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About JimL

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  1. IF you have access to the power supply capacitors, you can just measure the voltage on those. For the positive power supply you can measure the voltage at the "top" of the first or second stage plate resistors. For the negative supply, you can measure the voltage at the "bottom" of the output stage differential amp tail resistors.
  2. Well, you can always build a pro bias board and install a pro bias jack for the SRX to use the 007A. The amount of available power from the SRX depends on the power supply voltages and the amount of current that the output tubes are running. The basic SRX circuit uses 50k plate resistors vs 66k for the T1, so about 30% higher current-to-voltage ratio, but lower voltage supplies. Assuming +/-250 volt supplies for the SRX, that means about the same standing current but somewhat lower voltage swing - however still high enough for most listeners - probably peak SPLs around 110 dB. Not sure what yo
  3. Actually, since the T1 and SRX sound somewhat different, and the basic CCS mod is pretty similar (if you don't do the CCS on the output tail, which makes things more tricky, you could listen to both and modify the one YOU like better.
  4. The 10M90S/DN2540 cascode combination makes a very good constant current source for a simple and easy to build circuit. One drawback is that the current setting resistor can be quite variable due to the variability in the DN2540 from sample to sample, so a bit of experimentation is necessary. The original SRX Plus circuit used a separate -18-24V supply constant current source (CCS) on the tail of the input diff amp, but I realized that there was no need for a separate supply, since the this could be derived from the B- supply by using a 220k dropping resistor, which was added to the schematic.
  5. My friends who have listened to both say they think the SRX-plus sounds better than the T1. However, note that my SRX-Plus version uses the more powerful 6SN7GTA/B tubes which run at about 7 mA, whereas with the original 6CG7/6FQ7 tubes which appear to be what is in your version, you are probably going to be running them closer to the T1 standing current, around 5 mA. Both amps benefit from the CCS.
  6. If you modify it with the CCS mod (will need heatsinks), you'll have a really nice sounding amp, not that far off from much more expensive ones.
  7. A simple way to look at the Morgan Jones COSS graph is that the cascoded CCS requires a minimum voltage to operate optimally - in the case where both devices are DN2540 MOSFETs, at least 15 volts. As long as that criteria is met, no problem. Also, even at lower currents, the cascode performance of the 10M90S/DN2540 is excellent because the upper 10M90S device maintains the DS voltage for the lower DN2540 at around 3V even at 1 mA. That device is specifically designed as a current source, to keep a very constant GS voltage for currents between 1 and 100 mA regardless of gyrations in
  8. Getting back to the OP, it's hard to make recommendations for a turntable without knowing what the approximate budget is. It also matters whether the OP wants to get a package (turntable/arm with or without cartridge) or is interested in a separate turntable and arm. For example, the Rega, Project and Technics tables are good choices for a package deal of turntable and arm together, whereas if you are going with separate turntable and arm, you can pick and choose. Several years ago, for example, I bought a used Garrard 301 off eBay for $1000 (less than the going price), built my o
  9. Sorry for the delay in replying. Your quote is from Morgan Jones excellent book on valve amplifiers. He is talking about cascoded CCS using DN2540 MOSFETs as both the upper and lower devices in a CCS. Here the issue is, as he notes, that the lower device sits between the gate and source of the upper device, and sees a low voltage, and the COSS output capacitance is relatively high until the voltage across the CCS reaches about 15V. Part of the reason for this is that when you use a DN2540 for the upper device, the GS voltage is quite variable, between 1.5 and 3.5 volts. In this cas
  10. 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.
  11. The required wattage for the bias trimpots can be calculated easily. For the trimpots, the current is approximately 10 mA (approximately 5 mA per tube section), and the resistance is 2 kilohms, so the maximum power through them is I*I*R = (.01)*(.01)*2k = 0.2 watts. Using the rule of thirds, this means that any trimpot that is rated at 0.6 watts or more is fine.
  12. The safety resistors are a good idea even without the CCS mod. The easiest place to put them is at the PCB board - just de-solder the wire going to the output socket at the PCB and interpose the safety resistor between the PCB hole and the wire. Agree with Pars and Fitz comments re electrolytic caps. Replacing 100 uf caps with 330 uf caps should be fine as long as they physically fit - that includes the lead spacing, since the power supply caps are right up against the PCB so the lead spacing has to be the same. The brand shouldn't make a significant difference in sound unless the
  13. No particular brand recommendations. It is safe to install a larger capacitance as long as the voltage and size specifications are maintained. Larger capacitance is similar to slightly stabilizing the power supply voltages and allows the circuit to work a bit better. The amp should be off long enough to discharge the power supply caps, which you can check by measuring the residual voltages on the power supply caps - as long as there are only a few volts left you are fine. Note that because of variations in the AC line voltage and heat drift, there is going to be some c
  14. As electrolytic caps age they have a gradual decrease in capacitance and an increase in series resistance, but it is unlikely that you can tell that by listening. At some point as it approaches its end of life, the risk of the capacitor shorting out will increase, but again until it happens I don't know of any way to determine exactly when it will happen. That's why I recommend just replacing them all as routine maintenance. Kind of like replacing the oil in your car at specified maintenance intervals. If you don't do this, you are risking a catastrophic failure if the capacitor shorts out. Be
  15. For the electrolytic caps, if they are original they should be replaced as they are nearing, if not at the end, of their life. The most important things are the voltage rating needs to be the same or higher than the originals, and the size and lead spacing has to be similar so they will fit in the space. The brand is not really important as long as the electrical and physical parameters are similar. In some cases (i.e. the four big power caps) you can find caps with higher capacity that will fit in the same space, which can be an upgrade. In terms of tube replacement, both Hirsch a
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