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  1. Voja, I think you are seriously missing the point. The big problem with the transformer box is potential SAFETY issues. When spritzer talks about the construction not being up to code, what he is talking about is construction standards that are designed to protect the user from being SHOCKED, or worse, ELECTROCUTED. For example, he mentions that the XLR connectors are not spec'd for the voltages they are intended to carry, which is not aa good thing. The metal panels on the box are not all connected to ground which means that if a wire inside the box should somehow break and touch an ungrounded panel, and you then touch that panel, you would get a shock. That is why all modern appliances (including amps) with metal boxes have their outsides electrically connected to the ground wire on your three wire power cord, for your protection. You admit that you are not a technical person, so you should pay attention to someone like spritzer, who is. It doesn't matter a bit if the headphones sound absolutely perfect, if they could injure or kill you. End of discussion.
  2. 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.
  3. 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 you mean by ROI - the CCS mod more than doubles the effective power for less than $100 in parts for either amp, which is a pretty cheap to me. The difference between 250 volt and 320 volt power supplies is about 2 dB in ultimate volume, which you will likely never reach unless you are looking for permanent hearing damage.
  4. 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.
  5. 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. This eliminates the need for a separate -18-24v supply and also balances out the B+ and B- currents.
  6. 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.
  7. 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.
  8. 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 DS voltage. Remember that the Morgan Jones comments refer specifically to the use of the DN2540 as the upper as well as the lower device.
  9. 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 own plinth and bought a separate tonearm. But that's just me. Or for real DIYers, you can buy a Lenco L75 idler drive, toss the tonearm, build your own plinth and supply your own tonearm. Check out the Lenco Heaven website for more details. Another option that nobody has mentioned is a used AR turntable - the ES-1 model combined the excellent isolation of the original AR turntable (the original turntable was advertised as being able to take a mallet strike to the top plate while the record played on) with a decent modern tonearm at a good price. Sort of a beer budget version of the Linn LP12. I had one of those mounted with a Grace G-707 for several years and was quite happy with it.
  10. 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 case, higher is better, since a higher GS voltage puts the lower device into the saturation region, where the current changes very little with variations in voltage, and the device acts as a current source. Using a 10M90S as the upper device is one way to achieve this, since by design, the GS voltage of this MOSFET is at or above 3 volts for currents between 1 and 20 mA, whereas with a DN2540 as the upper device, the lower DN2540 could see a GS voltage as low as 1.5 volts which is much closer to, or below the point where it drops out of the saturation region. However, more significantly, since we are using the cascode CCS in the output stage, the standing voltage across it is about 320 volts. Even if we assume we are using the DN2540 for both devices (a bad idea since the total voltage across it could exceed its 400 volt maximum resulting in instant annihilation of the CCS), the only time the DS voltage would decrease to less than 15 volts, resulting in a higher output capacitance, is when the output voltage for half of the output stage reaches a peak of (320-15) x 2 (or supply voltage - 15, doubled because the other half of the output stage is at the opposite extreme voltage) = 610 volts peak = 430 volts RMS. With the typical Stax headphone such as an SR-007 Mk II with a rated sensitivity of 100dB SPL/100 VRMS/1 kHz, this would produce an SPL of 127 dB, close to the 130 dB threshold of pain. So basically, because the CCS sits at a high voltage all the time, it never sees the conditions that Morgan Jones is concerned about except when SPLs are so high that you risk permanent hearing damage. I wouldn't worry about it. See also my following comments below.
  11. 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.
  12. 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.
  13. 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 they are defective.
  14. 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 change in the output voltage on the order of a few volts over a few minutes time. The circuit is pretty stable and designed for long tube life, so checking the bias a couple times a year is probably adequate. The CCS mod is fine as published, the only changes are additions of 5.1 megohm safety resistors to the bias outputs after the last bias capacitor, which was left out of the original article.
  15. 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. Better to just put in new caps - it's cheap insurance. The parts cost for replacing all the electrolytic caps is less than $50 and you don't have to worry about it again for 10-20 years. So, stop f**king around and just do it. If you don't know how to solder get an electronics technician to do it. You shouldn't have to rebias the tubes unless the caps were already going bad and the power supply voltages were already going south. As far as the tubes go, yes, when they start to wear out, you will need to turn up the volume. Another way to tell is that you will need to adjust the bias to keep the output voltage near zero. The tubes you showed are examples of the short plate. The photo does show the CCS mod. The metal spacers in the photo are available from Mouser, as are all the electronic parts including the electrolytic caps and MOSFETs. The heatsink was bought on eBay for less than $10 and was cut down to a size that would fit the space available. Note that if you do the CCS mod you likely will have to rebias the tubes - there are instructions on how to do this on the internet.
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