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JimL

High Rollers
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Everything posted by JimL

  1. Easy way to check if it's a bad input tube is to swap input tubes between channels and see if the hum moves to the other channel with the swap.
  2. I'd power up using a variac on the AC line, monitoring voltages as you go so if there is a major error you can power down before it blows up.
  3. JimL

    kgst

    spritzer, how about using the kgsshvps8g power supply with the zener strings decreased to put out +/- 350 volts? That appears to be a more sophisticated PS than the KGST. Thanks in advance.
  4. So, since earspeakers apparently isn't doing the group buy anymore, is anyone else interested in doing it instead?
  5. JimL

    kgst

    Very nice job, Jose! What chassis and power switch did you use?
  6. It would help if you could type in to go directly to a particular page rather than having to advance a couple pages at a time. Maybe that's already available but I haven't been able to access it.
  7. So for Z101 and Z1100 you can use 100 volt zeners. S5277G diodes are specified as 400 v PIV, so you can use 1N4007 diodes there.
  8. On another thread KG recommends 2 oz traces for the tube filaments - makes sense because those are relatively high current - 0.6 amps each for the 6S4A (KGST) and 1.5 amps each for EL34s (Blue Hawaii). Since the filament traces are part of the board that means the whole board needs to be 2 oz.
  9. Excellent points by Birgir. When I built my version I did as he suggests and had the 4.7M resistor after the last cap - been awhile and I forgot that- early senility sucks sometimes.
  10. I did this using the schematic above, but kept one socket at normal bias, taking the voltage for it from halfway up the zener ladder. One thing I did was to glue a nylon 4-40 screw into middle hole in the socket I converted to pro bias. This prevents you from accidentally putting a normal bias phone into a pro bias circuit
  11. I seem to remember someone (KG?) suggesting using the back end of a 1 mm drill bit to loosen them up a bit, but for the life of me I can't find the post.
  12. Sounds good, I've already collected about 1/2 of the parts. By the way, I should only be in for 1 board set.
  13. JimL

    kgst

    Well, the standing cathode to plate voltage with plus-minus 350 v PS rails is about 335 volts, which is well within the max plate voltage spec of 450V for the A and B versions, and with the output current loads at 7-8 mA the power dissipation is about 2/3 of maximum. Moreover, the A and B versions were widely used in early 50s TVs, so they also have a peak cathode-to-plate voltage spec of 1500 V - that is right out of the RCA tube manual. This compares reasonably well with the 6S4A which has a peak plate voltage spec of 2000 V. When I was debugging and adjusting my prototype I often had 600+ volts on one of the 6SN7GTA plates for several minutes without getting any breakdowns or sparks. That was not deliberate - it's just that if the output tail current source is not within range one plate will sit near the positive PS rail and the other near the negative PS rail. As far as I can tell there was no damage to the tubes with this "abuse", they tested within spec on my tube tester afterwards. More to the point, with the original SRX using 50k output plate resistors and 7 mA through 6SN7GTA/B output tubes, run at 350V, if you swing the maximum peak-to-peak voltage, all of the tube current is going to drive the plate resistors with nothing left for the headphones. With a simple 10M90S current source at about 170 kilohms impedance, about 2 mA out of 7 mA of the tube current is going to drive the current sources, leaving 5 mA for the phones - much better. With the cascoded current sources at over 150 megohms impedance about .002 mA of the tube current is driving the current sources with nearly the entire 7 mA of tube current still remaining to drive the headphones. I would maintain that makes a significant difference in how well the 6SN7GTA/B tubes work in the circuit. Anyway, I would be very interested in your impressions, since I have not had a chance to compare its sound with any other amp, and of course a child is always beautiful to its parent. )
  14. JimL

    kgst

    Ah, thanks spritzer, missed that completely. On a related topic I have signed up for the KGST group boards, and unless I missed it somewhere I don't see part numbers for the terminal blocks on the amp and PS boards. Any suggestions that are available at Mouser? And somewhat off topic, what do you think of my revision of the venerable SRX design?
  15. JimL

    kgst

    Well, there are not a lot of parts on the PS circuit board, so there are only a limited number of ways for them to be connected.
  16. JimL

    kgst

    In case anyone is interested, it looks like the KGST power supply board is an updated version of the original Blue Hawaii power supply circuit that is published on the Headwize site, with a 10M90S current source replacing the 2SA1968 transistor and LED current source that drives the zener diodes, and an 8N80C MOSFET pass transistor in place of the IRF transistor. Someone correct me if I'm wrong.
  17. Doh! As Emily Latella would say, "Never mind!"
  18. BTW, are the new amp boards going to have provision for substitutes for the (apparently) out of print 2SA1468s?
  19. Is this just the amp boards or PSU also? I think I'd be interested in a set of 2 amp boards and PSU.
  20. luvdunhill: No, I haven't. Jung's measurements indicate that this provides an extra 10 dB improvement at 1 kHz and below, which is about a 3-fold increase in dynamic impedance. I actually built my current sources as mini hard wired circuits, so the extra complexity, although small, gets to be a bit much with resistors and capacitors hanging out in space - it's already looking a bit like a free-form solder sculpture if you know what I mean I don't think the increased impedance would make any difference in the current tail sinks for the differential amps. It might make a difference in the output current loads since the improvement is mostly at low frequencies, where the headphone impedance is the highest. However, even with the standard cascode 10M90S/DN2540 combination, nearly 90% of the signal current, and two thirds of the signal power is going to the headphones at 20 Hz, and by 100 Hz 99% of the current and 90% of the signal power is going to the headphones, so I don't know it it will make an audible improvement.
  21. PretentiousFood: I'm not quite sure how you have hooked up the FET to the tube tracer. If by degeneration resistor you mean a resistor going from source to ground, but you connected the gate stopper resistor to grid voltage rather than the bottom end of the source resistor, then this is not going to give accurate results for the impedance of a current source. The reason is that in a FET current source, the gate has to be connected to the bottom end of the source resistor. - see the current sources in the SRX schematic. This results in a negative feedback loop that increases the impedance, and also sets the static current. Here is how it works: suppose we try to increase the current running through the FET. The increase in the current increases the voltage across the source resistor, which in turn means that the gate voltage becomes relatively more negative which tends to turn off the FET, making the current decrease. The correct way to see its behavior as a current source is to treat it as a two-terminal device rather than a three-terminal device, which is what a tube tracer is usually doing. Instead of plugging in the FET like a tube, you have to use both gate and source resistors, and connect the gate resistor to the bottom end of the source resistor, then run the I vs V curves - you should get a dV/dI in the hundreds of kilohms, which is what both Pimm and Jung measured. The gate should NOT be connected to a constant grid voltage, which appears to be how you did the measurement. Jung specifically stated his measurements of the DN2540 were made at around 30 mA static current, where he measured about 300 kilohms, which is about 10x what you measured. If what I suggest is in fact the way you did the measurement then I apologize for the misunderstanding, however your results are so far off from his and Pimm's I suspect that that is not how you did it. To go one step further, the reason a cascode current source works so well is that a large variation in the drain-to-source voltage of the upper device causes relatively little change in the gate-to-source voltage of that device. Almost all of the variation occurs in the drain-to-gate voltage. The drain and source of the lower device in the cascode is connected between the gate and source of the upper device, therefore its drain-to-source voltage is nearly constant even while there is a wide variation in the voltage across the two device combination. Since the lower device controls the current, and since it is running at close to a constant voltage regardless of voltage variations across the cascode, and the lower device is also getting the negative feedback from the source resistor as described above, it should output a constant current - or very very close to it. And that is the definition of a current source. With the cascode, you should get a dV/dI on the order of a hundred megohms or more.
  22. Don't know if a 10M90S cascoded with itself is better than a single 10M90S, but a 10M90S upper device cascoded with a DN2540 lower device measures about 1000 fold higher impedance compared with the 10M90S alone - > 160 megohms vs about 170 kilohms. Whether that makes an audible difference in the T2, I don't know. My wild ass guess is it makes more of a difference in a lower current design than in the T2, where the standing output current is already overkill. Beyond a certain point, overkill just means the rubble bounces higher (always wanted to use that line ).
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