JimL

So, out of curiosity I looked up the ECL82 (6BM8) that spritzer suggested. It turns out the amplification factor when the pentode is triode strapped is about 6.8, which is nearly 10 dB lower than the 6SN7GTAB, leaving about 4 dB of feedback. If we assume the grid-to-plate capacitance is about 4-5 pf, this puts the open loop frequency response at -3dB between 22 and 27 kHz, which is where the feedback margin runs out, so the closed loop frequency response is about the same. By contrast with the 6SN7GTA/B the closed loop frequency response is -3dB at around 53 kHz. Voltage rating and power dissipation are not the only factors that need to be considered in selecting an output tube.

Frank, I also would like to congratulate you on the write-up in InnerFidelity. Very impressive work, indeed!

Thanks for the kind words, Frank. Coming from an experienced builder such as yourself, they are very much appreciated. I sometimes think of the original SRX as the electrostatic headphone amp equivalent of the Dynaco Stereo 70 - a simple, high performance circuit that can be modified for even better performance. Also, IMHO, any more complicated, more expensive design has to outperform it in some tangible way to justify its existance.

Didn't look at the rest of the board for the 6S4A cascode, which I assume is unchanged, but the wiring for the cascode current loads looks OK.

Well, the Miller capacitance can definitely be an issue. For example, with the SRX, the first stage has a very high output impedance because it's a cascode, so the impedance is nearly as high as the plate resistor. In combination with the Miller capacitance of the output tubes, with a 6SN7 output tube the open loop response is about 3 dB down at 11 kHz and the closed loop response is about 3 dB down at 53 kHz - that's measured with the SRX Plus (that's my name for the revised SRX). With a triode connected EL34, I measured the grid-to-plate capacitance at around 10 pf, so the open loop response is about 3 dB down at 8-9 kHz and with the lower maximum gain of around 10.8 in the output stage, the calculated closed loop response just makes it to 20 kHz before it starts rolling off if you use a current load - that's what Dr. Gilmore also reported on measurement. Also, the SRX input stage has puny standing current of about 0.55 mA/side. With a peak voltage of 16 volts into the output stage which should drive it into clipping, the 6SN7 has a Miller capacitance of about 84 pf, requiring 0.16 mA/side for a 20 kHz sine wave, but with music signal the max current needed drops to about 0.05 mA/side, which is more than acceptable. When you analyze the SRX circuit you can see where the engineering compromises were made, and how well chosen they were to produce a result that is "just good enough" but works very well in practice.

Incidentally, here is a link to the published RCA specifications, which includes plate curves going out to 600+ volts at 5-7 mA: http://frank.pocnet.net/sheets/049/6/6SN7GTA.pdf

nopants: Absolutely the 12SN7GTA is an option, exactly the same ratings as the 6 volt version, just need to use 12v filament supply. All NOS and cheaper since no commercial amp would use them, whereas the 6SN7 is one of the new darlings among tube audiophiles. Note that you have to use the GTA or GTB version as the plain GT doesn't have adequate voltage or power rating for this design, unless you derate the power supply to 250 volts and the output current loads to 6 mA/side. The 6BX7 could be a good tube in this design - amplification factor is 10, which is 6 dB less, but with the lower Miller capacitance the open loop response is over 20 kHz and the closed loop response is about the same. However, matching between sections could definitely be an issue, since as currently configured there is no way to balance the offset between tube sections - one would have to go to the ESX design with its adjustable bias for that. I have a vague recollection of someone posting in a DIY Stax amp thread a few years ago that the offset with random 6BX7 tubes was around 50 volts between tube sections, but I could be wrong. With my limited collections of 6SN7GTA/B tubes, that has not been an issue as I've found the offset somewhere between 4 and 10 volts between tube sections in the 1/2 dozen or so tubes I've tried. JoaMat: The T2 has a servo system to control offset. The + and - outputs are fed via 10:1 resistor divider chains to two op amps which act as integrators, so they ignore the audio signal and just monitor the offset. The op amps are protected by diodes from too high voltages. The divider chains are grounded so if the output offset is zero there is no signal to the op amps, but if one output is off, the op amps see a voltage. The op amps are fed with opposite polarities so the same input voltage will result in opposite output currents. The outputs of the op amps are near zero volts, and each output goes to transistor cascode current mirrors which translate the servo current to approximately +70 volts. This corrective current goes to the upper cathodes of the input differential cascode tubes, which shifts the DC output voltage of the input stage. Since the whole amp is DC coupled, this also alters the output voltages. At least, I think that's the way it works.

Sure. So the current loads are there for obvious reasons as spritzer has often posted about over the years, and I discuss in my new post on output stage requirements elsewhere on the DIY section. The current sink is there to improve the balance of the output stage (it does the same for the input stage which is useful because I'm running it single-ended on the input side) and also, in combination with the current loads, completely isolates the output tubes from the power supply and from the other channel. This improves channel separation, and improves the differential operation of the outputs. All the output tubes see is the signal from the input stage, the headphones on the output side, and the other tube section in the channel via the cathode current sink enforcing balance. This results in optimum function of the output tubes (modest, ain't I? ) However, it is impossible to build output loads and sinks that match each other down to the last electron. So instead of trying to match them, I deliberately set the current sink to soak up about 3 mA more current than the two current loads combined put out. To absorb this extra 3 mA, I put in the resistor string that runs between B+ of +325 volts and the current sink, which is sitting at about -310 volts. Varying the resistance of the string varies the cathode-to-plate current, allowing the plate voltage to be set close to ground. Now, this circuit does not allow balancing the plate voltages independently, so that the tube sections have to be well matched. In practice, with NOS tubes from the 1950s I've been able to get the output voltages within +/- 3 volts of ground with a couple volts drift, and with tubes from the 1980s I've been able to get the output voltages with +/- 6 volts of ground, which is more than good enough.

Sure. The equation relating capacitance, charge and voltage is: V = Q/C where V = voltage, Q = charge and C = capacitance Taking the time differential of both sides we get: dV/dt = (1/C)*dQ/dt where dV/dt = voltage change/time = slew rate, dQ/dt = change in charge/time = current = I So: dV/dt = I/C, or C*dV/dt = I For my example, dV/dt = 30 V/microsecond, C = 100 pf, plugging everything in gives I = 3 mA for 1600 volts peak-to-peak. I used to be a physics major in a previous life. ) The reason you got a different answer for slew rate is that you are calculating for a full-power, 20 kHz signal. However, music does not contain full-power signals at 20 kHz. As Nelson Pass and Peter Baxandall found, the music power spectrum rolls off above around 5-6 kHz at a rate of approximately 6 dB/octave. Because of this, for a 100 watt amp that can swing 80 volts peak-to-peak the fastest slew rate with a music signal is 1.5 volts/microsecond up to clipping, whereas a sine wave 20 kHz signal at clipping would slew at 5 volts/microsecond for the same amp. The fact is that music signals are not that fast compared to some test signals. Multiply those results by 20 for a 1600 volt peak-to-peak signal at 20 kHz gets you to 100 volts/microsec, which is what you calculated, or 30 volts/microsecond for music signals, which is what I got. You'll get the same answer as me if you plug in a full-power signal at 6 kHz, which is what Baxandall said was needed to reproduce music. Now, the calculations in the second post assumed a signal of 800 volts peak-to-peak, which is 400 volts peak, whereas your calculation was for 800 volts peak, hence your answer is twice as high as mine. My number also includes the 1.5 mA current into the capacitative load of the headphone, and because that current is 90 degrees out of phase with the current to the resistor, the overall answer 8.1 mA (rounded off) rather than 8 mA for the resistor load alone.

Another quick example. This time, comparing the SRX revised with 6SN7GTA, +/- 325 volt PS, 14 mA/channel with current sources vs the ESX with triode strapped EL34, +/- 400 volt PS, 16 mA/channel with 50 kilohm plate resistors. The EL34 stage dissipates 40% more power than the 6SN7 stage. Identical circuit except for PS voltages, output stage tubes and resistors vs current sources. The more powerful stage should win, right? Driving both amps to 800 volts peak-to-peak with the fastest music signal, producing 106dB measured into an Omega 2 (per Kevin Gilmore), the ESX requires 8.1 mA signal current, > 50% of the idle current the SRX revised requires 1.5 mA signal current, 11% of the idle current. A current source load can allow a smaller, weaker output device to outperform a bigger, stronger output device with load resistors.

There seems to be some consensus on the voltage requirements for electrostatic headphone amps. Nearly all of the commercially available designs put out between 1000 and 1600 volts peak-to-peak, a range of about 4 dB. The legendary Stax SRM-T2 was specified to put out a bit more, close to 1800 volts peak-to-peak, which is 1 dB higher. This would be equivalent to almost all loudspeaker amplifiers putting out between 50 and 125 watts, with the T2 being like a 160 watt amplifier. However, there hasn’t been much discussion on the current demands for electrostatic headphone amps. Output stage currents in commercial amplifiers have run between 2 mA/channel (Koss ES950) and 36 mA/channel (Blue Hawaii). Back in 1978, Nelson Pass published in The Audio Amateur (issue 4, p. 12) some measurements he had done on the slew rate of music signals. He tried out various cartridges and LP records, and using a 100 watt amplifier with a 30 volt/microsecond slew rate, reported that the highest slew rate he found with music signals was 1.5 volts/microsecond up to clipping levels. The late Peter Baxandall also published some years ago in Wireless World that music signals required an amplifier slew rate sufficient to drive a 6 kHz sine wave to clipping with low distortion, which works out to pretty much the same thing. A 100 watt amplifier has a peak-to-peak output of 80 volts. The Blue Hawaii, to take a current state of the art amplifier, has a peak-to-peak voltage at clipping of close to 1600 volts, which is 20 times higher, so the fastest music signal would have a slew rate of 30 volts/microsecond when the Blue Hawaii is driven to clipping. So how much current does an electrostatic headphone amp need to produce a slew rate of 30 volts/microsecond? A typical electrostatic headphone approximates a load of about 100 pf - Stax specifies most of their current models between 94 and 120 pf. The amount of current required for 30 volts/microsecond into 100 pf would be 3 mA. This is the amount of current that the amplifier has to supply to the headphones alone in order to play the fastest music signals up to clipping. Since amplifiers don’t sound their best at the very limits of their capability, for any real amplifier, there should be additional capacity in both slew rate and current over the bare minimum required. John Broskie has suggested on his TubeCAD website that for low distortion the maximum signal current demand on a tube be a fifth of the standing current. This calculation also assumes that the amplifier itself does not consume any signal current. But that is not always true. Take the Egmont, a basic, inexpensive tube electrostatic amp circuit. It uses 66k resistor loads in its output stage. With +/- 260 volt supplies the output stage runs at 7.9 mA current. If we drive the headphones to 1000 volts peak-to-peak using our fastest music signal the headphone consumes 1.9 mA, but the resistor consumes 7.6 mA, using all the current the output stage is theoretically capable of supplying. The reason that an amp with a total current of 7.9 mA can supply both 1.9 mA to the phones and 7.6 mA to the resistor loads is that the current to the headphone is approximately 90 degrees out of phase with the current to the resistors – remember the geometry of a right angled triangle? The headphone and resistor compete for the available current, and since the resistor is lower impedance than the headphone, the resistor hogs most of the current and the headphone is left with the scraps. Furthermore, the amount of signal current soaked up by the resistor depends on the magnitude of the signal, whereas the amount of current going to the headphones depends on the speed of the signal, so the ratio of 1.9 mA to the phones and 7.6 mA to the resistor is even worse almost all of the time. In fact, this is a problem for any electrostatic headphone amp that uses resistor loads in the output stage since the resistor sets both the standing voltage and the standing current. Massively increasing the voltage and current so that no user will ever come close to reaching its limits doesn’t really solve the problem, it just pushes it farther away. And then, a further problem is that devices and components which can withstand that amount of voltage, current and power are expensive, which rather defeats the goal of an inexpensive design. Now, take my revision of the Stax SRX tube design using current loads. The output stage runs at a higher current and voltage: 14 mA current with the power supplies run at +/- 325 volts. More importantly, the cascoded current loads on each plate measure over 160 megohms impedance, thus requiring a mere 4 microamps to drive them to clipping, so 99.9% of the total standing current is available to drive the headphones. The maximum current required to drive the headphones at clipping is about 2.4 mA, less than a fifth of the current available. To further illustrate the value of a good current source, let’s go back to the Egmont. With the output tubes in that design delivering the same peak signal current of 2.4 mA, it would produce about 300 volts peak-to-peak with about 2.3 mA going to drive the resistors and 0.6 mA to the headphones. For the same signal voltage into the headphones, the Egmont output tubes have to produce 4 times as much signal current. Now these are “back of the envelope” calculations. But at least, now we have a reasonable estimate of how much signal current an electrostatic headphone needs to faithfully reproduce the fastest music signals. And, it is clear that replacing resistor loads with current sources is a much more efficient method. Finally, let me make a brief comment about a related matter. It is sometimes said that electrostatic headphones require voltage but no power. This is false. It is true that electrostatic headphones resemble capacitors, and with a capacitor, the drive voltage and current are 90 degrees out of phase so that no power is consumed. However, remember that a capacitor is a simplified model of a stat headphone. In fact, electrostatic headphones have to consume energy, because we can hear the sound they produce! Sound is a form of energy, and by the law of conservation of energy, one of the most fundamental laws of physics, that means the headphones have to consume energy.

I grew up, so to speak, with Gould's '55 recording, so my mind is permanently warped by that. I am working on the Goldbergs myself on and off, but I don't have a chance in hell of duplicating his original speeds, so I try to ignore his interpretation when I'm playing them myself.

Confirmed, pongo5 told me he used KG's board files. I believe he also used the KGST PS board. However, the circuit is simple enough that it can be wired point-to-point, which is what I did

Good point. When someone makes a crap amp it poisons the well. But when properly used with current sources it's a good tube within its limits. Not as powerful as a 6S4A, not to mention a triode strapped EL34, but the very efficient current sources really help make up for the relative lack of power.

So there's a difference between how industry operated then and how it works now? Keeping costs down has ALWAYS been a part of it. But my limited experience with random 6SN7GTAs from the 1950s is that in the SRX circuit the plate voltages between two sections of the same tube are within 5-10 volts of each other and with Sylvania JAN 6SN7GTA tubes from the 1980s is that plate voltages are within 15 volts of each other. So are tolerances between two transistors nowadays better? Gimme a break! Here is a quote from Robert Tomer in "Betting the Most out of Vacuum Tubes" published in 1960, "It has been correctly stated by competent authorities in the field of industrial management that it is cheaper to make a quality (uniform) product, than it is to make a poor quality (non-uniform) product....Rejects add enormously to cost, and therefore, the fewer the rejects, the lower the manufacturing costs. ...no manufacturer can afford to produce a product that doesn't meet good quality standards." That was a common belief at the time. As to your comments about lousy quality control in tubes, you're chronology is off. In the late 60s and 70s when tubes were considered obsolete and the big manufacturers were using the old machines to produce tubes, and/or selling the machinery to Asia, etc., yes, quality deteriorated. But in the 50s when tubes were THE electronic devices quality control was still good. That's why a lot of tube buffs prefer NOS tubes from pre-war to the 1950s and wary 60s to tubes made later. The point is that regardless of tolerances the tubes fulfill the basic specs for voltage, current and power rating. Companies don't make money if their stuff breaks down frequently when used within spec. That's basic engineering regardless of era. And regardless of what you think of RCA, GE, etc., the 50s was when the Japanese sent their people over here to learn how to do quality control. And even though novels were new, they were still designing circuits with 6SN7GTA/B in the 1950s - witness B&W TVs. BTW, I made a mistake saying that the pre-war plates were smaller - did that from memory but I looked at some pre-war 6SN7s and some 6SN7GTAs and they are about the same size. However, when I compared a Sylvania 6sn7GT with a late 6SN7GTB, although the plates are basically the same size, the GTB had a doubly long "tail" where the plate was stapled together compared with the earlier tube. That tail does give some extra cooling capacity. To Earspeaker's point, NOS tubes generally don't go bad unless they've developed a vacuum leak. People are using NOS tubes from before WWII which still meet specifications and work perfectly. In fact until recently tube audio geeks uniformly recommended old tubes from the 50s and before as sounding better and lasting longer than new tubes. So here's the bottom line: Even if every 6SN7GTA/B out there just barely meets its specs, in the revised SRX with +/- 350 volt PS the standing voltage is 335 volts cathode-to-plate voltage which is less than 75% of maximum, and 7 mA standing current, so the combined power dissipation is 63% of maximum. If you're worried about it, use JAN (Joint Army Navy) military tubes that have been tested to meet specs. But even with commercial tubes, run as conservatively as they are in this circuit, the only way they would blow up is if they were defective in the first place.

Actually, almost anything he recorded you can hear him humming along. My sister and I once sent a letter to High Fidelity magazine after he recorded a piano transcription of Beethoven's 5th Symphony that he make a record of the 9th Symphony where he could hum the choral parts while playing the orchestral music. It was published in edited form.

I would add one other comment - whether or not you see or read of any design changes between the GT and GTA/B versions, remember that when the 6SN7GTA and B were designed in 1950 and 1954 there were no transistors, just tubes. They were designed and built by RCA, GE, Sylvania, all the big manufacturers, and used in all sorts of electronic instruments, industrial electronics, military equipment, etc. Engineers design using manufacturer specifications. Do you REALLY think that if the tubes were under designed, unreliable and didn't meet its specs hat it would have become as popular as it did? Sorry, and no offense, but cynicism or no cynicism that makes NO FUCKING SENSE!! And I mean that in the nicest way.

Well, the plates on a 6SN7GTA/B are larger than those of the early non-A/B types, and about the same size as the ECC99. Remember when the GTA/B were designed in pre-transistor days - if they couldn't meet the published specs we would have had a lot of consumer TVs blowing up and the tube company would have gone out of business. Now, later there were higher voltage and power tubes for color TVs such as the 6BL7, etc., but color TVs needed the higher voltage and power. No indication as far as I know that the GTA/B were in any way lacking for TV applications at the time they were in widespread use. Of course in the old days consumer equipment was designed conservatively for long life and reliability, but still, designers probably designed to, say 70-80% of max voltage and 65-70 of max power, which is what I did for the SRX revision. I grew up in the 50s when B&W TVs were very common and there were no issues with TVs blowing up, period. Also remember that later 6SN7s probably used the bigger plates just for convenience sake - no reason to keep the older lower power design in production when the A/B had the same specs. You need to compare the plates on a pre-WWII 6SN7 to a 1960s GTA/B. The 1950s GTs probably used the same plates as the GTA/B for simplicity of manufacturing - in fact, on Tubes Asylum one poster stated he saw a carton of 1960s TungSol 6SN7s of identical construction with some tubes labelled GTB and others labeled GT. The peak plate voltage is simply an indication that the design will take that kind of voltage w/o failing, sparking, etc. For stat amps where the highest cathode to plate voltage they will see is less than 700 volts (for a +/- 350 volt supply) this means that there shouldn't be any issues in terms of the tubes being damaged by too high a voltage. So when someone says the 6NS7 is a 450 volt tube running at 700 volts - NO! It's a tube capable of withstanding 1500 volts running up to 700 volts. And in the SRX circuit it's sitting at about 300-350 volts at rest - perfectly fine. I know you like to build lots of stuff - I suggest you build this with 6SN7GTAs, then if it sounds lousy or blows up, come back to me and we'll look into why.

Thanks for your comments, pongo5. Spritzer: Actually the 6SN7GTA/B is a pretty good tube for this amp. The RCA tube manual specs are: Max plate voltage: 450 volt Max peak plate voltage: 1500 volts NOTE!!! Max plate dissipation: 5 watts/plate, 7.5 watts both plates combined. Amplification factor: 20 Note that these tubes were used in a lot of early TVs for vertical oscillators and deflection oscillators, same kind of duty as 6S4A tubes were designed for. - hence the peak plate voltage rating. When I was debugging my SRX the tubes would sometimes be sitting there for a few minutes with 600+ volts from cathode to plate - no harm done. They tested exactly the same on a tube tester afterwards as before. By comparison: 6S4A Max plate voltage: 550 volt Max peak plate voltage: 2000 volts Max plate dissipation: 8.5 watts amplification factor: 16.5 ECC99: Max plate voltage: 400 volts Max peak plate voltage: none specified Max plate dissipation: 5 watts, dissipation for both plates combined not specified. Amplification factor: 22 EL34 Max plate voltage: 800 volts (Mullard specifies 600 volts when triode strapped) Max plate dissipation: 25 watts Amplification factor (triode strapped): 10.8 The 6SN7GTA/B is at least as good a tube as the ECC99 if not better. - higher plate voltage, equal power dissipation, specified peak plate voltage of 1500 volt which is more than it will ever see in the SRX circuit. Doesn't match the 6SA4 (great choice by KG) in power but it's not far off in voltage specs, I've found matching between tube sections to be excellent (generally less than 5-10 volts difference in circuit) and in the SRX I'm running it at about 4.5 watts total dissipation - 325 volts and 7 mA/plate. Doesn't sound like much, but with the cascoded current sources, driving the current loads to clipping requires less than 5 MICROamps, so 99.9% of the standing current is available to drive the headphones. This makes a major difference compared to using resistor loads (blechhh!) where the headphone (high impedance) is constantly fighting a losing battle versus the load resistor (low impedance by comparison) for the signal current. The EL34 used in the ESX variant has a lot more power, but because of its lower amplification factor when triode strapped, plus its higher Miller capacitance, the closed loop frequency response barely makes it out to 20 kHz before rolling off. And that's with current sources, if you use load resistors it starts to roll off earlier. With 6SN7GTA/B tubes due to the higher amplification factor and slightly less Miller capacitance, the open loop response rolls off above 11 kHz, and the measured closed loop frequency response is flat to 20 kHz and r-3 dB at 46 kHz - that's at 100 volts RMS output into a 100 pf dummy load. The 6S4A does very well here, slightly lower amplification factor but less Miller capacitance, the open loop response rolls off at about 20 kHz (calculated). The issue with frequency response occurs because the input stage is a cascode, which has the disadvantage, as a driver stage, of having a high output impedance. In combination with the Miller capacitance of the output stage, this results in the open loop response of the circuit rolling off at higher frequencies. For example, with the 6SN7GTA/B it is -3dB at approx 11-12 kHz. With the EL34 (I measured the grid to plate capacitance as about 10 pf it rolls off at about 8 kHz. With the ECC99 it rolls off around 9 kHz. The circuit doesn't have a lot of excess gain to begin with. With the 6SN7 there is about 14 dB feedback (5-fold) at lower frequencies, rolling off to no feedback at around 50 kHz. With the EL34 the feedback is around 8 dB (2.5-fold) at lower frequencies. If you do stupid stuff like Mikhail did with substituting tubes for the input section you may wind up with no feedback and the circuit running open loop. So if you're wondering whether I thought about tube choice a bit - yes, I did. If you use ECL82s with the pentodes strapped as triodes you need to see if the amplification factor is high enough and the Miller capacitance low enough to make a good combination. I've just resubmitted my article on this to AudioXpress - apparently they didn't get my previous submission - where I discuss this and other things in infinite gory detail.

Sorry, I don't know the answer to your question, but why not just reuse them (the VDRs)? Or, since you're in GB, why not ask Quad directly?

I'm sure spritzer will be along to comment on sonic merits of the two amps but if it were me, I'd go for the 717. From what I've read the 717 has a similar circuit design to the KGSS, with the major difference being a much simpler power supply. However, sound quality aside, the HUGE advantage the 717 has is that if and when you get a KGSSHV and want to sell your old amp, you should be able to get a good price for a 717 on eBay as it is a commercial amp and a known quantity. Everybody who has even a passing acquaintance with electrostatic headphones has heard of Stax. The exstata is going to be much tougher to sell as it is a DIY amp and few people outside of Head-Case or Head-Fi have ever heard of it. Anybody can go on ebay, look at closed auctions and get an idea of what a Stax amp sells for. Where do you go to research that info for an exstata?

That's cool. At some point overkill doesn't make any difference - it just makes the rubble bounce higher. Sounds like the using the KGSSHV PS is technically better but may not make much if any sonic improvement.

WHAT!!?? Isn't overkill the motto of the Stax Mafia? Here's spritzer on 4/16/14 on the KGSSHV design: "The whole goal of the KGSSHV was never the high voltage, that was just a side product of the parts available. The main focus was the new PSU design and indeed the CCS for the third stage..." So if the PSU for the KGSSHV is an improvement over the KGSS PS (which is basically the same as the KGST mini PS), shouldn't using a modded KGSSHV PS (with decreased output voltage) be an improvement for the KGST, assuming you have the space to do it?

If you look at the posts just before Birgir's post #662 of 1/10/15, I think you'll find that Birgir was talking about building a Blue Hawaii (#652) nopants was talking about building his Blue Hawaii (#653) and asking about whether the KGSSHV supply would work for that (#661). I believe Birgir was addressing nopants question (#662), and not saying that the KGSSHV PS was not suitable for the KGST. At least that's the way I read it.