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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), used instead of the 6CG7/6FQ7's in the SRM-600 variant of the SRM-T1 circuit, 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:


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


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:


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 6CG7/6FQ7 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?

Edit (May 12th, 2021): Another tube modification I have seen being suggested (e.g. by spritzer, once more) is to replace the 6CG7/6FQ7 tubes with the USSR/Russia-made 6N6PI (which is said to differ from the 6N6P tube only by a so-called "(im)pulse" operation - no idea what that means) instead of the ECC99. The case for this is that the 6N6P(I) has the same pin layout as the 6CG7/6FQ7's apart from pin 9, which is connected to a screen separating both triode sections in the 6N6P(I) which does not exist in the 6CG7/6FQ7. This screen seems to be disconnected from the remainder of the internals of the tube, which is consistent with spritzer's claim that the 6N6P(I) has the same pin layout as the 6CG7/6FQ7, period, unlike the ECC99 for which pin 9 is connected to a section of the heater. Therefore, tube rolling the SRM-T1S with the 6N6P(I) does not require rewiring the tube sockets as we need to do with the ECC99.

However, according to spritzer in the link above, we still need to replace the 33K/2W plate (anode) resistors with 30K/2W ones since the heater current on the 6N6P(I) is 50% larger (900 mA) than on the 6CG7/6FQ7 (600 mA). Interestingly, it is the same resistor substitution used for the ECC99 (which has a 800 mA heater current)... If one were not to apply the CCS mod as well. Otherwise, one just looses all the plate resistors altogether, which are replaced by the CCS's. If I understand correctly, given that both the 6N6P(I) and the ECC99 require the same plate resistor substitution, the former must use the same CCS current as the latter, as derived from Ohm's law and the 320V rail voltage connected to the anodes (320V / (2 x 33kOhm) = about 4.8-4.9mA per plate goes to 320V / (2 x 30kOhm) = about 5.3mA per plate). Is this correct?

What I fail to understand is which is the possible audio improvement one may get from tube rolling the SRM-T1S with the 6N6P(I) instead of the ECC99, if any. Again, if I correctly understood Kevin Gilmore's criticism of the SRM-T1(S)'s design, the main problem with using 6CG7/6FQ7 tubes in this circuit is that they end up being driven by a plate voltage too close to its maximum spec (330V, formerly rated 300V), which causes loss of high-frequency response at higher volumes despite the fact that the tube itself is not so strained since it is only being driven at 320V x 9.8mA = 3.1W = 55% of its maximum plate power dissipation (5.7W on both plates together, according to more recent datasheets). Given that the loss of performance caused by this mild voltage overdrive is a lesser effect than the loss of output current in the plate resistor array (solved by the CCS mod), the ECC99 nonetheless solves the former problem since its maximum plate voltage is 400V.

The plate dissipation power with either the ECC99 or the 6N6P(I) is 320V x 10.6mA = 3.4W, whereas the maximum plate dissipation power on the ECC99 is stated in its current datasheet from JJ as 3.5W - per section? It does not say... Older datasheets set it at 5W, some forums reported 8W on both plates but that info is most likely outdated. Update - June 8th, 2021: I've contacted JJ directly to clarify this, and according to them the maximum plate dissipation power of the ECC99 tube is indeed 3.5W per section and 7W on both plates together. That puts the plate dissipation power of the ECC99 in the SRM-T1S at about half of its maximum, which should make for great tube lifespan just like the 6CG7/6FQ7. That kind of answers my second question above. I conclude that the only technical disadvantage of using ECC99's in the SRM-T1S is having to rewire the tube sockets in a not easily reversible (if at all) way, so one is basically stuck with ECC99 tubes if one decides to follow that route. The question of whether the ensuing sonic improvement (if any) is worth it remains, of course.

The 6N6P(I), on the other hand, has about the same, slightly insufficient (300V) maximum plate voltage as the 6CG7/6FQ7 and therefore is potentially subject to the same voltage overdrive problem as the 6CG7/6FQ7. Tube longevity should be ok, though, since the 6N6P(I)'s maximum plate dissipation power is 4W in the 6N6PI variant (couldn't figure out either whether this figure is for a single section or both) and 8W for both plates in the 6N6P variant. Why should I expect the 6N6P(I) to perform sonically better in the SRM-T1S circuit than the 6CG7/6FQ7, let alone the ECC99? The only advantages I see in the 6N6P(I) for the CCS-modded circuit are:

  1. Tube rolling with the 6N6P(I) instead of the ECC99 is (at least more easily) reversible (the only other operation needed besides exchanging tubes and rebiasing should be adjusting the plate current in the CCS's trimpots),
  2. Possibly a (slightly) longer lifespan than the ECC99 due to the more conservative plate dissipation power (up to a rated service life of 500 hours for the 6N6PI variant, against the 2000-hour figure of the 6N6P variant - no idea why there is such a difference), and
  3. 6N6P(I) tubes are usually cheaper and easier to find than adequate 6CG7/6FQ7's, especially in matched pairs for easier rebiasing,

none of which really being related to audio quality. Am I missing something?

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.

Edited by plaurids
Added new info regarding 6n6pi tubes and ECC99's maximum plate dissipation power
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  • 3 months later...

OK, got all the replacement caps, safety resistors and a nice CCS mod circuit - what I'm lacking now is time to open the energizer and change the components...

Also just got an additional incentive to recap and mod my SRM-T1S - managed to get a fine used Omega SR-007A (serial number SZ2-1636) in addition to my Lambda Signature SR-407. Currently burning in the newly acquired cans, but already amazed at how different the 007A and the 407 sound from each other.

The most obvious differences to me are the noticeably stronger bass and somewhat depressed upper midrange of the 007A as compared to the 407. Resolution, instrument separation and imaging are clearly superior on the 007A (I didn't think that was possible), and the soundstage is wider (probably partly due to the thicker earpads). However, the aforementioned relative "darkness" in the mids and a bit of excessive "blast" in the bass (both to an extent depending on the recording) make the 407 sound more natural to my ears (personally, I'm not really a bass buff), at least when both are paired with my yet unmodded SRM-T1S, but 80% of the time the 007A is actually more pleasant to listen to (the 407 seems to be less forgiving with respect to less than ideal recordings). And yes, I do need a bit more of volume on the 007A - I find myself pushing the volume to 10-11 o'clock on the 007A and to 9-10 o'clock on the 407.

Comfort-wise, the 007A is light-years ahead of the 407 - the latter's headband assembly doesn't have the ratcheted height adjustment of higher-end Lambdas and cannot keep my head's fit for long, which is really annoying. Another problem I have with using the 407 for longer periods of time is that it squeezes my head a bit too hard. (update - June 22nd, 2021) In view of the aforementioned issues, I've replaced the original headband assembly of my 407 with the one from the L700 Mk II. I was afraid that the color mismatch would make the 407 look weird, but it actually looks quite nice in my opinion. The new assembly is sturdier (thanks to a large extent to the new metal driver holders) and keeps the height adjustment where I set it as it should, but it still presses my head a bit too hard for me to keep using it for longer periods, even though it's better than the original assembly in that respect as well. As for the 007A, once I figured out the proper earpad angle for my head, I simply forget it's there.

Can't wait to hear how the 007A will sound after the mod on the SRM-T1S.

Update (June 22nd, 2021): should be able to do the recap and mod of my SRM-T1S by the end of July. In the mean time, I decided to try spritzer's port mod on my 007A. My initial impression matches what others reported - the port mod evens out the response, the bass now sounds a bit more natural and (more importantly) that depressed upper midrange which was my main gripe with the 007A seems to be gone. Another effect is that I seem to get a slightly higher volume in good recordings. I guess sealing the port slightly increases the efficiency. However, the modded 007A seems to have become less tolerant with subpar recordings (especially ones with at least a fair amount of dynamic compression), becoming closer to the 407 in that respect. This leads me to conclude that the port aimed not only at eliminating the "Stax fart" from the original Mk I (yes, my 007A is "farty" now), but also at making the 007A more agreeable to the majority of modern recordings when used with "lesser" energizers like the (stock) SRM-T1S, which is clearly better matched to the 407. This will probably change after the mod, since the stock SRM-T1S sort of underpowers the 007A.

Edited by plaurids
Added sound impressions after port modding the 007A, added comfort impressions
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On 1/21/2021 at 2:36 AM, naamanf said:

Needs Blackgates.

Oh, are you referring to the electrolytic caps? I've already bought the EPCOS / Panasonic combo listed above, I figured it would make most sense since these are all power supply caps and they seemed to outperform or match most other choices for that application. Not to mention that Black Gates are discontinued and hence hard to find in the specs I need. Actually, I did look for Black Gates while researching this, but Hificollective is the only place I could find that still has some of those in stock.

For instance, Black Gate WKz caps would theoretically replace the C11-14 snap-ins but their physical size is too large for them to fit in the PCB, and the ones with the appropriate rated voltage (500V) are only 100 uF at most, so at best one doesn't gain any extra filtering muscle compared to stock. The EPCOS caps, on the other other hand, are 470 uF and still are smaller than stock. Likewise, I couldn't find any Black Gate caps that can match or outperform the specs of the Panasonic caps for the replacements I need within the allowed physical dimensions for the SRM-T1S's PCB.

Honestly, I couldn't get data on DF and ripple current for the Black Gates (which are the other specs that count the most for power supply caps), but I doubt they outperform the EPCOS / Panasonic combo within the allowed PCB dimensions and lower bounds on the capacitances and voltage ratings in that respect either.

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  • 6 months later...

I've finally managed to take some days off from work and open up my SRM-T1S for the component changes. To summarize, this is what I did:

1.) Recap. The stock power supply caps were replaced as follows (Mouser links to all caps listed below can be found in previous posts of this thread):

  • 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
  • C11-14: 4 x Nichicon KX series - 330uF, 400V, 105oC, DF=0.15 (at 120 Hz, 20oC), ripple current = 1.44A (RMS, 120 Hz), life = 2000h, dimensions (LS x D x L): 10mm x 30mm x 50mm
  • 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
  • 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

I didn't use the EPCOS / TDK B43547A9477M000 (B43547 series - 470uF, 400V, 105oC, DF=0.15 at 120 Hz, 20oC, ripple current = 2.71A RMS at 100 Hz, 105oC, 8000h life) for the big C11-14 caps as originally planned because I've encountered the same problem as thebrunx - the leads of the stock C11-14 caps were off-center and, thanks to that, the C11 and C13 leads were closer to each other than the cap's diameter. Due to that, the EPCOS caps cannot fit, but the Nichicon do (barely). Is the 2000h life of the Nichicon caps a problem in the long term? What kind of lifespan may I expect for the new caps on the SRM-T1S?

2.) Added safety resistors to the outputs:

  • Left +/- (resp. yellow / green), right +/- (resp. red / white): 4 x Vishay CCF025K11FKR36 2W 5.11Kohms 1%
  • Pro / Normal bias (resp. gray / blue): 2 x Vishay CMF654M9900FHEK 1.5W 4.99Mohms 1%

Notice that the wire color assignment for the output terminals in my SRM-T1S differs from e.g. the Stax Pro bias diagram in this post on adjusting bias on Stax tube amplifiers.

3.) Replaced the output plate resistors (8 x 33 Kohms) with four constant-current sources (CCS), following the mod design proposed by JimL. I didn't make the CCS array myself; I bought it ready-made from here. However, I did measure the currents before installation at 4.9mA for each CCS in the array using the testing rig proposed by JimL in the AudioXpress July 2017 article where he published his CCS mod.

The recapping had its moments of stress. The copper trails in the SRM-T1S's PCB are quite fragile and it took me a while to find the right temperature for the soldering iron, particularly when desoldering the big C11-14 caps, which not only had a lot of solder to melt but also the cap leads were bent sideways, so I had to keep the soldering iron for longer than reasonable until I could pull off the caps (by the way, the C11-14 caps were also glued to the PCB). In the process, I ended up peeling off a bit of the trail in two places - I've managed to reattach the trail at one place (right below C12), but at the other (right below C23) I had to replace the trail with a small piece of wire (AWG22 pure silver, PTFE-insulated). Just in case, I've measured the conductivity of the reattached trail and it's working fine. All new components were soldered using 3%-silver, lead-free solder.

When it came to turn the energizer on and rebias the tubes, a problem emerged whose cause I've been unable to pinpoint so far. The energizer did turn on and didn't explode (phew!), but it seems to me that the delay circuit is now taking longer than before to "release". So far, no big deal (I think, but it may be important in what follows, so I'm taking note of it). When rebiasing the tubes proper, the voltage between the channel terminals (Left +/-, Right +/-) and the ground terminal (e.g. in the back), which is adjusted by the Balance (TVR2) trimpots, can be set near zero on the right channel as it should but stays at about +/- 280-320V on the left channel, depending on the left channel's TVR2 adjustment. The left channel's Offset (TVR1) trimpot can be set "around zero" (sort of), but it swings with a much larger voltage amplitude than the TVR1 on the right channel - the voltage fluctuations are certainly far above the 15V bias tolerance interval. The latter problem alone makes rebiasing the left-channel tube near impossible. Like the TVR2, the TVR1 adjustment on the right channel is behaving normally.

What I could observe so far is the following:

  1. The problem is not caused by the new power supply caps, since judging by the schematic they should affect both channels equally;
  2. The safety resistors are measuring properly after installation - no shorts, no missed contacts;
  3. A problem with some of the CCS's is possible but unlikely - I don't know if a faulty CCS circuit could cause the problem I'm observing, but if that were the case, it would mean that precisely the two CCS circuits connected to the left-channel terminals are faulty;
  4. I tried to swap the tubes, but the problem stays in the left channel. The same happened after I replaced both stock tubes with a pair of Japanese, short-plated Raytheons (not a matched pair). Therefore, it doesn't seem to be a tube problem;
  5. The tube socket leads on the left channel are not cut off;
  6. All four red leds on the PCB are lit - the only odd thing (which I don't know if it's related or not) is the additional time the delay circuit is taking to release the energizer.
  7. The 2Kohm TVR1 and TVR2 trimpots on both channels are measuring properly (with the amp turned off) at several adjustments on the circuit and don't seem to be mechanically damaged. However, sometimes when turning the left channel's TVR2 trimpot, the voltage drops to reasonable levels for a few seconds and then goes back to 280-320V. This seems to indicate a faulty trimpot, despite the fact that resistance measurements with the energizer turned off seem to indicate normal operation.

Needless to say, the energizer was working normally before I started tinkering with it. Any ideas of what could be causing this? In view of 7.) above, I'll try to desolder the left channel's TVR2 trimpot from the PCB and measure it alone to see if it's really faulty. If that's really the case, I'll probably replace all TVR trimpots with e.g. 2Kohm, 1/2W Bourns 3386F trimpots, which should be fine. I couldn't find 2K trimpots with the same PCB footprint as stock and power rating higher than 1/2W, even though JimL recommended 0.6W of higher based on the rule of thirds. The only other component which could possibly affect the left channel alone in the TVR2 measurement is the left-channel C26 capacitor (ERO, now Vishay Roederstein, model MKP1841 - polypropylene, 0.01uF, 630V), which is connected to ground, but I think this one failing is highly unlikely.

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  • 2 weeks later...

I've found the culprit. A tiny solder blob which seems to have landed out of nowhere was shorting two solder points in the CCS module's PCB. Removed it, resoldered the module to the unit's PCB... and presto! Left-channel voltages back to normal! Phew!

Rebiasing with the stock tubes took a couple of hours (3-4 rebiasing sessions with about one hour in between to wait for the tubes to drift). I'll wait a couple of months to see if it's worth it to start tube rolling.

In the meantime... Currently testing my SR-007A with the newly recapped and CCS-modded SRM-T1S. Amazing! It seems a straitjacket was removed from the cans! The CCS mod really makes a difference.

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  • 8 months later...

I am a old audiophile 75 years now. I had stopped listening to music for for a number of years .  About 9 months ago I decided I would like to listen to some music.

The only way I had to do this was with my computer. I purchased a pair of Audeze headphones a Schitt Valhalla 2amp and dac and signed up for Spotify. Enjoyed listening again but I remembered listening to my Stax SR-X Mrk 2 headphones . I was able to get a pair of them NOS and a used SRM-/MRK2 again enjoyed this even more then the Audeze Schitt. Saw a SRM-T1S on ebay and as I have alwas been a tube guy I purchased it. Used it for about 2 months and one of the tube died. Replaced both tubes and it worked fine for about a month and one channel developed sever distortion. If I turn the amp off while listening the distortion goes away untill the amp sopts playing . Any help as to what may be going bad would be greatly appreciated . Wish I had the skill to do the thing recommenced in these pages but I do not. The most I have ever done it assemble some Dyna kits back in my younger days. 

Thanks for reading my post

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Did you check the offset and balance of the channels?  It's odd that it acts up but clears up when it is just running on the charge stored in the capacitors.  Might point to an over voltage situation somewhere or some part acting up when under full load. 

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Thanks for the reply I would not know how to check the offset and balance. Is there a service manual I could download or purchase that would tell me how to do it. I am going to put the old tube that went out back in as I remember what socket it was in and see if it's the same channel that has the distortion. My thought on doing that is that it may be the amp that took out the tube and not just a tube going bad.


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Hi sc351cobra,

what spritzer meant is that you probably need to check the bias of the tubes since you've changed them - offset and balance are the adjustments to look at (by probing adequate pairs of output pins with a voltmeter) when doing that, and they're adjusted through a pair of trimpots for each channel. These are tiny and are best (and most safely) accessed using a plastic-tip screwdriver. A nice set of instructions for rebiasing the tubes on the SRM-T1(S) can be found here and here. In my case, last time the bias of the tubes was off, 3-4 rebiasing sessions with 1-hour intervals to account for tube drift was enough to get stable adjustments, but that may change depending on the tubes you're using and their age. The important part is to get stable measurements as close to zero volts as possible (at worst within a 15-volt interval around zero) without too many attempts - if it's taking too long to reach that, then the tubes have probably reached EOL and should be replaced.

According to the SRM-T1S's manual, possible sources of distortion are too high a volume adjustment and a voltage drop in the power supply. However, both of these should affect both channels equally (supposing you are changing volume of both channels simultaneously at the same rate, as usual - the SRM-T1S's volume knob allows the user to change them separately, and I don't know how you're proceeding with that). Even if the power supply caps need replacement (which is something it's generally recommended to do about every 10 years or so), a bad power supply cap should also affect both channels equally if you are using a pair of good tubes with the same brand and age (preferably a matched pair for easier rebiasing). The fact that you are only getting distortion on one channel and the former gradually disappears as the power supply caps discharge after turning the unit off is really odd indeed. The problem I had above when doing the CCS mod shows that if there is a short circuit somewhere on the PCB by any reason then there may be some unreasonably high voltages on just one of the channels, which on their turn will cause distortion and may shorten the life of the tube on the said channel. If that's the case, it'll definitely show up in the bias measurements as unreasonably large voltage swings as you adjust the bias trimpots (e.g. well above 15 volts with even the slightest turn of an offset trimpot), and then you'll have to look for that on the signal path through the PCB. I wouldn't exclude it since you bought your unit used, but I think a bad tube is more likely. In any case, checking and trying to adjust the tube bias is the first step.

If your problem doesn't disappear after rebiasing (supposing the latter is successful), then I'm out of ideas (unfortunately, my knowledge and experience with electronics are quite limited as well - I'm trying to help based on what I've learned so far). Hopefully, more seasoned people on the matter (like spritzer) may know better and provide a more educated answer if that's the case.

Hope it helped.

Edited by plaurids
Added considerations on possible causes for distortion on only one channel
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Thanks for the information and it could well be a tube bias issue . It is a right channel problem I know that now after putting the bad tube back in. I then put the tube that was in when the distortion started and have been listening for about an hour and the distortion has not come back. I will read the information about bias and if I feel confident I can do it I will give do at least that much. Right now I'm just waiting for the distortion to come back although I am very much afraid it is intermittent .


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6 hours ago, sc351cobra said:

Just wanted to add that with my home audio tube amps I would keep a eye on the bias and keep it adjusted with the exception of my Audio Research D115 I never had the nerve to do the bias adjustments on it for fear of electrocuting myself


Your reservation in trying to rebias amp tubes yourself is understandable, especially in the case of e-stat energizers (which use even higher voltages than standard amps), but I assure you it's safe provided you're careful. In the case of the SRM-T1S, you can just adjust the voltmeter range, stick the voltmeter probes into the earspeaker pin sockets appropriate to the measurement to be made (that's the easiest and safest way) and turn the voltmeter on. This can be done with the energizer turned on without any problems. Please remember as well to get a fine plastic-tip screwdriver for adjusting the trimpots - I'd avoid getting anything made of metal close to the PCB while the energizer is on - and avoid touching the power supply caps (especially the four big ones). Finally, bear in mind that the trimpot adjustments for the SRM-T1S are usually tiny and the tubes in this circuit usually do drift quite a bit after the first rebiasing sessions.

If even so you feel insecure about doing it yourself, you can ask a technician to do it.

Edited by plaurids
Added safety tip on power supply caps
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Thanks for the help guys but the amp has now gone out in the right channel and output level of the right drops off at the same time channel goes out. The amp yesterday worked fine for over 2 hours today maybe 3 minutes. So I'm searching for for someone to repair and recap it in my area. I'm going into this with a lot of dread unlike in my younger years when I was a active audiophile and knew people in the area who worked on tube amps that has past me by along with my age. But then today is my 75th birthday so I'm happy to have made it this far.

Just a note for Spitzer strand as it may seem as soon as I turn the amp off everything clears right and both chanels work wile the caps drain.

I do have a solid state SRM-1/MK2 that I am listening to right now but sure miss my tubes.

Plaurids thanks for the information on bias think I would be able to do that with care but I'm pretty sure the amp needs more then bias adjustment. But if I am able to get it repaired I will be checking the bias myself.

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2 hours ago, sc351cobra said:

Just one more question are there any new tube amps that I can use my SR-X MK2 headphones with?


No idea. The SR-L407 that came together with my SRM-T1S is the oldest of my two Staxes, and never had any experience with an e-stat older than that.

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