SSE voltage readings

[mpri]

Hi, quick question. Is there a schematic of the SSE amp with voltage readings for a given B+ available? Specifically I am looking for known working voltages around the CCS. I’m having an issue with my SSE and am seeing very different voltages on the 10M45 CCS. On the right between the A and G I have only a 5V difference and the left I have a 24V difference. There is also a big difference on 12AT7 plate side of C11 and C21, about 21V. In addition I’m seeing a slight DC leak on C11, anywhere from .8 to 1.2V to the grid of the output tube. Is a small leak like that an issue?

Any assistance would be appreciated.

Thanks,
Mike

 

[mpri]

Ok, how about specifically, is 315-326V too high for the plates of the 12AT7? Is a 3-4V drop across R13/R23 too much?

Thanks,
Mike

 

[w5jag]

You really have to measure the current, don't you? The CCS will do whatever it takes to put the preset current into the tube. The max plate voltage specified for the 12AT7 is 300. It's cathode biased, so you would subtract that. It's not really voltage that kills the tubes, it's power.

Tubelab could better answer your questions, but here is all I know about the 12AT7 driver stage, fwiw:

With a generic chinese 12AT7, and the stock compenents in the CCS, at 300 volts, the current into the CCS was 16 ma.

When I jumpered the dropping resistor, it only took 260 volts to achieve 16 ma through the CCS.

In looking at my notes, I don't see where I actually measured the voltage at the plate. I would expect that to vary tube to tube because it wants to deliver a specific current, not voltage.

With a 1 Khz sine wave into the CCS at 1 v p-p, the output sine wave does not clean up until there is at least 230 volts into the CCS. That got 40 v p-p out.

At 250 vdc into the CCS, 1.8 v p-p in, yielded a maximum 70 v p-p out before clipping.

At 300 vdc into the CCS, 3.8 v p-p in, yielded a maximum 140 v p-p out before clipping.

At 350 vdc into the CCS, 4.0 v p-p in, yielded a maximum 190 v p-p out before clipping.

I didn't see the IMD change much as I varyed the voltage, but the load made a difference - it was lower into a Hi-Z load, by quite a lot, than it was into a 600 ohm load.

The SSE driver stage is robust. This may or may not help you - it's all I know about that stage at this point. Maybe if you were more specific about your problem, someone with more knowledge could help you.

I haven't measured leakage across my coupling caps, but I would ASSUME any leakage is undesirable until someone with more knowledge says otherwise.

Win W5JAG

 

[mpri]

Thanks for your info W5JAG.

Specifically the issue I have may have started from day one when I built the SSE 4 years ago. I’ve always had a volume difference between the left and right channels, left channel always dominant. I checked and rechecked every component and both sides compare equally within the component tolerance. Back then both channels sounded very good, just one side louder than the other, so since every else seemed OK I installed a dual concentric volume control so I could balance left and right as needed. At lower volume both were the same, as it went higher the difference grew. It worked well for 4 years. Now the weak right side has developed distortion, severe to the point where I have to use 1K bias resistors for R17/R27 to get good sound with cheap Chinese 6L6GC’s. I can’t even use my newer Tung-Sol 6550’s anymore at any cathode bias without distortion, which use to sound great. Also over the years it seems some of my tubes have gone bad. I’ve gone through 2 or 3 12AT7’s and 2 5AR4’s. Bad luck? I don’t know, but they were NOS, not the new stuff.

So I started taking voltage reading and found a somewhat discrepancy between the right and left channel (right being higher) and thought maybe that could point to what the problem could be.

My initial thought was that the right side CCS has a 1V difference between input and output and the left has 12V difference, which seemed odd. However in the end both output tubes do have the same 386V on the plate, so… I don’t know.

I’m seeing the same voltage differences regardless of what input source or 12AT7 tube or output tube I use. Maybe the right CCS bad? But it seems like I'm running more tha 10ma on both CCS's at the 330 ohm resistor. But only the right channel sounds distorted.

Right channel CCS:
P 330V, G 325V, A 329V, 12at7 PLATE 326v
Left channel CCS:
P 330V, G 310V, A 317V, 12at7 PLATE 313v

Additional info:
All components except motor run caps and iron are from Mouser and Digikey
PT: Allied 6K7VG
C1: 35mf 440V motor run cap
Triad C-14X choke
C2: 100 mf electrolytic 500V + 50mf 350V motor run cap
B+: mostly 415 (my line voltage is 115V-117V)
OPT: Transcendar TT-212-OT (older, like 312)
C11/C21 Mundorf Supreme .22 1200V
Power Tube: NOS Sylvania & JAN 5AR4 (several)
Driver tube: 12AT7: NOS Sylvania, NOS RCA, JAN
Output tubes: Can’t blow up Chinese 6L6GC, =C= 6L6GC (still seems Ok @ 1K), Tung-Sol 6550 Seems dead now at any cathode bias.
I use Triod mode only
Have not drilled out any pads on the PC board to inset components
5 selector cathode bias switch: 1K on board, 750, 630, 560 and 470 – only 1K is useable now!
25K dual concentric volume control

Thanks for any help anyone can give me,
Mike

 

[w5jag]

Do you have any way to check the cathode bypass capacitor on the weak side output tube?

Win W5JAG

 

[mpri]

W5jag,
Would that be C12, on the right side? What would be the best way to check it?

Thanks,
Mike

 

[w5jag]

If you don't have a way to test the capacitors, I would substitute known good parts for each one of them to find the cause of the problem.

If you have a leaky coupling capacitor on that weak side, I'd start there. I don't know what a Mundorf Supreme is. I'd put in a plain vanilla .22 uF and see if that fixes your problem. If not, move on to the cap bypasssing the cathode resistor.

Win W5JAG

 

[mpri]

Thanks W5jag,

I do have some Auricap's that I can use to swap the cuppling caps with. I will give that a try. I appreciate your help.

Mike

 

[mpri]

Well holy cow, the MCap brand coupling cap at c11 did go bad after 4 years of use. Replaced both c11/c21 with Auricap’s I had on hand and the distortion is gone. Nice! And all voltages between the right/left channels are just a volt or two apart. I like it. It didn’t fix the nagging volume difference between channels, but with separate volume controls that is at least manageable. I’ll look at that next. But for now I can use the 6550’s with a low bias resistor again with no distortion. I'm happy.

Thanks W5JAG for the suggestion!

Mike