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srx revisited

kevin gilmore

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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.



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.

Edited by JimL
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Really like what you've done with this circuit.  I've come close to building the original several times.   I've always had an attraction to dirt simple, but high performance tube designs that could be built in a weekend by a high school physics student using mostly  scrounged parts.


I don't have anything to add to to your extensive documentation except my output tube experience with several Egmont variations using simple 10M90  CCS.


As has been mentioned, the 6BL7/6BX7 have huge section to section variations. Out of 30+ samples I could not get any that matched closer than 10V. Some varied as mutch as 50V. Too bad, because on paper, they look like excellent candidates for this application, not to mention they are cheap.


By trial and error based on listening, I settled on 400V rails with 20mA current. Overkill I know, but I like small power tubes in non-traditional applications. I used triode-connected EL84's, 7591's, and 6v6. Running open loop, it was easy to hear the differences.  I would bet your cascode CCS is a mutch better approach. The 50's era 6SN7GTA/B is a great tube, I've run them no problems at 400V at full dissapation. They are also still plentiful and a great bargain. I especially like the Sylvanias.


Thanks for your contribution and let us know when you article comes out.

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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.

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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.

Edited by JimL
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  • 4 weeks later...

Although I said there was no way to balance output tube sections in a previous post, I realized recently that I was mistaken.  In the amp I built there is no way to adjust because I built the output loads with fixed resistors, however since the published schematic has adjustments in the output current loads, this can in fact be used to vary the cathode-to-plate voltage individually for each output section by slightly altering the current running through each section.  The concept is, start with the output current loads fixed, adjust the cathode current sink to approximately zero the output plates, then vary the current loads to adjust the offset between + and - voltages for each channel.  The three adjustments are interactive, so, it would probably be best to adjust one of the current loads to partially decrease the offset, the adjust the other in the opposite direction to further decrease the offset until that is balanced, then adjust the current sink to zero both plates.

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  • 2 weeks later...

I submitted the article again last month - apparently they didn't get it the first time.  Haven't received word on when the plan to publish, but I believe it's going to come out in two parts, with the power supply in the second part.  It's a pretty basic supply, derived from two articles in TubeCad but using MOSFETs instead of tubes, plus a stabilized TL431.

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Let’s look at the input stage in more detail – I don’t feel too guilty about putting this here since this discussion is more detailed (and longer) than the AudioXpress article – word count limitations and all that.  As originally designed by Stax, it is a long-tailed cross-coupled cascode phase inverter with the right tube grid grounded.  This can easily be converted to a balanced differential stage by un-grounding the right tube grid and using it for the negative input of a balanced signal.


Unfortunately, if we put a signal into the left tube gird with the right tube grid grounded, we don’t get a completely balanced signal coming out.  There are a number of things that can be done to improve the output balance. 


First, the higher the gain of the section, the closer we can get to a balanced output.  A cascode has a high gain, so that helps. 


Second, the cross-coupling between the plate of the lower tube and the grid of the upper tube improves the balance, as explained by D.R. Birt.  He describes it in an article in the June 1960 issue of Wireless World.  He notes that this cross-coupling does not alter the overall gain of the cascode, thus for purposes of calculating the gain, one can neglect the cross-coupling and use a simple cascode gain calculator.  In addition, since each upper tube section is being driven by both lower tube sections, the resulting push-pull action should help cancel out any even order distortion generated in the lower tubes – and the 12AT7 in particular has a significant amount of second order distortion.


Third, the higher the value of the cathode resistor, the better the AC balance, thus, replacing the cathode resistor with a current sink improves the output balance.  A simplified way of looking at it is, if a signal electron travels down the left input tube, when it hits the current sink (which ideally is an infinite AC resistance) it bounces over into the right input tube where it travels up. Not only is balance improved but this maximizes the fidelity of signal transmission between the two sides of the phase splitter. With an ideal current sink at the cathodes, the imbalance is approximately 1/(stage gain).  With cascoded 12AT7 tubes, the stage gain is over 100, so the imbalance is less than 1%, which approaches the degree of balance using matched resistors, and significantly exceeds our ability to match tube sections – not to mention the ability to keep them matched as they age.


Finally, the balance pot between the two cathodes helps static balance by allowing the output plates to be adjusted to sit at the same voltage.  The capacitors connecting the cathodes bypass the pot at audio frequencies, preventing the adjustment from altering the gain from one side versus the other. Really quite a sophisticated design for something that appears so simple.


Now, what are the advantages and disadvantages of this circuit?  The advantages are simplicity with high gain and good output balance.  There are two disadvantages, however. 


First, in order to achieve high gain, the output resistors are high impedance, which means that the current running through the circuit is low, so the current drive to the output stage is puny.  For 300 kilohm plate resistors and +/-325 volt power supply, each tube has less than 0.55 mA current available to drive the Miller capacitance of the output stage.  We will discuss Miller capacitance further in the next paragraph.


Second, a cascode has a high output impedance, roughly equal to the value of the output resistor(s).  Now, remember that, in AC terms, the output resistance comprises not only the plate resistor, but also the grid resistor in the following stage, which is in parallel with the plate resistor.  With 300 kilohm plate resistor, 500 kilohm output grid resistor, and the output tube resistance, the output impedance of the circuit is about 173 kilohms on each side.  This high output resistance is driving the output stage, which has a Miller capacitance equal to (μ+1)*(grid to plate capacitance), where μ = the gain of the output stage. 


For a 6SN7GTA output tube, the grid to plate capacitance is approximately 4 pf and μ = 20, for a triode-connected EL34, the grid to plate capacitance is approximately 10 pf and μ = 11, so the Miller capacitance is approximate 84 pf for a 6SN7GTA and 120 pf for a triode connected El34.  Note that the Miller capacitance of the output tubes is the same order of magnitude as a set of electrostatic headphones.  However, since the output tubes also produce gain, the amount of current needed to drive them is proportionately less than the current needed to drive the headphones.  On the other hand, with the high output impedance of our input stage, this means that the open loop frequency response is -3 dB at 11 kHz for a 6SN7GTA output tube and -3 dB at 8 kHz for a triode-connected EL34.  This circuit NEEDS overall feedback to achieve a flat frequency response over the entire audio band.  That means for the entire circuit to get to 20 kHz closed loop without rolling off, it needs 6 dB open loop gain over closed loop gain with a 6SN7GTA, and 9 dB open loop gain over closed loop gain with an EL34.


Now, before you throw up all over your shoes because of this crappy open loop frequency response, let me point out that the Dynaco Stereo 70, a class B Stereophile recommended amplifier back in the good old days of the late J. Gordon Holt, has an open loop frequency response that is -3 dB at around 7 kHz.  Yep, 7 kHz.  Still sounds pretty good.


Anyway, we have two good reasons to want the input stage gain to be high: first, it maximizes the balance from an unbalanced signal going into the balanced output stage, and second, it provides the extra open loop gain needed to  flatten the frequency response up to and beyond 20 kHz.


Incidentally, in a cascode, the lower tube is the major determinant of the gain, however the upper tube does most of the voltage swing.  Some people think a cascode is a two stage design with the lower tube as the “input” and the upper tube as the “driver.”  However, in a cascode, the same signal current runs through both stages.  In the usual two stage input/driver design, the input stage performs much of the voltage gain and runs at a low current, while the driver stage generally runs a higher current to “drive” the output stage.  Thus, it makes no sense to use a high current tube as the upper tube of a cascode, if the circuit as a whole is running at a low current, as is the case here.  Using a high current tube, say a 6BX7, as the upper cascode tube in this circuit just means that you are running it at a trickle current where it is non-linear.  You may like the sound of distortion, but it is by definition malfunctioning.


As a general rule, a well designed tube circuit is optimized for the tubes that are used in it.  The notion that any tube that you can jam into the sockets with or without the use of a socket adapter is just hunky dory (aka tube rolling) as long as no smoke or sparks occur is, to put it bluntly, stupid, unless you like the sound of a malfunctioning, high distortion circuit if you use the wrong tube.  In light of this, let us consider some candidate input tubes.  Stax used 12AT7s for both input tubes in the cascode.  The calculated gain using these is around 42-44 dB, depending on what parameters you use.  The 12AX7 and 6SL7 do approximately as well, yielding calculated gains of around 41-43 dB.  On the other hand, using 6SN7s gives a calculated gain of around 37-41 dB.  These numbers don’t seem to be that much different, however remember that 3 dB represents a 41% increase in voltage gain.  Also, it is important to note that the 12AX7 and 6SL7 are designed to be linear at low currents, whereas the 6SN7 tube really wants to have about 10 times as much current to be in its linear range.  So using a 6SN7 in this circuit produces a non-linear result with reduced gain - like I said, a malfunctioning circuit. 


So for best results, the input circuit should use 12AT7 tubes, which is what it was designed for, with the 12AX7 or 6SL7 as possible acceptable substitutes.  Now, as I mentioned before, since the upper tube in the cascode does most of the voltage swing, it is not unreasonable to have the 12AT7 as the lower tube, which determines the gain, and a 12AX7, 6SL7 or 5751 (roughly a miniature equivalent of the 6SL7) as the upper tube as they are designed for linear voltage operation at low currents.  I confess I have not tried this as I much prefer listening to music to listening to different tubes.

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Let’s finish off by looking at the output stage and how it interacts with the input stage.  The original SRX used a 6CG7/6FQ7 dual triode in a simple balanced differential output. This tube has a maximum DC plate voltage rating of 330 volts, maximum plate dissipation of 4 watts per plate but a combined dissipation rating of 5.7 watts for both plates combined, and an amplification factor of 20.  In the SRX this was run at a DC plate voltage of about 250 volts and 5 mA/plate for a total dissipation of 2.5 watts, which is quite conservative at 75% of max plate voltage and 44% of maximum plate dissipation.


The 6CG7 was designed as a nine-pin equivalent of the 6SN7 (read: cheaper).  If we want more power, we can go to the 6SN7’s bigger brothers, the 6SN7GTA and GTB, which are rated at 450 volts maximum DC plate voltage, 1500 volts peak positive plate voltage, and power dissipation rating of 5 watts/plate and 7.5 watts combined. Note the difference between the maximum DC plate voltage, 450 volts, and the maximum peak plate voltage, 1500 volts.  Even if we run this tube at the maximum DC plate voltage, and drive it to its theoretical maximum sine wave output of 900 volts peak-to-peak, it is not going to spark over.  This is quite unlike solid state, where even a momentary spike above the maximum rated voltage will let out the magic smoke. 


Substituting a 6SN7GTA will duplicate the gain parameters of the original design, but allow us to increase the power supply voltage and the standing current to 325 - 350 volts and 7-8 mA/plate, while still staying well below the maximum combined power dissipation. With the increased current, the cathode resistor needs to be decreased to about 1 kilohm.  And, the RCA data sheet published in 1954 shows static plate curves going up to 650 volts with good linearity, which means that a differential stage can swing 1300 volts peak-to-peak with reasonable distortion. With current loads the Miller capacitance is about 84 pf, and two dual triodes (one per channel) need about 7.5 watts for the heaters.


Another possibility is KG’s favorite small power tube, the 6S4A.  This has an amplification factor of 16 = 24 dB, but has a significantly lower Miller capacitance, about 41 pf.  Heater current is 0.6 amps per tube, or 15 watts total.  Because this is a single triode, matched pairs are needed for each channel.  Interestingly, even though the 6S4A has a higher maximum DC plate voltage of 550 volts and power dissipation of 8.5 watts, more than the 6SN7GTA, the RCA data sheet only shows 6S4A plate curves up to 470 volts, and TungSol shows plate curves up to 550 volts.


What about a triode-connected EL34?  With 800 volts maximum plate voltage (although Mullard lists 600 volts max for triode connection), 140 mA and 25 watts maximum plate dissipation it dwarfs the other two.  But it has a lower voltage gain of 11 = 21 dB, as well as a higher Miller capacitance around 120 pf, and pulls 1.6 amps heater current, so 6.4 amps total, or about 40 watts just for the heaters. Again, matched pairs are needed for each channel.


To simplify the comparison between these output tubes, let us assume +/- 350 volt supplies, 12AT7 input section with 42-44 dB gain, and a theoretical maximum 1400 volt peak-to-peak output. We will use the same slew rate criteria as in the output stage current thread, assuming full power 6 kHz sine wave as a worst case signal, and we will also assume that that the output stage uses current loads to improve function and maximize the voltage gain of the stage.


For a 6SN7GTA output stage, the input stage has to deliver:

Peak voltage = 35 volts

Peak current = 0.22 mA

Total open loop gain:  68-70 dB

Open loop roll-off frequency:  11 kHz


For a 6S4A output stage, the input tube section has to deliver:

Peak voltage = 44 volts

Peak current = 0.24 mA

Total open loop gain: 66-68 dB

Open loop roll-off frequency:  22 kHz


For an EL34 output stage, the input tube section has to deliver:

Peak voltage = 63 volts

Peak current = 0.44 mA

Total open loop gain:  63-65 dB

Open loop roll-off frequency: 8 kHz


These calculations include the current used in the input stage plate and output stage grid resistors.  Notice that the 6S4A and EL34 tubes need higher voltages from the input stage for the same output because of their lower voltage gains.  Also note that the differences in open loop roll-off frequency due to the differences in Miller capacitance between these tubes.


Now, remember that one of the limitations of the SRX input section is the puny current, approximately 0.55 mA per side, or 1.1 mA total. Usually, amp designers keep distortion lowest in the input and driver stages, with the maximum distortion occurring in the output stage.  John Broskie at TubeCad suggests that a good rule of thumb for low distortion in tube circuits is that the maximum current drive required be 20% of the total current or less.  With the 6SN7GTA as the output tube, the maximum current demand on the input stage just meets this criteria, the 6S4A comes very close to meeting it, whereas using the EL34 as the output tube, the maximum current drawn from the input stage reaches 40% of the total input stage current.


Next let’s look at the closed loop frequency response.  Since the open loop gain rolls off at roughly 6 dB/octave above the frequencies calculated above, the closed loop frequency response will roll off when the closed loop gain equals the open loop gain.  The SRX has a closed loop gain of 54 dB, so using 12AT7 input tubes, the -3 dB frequency is roughly as follows:


6SN7GTA:     55 kHz

6S4A:              90 kHz

EL34:              22 kHz


So the 6SS7GTA and 6S4A seem to be the best choices for output tubes, with low distortion operation of the input stage and good closed loop frequency response. The 6S4A provides 1-2 dB higher power potential and more extended high frequency response, but has double the heater current, and because they are single triodes, they need to be matched pairs for each channel.  The 6SN7GTA is the easiest to drive, has significantly lower filament current and, at least in my unselected samples, close matching between tube sections, with plate voltages within 5-10 volts of each other. The measured frequency response of the SRX Plus using 6SN7GTA tubes matches the calculated response, and is nearly identical to the measured response of the original KGSS.


One more example: if we use 6SN7s in the input stage and EL34 output tubes, the total open loop gain is only 58-62 dB, so there is only 4-8 dB of feedback available at low frequencies, and the -3 dB point is around 14 kHz.  This is essentially the ES-X repair/restoration that spritzer reported on in 2010.  Is it any wonder that he noted that “the HF does lack some sparkle and presence…?”  The low feedback might also partially explain his other comments that “the midrange doesn't have the projection of the BH and is a bit thicker than it should be. The bass goes deep but is a bit too round and boomy…”  His later substitution of 7F7s (6SL7 equivalents) in the input stage was “the biggest” improvement, which is not surprising since it significantly increased the negative feedback margin and bumped the -3 dB point up to around 22 kHz, bringing the circuit closer to its optimum function. 


We see that both the choice of input tubes and output tubes can significantly affect the function of the circuit as a whole.  Sub-optimum tube choices in either stage can significantly worsen overall performance.  The reason that these tube choices make such a difference is that the circuit is so economical – it uses just enough stages and just enough tubes to do the job, and no more.  But that means that each tube must contribute a high gain. There’s nothing wrong with a 6SN7 as an input stage tube, or an EL34 as an output tube, but they are not the best tubes for this circuit.


Thus far I’ve discussed the basic SRX output stage, now let’s move on to the modifications in the SRX Plus.  In the output stage current thread, I show the benefits of using current loads instead of plate resistors.  The SRX Plus does this, but in addition the cathode resistor is replaced by a current sink.  This improves the differential balance just as it does for the input stage.  But it has another benefit – the output tubes are now totally isolated from the power supply, and from the other channel.  All they see is the input signal, the other differential tube, and the headphone load.  When the current loads are high enough impedance they are virtually “invisible” to the output tubes – all the signal current goes to drive the headphones, maximizing their efficiency.  This provides the most stable possible environment for the output tubes, and maximizes channel separation because the output tubes in one channel cannot “see” the other channel via the power supply.  The result, quite simply, is the best performance the circuit is capable of.


Now, the reason for using cascoded current sources in this design was not to have better sounding current sources, whatever “better sounding” means, it was to have non-sounding current sources.  They should set the circuit parameters while being sonically inaudible.  My argument is, the closer a practical current source approaches the ideal of infinite impedance with no noise, the less it can contribute to the sound.  A cascode current source comes respectably close to that goal with a minimum number of parts, and allows the active device to have a voltage swing within 15-20 volts of the B+/- supplies.


However, we run into a problem when we have both current loads and current sink, and that is that current can neither be produced nor destroyed.  So, to provide a path for unmatched current to drain harmlessly, there is an adjustable resistor string between B+ and the cathode current sink.  As far as I know this little bit of circuitry is unique.  This resistor string is effectively in parallel with the current sink for AC signals, and while this does compromise the impedance of the current sink, at 220 kilohms, it is still much higher than the original 1.5 kilohm resistor, and in fact, is as high or higher than a simple 10M90S current sink. 


Also, adjusting this resistance varies the DC offset of the output plates, allowing them to be zeroed.  And, as mentioned in a previous post, adjusting the current loads can help balance the voltage between the plates, although, with the 6SN7GTAs I have tried, this isn’t really necessary as the plate voltages have been within 5-10 volts of each other with my fixed current loads.


The tubes are run quite conservatively.  The 12AT7 input tubes are run at less than 30% of maximum DC voltage and less than 10% maximum power dissipation, so they are barely turned on.  The 6SN7GTA output tubes are run at less than 80% of maximum DC voltage and less than 65% of maximum power dissipation, so they are coasting.  NOS (new old stock) industrial tubes running under these conditions can be expected to last 5000-10,000 hours.  You can further improve tube life by running the filaments at slightly lower than rated voltage, e.g. 6 volts instead of 6.3 volts, or 12 volts instead of 12.6 volts.


Now, since we are using both tubes and transistors, is this a hybrid amp?  Well, not in the sense of, say, the Blue Hawaii or T2, where both tubes and transistors are in the signal path.  In the SRX Plus, the solid state is all current sources which are used to set circuit parameters and maximize the performance of the tubes, which do all the handling of the signal voltage and current.  So this is still a tube amplifier, but with solid state support elements. If it has any sonic character it should be tube-like.


A couple comments about tubes versus transistors.  One of the enduring controversies in audio is whether one sounds better than the other, which I’m going to sidestep.  I do think that tubes make a good match for electrostatics because they are high voltage low current devices. But there are practical considerations. 


First, while tubes physically somewhat fragile, they are electrically more rugged.  When I was adjusting my SRX Plus, there were times when one plate on a 6SN7GTA output tube was sitting at +300 volts while the other was around -300 volts, meaning that one cathode to plate voltage was over 600 volts for a minutes at a time with no harm done, despite a maximum DC cathode to plate rating of 450 volts.  Do that with a set of transistors rated for 450 volts and you’ll need to replace the remaining fragments.


Second, the tube types used in the SRX Plus should not become obsolete in the forseeable future, as has been the case with a number of high voltage transistors.  These tube types are used in guitar amps, which have a continuing demand that far exceeds the demand for audiophile tubes.


So, is all this worthwhile?  Well, the ES-X is the nearly identical circuit except using EL34 output tubes, which are less well suited for this circuit, and here’s what spritzer had to say about that amplifier in 2010:


“The T2 and the Blue Hawaii are the best you can get and for good reason.  They cost a lot to build (the T2 quite a bit more than any BH) but the money is well spent.  Stable PSU and enough current reserve to never leave the headphones wanting.  A KGSSHV run at full tilt would also qualify here. 


One step further down the ladder would be the current version of the ESX with a CCS loaded output stage.  The amp with just the old plate resistors is very good but running the tubes properly would make a world of difference.” 


In my opinion the circuit is better than most.  It is simple yet sophisticated, and has the inherent linearity of triode tubes.  The dual differential design means it is inherently balanced, and it will stay that way.  With its low parts count it is inexpensive and simple enough to be built point-to-point.  Given that the output stage of an amplifier is where there is the most potential for distortion, the use of cascoded current loads means it should perform better than designs that use load resistors or inductors such as the Egmont, RSA A10, or Woo WES, or designs that use poorly designed current sources such as the eXstatA or Liquid Lightning (modest, aren’t I?).  The SRX Plus lacks the sheer power of a T2, Blue Hawaii or KGSSHV but it comes respectably close – within a couple of dB - for a lot less.  If those are Stereophile Class A, then the SRX Plus is Class B, “the next best thing to the very best sound reproduction,” and at a bargain price.


So there you have it, a detailed analysis of the SRX Plus, an optimized version of the Stax SRX tube amplifier using solid state support.  Any regulated power supply such as the KGST or mini KGBH PS adjusted to 325-350 volts should work fine with this.  When the article comes out I can discuss the shunt regulated power supply, but I can tell you it’s a pretty basic supply.  Happy building!

Edited by JimL
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So there you have it, a detailed analysis of the SRX Plus, an optimized version of the Stax SRX tube amplifier using solid state support.  Any regulated power supply such as the KGST or mini KGBH PS adjusted to 325-350 volts should work fine with this.  When the article comes out I can discuss the shunt regulated power supply, but I can tell you it’s a pretty basic supply.  Happy building!


Thanks for that tidbit, I think it and the Lamda Sig are destined for office use at this point

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With cascoded 12AT7 tubes, the stage gain is over 100, so the imbalance is less than 1%, which approaches the degree of balance using matched resistors, and significantly exceeds our ability to match tube sections – not to mention the ability to keep them matched as they age.


Funny story. I'm using Philips JAN 12AT7s, and hadn't really given it much thought. I'd read here and there that many of the JANs being sold were in fact rejects, so, for fun, I popped them into the tester and came across this gem:




Interestingly, the lower current section actually matches the datasheet, so it's definitely not a consequence of aging. The dashed section has about twice the rated gm; clearly, the circuit is very good at forcing AC balance.



I'm getting current production, next time. :)

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Interestingly, the lower current section actually matches the datasheet, so it's definitely not a consequence of aging. The dashed section has about twice the rated gm; clearly, the circuit is very good at forcing AC balance.





The only thing that matters to AC balance is the matching of the plate resistors, and that you have a tail-CCS.

DC balance is subject to tube parameters, but who cares in a cap coupled circuit? 

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How accurate do the resistor values need to be? Thrown off a bit by the 1.43k and 102 ohm values. I figure with the trimmer in line, 1.5k and 100 should be fine?


Also, what value dropping resistor are you using to drop the -HV to -20? 


This should be fun though, first time P2P-ing something- thinking about mounting the 12* tubes inside and letting the 6SN7* stick out

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Those values should be fine.  In my build I actually made a test jig and measured the closest value to give the desired current, then soldered in the closest 1% value resistor.  For the schematic, I took the actual values used and calculated a fixed resistor plus trimmer that would cover the values, plus some extra in case.


On the subject of tubes, old tubes that have never been used, aka New Old Stock (NOS) used to be preferred by many tube buffs.  Back in the 40s to 60s when tubes were the only, or the predominant, amplification device, they were made in the millions by big industrial firms like GE, RCA, Sylvania, Telefunken, Siemens, Mullard, Amperex, etc. with decades of development behind them, and used techniques that were not always documented.  They were reliable and long lasting because of competition between these companies.  Industrial tubes were routinely expected to last 5000 to 10,000 hours, and the Telefunken ECC83/12AX7 was legendary for having a 100,000 (that's one hundred thousand) hour life span, one reason that even used Telefunken tubes still sell at high prices.  So tube buffs felt that they were not only the best sounding but the most reliable and long lasting.  New production tubes generally come from China, Russia and Eastern Europe, often made on equipment sold by western firms that were getting out of the tube business, and for a long time had the reputation of having less reliability, a shorter life span and worse sound.  However some of the newer production tubes appear to have good reliability and good sound, in some cases comparable or even preferable to NOS tubes.  For example I have seen some favorable reviews of the new Tungsol 6SN7GTB and 12AT7 tubes, which are made by the Reflector plant in Russia.  The 6S4A tubes are NOS only as they were primarily designed as TV tubes and of course nobody makes tube TVs any more so there is no demand except by fringe types like us.


There are a number of reputable tube dealers on the internet.  I have personal experience with Brent Jessee (NOS), Jim McShane (NOS and new production) and Upscale Audio (NOS and new production), but there are others as well.  I have also bought tubes off eBay - but I check every tube I buy on a Hickok tube tester to make sure it is OK.  The ultimate test, of course, is in circuit but a tube tester at least makes sure it isn't a dud.

Edited by JimL
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