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    Trying to be an audiophile with a tight room and a tight budget...
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    São Bernardo do Campo
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    Adjunct Professor of Mathematics at UFABC
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    Stax Omega SR-007 Mk I, Stax Omega SR-007A, Stax Lambda Signature SR-407, Moondrop Blessing 2 IEM + Dunu Blanche cable, AKG K701
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    Stax SRM-T1S
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    RME ADI-2 DAC + iFi DC iPurifier + Oyaide Continental USB cable, Ifi Micro iDSD Black Label DAC, Fiio X7II DAP, Sony MDS-JE500 Minidisc Deck
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    M-Audio ProFire 610, Focusrite Scarlett 2i4 1st gen, RODE NT1 Condenser Mic

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plaurids's Achievements

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  1. Just a short-ish recap (badumtss) of everything I've done so far to improve my SRM-T1S, and a roadmap for future updates already in motion. All electrolytic caps replaced and safety output resistors added in August 2021, as described in detail here. All plate resistors replaced by CCS's in August 2021. The CCS circuits I used were designed by James Lin (aka JimL here on Head Case) and built on demand by Dominik Stritt (aka Firschi in the Head-Fi forum). Power supply resistors R31-34 used to derive the +/-48V rails from the HV rails replaced (new resistors are Dale on R31-32 and Philips on R33-34) about a month ago, as described here. Stock (Gold Aero Stax Signature = most likely rebranded Sylvania) 6CG7 tubes replaced by a pair of Japanese short-plated Raytheon 6CG7's about a month ago. Upgraded power cord to a 2-meter-long Audioquest NRG-Z3 this week. Have said plenty already about 1.)-3.) in previous posts (some of the linked above for convenience), so just a few words about upgrades 4.)-5.) Regarding 4.), recall that Raytheon tubes are usually rebrands from other manufacturers. Up to recently I couldn't determine which Japanese brand was the original maker of my Raytheon 6CG7's. This week, however, I managed to get a matched pair of Toshiba short-plated 6CG7's, which according to hirsch and spritzer are the best match (ahem) to the SRM-T1(S), and their insides look exactly the same as my Raytheons', so I reckon the latter are indeed actually made by Toshiba (yay!) as I had hoped them to be, based on hirsch's suspicions in the link above. The Raytheons indeed sound better than the stock tubes, even when they were in better shape, so I infer the Toshibas will perform similarly. Will tube roll again in the near future to confirm that. (btw, the date code of the Toshibas seems to be "3L", next to "Tokyo Shibaura Electric" = extended name of Toshiba, with a 10 engraved on the glass bottom. Couldn't figure out the manufacturing period from that info yet) As for 5.), I've decided to have a stab at some of lupoal's suggestions (more on that later). To be honest, the power cord solutions I've previously used on my SRM-T1S were all unsatisfactory in many aspects, even though I hadn't noticed any RF interference with the last power cord I used. lupoal's comments made me revisit this discomfort of mine. Last week I happened to find a mildly used Audioquest NRG-Z3 power cord for sale at a reasonable price (these are usually crazy expensive), so I decided to hit the hammer on it. From pure safety and connectivity viewpoints, it's a far better solution and now I'm dead certain that no RF interference will find its way through it, but couldn't tell so far if it had a noticeable sonic improvement (I guess not). Now I'm planning the following upgrades / experiments. All needed parts (except for 4.)) are either already here or at least already ordered and under way. Try to use 6SN7GTB tubes in the output stage, replacing the 6CG7's. More precisely, I've got a matched pair of US-made NOS Tung-Sol 6SN7GTB's (more details on these below), which will be attached to the T1S circuit using a pair of custom made, angled socket adapters so that I can keep the top cover. These will be using an increased plate current of 6mA/section by recalibrating the CCS's, as privately recommended by Dominik (who also made the adapters), see also here for a similar recommendation of his using ECC99 tubes (in German, sorry). The higher maximum plate voltage and plate dissipation power of the 6SN7GTB should in principle allow for a higher output power (approx. 22% more) while keeping farther away from the tube's voltage limits, which should turn into a superior frequency response with my SR-007's while keeping a rather conservative plate dissipation power (about 51% of the total maximum). Hopefully this will be enough to mitigate the T1S's deficiencies when driving the SR-007's. The higher plate current will require replacing the 2.2kOhm R20 cathode resistors in series with the TVR2 trimpots with 1kOhm resistors in order to achieve 0V cathode bias. Replace the TVR1-2 trimpots with multiturn Bourns 3339H-1-202LF 2kOhm trimpots, as suggested by factory2nd above. This is mainly for convenience when rebiasing. These have a power rating of 0.5W each - although JimL had suggested going for at least 0.6W based on the rule of thirds, my research couldn't find trimpots (be they single-turn or multiturn) in the market with a compatible footprint and a power rating of at least 0.6W, which led me to believe that the stock trimpots are probably rated at 0.5W as well (no information on this in any of the schematics I could find, though) and so it's probably OK to use these Bourns trimpots as TVR1-2. Replace the stock (1.5A) fuse with a (properly directed) Gustard 1.6A 5x20mm fuse, as suggested by lupoal above. The choice of current rating is the closest one to stock available. Let's see what happens... Unlike lupoal, I don't see the need to replace the fuse holder, though. The stock fuse's size seems to be the same. Moreover, as a rule, I find my T1S's connectors and fuse holder to be of pretty good quality (unlike in pictures of many other T1-like units I've seen around), so I don't see a point in replacing any of these. I might try using spikes instead of the stock feet in the future, still thinking about it... If I decide to go this route, the best option seems to be the Japanese Kryna T-Prop TP-4 M3 spike / insulating cup assembly (4 = number of devices = "mechanical diodes" as Kryna calls them). Particularly, these use M3 screws just like the ones securing the stock feet, so no glue involved in attaching these. I might try rebiasing the 6CG7 tubes with the 1kOhm resistors at R20 just to see if bias can be achieved in practice at 4.9mA/section, as suggested by my calculations above (probably with a bit less leeway from TVR2's extremes). If so, this will simplify going back and forth with these and the 6SN7GTB's - only adjusting the CCS's will be needed, if I wish to change the plate current at all. If that works, I'll try next just swapping these tubes while keeping that lower plate current to see how the sound changes with just that. If I like the result with the 6SN7GTB's better, I'll proceed and recalibrate the CCS's to achieve 6mA/section. Finally, a few words about the particular Tung-Sol 6SN7GTB's I've got. Each one has a tall bottle, black base, white labels, triangular black plates and a top getter (couldn't tell which getter type from the vendor's pictures due to the chrome dome, will update here when the tubes arrive). The label reads 322KT3 on both tubes - "322" is the EIA code for Tung-Sol, whereas "KT3" indicates the 12-month warranty period. This is an internal factory code for which no listing seems to be available on the Internet. According to the bits of information I've managed to gather (mainly here, here and here), the first letter indicates the last digit of the year (within a certain production period) and the second letter indicates that year's quarter offset by a month (plus), and in my particular case (KT) this seems to point to an August-October 1960 manufacturing period. No idea what the last digit "3" stands for. At the very least, I know these were made in the same period. If my above assessment of the tube code is correct, the venerable Head-Fi 6SN7 reference thread indicates that these tubes ought to be quite good... Let's see.
  2. This means other components in the would-be SS circuit must be microphonic to some degree and thus are smuggling the transformer's mechanical vibrations into the signal path, albeit not to the same degree as (some) vacuum tubes... Maybe are some of the caps the culprit, behaving like tiny condenser microphones? Mind you, I'm assuming here that in this hypothetical circuit there are no ceramic caps, which are known to be piezoeletric and thus indeed microphonic. Even so, if that is happening at (or even before) the initial amplification stages, the effect could indeed become noticeable. However, properly chosen caps shouldn't cause these problems to begin with... Regarding 6SN7 microphonics, the links on my previous reply seem to indicate otherwise, but I do agree that those seem to be largely anecdotal evidence, so I'm inclined to believe in you on that. After all, 6SN7's are used in a huge number of hi-fi amp designs. Moreover, the effect of tube microphonics (if present) tends to be minimized as one moves towards the output amplification stage since any mechanical pickup into the signal path at that point will be much less amplified towards the output. Small tubes are usually employed as initial driver stages for larger output tubes (EL34, 300B, KT66, etc.), where minimizing tube microphonics is hence more critical. For a hybrid, tube output-only design such as the T1 and its siblings, this is good news. On the other hand, the caps used in T1-type circuits don't at all seem prone to be microphonic, they are either electrolytic (in the power supply and the delay circuits) or polypropylene / polystyrene. Moreover, Stax had care in using only tantalum-film resistors in the signal path (which eliminates any microphonics stemming from residual capacitance) and non-magnetic parts wherever possible (even the screws and the metal standoffs). In view of that, I'm forced to conclude that the major potential source of microphonics in T1-type circuits is still the output tubes, however small it may be, so it boils down here to tube lottery, so to speak. Recall as well that we're talking about subtle audio effects here... Even a tiny 50/60 Hz mechanical pickup from the transformer could in principle affect the noise floor and thus the low-volume output resolution, with ensuing impacts on decay, imaging and soundstage. US-made NOS Tung-Sol 6SN7GTB's can be quite expensive these days, as they're in high demand. That's why I'd decided to try the new, re-issued Russian ones... However, I'm already seeing the impact of the Ukraine war on Russian tube prices and availability, my latest research shows there is no longer such a big price difference among re-issued Russian 6SN7GTB's and Sylvania / Tung-Sol NOS ones, and the former are getting harder to find. I don't know... I may end up getting the US NOS Tung-Sols after all. Moreover, as I mentioned at the end of another of my previous posts, Western Electric (the makers of their legendary 300B triode) is planning to start making 6SN7's in the near future, and their prototypes are reportedly quite good. The only question mark is how expensive these will be... I'm a scientist by trade, so I'm curious by nature but also a skeptic. I also try to temper my curiosity with method because this is an expensive hobby, spending can easily get out of hand without a corresponding improvement in sound quality, so demystifying and understanding what's going on matters to me. In other words, my being an audiophile isn't just for listening pleasure but also a scientific (pleasure) pursuit. There is also a practical obstacle. Making hi-end power cords is much harder here in Brazil because of our power outlet standard (ABNT), which is different from the US and European ones. There are simply no hi-fi audio-grade plugs available for that standard, and using plug adapters (which are mostly crappy) kind of defeats their purpose. I'm not at all willing to restyle my office's wall outlets just to be able to use such power cords without adapters, I need them kept to Brazilian standard for other purposes. Recall as well that these outlets are ungrounded and there's nothing I can do about it since I live in an apartment. Supra LoRad 3x2.5mm2 shielded audio power cable is not difficult to find around here, but it's very (even if not the most) expensive... Finally, investing in hi-end cables and power cords is firmly into the territory of diminishing returns. My experience with the cable and power supply purification upgrades I've made so far is that they do represent a noticeable, but subtle improvement, and only so because the gear was already so good to begin with (again, my audio chain is already very quiet and low-volume resolving as it is, without any noticeable interference pickup). Otherwise, any effects would be overwhelmed by other defects (no pun intended), and no one would notice a difference at all. By the same logic, it makes no sense to get "best of the best" cables if one's whole audio chain is anything less than that... The SR-007 Mk1 may still qualify for that rank, but I wouldn't call the T1 (even with a CCS mod) "best of the best" by any stretch of imagination - again, bang for buck is the name of the game here. If someday I get a BHSE or a DIY T2, then maybe going for flagship cabling will bring meaningful improvements. In other words, I don't regret making such an investment, but I find it difficult to justify beyond a certain cost. This means I'll have to think about your suggestion... but thanks again anyway!
  3. The value of R20 doesn't really have to do with using the CCS mod or not. It has to do with achieving 0V cathode bias at a certain operating point for the output tubes, regardless of whether you load the plates with resistors or CCS's. Let me take the opportunity to be a bit more precise about what I wrote in my previous reply: if you look at 6SN7GTB's datasheet (just google it), if you keep the stock operating point of +320V plate voltage and a plate current of 4.9mA/section (be it with 2 x 33k resistors as stock or a CCS replacing them), you see that the corresponding grid voltage is approximately -13.4V. At this operating point, the grid current should be negligible and we may take the cathode current as equal to the plate current = 4.9mA. To achieve 0V cathode bias, we need to compensate this with an appropriate resistor between the cathode and the -350V rail, which is done by R20 and the TVR2 trimpot in series. The appropriate resistance value is then given by Ohm's law: 13.4V / 4.9mA = 2.7kOhm, which is practically the same setting as if you were using stock 6CG7 tubes and therefore maintains a leeway of about 500 ohms when adjusting TVR2. This is not surprising since the 6CG7 is derived from the original 6SN7 (not GTB). Of course, such a value is only approximate and is subject to fluctuations due to circuit component variations and aging, that's why we need a trimpot here in the first place as you know well. If you replace R20 with a 1k resistor while keeping the same plate current, this means you only get a elbow room of about 300 ohms for adjusting TVR2 as tubes drift with age and use. This means you actually lose a bit of control/stability if you put a 1k resistor on R20 in this scenario. On the other hand, if you increase the plate current (again, by either changing the plate resistors or replacing them with appropriately set CCS's), the stock R20 may no longer be favorable in that regard, as explained in my previous post, so then it makes sense to change it. Going back full circle to my first statement, the gist here is this: if you keep a plate current of 4.9mA/section as in stock config, you may keep R20 as stock as well, regardless of whether you use the CCS mod or not. So you're using those ceramic cones as spikes, I suppose... Always wondered about their actual sonic effect. After some research, my understanding is that they should only matter if the tubes have an appreciable microphonic behavior (so they start picking up surrounding mechanical vibrations such as transformer vibration and so on), and this actually depends on each individual tube sample. Indeed, the 6SN7 seems relatively prone to be microphonic, especially for older samples (see also this discussion), but badly microphonic 6SN7's should still be rare. I don't know. Anyway, it's good to be aware of that... The fuse upgrade is new to me, never heard of it. I don't really feel motivated to mess with cables any further... I'm using a pretty standard, computer-type ungrounded power cable (I don't have grounded power outlets in my apartment, unfortunately) with my T1S, and my audio chain is the following: Computer -> RME ADI-2 DAC -> CCS-modded SRM-T1S -> SR-007 Mk1 / SR-007A I use an Oyaide Continental pure-silver USB cable with rhodium contacts between my rig and my DAC, which is connected to the T1S using studio-grade custom balanced XLR cables. The DAC's power supply is actively filtered using an ifi DC iPurifier. As it is, the chain is dead quiet, without any perceptible floor noise and with plenty of low-volume resolution. My experience with tube rolling my CCS-modded T1S is pretty limited so far... until a couple of months ago I was just using the stock Gold Aero 6CG7 tubes selected for Stax (according to my research, mine are probably rebranded US-made Sylvanias), but they've started getting a bit unstable as of late (reaching EOL, maybe?), so I started using Japanese short-plated Raytheon 6CG7's a couple of weeks ago. Not entirely convinced of any improvements yet, still evaluating - my mess-ups during rebiasing kept me about a month without listening to my Stax earspeakers, so my previous impressions are a bit blurred. Currently I have my sights set on a pair of short-plate Toshiba 6CG7's (which are supposed to be the best match to the T1(S)), but will probably add brand-new Russian Tung-Sol 6SN7GTB's to my tube repertoire as well, planning to use angled adapters to allow me to keep the top cover. Will try comparing these without changing the plate current at first, just as (I guess) you're doing, and then update this thread with my impressions. Judging by your impressions, the Tung-Sol 6SN7GTB's (btw, are yours NOS or brand new?) should be a good match to the SR-007's, given the latter's strong bass and warmth. Which earspeakers are you using with your 006tS?
  4. Hi lupoal, Never heard of such a mod, but more experienced people around here might have... Recall that the role a power supply (PS from now on) must fulfill to ensure optimal sound quality is to deliver as pure and stable a DC current on all rails as possible. No idea on how upgrading the rectifier bridges D7-8 the way you've suggested contributes to this, if at all. The first question that comes to my mind is whether such diode bridges fit into the PCB or not. Wiser people in this forum might have something more educated to say about it, sorry... What is within my limited knowledge on the matter is this: usually the PS improvement which is recommended for the T1 and its siblings is replacing all electrolytic caps, preferably with ones with higher ratings for better ripple suppression right after the rectifier bridges D7-8 (C11-14) and at the +/- 48V rails (C9-10). Anything appreciably better than that, I think, could only be achieved by an actual regulated PS, which unfortunately cannot fit into this PCB. Some bold people seem to have tried going full BHSE style and constructing an external regulated power supply, but the amount of work, time and money involved in this over just buying something like, say, a KGST seems a bit silly to me. The whole idea of all these mods is getting more bang for buck. The theoretical advantages of using 6SN7GTB or ECC99 tubes over stock 6CG7/6FQ7 are: The maximum plate voltage of these tubes is larger - 400V for the ECC99 and 450V for the 6SN7GTB, so you are comfortably away from these limits with the +320V plate voltage used in the T1(S) and the 006 variants, unlike the 6CG7/6FQ7 which are driven slightly over their spec* by these circuits. According to Kevin Gilmore, this causes loss of high-frequency response at higher volumes. This is a benefit you're already reaping. Their maximum plate dissipation power is also larger - 3.5W/section, 7W total for the ECC99 and 5W/section, 7.5W total for the 6SN7GTB. This gives us extra headroom for increasing the plate current while still keeping plate dissipation power rather conservative - e.g. from stock 4.9mA/section to (say) 6mA/section (more on that later) -, which mantains a long tube lifespan (55% for total maximum plate dissipation power for the ECC99 - same as stock config - and 51% for the 6SN7GTB at 6mA/section). Increasing the plate current, on its turn, increases the output power, thus benefiting hungrier, more inefficient earspeakers such as the SR-007. (* - more recent datasheets for the 6CG7/6FQ7 do rate its maximum plate voltage at 330V, but older ones do it at 300V. Hence, If you use older NOS tubes, you'll most likely exceed spec, but even for newer NOS tubes it's too close for comfort) Regarding (2.) above, it must be pointed out that changing R20 doesn't change the plate current, but rather its replacement is needed if you do it (see next paragraph for how to) because otherwise you may have trouble rebiasing the tubes, you'll be too close to (or maybe even beyond) one of the extremes of the balance (TVR2) trimpots to achieve zero-volt bias if you leave R20 as it is. If you're still using 4.9mA per tube section, you can (and in fact should, for the same reason) leave R20 alone as (I guess) you've done so far. If you actually look at R20 in the PCB, they are in the bottom side and are pretty tiny, there is no room at all for alligator clips in there without risking shorting something - a bad idea for a HV circuit such as this... To actually change the plate current, you need either to change the plate resistors R27-30 (x2), or to recalibrate the four plate CCS's to achieve the desired current if you're using the CCS mod. For example, if you want to use 5.3mA/section with plate resistors, you need to replace all 8 of them with 30k resistors instead of the stock 33k ones, just like in the SRM-600 which uses ECC99 tubes. Just watch out for their power ratings (2W each), they need to be kept the same! If you do so, the plate current will increase to (about) the desired value according to Ohm's law (recall that these are fed by the +320V DC rail). In the SRM-600, the plate voltage is +350V, so the plate current there is actually 5.8mA/section. BTW, how is your experience using 6SN7GTB's so far over stock 6CG7/6FQ7 tubes regarding sound quality? Have you noticed any improvements?
  5. True, but the current through R5-6 is DC only, there is no signal through them. That's what I meant.
  6. Whoa. That's pretty... steampunk. True about the +/- 48V rails, that's what really matters. Looking at the SRM-T1 schematic, R5-6 are not really part of the feedback loop, but rather of the power supply to the input stage, which is modulated by the feedback loop. Those resistors don't really enter the signal path. The non-electrolytic caps of the T1(S) circuit are of good quality and shouldn't need replacement unless the unit was really mistreated. Visually yours look ok. They are used only for the input RF filtering, for the driver membranes' bias stage and for safety. In the particular case of the feedback caps, these correct the phase shift due to the signal path's residual capacitance in parallel with the feedback resistors, which in principle may lead to feedback instability if left alone. These caps, however, probably could even be discarded (possibly with a higher-quality choice of components in the remainder of the signal path), as done in e.g. the Mjölnir Audio KGST energizer, which is essentially a CCS-modded T1 with a pair of separate 6S4A triodes replacing each 6CG7 tube and a better, properly regulated power supply, which altogether allow for higher voltage swings and higher plate currents. T1(S)'s design is direct coupled and there are no caps in the signal path, so upgrading these non-electrolytic caps should have virtually no impact on the sound quality. I wouldn't do it unless something goes wrong. As for the signal path resistors, I believe any sonic improvement coming from upgrading them would be rather minor, if any. The upgrades on the T1(S) and its offspring (SRM-600, SRM-006t, etc.) that make the most difference are: Changing all power supply electrolytic caps. This is basic maintenance and should be done every ten years or so. Upgrading their ratings is the only way to improve the supply's power filtering and is easy to do since modern caps are smaller for the same ratings. Replacing the plate resistors with constant current sources - the famed CCS mod. This removes the waste of current driving the earspeakers and greatly increases the effective output power, improving frequency response throughout the entire audio spectrum. This is particularly noticeable with higher-end, power-hungry Staxes such as the SR-007, but any earspeaker model will benefit from this. Tube rolling. Again, you'll end up doing it eventually since tubes have a finite lifespan. Here you can either stick to 6CG7/6FQ7's or try ECC99's (as in the SRM-600) or 6SN7GTB's (as in the SRM-700T). The latter two can be found brand new (JJ, Tung-Sol, etc.) but will require either rewiring the tube sockets or using appropriate adapters (in the case of the 6SN7GTB), as well as changing the cathode bias resistors R20 to allow for rebiasing with an increased plate current, as explained in former posts in this thread. As for the former, it's usually recommended for this particular circuit to stick to Japanese tubes, particularly Toshiba, NEC, Matsushita or rebrandings of these by e.g. Raytheon. These are all (N)OS, by the way. However, tube rolling enters deep into the realm of personal taste. I'd guess 6CG7/6FQ7's are plenty enough for the L300 since the T1 was originally designed to drive the Lambda series. All that being said, your choice of Bourns multiturn trimpots seems indeed like a good idea. I haven't considered it before because I thought a 1/2W power rating for TVR1-2 was a bit tight, especially if I increase the plate current to 6mA per tube section (recommended if one decides to use ECC99 / 6SN7GTB tubes), based on the safety rule of thirds: power rating = 3 x maximum power dissipation through the component in the circuit. However, from my research on trimpots with power ratings higher than 0.5W I no longer expect the stock TVR1-2 to have a greater power rating than that anyway. That makes me want to reconsider my choice... Rebiasing with the stock trimpots is really a pain...
  7. Hi factory2nd, welcome to the Stax journey! Visually your R31-32 look almost like mine and this (both rated at 8.2k instead of 7.5k, though) and your R33-34 look like this, which has the same ratings for R31-34 as yours. I do think these are stock, though, my guess is that Stax made a bit of a mess with these components along the T1(S)'s production run. C11-14 certainly look like stock, changing all electrolytic caps is a good idea. I've never touched the feedback resistors (R21-26 in the schematic) and caps (C5-8 in the schematic) because I've never felt the need for it. Same for the TVR1-2 bias trimpots - I did suspect their failing in the past but it actually wasn't the case. I've contemplated the possibility of replacing them with multiturn (e.g. Bourns) trimpots for easier rebiasing, but I couldn't find a multiturn model with a compatible footprint and the same ratings as stock. As a general rule, I wouldn't change any components that aren't prone to failure unless they do go bad (i.e. "if it ain't broken, don't fix it"). The only exceptions are the tubes (if you feel like tube rolling) and the plate resistors R27-30 (in case you decide to do a CCS mod on your T1). R31-34 are probably the resistors in the circuit under the most stress since they are used to derive the +/- 48V rails from the HV rails, but I've only ended up replacing them because one of them got a broken lead during testing, so it wouldn't hurt to do it. If you decide to do it anyway as well, I'd keep the same ratings as in the replaced components and use the R31-34 power ratings from the T1 schematic since these usually don't show up in the components themselves. Just out of curiosity, which earspeaker model do you intend to use this T1 with?
  8. A side (bunch of) question(s): as of late, I've been contemplating the possibility of trying 6SN7 tubes on my CCS-modded SRM-T1S using e.g. the angled socket adapters made by Stritt Audio. Not the original 6SN7, mind you, but the GTB version. I've managed to find brand-new Russian Tung-Sol 6SN7GTB pairs (currently used in the Stax SRM-700T) for a reasonable price. The GTB variant of the 6SN7 has higher maximum plate voltage (450V) and higher maximum plate dissipation power (5W on a single plate, 7.5W on both plates) than the original 6SN7, the 6CG7/6FQ7 and even the ECC99 tubes, while keeping the remaining characteristics (as far as I could tell, please do correct me if I'm wrong) the same as the 6CG7/6FQ7. My idea with using a pair of 6SN7GTB's is to use the extra elbow room on the maximum plate voltage to improve my SRM-T1S's behavior with my 007's at higher volumes and frequencies since these tubes won't be driven so close to their limit in that regard. Moreover, by JimL's reasoning on this Head-Fi post of his, I can also increase the plate current from 4.9mA to 6.4mA per tube section in the CCS module, while still keeping me under 55% of the maximum plate dissipation power on both sections. Hopefully, this should somewhat mitigate the lingering deficiencies of my CCS-modded SRM-T1S when driving my SR-007 Mk1 and my SR-007A. Still a far cry from the likes of the KGST and the BHSE, I know, but theoretically I should get some further improvements in the sound quality at a minor cost. At the same time, I'd like to try a more easily reversible mod than using ECC99's, should I dislike the ensuing sound. This is a real risk since spritzer recently discouraged me from trying ECC99's on my CCS-modded SRM-T1S, claiming that they don't sound therein as good as the original 6CG7/6FQ7. In view of that, I'd like to try a different tube flavor from a safer return point. Now for a quick recap of how to determine the approximate value of the cathode bias resistance and, from there, which value the R20 resistors in series with the balance (TVR2) trimpots should take for a good elbow room on the latter when rebiasing. The operating point for each triode section is 4.9mA for the 6CG7 and 6mA for the ECC99 at +320V. If I use the same operating point as the ECC99's for the 6SN7GTB, looking at datasheets for these tubes I get the following approximate values for the grid (bias) voltage: ECC99: -13.8V; 6CG7, 6SN7GTB: -13.2V. Curiously, I've got (practically) the same grid voltage for the 6CG7 and the 6SN7GTB if I use 6mA plate current for the latter. If I choose 6.4mA instead, the ensuing grid voltage for the 6SN7GTB goes slightly up to (approximately) -13V. To approximately determine the bias resistor value one needs to use the (absolute value of the) grid voltage and the cathode current per section as inputs for Ohm's law. For the latter, if I understood correctly the the theory and the datasheets I can neglect the grid current at these operating points and use the cathode current as being equal to the plate current = 4.9mA for the 6CG7 and 6mA for the ECC99 / 6SN7GTB. By Ohm's law, this results in the following approximate bias resistor values: 6CG7 @4.9mA / section: 2.7 kOhms; ECC99 @6mA / section: 2.3 kOhms; 6SN7GTB @6mA / section: 2.2 kOhms; 6SN7GTB @6.4mA / section: 2 kOhms. These values should match R20 and TVR2 in series with the latter properly adjusted. If one takes into account the variations on all components and how they change with age - most importantly, the tubes themselves - I understand why moving to 1k at R20 is needed if you go to ECC99 or 6SN7GTB tubes, for otherwise there is no practical elbow room in TVR2 for achieving proper bias. Likewise, keeping R20 at 1k if one returns to 6CG7's @4.9mA / section indeed seems also too close for comfort. I still do have questions, though: Can the SRM-T1S's power supply and heat dissipation handle the CCS module set to 6.4mA? Or is it better to be more conservative and keep e.g. to 6mA as suggested for ECC99 tubes? Most importantly, does the overall sound quality really benefit from such a mod, as compared to sticking to the best 6CG7's I can find (e.g. short-plate Toshibas)? If the 6SN7GTB route is successful, there should be yet further room for improvement by tube rolling, especially considering Western Electric may start soon producing 6SN7's to meet the demand ensuing from Russia restricting the export of vacuum tubes due to the Ukraine war.
  9. Well, this is embarrassing... It turned out I was just mistaking the bias pin for ground. Bias voltages are actually OK 🤦‍♂️🤦‍♂️ The silver lining is: the unit just got two new pairs of power supply resistors R31-34, which according to Birgir (aka spritzer) are prone to long-term failure anyway... Anyhow, after a proper session of rebiasing adjustments, indeed the stock (Gold Aero / Sylvania 6CG7) tubes are showing persistent instability, stubbornly drifting more than 5V within an hour even after five rebiasing attempts. According to the SRM-T1 schematic, it should be OK to stay within +/-15V, but these tubes were far more stable one year ago. Taking into account as well the recent need to increase the volume, the time seems ripe for tube rolling, so now I'm going with a pair of Japanese short-plate 6CG7 Raytheons. They definitely showed more stability than my stock tubes through today's battery of rebiasing sessions - recall that I typically do four rebiasing sessions with one hour in between to allow for drifting. I don't know whether the aforementioned relative brightness of the D3-D4 LEDs really signals a problem or not by itself. I'll assume it doesn't. I'll report on the sonic improvements with both the SR-007 Mk1 and the SR-007A in the following days. Thanks again to Birgir and Dominik for all the help!
  10. Have just replaced the R31-34 resistors in the following fashion to keep these parts minimally coherent after one of the R34's leads broke: R31-32: 8.2kOhm, 5W, 5% (Dale). R33-34: 8.2kOhm, 3W, 5% (Philips). These are the same ratings as in the stock parts, recalling that I couldn't figure out the latter's actual power ratings. I've used the only SRM-T1 schematic I have for that rating's reference, but there R31-34 are all rated as 15kOhm. Unfortunately, as expected, the bias voltage problem remains unchanged. Update (October 31st 2022): I messed up the pins last time I made the measurements, sorry. Actually, the problem manifests itself like this: the balance voltage measurements (i.e. measured between +/- and ground, and adjusted by the TVR2 trimpots) as one sweeps through TVR2 are (with the current tube pair): -235V to -330V on the left channel; -243V to -330V on the right channel. All balance voltage measurements were made with the ground (black) voltmeter probe in the ground pin. I'm currently somewhat in the middle TVR2 setting on both channels, measuring -283V (left) and -285V (right). The offset voltage measurements (i.e. measured between + and -, and adjusted by the TVR1 trimpots) on both channels can be brought down to +/- 2V around zero at best after drifting, which is still too unstable (it should be +/- 1V around zero at worst). As before: Tube swapping/rolling does nothing; No electrolytic caps are shorted or bloated; Couldn't identify any short points in the main PCB or in the CCS module's PCB; Currently looking for broken copper traces in the main PCB, particularly around C9-10 and R31-34 (where the +/-48V rails are derived from the HV rails), but haven't found anything yet. Will update again here if that happens. One aspect I hadn't paid attention to before, which was raised privately to me by Dominik Stritt (aka Firschi in the Head-Fi forum), is the relative brightness of the D3-D4 LEDs. All four are lit, but D4 (next to the offset TVR1 trimpots) is much weaker than D3 on both channels. Notice that in my unit these LEDs are marked the opposite as in the SRM-T1 schematic (i.e. the LEDs next to TVR1 are marked D3 in the latter). Tube swapping/rolling doesn't change the behavior of D3-4 either. For the time being, I'm performing all tests with the stock Gold Aero tubes, since the above problem makes impossible to be certain of these really being worn out. In retrospect, I cannot ascertain since when this has been happening. It certainly happened after installing the CCS module and the ensuing successful rebiasing more than a year ago, but I haven't made any measurements between then and my bias measurements about two weeks ago. I could seriously use some help right now, for I have no idea what's going on... Any suggestions are more than welcome!
  11. Hmm... The one I have is this low-quality PDF scan of the original SRM-T1 schematic, the only one I could find (the same as this one). It's reproduced with higher quality (but less info on the components) in JimL's AudioXpress article on his CCS mod (particularly, there R31-34 are rated at 15kOhm as well). Couldn't find one for the actual SRM-T1S (would love to, really), so it seems this is something that has really changed in the SRM-T1S circuit. Or maybe the schematic I have is for the first-generation SRM-T1...? Just to be sure in case I actually decide to replace R31-34, may I assume the power ratings are still the same (i.e. 5W for R31-32 and 3W for R33-34)? Regarding R44-45 (the small 150 Ohm resistors right next to the big C11-14 caps), I'm not sure about what you mean... Which voltage measurements should I watch out for being low? Anyway, I'll have a look at these resistors as well... That may take a few days, though, for I have quite a bit of work to catch up to... Thanks! Update (October 6th '22): In due time, R44-45 are measuring both 150 Ohms as they should, so they're not the culprit either. I also couldn't find any short points yet. I'll try to partially remove the CCS module to see if something in its PCB is shorted, as it happened a year ago. If there's nothing happening there, I'm out of ideas... Update (October 10th '22): no shorts on the CCS module's PCB, all measurements therein OK. Checked the safety output resistors, also OK. No apparent shorts elsewhere on the PCB. I'll try and replace R31-34 anyway, also because one of R34's leads was fragile and broke when I was putting these resistors back, but now I have no idea what could be the cause of the problem... I could really use an updated schematic for the SRM-T1S, any hints?
  12. The plot thickens... I confess I'd never paid attention to the ratings of R31-34 as compared to the SRM-T1 schematic... To my surprise, they are actually rated as follows: R31-32: 8.2kOhm, 5% tolerance (code letter J); R33-34: 8.2kOhm, 5% tolerance (4 code strips - grey, red, red, golden), and not 15kOhm as in the schematic. No idea about their actual power ratings. As for the measured resistances (detached from the circuit, of course): R31-32: 8.15kOhm; R33-34: 8.0-8.1kOhm. This raises a bunch of new questions: Why the difference in the ratings as compared to the SRM-T1 schematic? Did Stax intend to make this change in the SRM-T1S circuit? The R31-34 resistors in this CCS-modded SRM-T1S look exactly the same as mine, including their ratings. The R31-34 resistors measured within 5% of their ratings, which means they are (probably) OK (for their intended use). If so, why were the bias voltages normal when I installed the CCS module about a year ago, but now no longer so? This means that R31-34 are probably not the culprit, regardless of the difference in their ratings as compared to the SRM-T1 schematic. If so, what is actually causing the problem then? Since R31-34 are subject to a fair amount of power stress, it may be advisable to replace them anyway due to their age. Should I keep their ratings, or go for 15kOhm resistors as in the SRM-T1 schematic? I'll try to check again for short points in the next few days...
  13. Thanks spritzer - do you mean the big green ones (R31-32, 15kOhm, 5W) and/or the big blue ones (R33-34, 15kOhm, 3W) around the electrolytic caps C9-10 in my PCB? I'll detach them from the PCB and measure them tomorrow. Supposing one or more of them is the culprit, which kind / brand do you recommend? The SRM-T1 schematic states these should be metal oxide resistors (although R31-32 seem wirewound visually), which are usually available (e.g. at Mouser) with 5% tolerance. Is this enough? Are metal film resistors with the same specs OK for this as well? Or should I go for wirewound resistors to get a tighter 1% tolerance instead?
  14. Some bad news... First, a short prologue: the need to raise the volume on my CCS-modded SRM-T1S has increased in the last couple of weeks, to a point it could no longer be ignored. This meant that either the tube bias drifted and needed to be adjusted, or the (remember, stock) tubes were reaching EOL. Thus, today I decided to try a rebias session to see if the tubes could hold it together - if not, tube rolling is in order. First differential (+/-) measurements indicated a bias drift of 1-2 volts that couldn't be pulled down enough by pot adjustments, so I decided to proceed to changing the tubes. After a while with the energizer turned off, I decided just out of curiosity to see how the DC offset was measuring... and then it started. Both channels are now measuring DC offset between 150 and 300 volts, with a differential measurement of over 600 volts (!) Curiously, both channels seem to be measuring the same at similar pot positions. My previous experience with the CCS module installment suggests a short circuit is happening somewhere, but this time on both channels. As a first sanity check, I've checked if some power supply electrolytic cap (among the ones I'd replaced) or some of the polypropylene blue caps (far less likely, but just in case) were shorted, neither of which was the case. I've also tried replacing tubes, to no avail. Tomorrow I'll go through the PCB again with a good light to check for potential short points, but I'm getting worried some component has gone bad and needs to be replaced. Any ideas about what could affect both channels equally in the aforementioned manner?
  15. Ok, a short update on my impressions about the 007 Mk1 after roughly two months of brain burn-in. As before, this post will be updated again in a few days, this time because my 007A got a bit of mold on the earpads and needed a rather thorough cleaning. Sitting now inside a sealed plastic bag with a couple of bags of silica gel for a few days to remove any excess humidity that may still be lingering, so any further comparisons will have to wait. The fit / Stax fart issue has largely been solved. Bending the arcs in the headband assembly a bit further helped a lot, and I finally found the optimal angle for the earpads so that the drivers stay in place comfortably and with a good seal. This somehow largely reduced the Stax fart problem, not only (albeit mainly) because I no longer keep fiddling with the fit, but also because the farts are no longer so loud in the current optimal angle for the earpads. For reference, one needs to position the lateral sewings on the earpads facing front but very slightly downwards (at least for my head and ears). Most of my initial impressions on the 007 Mk1 still stand, and now I like it even more. Bass on the 007 Mk1 is actually very impressive, all the more because now the the bass details sank in. The optimal positioning of the earpads must also have helped somehow. As for the aforementioned shoutiness in the upper mids, there is something funny happening - either I got used to it or it got a bit damped somehow compared to two months ago. It feels more pleasant but I find myself pushing the volume of the energizer a tiny tad up in some tracks. In retrospect, since it has been more than a year since I did the CCS mod on my SRM-T1S, it may be time for checking for tube bias drift. I may even take the opportunity and start tube rolling, let's see...
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