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Hints on maintaining / improving my Stax SRM-T1S


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The current through the bias trimpots is the sum of the currents through the two tube sections, which is determined in the stock amplifier by the plate output resistors, the positive voltage supply and the fact that the plate voltage (ideally) is 0 volts. Since the nominal positive voltage supply is +320 volts, and the summed plate output resistor is 66 kilohms, by Ohms law the current through each tube section has to be 325/66k = 4.85mA, so for two tube sections (per channel), the total current is 9.7mA. 10mA is close enough for calculation purposes.

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Just a summary of what I have learned on planning the recap of my SRM-T1S unit - a bit on the TL;DR side and probably repeating a lot of boring stuff well known to experts in the forum (specially ones with little or no patience for capacitor esoterics - yeah, I did notice that the latter is a thing in the audiophile community 🙄 ), but it may be useful to others (or at least to me) in the future to have everything in one place. I still have a few doubts, though (those only interested in that may skip to the last paragraph below)... Thanks again to all who helped!

My current (stock) configuration of PS electrolytic caps is the following (cap numbering follows the schematic):

  • C9-10: 2 x 10uF, 50V (Marcon, currently United Chemi-Con), radial - dimensions (LS = lead spacing x D = diameter x L = length): 5mm x 10mm x 12.5mm
  • C11-14: 4 x 100uF, 400V, 85oC (Hitachi, currently AIC Tech), snap-in - dimensions (LS x D x L): 12.5mm x 30mm x 60mm
  • C21-22: 2 x 220uF, 10V, 85oC (Elna), radial - dimensions (LS x D x L): 3.5mm x 6.5mm x 11.5mm
  • C23-24: 2 x 47uF, 35V, 85oC (Elna), radial - dimensions (LS x D x L): 3.5mm x 6.5mm x 11.5mm

Surveying through a number of places (based on the above comments by Pars, Fitz and JimL) pointed that caps meant for power supply should privilege not only low dissipation factor (DF), but also high ripple current and long endurance (assuming capacitance and voltage rating the same) - high ripple current suppression capabilities are not relevant for signal-path caps (which assume that the DC power has already been filtered enough in the power supply stage) but are critical for power supply caps. "Audio grade" electrolytic caps are usually strong in the first characteristic but rather weak in the last two. They are also more often than not unavailable with the 105oC temperature rating recommended by Pars above (which seems reasonable since the circuit gets quite warm, as a pure class-A tube amp should). Taking that into account when searching through the Mouser catalog, it seems to me that the following substitutions should yield a good result within Mouser's current stock availability:

  • C9-10: 2 x Panasonic EEU-FR1H151B (FR series) - 150uF, 50V, 105oC, DF=0.10 (at 120 Hz, 20oC), ripple current = 0.82A (RMS, 120 Hz, 105oC), life = 6000h, dimensions (LS x D x L): 5mm x 10mm x 12.5mm - https://br.mouser.com/ProductDetail/667-EEU-FR1H151B
  • C11-14: 4 x EPCOS / TDK B43547A9477M000 (B43547 series) - 470uF, 400V, 105oC, DF=0.15 (at 120 Hz, 20oC), ripple current = 2.71A (RMS, 100 Hz, 105oC - EPCOS is a German manufacturer and mains power in Germany is 50 Hz), life = 8000h, dimensions (LS x D x L): 10mm x 30mm x 55mm - https://br.mouser.com/ProductDetail/871-B43547A9477M000
  • C21-22: 2 x Panasonic EEU-FR1A681 (FR series) - 680uF, 10V, 105oC, DF=0.19 (at 120 Hz, 20oC), ripple current = 0.66A (RMS, 120 Hz, 105oC), life = 6000h, dimensions (LS x D x L): 3.5mm x 8mm x 11.5mm - https://br.mouser.com/ProductDetail/667-EEU-FR1A681
  • C23-24: 2 x Panasonic EEU-FR1V181 (FR series) - 180uF, 35V, 105oC, DF=0.12 (at 120 Hz, 20oC), ripple current = 0.71A (RMS, 120 Hz, 105oC), life = 6000h, dimensions (LS x D x L): 3.5mm x 8mm x 11.5mm - https://br.mouser.com/ProductDetail/667-EEU-FR1V181

A small comment on the lead spacing of the chosen replacements for C11-14 is in order. As thebrunx pointed above and also comparing to recap choices on the SRM-T1(S) made by other people, it may be in fact necessary to slightly reduce the lead spacing from 12.5mm to 10mm. The size of the lead holes in the PCB seems to allow for that, and there seems to be no replacement snap-in caps available with 12.5mm lead spacing and up to 30mm diameter, so one has to make do with 10mm LS here.

One may also notice that the diameter of the chosen replacements for C21-24 is slightly larger (6.5mm -> 8mm). Again, this is due to the fact that there seems to be no available replacement caps with at least the same capacitance / voltage rating, the same lead spacing (3.5mm) and 6.5mm diameter - the smallest diameter available within the former constraints seems to be 8mm. On the other hand, there seems to be enough room in the C21-24 positions for 8mm-diameter caps.

Comparing this with an "audio grade" choice of caps:

So, one can see that the Panasonic + EPCOS selection outperforms or at least matches the Nichicon "audio grade" selection on all characteristics, apart from the DF of C9-10, for which the difference seems not to be of much significance (Nichicon KZ: 0.08, Panasonic FR: 0.1). I imagine that the latter is outweighed by advantages of the Panasonic cap in the other characteristics for power supply applications (is it?)... As for the price, yeah, the Panasonic + EPCOS selection is cheaper (notice that the "audio grade" caps linked above are priced in Euro) but not by nearly as much as I thought, so it really boils down to specs and quality for the intended purpose.

One concern I still have (warning: people who didn't bother to read up to here may do so from now on 😁 ) is the following: I understand that increasing capacitances in the recap procedure increases the filtering capabilities of the power supply circuit. However, won't doing this cause other problems if I increase the capacitances too much above stock values? Is it safe to increase capacitance as much as possible just within the allowed physical dimensions of the caps, as recommended in this thread?

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Now, regarding tube replacement / rolling...

I am pleased with the sound of my Gold Aero / Sylvania 6CG7 tubes (they probably can use some rebiasing, though, but that will wait for after recapping), but I have nothing to compare them against, and as I said above it's good to have spare tubes when mine decide to die out, at least. JimL forwarded above advice hirsch and spritzer gave on other threads about Japanese short plate tubes sounding best with the SRM-T1(S) circuit. Digging deeper on these threads revealed that Toshiba 6CG7's seem to be preferred, and that some of these have been rebranded by Raytheon.

  • How do the Raytheon-rebranded Toshibas compare against the "original" Raytheons made in USA in the SRM-T1(S)?
  • How about other Japanese brands, like Matsushita (i.e. Panasonic), Hitachi and NEC (some of 6CG7's made by them were also rebranded by Raytheon, by the way)? How do they fare against the Toshibas on the SRM-T1(S)?
  • Some USA made 6CG7's like the "clear top" RCA (i.e. without the chrome plating on the top of the glass most 6CG7's have) and Amperex (these were made by Philips, I believe) also seem to be highly regarded in general - even more than the Sylvanias by some -, but how do they compare against the Toshibas on the SRM-T1(S)?
  • Does the center shield present in the 6CG7 and absent in most of the 6FQ7's make any sonic difference?
  • Quite a few people recommend going for "black plate" tubes against the "grey plate" ones (like mine). Is there really a sonic difference?
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I've found a potential problem concerning a possible future replacement of the bias trimpots (TVR1-2) in the SRM-T1S. I couldn't find 2kOhm trimpots with at least 0.6W power rating as recommended by JimL above with the same PCB footprint as the original trimpots (three pins forming an equilateral triangle - 3 x 60 degrees). The closest PCB footprint I could find for these is with three pins forming a right isosceles triangle (2 x 45 degrees + 1 x 90 degrees - e.g. Bourns 3345W single-turn). Ideally, were I to replace the bias trimpots, I'd use multiturn parts to make the rebiasing easier as recommended by spritzer and others in other threads, but the only multiturn trimpots I could find with vertical adjustment and the same specs (2kOhm, >=0.6W) have a PCB footprint with the three pins in a single straight line (e.g. Bourns 3252W and 3290W). In view of that, I have the following questions:

  • Is it safe to bend the pins of the trimpots in order to make them fit into the existing PCB holes? I'm unable to see whether the original trimpots have bent pins or not...
  • May I leverage the multiturn option and use higher resistance trimpots so I can have more footprint options? For instance, may I use 3.3kOhm, 1W trimpots instead? These would be compatible with a 10mA current and the rule of thirds. By that logic, these would probably be the largest resistance trimpots I could use, since I couldn't find trimpots with a power rating higher than 1W. Even so, I'm not sure I'd be able to find options with the same PCB footprint...
  • In view of that, how much freedom with the "rule of thirds" do I have with the power rating? Allowing for lower power ratings (1/2W? 1/4W?) would significantly increase my choices...
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Another improvement I've seen people around (e.g. spritzer) suggest on the SRM-T1 is to replace the RCA input jacks and input wiring, but judging from pictures it seems that there is some improvements made by Stax on that area from the T1 to the T1S - the RCA jacks definitely look beefier and the input wiring looks nice enough (actually better than the output wiring)... but again, that's only a visual impression. I wonder whether it's sonically worth the trouble to make these upgrades... Anyhow, I'm using the XLR balanced inputs in my SRM-T1S for now, so this makes upgrading the RCA input jacks moot (but possibly not the input wiring).

I've also seen some people (e.g. spritzer again) recommend to change the tubes from 6CG7/6FQ7 to the modern ECC99 from the Slovakian brand JJ (apparently the only one who makes these), but that requires rewiring the tube sockets somewhat and changing the R20 resistors in series with the TVR2 bias trimpots from 2.2kOhm to 1kOhm, 1/2W in order to achieve proper bias for the tubes, as JimL suggested here:

https://www.head-fi.org/threads/stax-srm-t1-repair-re-cap-mod.796058/post-14276400

Rewiring the tube sockets for the ECC99's can be done as e.g. in these instructions (in German, sorry):

http://blog.prof-x.de/2019/03/02/stax-vacuum-tube-driver-teil-4-ecc99-tube-swap/

The author combines JimL's CCS mod with the ECC99 mod but he changes the CCS trimpots from 100 to 200 Ohms in order to achieve more easily the required 6mA plate current required to run the ECC99's in SRM-T1(S)'s CCS-modded circuit. The claimed improvement over the CCS mod with stock tubes is an extended frequency response at higher volumes, but given that the ECC99 mod is not easily reversible, should one regret doing it (particularly due to the pair of PCB trail cuts in the rewiring process), I wonder how much one gains with it compared with the CCS mod alone (possibly also after a tube replacement / rolling with some nice Japanese short plate NOS tubes such as Toshiba, Matsushita, Hitachi or NEC). Another thing that worries me about the ECC99 mod is this tube's longevity, which seems to be not so great in certain setups:

https://www.diyaudio.com/forums/tubes-valves/205364-lifespan-jj-ecc99-tubes.html

JimL claimed in the AudioXpress article where he published his CCS mod that SRM-T1(S)'s circuit is rather conservative with the stock tubes, despite Kevin Gilmore's criticism that the C6G7/6FQ7 tube being driven somewhat above its rated voltage by the SRM-T1 circuit also causes loss of high-frequency response at higher volumes, but how much strain does the SRM-T1 circuit put on the ECC99? More precisely, how is ECC99's lifespan on the SRM-T1(S), particularly with the CCS mod?

I suppose much (all?) of the above information has already been discussed at length in the (loooong) Stax threads in this and the Head-Fi forums along several years, but I've found it difficult to gather all of it in an up-to-date form, so I apologize in advance for the redundancies on my behalf in this thread and appreciate the community's patience with them. Again, it seems convenient (at least to me) to have everything I need in one place.

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