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Sunday night dinner - chicken enchiladas verdes - made with my chili verde recipe for the sauce and marinated chicken (in lime juice, olive oil, salt, pepper and oregano). Pre-bake Time to eat

Sunday breakfast. Sausage, potato and cheese omelet with bagel and cream cheese.

Indigenous People of Siberia Photographed for 'The World in Faces' (mymodernmet.com)

Thank you! The final mod, that of the front end, is only slightly more complicated. Here is the updated schematic with both mods: The list of changes vs. the original schematic: Replace R1, R2, R6, R7 with 100 ohm resistors Replace R5 and R8 with 332 ohm resistors Replace R9, R10, R11, R12 with 274 ohm resistors Replace R16 and R17 with 22 ohm resistors Replace R19 with a 562 ohm resistor (reuse one of R5/R8) Replace R23 with a 1 Megohm resistor Replace R24 with a 68pF 50V NP0/C0G ceramic capacitor Replace R33, R34, R35 and R36 with 0.22 ohm 2W or 3W resistors Replace R37 and R38 with one 47 ohm resistor connected between bases of Q23/Q24 and Q25/26. Make sure to connect the new resistor correctly - see the photo below - or your output transistors are at risk. Replace R47 with a 3.92k resistor Remove C2 and C3 Add a 4.7nF film capacitor and a 47 ohm resistor, connected in series, between the collectors of Q2 and Q3. Add another 4.7nF film capacitor and a 47 ohm resistor, also connected in series, between the collectors of Q7 and Q8. Place the new parts on the underside of the board if you like. Repeat for the other channel. That's it! The list of parts required to modify two channels: 8x 0.22 ohm 2W or 3W resistors 4x 22 ohm resistors 6x 47 ohm resistors 8x 100 ohm resistors 8x 274 ohm resistors 4x 332 ohm resistors 2x 562 ohm resistors (not needed if you can reuse R5/R8) 2x 3.92k resistors 2x 1 Megohm resistors 2x 68pF 50V capacitors, NP0/C0G ceramic or mica 4x 4.7nF film capacitors Some 18 AWG single core insulated wire and 2x 10 ohm 2-3W resistors for the output RL network As resistors's names are not marked on the board, here is a photo showing what to replace with what. Note that the additional parts from step 12 above are not shown - they are under the PCB. After assembly, take the usual precautions before powering the amplifier up, as if it were a newly assembled board. Turn the bias adjustment trimpots all the way counterclockwise, connect a current limited +/-21V power supply with the current limit set at 0.5A per rail. With power on, check the output for a possible oscillation, then adjust the bias and re-check for oscillations. The bias level needs to be at about 20mA per transistor - with 0.22 ohm emitter resistors, it corresponds to 4mV between the test points, which should be easy to measure with a DVM. Higher bias levels are possible, but the distortion will be slightly higher. Let the amplifier warm up for 10-15 minutes and readjust the bias - it should go down as the output transistors warm up. As with any feedback amplifier, capacitive loads may affect stability. In my testing, the amplifier remained stable with capacitive loads of up to 100nF. Consider adding the usual RL network between the output of each channel and the load to ensure stability. Make 20-30 turns of 18 AWG single core insulated wire on a 1/2 inch (12mm) former - a Sharpie will work - to make an air core inductor like this: then connect a 10ohm 2-3W resistor in parallel to it.

If I had the room, the SawStop PCS is the only one I would consider. I was leaning towards the little Pro cart one but its minimal cut capabilities did not seem worth the up charge, I guess the finger saving is the up charge. Glue Up Kit Holder for the wall. Need to run through the router and get it glued together.

I completed the full modification (output stage and front end) on both channels and listened to the amp briefly. The headphones were Grado GS1000 and Sennheiser HD595, the 8ohm speaker were B&W 602.5 floorstanders. The amplifier performed very well in each case and sounded immaculate. It is a HUGE upgrade over the original and over the output stage mod alone. I compared it against Musical Fidelity X-CANv8, and they performed equaly well. I took some measurements. Here it is delivering 1W and 5W with an 8ohm load: Here is the performance of the fully modified KSA-5 into a 32 ohm load: and into 100ohm:

I made the mapo tofu spaghetti tonight. And creamy Miso spaghetti too. Nom Nom Nom

I love those things, btw. I think they are about the best table saw accessory I have found. They are helpful for sheet goods, but where they really shine is in long rips of hardwood. Cutting trim pieces, for instance. as someone whose thumb is still whole because of a sawstop, I would pretty much refuse to touch a non-sawstop table saw.

Finally got a new humidor that I can use to transport today’s selections out to the patio. Today’s choice

I'm a couple of weeks into a month-ish sub for Disney+ (just the Disney $7 part). I wasn't exactly crazy about the first couple of episodes of Mandalorian but by the end of the 2-8 episode seasons, I was sold! Very true to the Star Wars world and lots of great guest actors and fx. The episodes are uneven so there are a few that I wasn't that crazy about but there were 4 directors so I wasn't that surprised. There were a few that really made me sit back and grin. If you're a Star Wars diehard like me, The Clone Wars cgi series is great for back stories. They can be very uneven too but as the series goes along (there are 123, 30 minute episodes) the animation gets to a very high standard for a commercial show (if you can excuse the "will cgi ever make hair not look like a helmet" problem). Anywho, I've always appreciated your music recommendations so I thought I'd give you my 2 cents on Disney+.

Let me know if you want any laser cut or etched funness, that's what got me thinking about it. With the lining material being 1/4" typically there's a lot that can be done with the laser and it's not like charring the edges would be an undesirable effect in a box that houses things to be smoked.

Having a large cabinet humidor has been my want list for a long time. I’ve actually been thinking about making one for a while. Local large commercial lumber supplier actually stocks the Spanish cedar so getting material shouldn’t be that hard to make it. I just need to step up my design game.

Hey @naamanf - A humidor might make a fun project for the CNC router and laser...

Nice! I just bought mine, but it was inspired by your previous mentions of how popular/delicious it was, that's why I mentioned it.

Mapo Tofu (amongst other things)

They are a lot cheaper than new fingers.

It keeps the material pressed down on the table and the wheels are angled to keep it pressed against the fence. Basically does the same thing feather boards do, helps make straight cuts and adds some safety against kickback.

Let me first modify the output stage. The mod affects only the performance with lower impedance loads, and even there it can take us only so far, but it is a start. The front end modification that increases the feedback loop gain will be posted separately. The changes are simple. Here is the schematic: R33-R36 are replaced with 0.22ohm 3W resistors, R19 is reduced to 470..560ohm to allow proper biasing, and R37-R38 are replaced by a single 47..51ohm resistor. The bias will need to be re-adjusted. Note that 50mA per transistor that resulted in 100mV between the test points in the original will now give 11mV - still easy to measure with a DVM. The performance improvements are as follows. With 1W and 5W into an 8ohm load: With 4Vpeak (2.8Vrms) into a 33ohm load the effect is much smaller: Distortion into 100ohm does not change appreciably, so I don't show it here.

Oh! https://www.waxingmoonhumidors.com/

Ramon Allones Specially Selected

With an 8 ohm load, Class A ends at 1.6V peak, so 1W performance is not so good anymore. Curiously, the Krell brochure mentions that the amplifier is good for 5W into 8ohm, but the owner's manual warns agains connecting it to any loudspeakers. I understand the 8ohm was meant for driving STAX via a transformer. As usual, at a higher output level the distortion percentage improves somewhat:

I recently picked up its lower grade sibling, nicely restored, from a local dealer and fixer, to add a tape deck to my system. I did buy it more for its looks than its sound... but who can blame me for that?

1) This isn't a headphone site 2) As an engineer, you should already know that nothing is perfect...

If you want to stroll down memory lane... 20 important Macs. https://sixcolors.com/post/2020/12/20-macs-for-2020-1-imac-g3/ Video version:

Yesterday's Meat

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: 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 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: 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), 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 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.

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...

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?

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: C9-10: 2 x Nichicon KZ Muse series - 33uF, 50V, 85oC, DF=0.08 (at 120 Hz, 20oC), ripple current = undisclosed (I've seen reports on KZ having about 1/3 of the ripple current rating of similarly spec'ed Panasonic FM, whose only difference to FR according to datasheets is a shorter life - no idea where the former data came from, it's not in the Nichicon KZ datasheet), life = 1000h, dimensions (LS x D x L): 5mm x 10mm x 12.5mm (this is the largest capacitance KZ with 5mm LS) - https://www.nichiconcapacitors.com/product/nichicon-kz-muse-33uf-50v/ 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 - https://www.nichiconcapacitors.com/product/nichicon-kx-330uf-400v-snap-in/ C21-22: 2 x Nichicon KW series - 470uF, 16V, 85oC, DF=0.20 (at 120 Hz, 20oC), ripple current = 0.42A (RMS, 120 Hz), life = 1000h, dimensions (LS x D x L): 3.5mm x 8mm x 11.5mm - https://www.nichiconcapacitors.com/product/nichicon-kw-470uf-16v/ C23-24: 2 x Nichicon Fine Gold series - 47uF, 35V, 85oC, DF=0.12 (at 120 Hz, 20oC), ripple current = 0.10A, life = 1000h, dimensions (LS x D x L): 3.5mm x 8mm x 11.5mm - https://www.nichiconcapacitors.com/product/nichicon-fg-fine-gold-47uf-35v/ 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?

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.

So, just a little technical explanation if anyone is interested. In the Megatron design, the driver 12AX7 and EL34 outputs operate within a global feedback loop with the gain set by the ratio of R21 (33) to R11+R12 (23+24). The operating conditions for the 12AX7 are set by the constant current loads Q1 and Q2 and the grid-to-cathode voltage which is set by R11. If you look at the tube characteristics, for a set grid-to-cathode voltage, as the plate current increases the plate voltage will also increase. However, because the plate current runs through R11, this makes the grid-to-cathode voltage more negative, which further increase the plate voltage. The result is that the plate voltage gets closer to B+, which can result in premature clipping of the driver stage. So in order to increase the plate current without increasing the plate voltage, we must decrease the grid-to-cathode voltage. This is done by decreasing R11. Note that this will also increase the overall gain and decrease the closed loop bandwidth, however, since R11 is only 1/3rd the value of R12, a 10% change in R11 will only result in a 2.5% change in gain and bandwidth. If we wish to maintain the overall gain and bandwidth, then we need to increase R12 so that the sum total of R11 and R12 are unchanged, .e.g if we decrease R11 to 2k, R12 should increase to ll.3k.

I spent most of this past weekend working with JimL and Kevin tweaking my Megatron.( I was just the coolie equipped with a soldering iron and a multi meter and Jim and Kevin supplied all the brain power. ) Kevin suggested that I share the information here. My goal was to raise the standing current to the plate of the driver tube (12AX7). It's set at 0.5mA in Kevin's original design. Kevin has mentioned in his June 28, 2012 post that the only thing stable at 0.5mA is the 2SA1968 which has since been obsolete. My build uses KSA1156 in place of the 2SA1968 so this tweak may be more applicable to similar builds. This tweak, at the end, turned out to be the few relatively simple steps below: 1. Reduce R6 and R7 (2.2K) to about 1.3K which sets the plate CCS at 0.85mA. I used 3K to parallel with the 2.2K to get to this value. This higher current also raises the standing plate voltage of the 12AX7 from around 245VDC to about 275VDC (this will vary some with the tubes used). 2. Reduce R11, R23 cathode resistor (3.3K) to about 2K so that it pulls slightly more current and brings the plate voltage down to around 245VDC. I used 4.7K to parallel the 3.3K to get to 1.99K. We need minimum 50V differential between the plate and the B+ (300VDC) to allow for the driver voltage swing without clipping. 3. Because the resulting higher current draw of the front end, you need to adjust R19, the drop resistor, to maintain the 300VDC for front end B+. I like my Megatron’s sound better with this tweak. YMMV. It may be possible to raise the driver current even higher subject to the plate dissipation and CCS transistor limit. I am happy and content with the outcome now but may experiment further in the future. I am also hoping someone may be interested in running a SPICE model and/or observe the outcome with a scope to validate.

RIP Pat DiGiacomo (aka Jocko Homo), passed away Nov. 4th. Good guy, knowledgeable engineer, but came off as being a bit cranky https://www.diyaudio.com/forums/in-memoriam/362564-rip-jocko.html

This one hurts, though he’s discussed this day often. Michael Apted, 1941-2021, ‘visionary’ director of Up series dies. https://www.theguardian.com/film/2021/jan/09/michael-apted-1941-2021-tributes-paid-to-visionary-director-of-up-series?CMP=Share_iOSApp_Other