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JimL

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  1. Trilogy H1 electrostatic amp

    If spritzer is correct that the output tubes are 6N6P, the data sheet says their maximum plate dissipation is 8 watts total for both anodes, max static plate voltage is 450 volts. Pretty similar to 6SN7GTA/B at 7.5 watts total and max static plate voltage of 450 volts. I note that the most recent ECC99 data sheet lists plate dissipation per side as 3.5 watts (down from 5 watts in the original data sheets) and max static plate voltage of 400 volts.
  2. Trilogy H1 electrostatic amp

    Just what the world needs. A variation of a 40 year old design, with passive power supply - for 5000 pounds??!!!?!?
  3. Trilogy H1 electrostatic amp

    Whatever, personally I think they're just variations on a theme. But, run your simulations with the SRX circuit, the Birt circuit, and the SRX circuit without the cross-coupling, but with fixed bias on the upper tube grid, and see what you get.
  4. Trilogy H1 electrostatic amp

    So, what is the circuit board at the bottom of the box? Regulated PS?
  5. Trilogy H1 electrostatic amp

    The first place I saw this circuit was in an electrostatic amp design by Joe Curcio using 6DJ8 tubes, which was published in the very first issue of Glass Audio (Vol 1, no. 0), 1988. That's where I found the reference to the Birt article. I had to go to the MIT library to find a copy of the article on microfilm. I have it in a pdf file so if you PM me, I can send you a copy. Here is a quote from the article (part II): "From a discussion of the general principles in last month's article, the requirements for the first stage of a practical amplifier are now fairly clear. We have seen that for good balance and low push-push [as opposed to push-pull] gain, [tubes] V1 and V2 should have a high gm and large anode impedance...... A better plan is to substitute cascode stages for V1 and V2. It is generally possible to achieve a higher gain in this way, and a screen grid supply is not required. The grids of the upper triodes of the cascode pair require ideally to be at a constant potential relative to cathode. This is not a difficult problem, as we may decouple the grids to cathode, and make the grid feed resistor large. Alternatively, a cross coupling arrangement can be used as shown in figure 9 [this is the cross coupling the SRX uses]. The operation of this circuit is relatively interesting. When a push-pull signal is applied, it can be seen that the drive to the upper triodes of the cascode pair is applied to both the grid and cathode, in anti phase. As far as the cathode circuit is concerned, this turns out to be equivalent to doubling the gm of the upper valve, and therefore the cathode impedance is halved and the voltage gain to this point is halved. However, the grid-to-cathode voltage of each upper triode is the same as it would be in a conventional cascode amplifier, and the overall gain is similar." My comments and clarifications are in [ ]. The article then goes on to plot the V-I characteristics of the cross-coupled circuit using 12AX7 tubes, which, surprise, look just like a standard cascode, i.e. pentode-like. You may not like to call this a cascode because it doesn't keep the grid of the upper tube constant, but considering that cascode was originally coined as a contraction of CASCaded triODE, that the connections of the two tubes are the same as a standard cascode (plate of lower tube to cathode of upper tube), the same current runs through both tubes, and that the V-I curves of this circuit are the same as a standard cascode circuit, I don't know what else you could call it. Just don't let those two extra lines confuse you. Note that the SRX input circuit is NOT an LTP into a common cathode because of the connection between the PLATE of the lower triode to the CATHODE of the upper triode ON THE SAME SIDE. The common-cathode-like connection of the PLATE of the lower triode to the GRID of the upper triode is on THE OPPOSITE SIDE. This is the cross-connection. Take it out, substitute a fixed voltage to the grid of the upper triodes and you have the Hedge circuit, which is a cascode differential pair (balanced) or cascode LTP (single ended). Given that the gain of the cross-coupled circuit is the same as the standard cascode, I calculated the gain as about 41-42 dB, and an output impedance per side of close to 147 kilohm, using the estimated rp and gain for the low current used in the SRX circuit. That's with my 250 kilohm plate resistors rather than the original 300k. If you use the standard published 12AT7 parameters, you'll get a slightly different answer. The input capacitance of the input circuit is low because of the cascode connection. The issue is the high output impedance of the input stage into the output tube Miller capacitance. With the 6SN7GTA/B, the Miller capacitance is about 84 pf, and the circuit rolls off above 11-12 kHz. There is about 14 dB feedback at low frequencies, and so the calculated closed loop gain rolls off above about 53 kHz, which is what I measured. "
  6. Trilogy H1 electrostatic amp

    First, ignore the cross-coupling for a minute. If you fix the voltage of the grid of the upper tubes, you'll see that you have a cascode circuit with a common cathode for the lower tubes and a "grounded grid" for the upper tubes. Now, with the cross-coupling, what you have is the plate signal from the left lower tube going to both the cathode of left upper tube and the grid of the right upper tube, and vice-versa. Remember that plate signals from the two lower tubes are out of phase. So what the upper tubes see is double the voltage signal between the cathode and grid that they would without the cross-coupling. It's still operating as a cascode, just that the voltage signal to the upper tube effectively doubles because the grid and cathode signals are out of phase. If we analyze the upper tube in isolation, it looks like it has double the gm (change in current out for change in voltage in) because the same grid signal now results in twice the variation in current due to there also being an anti phase cathode signal. This results in twice the voltage gain for the upper tube. However, for the lower tube, as it is feeding both upper tubes its current signal is split for the same voltage input, so its gm is halved. The result is that the overall gain is about the same as a cascode without the cross-coupling. So if there is no change in the voltage gain, what is the point of the cross-coupling? Well, if there is a difference between the output signals of the lower tubes due to a single ended input, then the cross-coupling tends to balance it out because the output of the lower tubes feeds both the upper tubes.
  7. Trilogy H1 electrostatic amp

    The SRX input IS a cascoded LTP, AKA Hedge circuit, with cross-coupling per D.R. Birt's article "Self-Balancing Push-Pull Circuits" June 1960 in Wireless World to improve balance for a single-ended input. The T2 also has cascoded LTP/diff amp input but CCS tail and plate loads but w/o cross-coupling. If it's a variation on the SRX, so much for "...a completely clean sheet, creating an all-valve, zero feedback, fully differential design with a direct coupled output stage." A limitation of the SRX input circuit is its high output impedance, which into the Miller capacitance of the output stage rolls off the open loop frequency response above 11 kHz, requiring feedback to flatten the response.. And, plate resistor outputs is so last century.
  8. Stax SRM-T8000

    Actually, I wouldn't be surprised if KG amps are vanishingly rare everywhere. As you note, third party amps are a fraction of the Stax market (IIRC, BHSEs are up in the mid-200s), and DIY amps are probably a fraction of that.
  9. Schiit Jotunheim

    Agree with spritzer. Now the original 6SN7 is the octal equivalent of the 6CG7/6FQ7 in the SRX Plus, but with only 4 tubes, they are pretty much limited to the Egmont topology, especially since they had both single-ended and balanced inputs. In fact, Jason Stoddard says, "It’s not a unique design—you’ll also see stuff like this also from John Broskie," Here is Broskie's stat amp. You will see it is very similar to the Egmont posted above, with a wrinkle - specifically, the resistor/cap combo in the input diff amp tail to do some Aikido style power supply noise cancellation. If Schiit was running the power supply at +/-400 volts I hope they were using the A/B variant of the 6SN7 tubes as that is over the rated voltage of the 6SN7. Note that the SRX Plus uses 6SN7GTA/B (max static plate voltage 450 volts, max power dissipation 7.5 watts combined) which increases the voltage and power limits a bit over the 6CG7 (max plate voltage 330 volts, max power dissipation 5.7 watts combined) in the original SRX, or the 5965 (max plate voltage 300 volts, max power dissipation 4.0 watts combined) in Broskie's design. Using the A/B variant of the 6SN7 in combination with the constant current sources gets the circuit off its knees in terms of drive capability, although it is not nearly as capable as the top Gilmore designs in ultimate drive capacity. Incidentally, if anyone is concerned with running 6SN7GTA/B tubes at 320-350 volts static plate voltage, RCA has plate curves running up to 650 volts plate voltage. 400 volts on the other hand may be pushing things a bit.
  10. grounded grid

    Nice to have the SRX-P included in the group. Of course, you can roll different 12AT7 and 6SN7GTB in that too, which may change the flavor a bit. I like the Sylvania GTAs myself.
  11. King Sound electrostatics

    I assume the Stax were better built too - somehow, the Kingsound looks like it's made from recycled tin cans.
  12. (RE)Building the Perfect Beast...

    IIRC the basic circuit is the SRX, which is fine. Mikhail just did everything he could think of to f**k it up.
  13. (RE)Building the Perfect Beast...

    Whoa! I thought you had already re-built it. Are you going with constant current output loads, or sticking with the resistor loads?
  14. Stax SRM-T8000

    Yeah, basically the same circuit, the only thing that changes is the outputs - transistor vs tube, resistor load vs constant current load...
  15. Phenomenon Libratum

    Possibly a very high impedance coating? The original Quad ESL had an extremely high impedance coating and hence took a while to charge up.
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