  Did you mill the stops yourself or job that out to someone else?  

Cut all the round holes myself with a table drill. The AC socket hole and the ventilating slots came standard on the chassis.

[mwl168]

Since I started trying different tubes on my SRX Plus I've noticed that, while the amp is dead quiet with some tubes, some tubes will result in audible hum with no input signal present. After some reading and experiment I decided the hum is most likely caused by leakage current between the heater and cathode of the input tubes. I decided to elevate the input tube AC filament supply.

The photo below shows the filament elevation circuit I added this morning. The 12.6vac filament is elevated to about + 56VDC reference to ground. The amp with the same set of tubes went from a bothersome hum to dead silent (knock on woods)!   

[Blueman2]

@mwl168, I found exactly the same thing.  Once I elevated the heaters, the hum was eliminated completely.   12AT7s are at 60V (voltage divider off B+ rail) , and 6SN7s are at -340V (tied directly to B- rail).  In all, I did 4 things to make mine totally silent:

 

•  Used voltage divider to elevation the filament voltages.  +60V for 12AT7s and -315V (or in my case, I just tied it to B- so -340V) for the 6SN7s
• I switched to different set of tubes.  The ones I started with just hummed.  After switching, much less hum
• More careful grounding of components
• Finally, to get rid of slight hum when volume control was at 11:00 to 5:00 position, I switched from a 50K pot to a 10K pot, which totally eliminated any hum when the volume control was in the mid/high region.  

 

Edited August 15, 2020 by Blueman2