SRX-Plus Build Thread

[JimL]

I used something like that ground loop breaker for my phono preamp.

[Blueman2]

I need some advice/help on tuning my latest SRX-Plus amp build.  I tried to duplicate what I did on the first amp I built which I was able to adjust per the settings in mwl168's post earlier in this thread.  I have the 7ma adjusted on the output current loads and 17ma on the output current sink.  I then try to do step 6, which is

Quote

6. The goal of this step is to set the two plates of each of the upper 12AT7 to be equal and about mid-way between B+ and ground. So if you are using B+ of 360VDC, the mid way is 180VDC.

First attach the DMM probes to each of the two .22uf/1000V coupling caps where they are connected to the plates of the upper 12AT7 and measure the DC voltage between them. (in my case. I purposely left the leads of the coupling caps sticking out a bit so I can clip the DMM probes on them. With the 5K trim pot set at its mid-point, the measured DC voltage between the two plates tells you how well matched are the two triodes of the 12AT7 tubes used. If they are well matched it should be within 5 VDC or so. If it's more than 20VDC you may have problem**.) Adjust the 5K trim pot so the measured voltage is close to 0VDC.

When I do this, the voltage difference between the two plates swing wildly, eventually settling on a 120V delta between them!!  When I power on, it shows about 8V delta for 10 seconds.  Then when the tubes kick in, the voltage goes from about 100V to 0 to -120V delta over a period of about 30 to 45 seconds.  On my first amp, it will swing briefly, but will always settle down on a value less then 10V delta.   Of course, the 1K pots cannot come close to resolving that big of a delta.  I replaced all the tubes with known good ones from my other amp and get exactly the same result.  

I also ran the same test with the other channel, and get exactly the same result!!  So if I made an error, I made it perfectly across both sides!  Any idea what could be causing this?  Interestingly, the amp sounds just fine when listening to it with my 507's.  But having such a huge plate voltage delta is a concern to me.  

One possible cause could be that when doing a low voltage test (1/5 power with a variac) I realized I have reversed the B+/- rails.  I corrected that before the full power up.  Could that have fried the silicon and be causing this issue?

Edited October 26, 2017 by Blueman2

[jose]

Do you have the cold channels bridge to ground?

 

Edit: and volumen pot to 0

 

Edited October 26, 2017 by jose

[Blueman2]

jose, yes I have both cold channels tried to ground.  No volume control on it right now.  

[JimL]

First, I would take the 12AT7 tubes from the bad amp and substitute them one at a time into the good amp.  If the tubes balance in the good amp, then they are not the problem.  While the tubes are out, you can re-check the plate resistors to make sure you didn't somehow put in the wrong values.  Assuming they are good, that leaves the tail current sources. If that is the problem,  I would replace all the DN2540s in both input tail current sources.

[mwl168]

6 minutes ago, JimL said:

First, I would take the 12AT7 tubes from the bad amp and substitute them one at a time into the good amp.  If the tubes balance in the good amp, then they are not the problem.  While the tubes are out, you can re-check the plate resistors to make sure you didn't somehow put in the wrong values.  Assuming they are good, that leaves the tail current sources. If that is the problem,  I would replace all the DN2540s in both input tail current sources.

^ that. And also, what's the plate voltages reference to ground when you are seeing the 120V delta between the plates?

[Blueman2]

Thanks for the quick reply, Jim.  I did exchange the tubes to ruled that out.  I am using 300K plate resistors and they test right.  So I will replace the DN2540s.  Should I focus on just the 2 per channel on the input tail CCS (the 2 near the 500 ohm POT)?  I was not sure if you meant to also replace any of the DN2540s that are associated with the 10m90s.  Thanks!  

mwl168, the plate reference to grounds starts at 360V on power up (which is my B+ rail), and then goes all over the place.  Of the 4, one goes steady to 90V (I think this is the only one acting correctly), and the other 3 sort of go all over the place and never seem to settle down, often at 220V or so.  

Edited October 26, 2017 by Blueman2

[Blueman2]

BINGO!!  One of the DN2540s was indeed bad.  I had not tested those as well as I had tested the 10m90s because I had it in my mind the 10m90s were the place to start.   

JimL, you saved me many hours of replacing all the silicon which was going to be my next step.  LIFESAVER!!!!  

Thanks!

Edited October 29, 2017 by Blueman2

[JimL]

The advantage of a simple circuit is there are only a limited number of things that can go wrong.

[Blueman2]

Oh, and jose also brings up a good point.  I am now able to balance the amplifier perfectly and the voltages are now stable (I could not get it stable with the bad DN2540).  But as soon as I tie COLD INPUT to GND, I am back to delta between plates being around 90V.  I assume this is normal?  Because everything seems just fine when COLD INPUT is left floating.  I assume settings should be done with nothing attached to inputs or outputs?  

In any case, JimL, thanks for the lead on the DN2540.  Amp is now stable and sounds GRRRRRRRRRREAT.  Apologies to Tony the tiger.   

[JimL]

Hmm. Well, no. My prototype is single ended with the negative input tied to ground (I don't have any balanced sources, only single ended), and the plates voltages are equal. In fact, the original Stax SRX circuit has the negative input grounded.  If you have a single ended input, you should balance the plates with the volume control turned all the way down.

Edited October 27, 2017 by JimL

[Blueman2]

Just now, JimL said:

Hmm. Well, no. My prototype is single ended with the negative input tied to ground (I don't have any balanced sources, only single ended), and the plates voltages are equal.

OK, then I must have another problem.  Because I can balance the amp with inputs are open/floating, but as soon as I tie either cold input to ground, I cannot even come close to balancing the 2 plates.  same on both left and right channel.  

But you are right, because I was able to balance the first amp I built just fine with cold tied to ground.  I wonder if I have more bad DN2540?  I did a quick and dirty test on them, and aside from the one that showed open circuit between Drain and Sink, all the others show the expected 10-17 ohm resistance.  Very odd.  

[JimL]

Or you have more 12AT7 problems.

[mwl168]

I seem to remember that the heater supplies to the tubes need to be floating for the SRX Plus. Is this how you have them wired? Not sure if this is related to your problem with grounding the inverted side of the input but just taking a shot in the dark.

[JimL]

Yes, I recommend floating the heater supplies. The input tube and output tube heater supplies also need to be separate, even if you rewire the input heater supplies to 6 volt.

[Blueman2]

I do have both filaments elevated.  12AT7s are at 60V (voltage divider off B+ rail) , and 6SN7s are at -340V (tied directly to B- rail).    I did this to eliminate hum.  I do of course have have 2 separate 6.3V windings (I modified my board to use 6.3V for 12AT7s) so they each are elevated to their own levels.  But this duplicates what I did on my first amp, and on that amp I found elevating the filaments actually made balancing the amp much easier.  But let me try removing the elevation and see what that does. 

I did check all the DB2540s on the board.  All show about 11 ohms across Drain to Source.  They should be showing about 17 ohm according to the spec sheet, and indeed the new one I put in (to replace the one that showed open circuit across D to S) does read 16 ohms when installed.   All are from the same mouser order.  I will put -10V across gate to see if they are working correctly just to be sure.  

Right now, I am pulling all the tubes from the known good amp and putting them in the new amp in the same positions just to rule out tube issues, as JimL suggested.   

[mypasswordis]

Elevated and floating are not the same thing.

[Blueman2]

Just now, mypasswordis said:

Elevated and floating are not the same thing.

They are essentially the opposite, right?  Floating means there is no reference DC voltage applied and the filaments will float to whatever makes them happy.  Elevated means I have tied them to a known reference DC voltage, such as +60V for the 12AT7s.  Is that right?  

[mypasswordis]

Yep, that's right. Don't have the schematic right now but did you also check the resistors/pots in the input stage current sink?

[Blueman2]

Will do.  The amazing thing about this very simple yet elegant SRX-Plus design is that even with it being way out of spec for plate voltages, the damn thing still sounds INCREDIBLE!  It might not last long in this condition but I cannot help but be amazed at the quality of sound compared to the SRD-7 I was used to.  

[Blueman2]

13 hours ago, JimL said:

Or you have more 12AT7 problems.

Damn it, JimL.  Must you always be right?  First, you intuitively guess a bad DN2540 on the input tail CCS and were right.  Then you intuitively guess 12AT7 issue and, again, were right.  I put in all of the known good tubes from my prior amp, and the new amp is now acting and adjusting perfectly.  Rock solid and steady, even with SE config (cold input tied to ground).  And even with the filaments still elevated.  

I did not think at first to try that because I had already put all those good tubes in the new amp and it did not help.  But that was when I still had a bad DN2540.  

JimL, you absolutely rock the STAX world!!  

mwl168, jose and mypasswordis, thank you too for your help.  I very much appreciate this wonderful community.  

Edited October 27, 2017 by Blueman2

[Blueman2]

On 4/29/2016 at 2:45 PM, mwl168 said:

7. Let the amp warm up for another 15 - 20 minutes for the tubes to reach their stable operating state then repeat the adjustments again. Measure the balance via the headphone jack like you do with other Stax amps. If the two triodes in each of the 6SN7GTA/B are well matched you should see around 1 or 2 VDC or so. And this is perfectly fine.

8. Measure the offset via the headphone jack like you do with other Stax amps. Adjust the 20 ohm trim pot (output current sink) to bring the offset as close to 0 VDC as possible.

9. Assuming all adjustments are carried out without problem, you are done and ready to listen to music now :-) This is an all tube amp without servo so the offset and balance etc. will never be spot on and will vary and drift a bit every time you power on the amp. That’s to be expected.

10. There is an optional step which I have not done myself - you can adjust the 2 100 ohm output CCS trim pots to bring the balance of each channel to 0 VDC. I have not felt the need to do it (maybe I got lucky with the tubes I have). If you choose to do so, try to adjust both trim pots in a converging fashion instead of adjusting only one of the two pots. You'll also need to "coordinate" this with adjusting the 20 ohm pot in step 8. 

 

mwl168 or others,

With everyone's help, I was able to get my second SRX-Plus so stable that I went total OCD and decided to balance everything within an inch of its life.  I ended up adjusting the 100R output CCS trim pots to get balance between + and - of each channel to be equal, and used the 20R pot to get offset to 0VDC.  So after about an hour of warm up, all 4 outputs read less than 1VDC (measuring from GND to each of L+, L-, R+, R- outputs).  I then adjusted the 500R pot to get upper plate voltage back to exactly 1/2 of B+.  Fine tuned the 5K pot to keep plate voltages equal.  So from these measurements, I think I have it dialed in about as good as I can. 

However, this setup leaves me with about 3.3ma of 'extra' current sink between output current sink and source rather than the target 3.0ma .  I assume that is OK, right?  Here are my final measurements across the test resistors:

Output current loads:

• R+: 6.83ma
• R-:  7.00ma
• L+: 6.50ma
• L-:  7.01ma

Output current sink:

• Right: 17.18ma
• Left:   16.78ma

Input current sink (340V B+ rail, 300K plate resistors):

• Right: 1.07ma
• Left:   1.06ma

Does this all look OK given that the outputs are now balanced and offset to 0, and upper plates are equal and 1/2 B+?   

 

Edited October 29, 2017 by Blueman2

[mwl168]

JimL is the authority here and I am sure he'll chime in.

I am certain what you described is just fine. This is a tube amp with no servos employed. All these operating points will vary and drift a bit every time you power up the amp.

[JimL]

3 hours ago, Blueman2 said:

mwl168 or others,

With everyone's help, I was able to get my second SRX-Plus so stable that I went total OCD and decided to balance everything within an inch of its life.  I ended up adjusting the 100R output CCS trim pots to get balance between + and - of each channel to be equal, and used the 20R pot to get offset to 0VDC.  So after about an hour of warm up, all 4 outputs read less than 1VDC (measuring from GND to each of L+, L-, R+, R- outputs).  I then adjusted the 500R pot to get upper plate voltage back to exactly 1/2 of B+.  Fine tuned the 5K pot to keep plate voltages equal.  So from these measurements, I think I have it dialed in about as good as I can. 

However, this setup leaves me with about 3.3ma of 'extra' current sink between output current sink and source rather than the target 3.0ma .  I assume that is OK, right?  Here are my final measurements across the test resistors:

Output current loads:

• R+: 6.83ma
• R-:  7.00ma
• L+: 6.50ma
• L-:  7.01ma

Output current sink:

• Right: 17.18ma
• Left:   16.78ma

Input current sink (340V B+ rail, 300K plate resistors):

• Right: 1.07ma
• Left:   1.06ma

Does this all look OK given that the outputs are now balanced and offset to 0, and upper plates are equal and 1/2 B+?   

 

No problemo.

[Blueman2]

Thanks, guys.  On my new amp (above post) , I was not too far off in terms of output offset or balance.  But on my older SRX-Plus, which is what I have been listening to most, the output offset voltage was over -120V on all outputs.  Way off.  So I did the same adjustments as I outlined above, and WOW!!  The amp had a much better tone.  Broader soundstage and more clarity.    I wish I had done these adjustments before!  I thought they were mostly for tube life, etc.  But unless I am mistaken, it does have a very noticeable impact on sound.  

Enjoying my finely tuned SRX-Plus right now as I type.  I just cannot turn this thing off.

Edited October 29, 2017 by Blueman2