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Steps 1-3 and step 5 are correct.  Note that my design uses 250k plate resistors for the input stage cascode whereas KG'b board lists 300k resistors.  If you use 300k the input stage current should be 1.1 mA, if you use 250k resistors it should be 1.3 mA.  


Step 4 should be: set the INPUT balance to 0 VDC by adjusting the 5k pot.  You are trying to get the upper 12AT7 plate voltages to be the same.  An easy way to do this is to connect your meter leads between the "bottom" of the two 12AT7 plate resistors and adjust the 5k pot to zero volts.  


As for the post from 5/06/15, this is to balance the output voltages to close to zero.  Basically you do the step 5 adjustment which should get the outputs within 10 volts of zero if the 6SN7GTA is reasonably well balanced.  What this means is that one output may be +7 volts while the other is -5 volts, for example.  Honestly, this is close enough.  However, for those compulsive obsessives, if you want to get even closer you can adjust one or the other of the plate loads so the output voltage is the same for the plus and minus outputs, then re-adjust the current sink to zero everything.  Now, because you are dealing with tubes, the plate voltages are likely to drift by several volts over time even if you can get it zeroed- unlike solid state, it's not going to stay stable, but 10 volts or so is not going to make any difference whatsoever.  Furthermore, every time you turn it on the final voltage will be somewhat different, but usually only by a few volts.  So one time it may be +5 and -3, another time it may be -2 and -10, another time +4 and -4, and also over time during one turn-on cycle you may see all of those values during a couple hour period.  Usually the off set between plus and minus outputs is pretty constant but the actual value (with respect to ground) may vary.

Edited by JimL
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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. 

not the prettiest build, but i'm learning. goal was to make it work, and worry about the looks later. metal works was something new for me, you need to have good tools. these are my first two amps i h

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Many thanks for the detailed explanation JimL. Very much appreciated. 

I should have read your posts more carefully.

One more point of clarification; based on you post on May 22, 2015 on the SRX Revisited thread where you explained the input stage in detail, do I understand correctly, that the "lower tube" refers to the 12AT7 that accepts the input and the "upper tube" is the 12AT7 that drives the output stage?


Edited by mwl168
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I have been listening to my newly built SRX Plus for the past 2 hours or so. 

I am using a reconfigured KGSSHV PSU to supply the HV (+/- 330 VDC) and a -15VDC for input stage current sink. 

The build was pretty straight forward. I did change the value of the resistors in series with the trim pot of all the cascoded CCS as advised by both Congo5 and MLA. Specifically:

Output current source: 200R in place of 250R

Output current sink: 75R in place of 102R

Input current sink: 1.2K in place of 1.43K

The DN2540 has quite a bit of variance from sample to sample even from the same batch so be prepared to adjust these value for your build.

I am running 7mA output plate current, 17mA for output current sink and adjust the input current sink (the 500R trim pot) so the upper 12AT7 plate voltage is half way between ground and B+ (167VDC for my build) and adjust the 5K trim pot so both the plates of the upper 12AT7 sit on the same voltage.

I do have a very slight buzz on one of the channels that is not attenuated with volume. I need to trouble shoot it. It happens to be the channel where my AC heater supply wires pass below but there could be many other causes.

Thanks to JimL for coming up with this modified SRX circuit, Kevin for laying out the PCB and JimL, Congo5 and MLA for helping me along my build.


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I forgot to mention that I am using 4 made in China Golden Dragon 12AT7 and 2 GE 6SN7GTB I randomly took from my parts bin for this initial testing. I don't know if they were matched or not. But simply by adjusting and balancing the upper 12AT7 plate voltage (they were off by roughly 15VDC with the 5K pot set in its mid-point) and leaving the output current source and sink at their pre-dialed current setting, the output balance are within 2 VDC and the offset under 4 VDC once the amp is warmed up and that's where I left it.

Maybe I got really lucky with the tubes I use. 

Edited by mwl168
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Re: buzz.  Obvious things to do are 1) make sure the filament (heater) wiring is twisted tightly, and 2) keep the filament wiring away from the inputs and run them so they are perpendicular to any signal traces if possible.

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Thanks JimL!

What you advised were the first things I tried. After some experiment, it seems to come down to one of the 12AT7 tubes after I started swapping tubes around. Once I knew the amp is working, I did rearrange and tidy up the wiring up a bit in my testing setup. I am still not 100% sure though but happy to report it seems to be buzz-free at the moment.  

As is customary, here are some pics of my build. As you can see I put the trim pots and the 100R resistors on the top to make it easier for testing and tuning. The gizmo next to the amp board is something I rigged up to lower the AC filament voltage. 

I am really liking the sound of this amp even with the cheapo tubes I am running with at the moment.


SRX Plus -1.JPG

SRX Plus - top.JPG

SRK Plus - Bot.JPG

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Now that the SRX Plus has been running for a few days and seems stable, I thought I would share some information that others may find helpful for initial testing. Congo5 was kind enough to share similar information with me that really made my testing much straightforward. 

As I mentioned in earlier post, I adjusted the value of the fixed resistors in series with the source pin of the DN2540 and the trim pots based on the recommendation from both Congo5 and MLA. So for the 7mA output current source, 17mA output current sink and 1.1mA (based on 300K 12AT7 plate resistors) input current sink as JimL recommended, here are the approximate mean of the trim pot values I end up with:

Output current source (100 ohm):  35 ohm (200 ohm fixed resistor)

Output current sink (20 ohm): 12.5 ohm (75 ohm fixed resistor)

Input current sink (500 ohm): 410 ohm (1.2K fixed resistor)

To adjust for balance of the input stage, I put the DVM probes on the two .22uf/1000V coupling caps where they are connected with the plates of the upper 12AT7 then adjust the 5K trim pot until the meter reads approximately 0 VDC which sets both the plates of the 12AT7 at the same voltage relative to ground. I then re-adjust the 500 ohm pot as needed so the 12AT7 plate voltage sits at mid point of the B+ and ground (167 VDC for my SRX Plus which runs on 330 VDC rails).  

As to how does it sound? It's a different sound signature than all other KG ES amps I have built (I don't have a Megatron or DIY T2). I am guessing it's the nature of an all-tube circuit. It's surprisingly spacious and punchy sounding to me for such a minimalistic circuit. I think the cascoded CCS is really doing its job. 

I like it very much. I hope more people will build it.  

Edited by mwl168
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Not possible on the current PCB to install the 10M90S resistors on the other side.

As is, the heatsinks do not get too warm running 330VDC rail and 7mA output current in open air. I measure about 51 degree Celsius (about 123 F) after 4+ hours of continuous running. It'll obviously run warmer once cased. I am using 38.1mm tall heatsink (Mouser part # 532-529801B25G) and there are taller ones available. 

Edited by mwl168
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Each of the output current source heatsinks has to handle less than 2.5 watts apiece, or roughly 10 watts for the amp board, so it's not as if there is a huge amount of wattage trapped inside the box.

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On 3/11/2016 at 5:42 PM, JimL said:

Re: buzz.  Obvious things to do are 1) make sure the filament (heater) wiring is twisted tightly, and 2) keep the filament wiring away from the inputs and run them so they are perpendicular to any signal traces if possible.

A quick update on the buzz. I finally traced the buzz to noisy 12AT7 tubes. Once I replaced the noisy tubes the amp is silent without input signal.

As JimL advised, I twisted the AC filament wires tight, keep them away from the input stage as far as possible and also arrange them so they are perpendicular to the signal traces.  

Edited by mwl168
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Accidentally left the SRX Plus running overnight. So as it sits now it's been running continuously for more than 14 hours. Happy to report that it appears to be very stable, the offset and balance all measured as expected and all the heatsink temperatures are steady as well. 

No, it does not sound better than say, after a couple hours of warming up but it's very good to know that the amps is very stable and my ignorance did not set the house on fire.

Edited by mwl168
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Yes, I left mine on overnight also, when I was testing my build, except that I did it on purpose.  The current and voltage parameters were chosen so that it should need no more care than a typical TV.  You fall asleep in front of the TV and wake up in the morning - no big deal.


Incidentally, here are the plate curves for a 6SN7GTA from the RCA specifications:


Notice that the plate voltages run up to over 650 volts, which is higher than the maximum voltage that these tubes will see in the SRX Plus design (assuming +/-325 volt supplies).  And incidentally, the largest consumer use for these tubes was in black and white TV sets.  If they hadn't been reliable the screams from consumers would have been epic.

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Thanks JimL. 

An additional data point to add; while I was testing and using the SRX Plus, I also measured the tube temperatures using an infrared gun pointing at the plate of the tubes. As perhaps a testimony to JimL's point; both the 12AT7 and 6SN7GTB were running at surprisingly (to me) low temperature. IIRC, the 12AT7 was at around 65C and the 6SN7GTB at around 120C.  These temperatures measured the same after I accidentally left it on for 14 + hours.

EDIT: corrected the temperature unit of measure

Edited by mwl168
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Is the "TIL731" similar to "SPX431"?

I can´t find any reference with TIL731

Possible PSU bom:


Part   Qty.   Mouser Ref.     Comments        
0,1uF/1Kv   4   505-MKS4.1/1000/10            
0,1uF/1000v   3   539-160104J1000N-F            
22uF/450v   2   871-B32774D4226K            
2,2uF/50v   1   647-UBT1H2R2MPD            
100uF/16v   1         Posible cambio a 100/25v   647-UKW1E101MED
330uF/450v   2   647-LKX2W331MESB45            
IXCP10M90S   2   747-IXCP10M90S            
STF19NM50N   2   511-STF19NM50N            
DN2540N5G   1   689-DN2540N5-G            
TIL731*   1         SPX431 ¿?*     701-SPX431AN-L/TR
STTH512FP   4   511-STTH512FP            
1N4007   2   512-1N4007              
BZX55C15 Zener 15v 1   78-BZX55C15            
Z4KE150A Zener 150v 1   625-Z4KE150A-E3            
1N5952BRLG Zener 130v 3   863-1N5952BRLG            
10 Trimpot 1   652-3296W-1-100LF            
100k   2   660-MF1/2CC1003F            
100k 1,5W 1   71-CMF65100K00FKEK            
3k*   1  
*Xicon 350v 3,32K KOA   660-MF1/2CC3321F
4,7M   1   660-MF1/2DCT52R4704F            
806k*   1   71-RN65D-F-806K* *Vishay 350v          
1k   3   660-MF1/2CC1001F            
15k   1   660-MF1/2DC1502F            
15,4k   1   660-MF1/2CC1542F            
10k   1   660-MF1/2CC1002F            
100   1   660-MF1/2CCT52R1000F            
30*   1   273-30-RC*   *Xicon 350v          
10   1   660-MF1/2DC10R0F            
1M   8   660-MF1/2CC1004F            
82k*   1  
*Xicon 350v 82,5K* KOA   660-MF1/2DC8252F
200 1,5W 1   71-CMF65200R00FKEK            
133k 1W 1  
365k 1W 2  
590k 1W 1  
3 pin conector   2   651-1729131              
2 pin conector   2   651-1729128              
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