Discussion in 'Tube Audio' started by kward, Jul 7, 2018.
Thanks Primo. Very good information. I'll look into those suggestions as I start to prototype mine.
Te nada! El gusto es mio.
Look for posts by Dr lowmu
Good question. Personal Background.
My first audio mentor was Robert W. Fulton, of Fulton Musical Industries . He was responsible for the FMI-80, FMI J-Modular, Premiere speakers ( 12HZ to 100 kHZ ) . I first met RWF in 1978, and in 1981, I built my first DHT amp, 2A3 P-P, from an AudioNote Kit for Audio and LP Reviewer Neil G. Levinson. In 1982, I decided to do my own design. The very first thing RWF told me was " to make all the chokes to the 2A3 Finals 20 Ohms or less, IF I can find them. " He also suggested two L/C sections in series, as being superior. A six chassis 700 pound monster evolved, 1982-1986 .....P-P-P 6B4Gs.
Sadly, RWF died in 1988, and pre-internet, I corresponded with other amateur audio designers and DIYers, People like Art Loesch, Herb Reichert, JC Morrison, Jim Dowdy, and Dennis Fraker. I shared freely with them " 20 Ohms or less " on Ls.
Reichert's Flesh and Blood 300B amp had my "only $8.00 surplus - priced potted 3 HY at 24 Ohms DCR choke ", as part of his final design in that Sound Practices magazine article.
Dennis Fraker introduced me to a PhD EE - audiophile, who was head of the EE Department at the University of Alabama. Dr. Charles E. Halijac gave me a scholarly run down on DCRs in power supplies, and how conventional EE supply designs " by the book " were " outmoded and out-to-lunch ", something I already sorta knew, and it alluded to RWF's " 20 Ohms or less ".
I think, in around 1998, Dennus Fraker, who was my brightest and best audio friend, and a " second audio mentor", developed his first Serious Stereo DC 2A3 SE audio amp. It used two 10 Ohm C-2708 Stancors, and two L/Cs, ....right up my alley. But Dennis' implementation was FAR FAR advanced, from just using Low DCR Chokes. He also BEAUTIFULLY devised a scheme where the chokes and the amount of capacitance, in uFs, was also low. This was HIS own LSES supply, totally unique in the history of audio. LSES stands for Low Stored Energy Supply.
From Dennis' second year showing at RMAF / Denver, 2006, I attended those shows annually, to hear his SE 2A3 amp implementations, and anything else that sounded good to me, in audio. Fun. I applied his approach to my SE amps, and enjoyed unique results. When I A-Bed my SE 45 amp, with LSES, against, say, a local audio friend's Yamomoto A-08, my implementation put that Yamamoto SE 45 amp to TOTAL shame !! We both heard it instantly, we KNEW, and we were encouraged.
I eventually wanted to share this supply , based on a MONO 45 or 2A3 amp, with good people on the Audio Asylum. There, I had mixed results. The " 100 year old textbook theory guys, " ALL of them never EVER hearing Dennis' amps at a RMAF show, wanted to criticise me for parting with their beloved theory of " critical inductance ". Two very very sharp EEs, Messers John L. Hasquin, and John Swenson, embraced the design concept, developed solid engineering theory to explain why it was so good, and developed their OWN version of LSES, called by Swenson, a FLYWHEEL. These two actually built a circuit and LISTENED to what they built...amazing !! If anyone wants to read their chronological AA Posts, I have them saved and can email them to you.
I did try Swenson's Flywheel , ( which used Fraker's design concepts in different way ), and eventually, I personally went back to pure LSES, ala Dennis, - upon HEARING his RMAF exhibits annually, and have stayed there, and its been developed additionally, in subtle ways.
So, ALL OF THE ABOVE, represents 37 years of my DIY audio life, dedicated to this subject ( 2018-1981) .
In a separate post, I will address your questions, on HOW I would employ LSES in 2018, and what constitutes a good SE amp power supply, in my experience. This post, addressed my personal background for folks to comprehend.
Have fun, I always do.
'Am presently prototyping a SE two stage Direct Coupled 6AQ5 amp, ultralinear. That tube itself, deserves a great circuit implementation, IMHO.
My one qualm with the LSES idea, is that by using a low-value choke and low-value cap- you're creating a resonant circuit that's likely got a corner frequency within the audio band.
I've heard tube amp power supplies with this issue- and my usual complaint, is reduced low bass energy and impact- and what bass is there, tended to be tubby and ill-defined. I've modified such designs, with additional capacitance and/or inductance- getting the resonant frequency below the audio band, and reducing the Q of the resonant circuit to a low enough level to not overshoot- and the bass has almost always improved.
Incidentally- I HAVE heard Dennis Fraker's designs- at RMAF 2017. I actually directly compared his room, with another room, also using a monitor speaker based on the Altec 604, among other rooms I sought out, with tube amplification, anywhere I could find them. I thought Dennis' system (with the SE amps and Altec 604s) didn't have nearly the bass weight or definition of the other 604s, in fact. The sound in Dennis' room was sort of "thin", by comparison, to my ears.
I didn't even know about his power supply designs at that time- but if they're designed in the way described above, the sound I heard actually makes more sense to me...
Of course, I will admit- I'm more attuned and drawn to the bass authority, reach and clarity, generated by Kevin Hayes' VAC amps. It's hard for ANYTHING else to compete with that, IMHO... and that includes all other amp designs of all types, tube or solid state, that I've encountered, period...
I am slowly making progress on this thing. I've got a jig built that will allow me to prototype the basic topology of the amplifier and let me play with biasing, B+ voltage, and resistor value choices. It'll be slow going for a while though, since we gotta get our daughter off to college in another few weeks.
Can't do much else on honing in on the circuit until I order the output transformers. That will take, what, 6 weeks once I place the order from Edcor....
Here's the jig:
Just curious, which Edcor iron did you order?
I am not sure you can drive the KT88 grid load with a 12AT7. I designed a similar SE amp using orig. 6550s but used a 6DJ8 into 47 k ohms.
It has an output Z of 6k ohms, x10 for bridging comes out to 60 k Ohms. A bit of a mismatch but it works well. BTW, it produces 7 w RMS and about 12w peak, which drives my 90 dB speakers well enough unless a huge bass signal comes through. Then it just goes SPLAT for a second.
Paul they will be the CXSE-25-8-5K.
Triode, what is the difficult part of a 12AT7 in driving a KT88? Is it voltage swing, or maybe recharging the coupling cap quick enough without causing blocking distortion? This splat you describe sounds like blocking distortion? I might have to resort to buffering. I'll find out soon enough when I prototype it.
In an RH type SEP amp, where fb is from the plate of the power tube to the plate of driver tube. The 12at7 has no problem driving the kt88. So, while Kward's circuit is not exactly that it is similar but no Schade FB. I have made many RH type amps with the 12at7, and also others with different driver tubes like 6sn7 and 6sl7.
The Tubelab Simple SE amp uses a 12AT7 to drive KT88 finals amongst others. I don't know if it does so optimally but folks seem to like it.
Thanks. I'm thinking it should be okay since 12AT7s are used routinely to drive output stages in power amps (mostly guitar I think, but some hifi as well). I am concerned about the size of the coupling cap as this could increase the propensity for blocking distortion. I might resort to partial cathode bias so that I can use a larger grid leak resistor which would allow me to use a smaller coupling cap.
Finally got the Edcor CXSE-25-8-5K outputs delivered. These are big heavy lugs of iron. Size and weight-wise, they look like about the size of Dynaco A431 iron (something I'm familiar with).
Measuring them on the bench:
Unloaded: 4.2K primary
Loaded (8.0 ohm non inductive load): 4.7K primary
Screen tap: 39.4% of voltage at plate
Frequency: 60 Hz
Unloaded: 4.3K primary
Loaded (8.0 ohm non inductive load): 4.8K primary
Screen tap: 40.0% of voltage at plate
Frequency 60 Hz.
So...screen taps are right on the money, but these samples are not quite 5K primaries as advertised. Nonetheless, close enough I think, where I can work with them. The load line will be slightly steeper than I anticipated so I may need to adjust the desired plate voltage a smidge to get equal swing on either side of the quiescent point.
Anyway, next step is to mock up one channel and see if I can determine the optimum plate voltage to use. Once I know that I can hone in on a workable power transformer.
I have a day off today (Labor day) so I get to work on some hobbies! I've mocked up a single channel of this amp. Here's a picture of the rat's nest:
Basically what I have is 1/2 of a 12AT7 for gain stage, capacitor coupled to a Tung Sol KT120 output tube. I decided to go with the KT120's because I have a quad of them I recently purchased. I don't have any KT88's in house, and I don't want to fork out the money for a nice pair at the moment, so I think I will make this a KT120-based amp instead. Using the bench power supply, I then adjusted plate and negative bias voltages so as to obtain the lowest distortion reading measured on the output across an 8 ohm resistor load. Here's what I got:
B+ voltage: 450V
Bias voltage: -49.6V
Cathode current at idle: 90 mA.
Screen current at idle: 10.3 mA
Plate dissipation at idle: 36 watts (60% of design center max dissipation for a KT120 tube. If using a KT88, this would be approx 85%)
Power output into a dummy 8 ohm load: 14 watts before significant wave form rounding on the top sides of the waves (but it's only a single channel driven so far. I expect it will drop to maybe 12 watts when both channels are driven)
Distortion at 14 watts output at 1KHz: 1.5% THD (but no feedback is applied yet).
I'm noticing the output waveform clips cleanly on the bottom side of the wave form, but the top side rolls the sine wave rounder and does not ever produce a sharp clipping corner until way way into overdriving the output stage. Not ever having built or even worked on an SE amp before, it was a little disconcerting initially to see this behavior--not like a PP amp at all. But on the other hand, I think this is expected behavior given this kind of topology in a SE amp--tube behaves differently when it's forced into cutoff on the down side vs. forcing into grid current on the upside.
Anyway, the next thing to do is add some feedback and take a look at the HF performance of these Edcor output transformers....
Is this OK to do with a 25W SE tranny? I've never seen a clear explanation of how the power rating is arrived at with one of these when you can have high idle currents seperate from the signal power. Some have a max current rating for the primary but the power into the primary would be relatively steady. Perhaps it's the voltage drop across the tranny (x) the current that is what the primary is subjected to. That would be pretty small W then converted to big W via the turns ratio. Seems like something for nothing but I don't know.
Specs on this particular transformer say max power handling capacity is 25 watts and max current through the primary is 200 mA.
The DC (quiescent) plate power dissipation at idle is 36 watts as quoted above. That's calculated as P = I * E, where E is the voltage drop across the tube (450V DC) multiplied by the current through the plate (about 80 mA DC). So the current going through the primary at idle is 80 mA through plate + 10 mA through the screen = 90 mA total.
The AC (RMS) power generated at max output power is calculated as P = I * E as above, but I and E are the RMS values of the voltage and current swings which generate that max power output. Peak to Peak current swing at max power output is approx 160 mA (got that from the load line I drew), or 56 mA RMS. Peak to peak voltage swing at max power is about 850V (again from the load line), or 300V RMS. So AC plate power generated is 300 V * 56 mA = 16.84 watts.
The transformers are rated for 25 watts but only 16.84 watts are consumed. The transformers are rated for 200 mA current through the primary, but only 80 mA DC + 56 mA RMS = 136 mA total is consumed. So yes I do believe these output transformers can handle both the power and current demands of this amp simultaneously, and actually with a good bit of headroom to spare. This was the reason I picked these particular output transformers so that I would not be bandwidth limited even at max power the amp will produce.
Hi Kev, did you give a listen to the circuit? In my SEP KT120 amp i am using 6sn7 for the driver but the original design was for a 12at7. The circuit calls for a 100K feedback resistor from the plate of the power tube to the plate of the driver tube. This amp also uses fixed bias and i am running the cathode current at 110 ma . The Edcor i use is the same size as yours but 3.5K primary impedance if i am remembering correctly. I did an impromptu amp shootout with a SEP 7591 amp from a "Airline" console. This amp is the stock circuit of 12ax7 driving the 7591 with a beefed up power supply. I was really surprised how much better the big RH amp sounds with plate to plate fb. The airline amp has global NFB. After making many SEP i am pretty much convinced that Schade type FB is better sounding than global NFB. I guess with your current test bed it would be interesting if you could compare the sound quality between the 2 different FB circuits.
With the application of 10 dB of global feedback encompassing the output transformer, I have the following situation:
At 14 watts output power (still single channel driven), THD is 0.52% at 1KHz. This is actually a bit lower than I was expecting, but might rise a bit when the power supply drives both channels simultaneously. I'm driving the amp right now with my bench PS, which regulates both high voltage output and negative bias voltage output. So this is the best I think I can expect.
Input sensitivity is 1.63V. This is a bit more sensitive than I was expecting. Apparently I have mis-calculated the closed loop gain of the 12AT7. I'll have to go back and measure this when I get more time.
As mentioned, the only thing added in the FB loop is the feedback resistor itself (no phase adjust cap or any other HF compensation yet). Here's what an 8 KHz square wave at 1 watt output into a purely resistive load looks like so far.
This seems extremely promising. The tops of the square waves are pretty flat after the initial ring settles. There appears to be very little winding resonance up in the higher octaves above 20 KHz and this is a very good sign. It should be quite straight forward to add some HF compensation now to tame that initial overshoot.
So things are looking on the up and up!
Next thing to do will be to add some HF compensation, and then validate HF and LF stability. If I get that far and things still look good, I think I will have validated this design enough for me to take courage to start building the actual amp.
But a nagging feeling persists that something could rear its ugly head, so I am not claiming victory just yet...
Primo--no I haven't listened to the amp at all yet. I want to try that Schade style feedback at some point. Maybe I could use a 6FQ7 in place of a 6SN7 on the test jig. I'm stoked right now though that the Edcor OPTs seem to be really well wound. Not at all like my Edcor PP output transformer experiences...
The 16.8W of output signal power are surely what the secondary has through it. But now I wonder how the true power through the primary is figured since the idle current on the plate is 80ma, through the primary, it seems that would be the swing point +/- the AC current, not 0ma as a normal RMS would be figured from. So the true current for figuring power through the primary itself would be north of the 56mA. If the current through the primary goes from 0mA to 160mA wouldn't you need to use the average current of 80ma (x) the swing voltage? That would be near your OPT rating. 24W in gets you 16.8W out.
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