Which high power SET

I'm looking for a high power SET to drive harbeth shl5, using Shindo masseto pre amp.

Have short listed Verdier 845, Audion black shadow and Wavac MD805m. All three should drive harbeth sufficiently. Any opinions on the above 3 amps are most welcome, especially thoughts on matching with shindo pre amp.
That's a very nice list you have. There's a Shindo GM 70 SET available here on audiogon, surely a fine mate with your Masseto. Yes, some excellent choices to pick among.
You might consider Japanese Triode. They have a nice TRX-M845 SE amplifiers. May be a little to much to spend to drive your Harbeths.
You could go PSE, Parallel Single Ended if you want/need higher power. Possibly not as pure as true SE, but opens up more choices for amps.
I own Cary 805 monos which are also probably worth a listen if there's a dealer in your 'hood.
i owned a wavac 572 years ago w/ watt puppy6s. bass was always an issue, clipped a lot, but never had that midrange tone in my room before.

haven't since either. and i'm still trying.

volume pots in signal path are best removed. found build quality was fair at best, and also had a burned wire requiring a fix (really amps are simple enough circuit anyone local can fix it). cary being in states would matter to me just in case...
hifiharv ;
Any suggestions on that PSE suggestion ?
Feel free to PM me , if you wish .
I'll bet a dollar sounds real sells the brand he recommends (and didn't disclose it.)
I hate to pop your bubble but OTOH, I will probably save you a lot of heartache too so:

The Harbeth is a revealing speaker but not particularly efficient. Its efficiency (and efficiency matters more to tube amps than sensitivity FWIW) is low- only about 85 db. For those that think it is higher keep in mind that Harbeth rates in sensitivity and the impedance of the speaker is 6 ohms. When you do the math 85 db is about right.

You will find that you are better off with a push pull amp that can make some power. There are several reasons for this, not the least of which is that the more powerful the SET is, also the less bandwidth it has. In this case any 20-30 watt amp is going to fall on its face right away.

To take advantage of an SET's characteristics (low distortion at lower power levels) you really do need some efficiency, at least 10 db higher than what you have for the amps mentioned. The only caveat here is if you plan to listen in a small room and in the near-field. Otherwise, seek a 100-200-watt amplifier for the average American listening room.
Atmasphere makes a good point and his reasoning is sound. However if there's any chance you can hear one of those premium SET amplifiers with the speaker, please do so. From personal experience I can say that expectations aren't always reality. Those are "very good" SETs on your wish list. Depending on your taste, music genre,room size and listening volume they could be just what you want.Compare them to a higher powered(pentode tubed) push pull amp and decide. You may be pleasantly surprised by what you hear.DHT tubes are capable of producing beautiful natural sound.
Good Luck,
I'm using a push pull 30W and it's more than enough for my listening tastes. Hence the thought of SET for more refinement. To rephrase my question, which of the 3 on the list would be the best match for shindo masseto pre amp?
I doubt that there are many here, or for that matter anywhere, who have heard your particular combination of gear with the three candidate amps. I have only heard the WAVAC and the system sounded quite good so it is a possible choice.

I own a low-powered SET amp (parallel 2a3) as well as a pushpull triode amp (45 tube) and a pushpull pentode amp (349 tube). All of these amps sound different and quite good. I don't think one should be completely committed to one particular approach as being inherently better than another. At this time, the pushpull pentode amp is in my system because it delivers the most realistic sounding midrange and punchier bass (though not as nuanced bass as the parallel 2a3). In other words, if you are looking for something better than your 30 watt pushpull amp, candidates for "better" may be another pushpull amp from a different designer or one using a different tube or design implementation. I would personally be wide open to trying different options and I would insist on hearing these options in my particular system.

I have heard 211 and 845 SET amps in various systems and they can be very good sounding. But, at least when run in extremely efficient systems, these amps don't deliver quite the same sound as a 45 or 2a3 SET (the amps I generally prefer). I suspect that every choice has certain tradeoffs and one must listen and decide which tradeoffs to make.

I think that where some power is really required, but not extremely high power, OTL amps should be in consideration. These amps are amazing when it comes to perking up the sound--they are fast, dynamic and have oodles of "presence" without being overly harsh and unpleasant. Some may not exactly fit into the category of being "refined" sounding, but, you may change your priorities once you hear them. Like any kind of topology, diffent implementations are voiced differently so you would have to hear the different choices. I happen to like some lower-powered SET amps because they can deliver a lot of the same kind of lively sound of OTLs (taking into account there MUCH lower output), but one really must have ultra efficient speakers.
Many good points you've made as usual. I agree that no single amplifier or particular topology is perfect, they all will require some degree of compromise. The key to ultimate success and long term happiness is knowing what type of sound/presentation you truly want. I stuck with the SETs since this is what Edoit expressed interest in. I believe that he's headed in a very good direction with his list of candidates.

Edoit you say that you current 30 watt amplifier is enough power and pleases you, that's important information to know. Based on that I'd strongly urge you to listen to the 845 SETs you're curious about. Given your comments it suggests that high quality 30 watts is satisfactory with your speakers. Quality will often win out over quantity with the appropriate speaker match. As Larry said an OTL is a viable option (they all sound different depending on the brand).Personally high quality SET amplifiers are the closest I've heard to mimicking the live venue experience. That's why I'm very supportive of your pursuit of them. Naturally YMMV.
Best regards,
Edoit, I think you will find its in your best interests to do an audition on your speakers in your home if you can.

SETs with the same power as your 30-watt push-pull amp may not come off as just as refined. I alluded to the issue, but here it is in plain black and white:

If you want to really experience what an SET does, the efficiency of your speaker should be such that the amp *never* is asked to make more than about 20-25% of full power. If you follow this rule you will get the most bang for the buck out of your SET investment dollar. The reason has to do with how SETs make distortion- above that power level the higher order (harsher) distortion elements come into play. As the power demands are decreased, the distortion goes to unmeasurable and is the reason for the 'inner detail' claims you see describing a lot of SETs. This has to do with how distortion can mask detail per the ear's masking rule.

Since this speaker also has some bandwidth, you may also notice that an SET of similar power simply does not have the bass delivery, as it is very difficult to make an output transformer that can deliver bass and full treble extension at that power level.

I'm not a fan of pentode amps FWIW, being a manufacturer of triode amps myself :)

I think you have sussed out that SETs have a following for good reasons- I think if you look into it, you will find that the most successful systems have lower powered SETs (which have more bandwidth) and much more efficient loudspeakers. My speakers are 98 db 1 watt/1 meter, and I find that 30 watts is a minimum in my room, which is an average size. I do have a type 45-based amplifier (push-pull, makes about 6 watts) and it sounds better than the 45 in SET mode on the same speaker; but even so its hardly enough power to do the job!

Now for the math: the 6 watts that I have on my speaker translates to the same sound pressure level on your speaker with about 120 watts. Think about that. There is no point to buying any amplifier and then pushing it as hard as you can.
There are enough existing variables involved that a solid argument can be made for numerous opinions. Edoit's typical daily listening levels will determine so much about successful amplifier selection. We all can agree that an in home audition is the ideal solution.
I agree with Atmasphere, before writing a check for a 30 wpc SET, audition in your system. SET sounds best not pushed due to their distortion.

Larryi also has good points. Have an open mind and don't get hung up on a particular type of amp. They can all sound great but different.

I would add a higher power amp in your list and hear how your speaker can sound with more current and power. My bias is for bigger amps ... gets old fast with under power amps. GL!
06-24-14: Edoit
To rephrase my question, which of the 3 on the list would be the best match for shindo masseto pre amp?
My suggestion is that a more important consideration is which of the three amps is the best match for the speakers. As you can see in the impedance plots for the SHL5 shown here, its impedance is in the vicinity of 6 ohms in the mid-bass and upper treble regions but rises to a peak of about 34 ohms at a little under 2 kHz, which of course is a particularly important part of the spectrum. The interaction of that impedance variation with the relatively high and also model-dependent output impedance of a SET will cause significant tonal balance variation as a function of which amplifier is used.

It follows from that, also, that a given SET amplifier will likely sound significantly different when used with your particular speakers than when used with many or most other speakers.

One more reason, in addition to those that have been mentioned, to try to audition in your system prior to purchase, and for why a SET might not be the best choice for use with your speakers.

-- Al
I have heard the Wavac. I have also heard some other amps recommended in this thread. What is important to note is, SET or PP, the output tubes have a major contribution to the sonic signature. Most of the big tubes, especially 845, 805 etc have a certain "artificially big sound" about them. They lack the musical subtlety compared to the smaller tubes. This is true even with the bigger Wavac. You hear all the details and of course have lots of power but the sound is more mechanical. It is more "sound" vs music. In that regards I discovered that even a well designed Push pull amp sounded way more musically satisfying if the output tubes are one of the smaller tubes (2A3, 45, 300B, EL34, EL84, Kt88, 6L6GC, 6c33c etc). I have not heard a well implemented 211 tube based amp so I cannot comment on that. But 845 and 805, not for me.

I suggest you also look at Unison Research Performance amp. It is more like a high powered SET without using big transmitter tubes.
Hi Edoit,

You're getting a lot of great advice here from some smart audiophiles. The common threads appear to be: 1) don't become overly enchanted by the idea of any single topology; 2) your speakers are ill-suited for SETs.

SETs can sound great; they also have real limitations. I've been running a sweet 2A3 DHT SET into 99db speakers (easy 8-ohm impedance, compression drivers) since 1999. It sounds really good in small spaces for quiet near-field listening. In my critical-listening room, however, where vinyl rules and I want realistic SPLs, it pales. That's with speakers designed for low-watt tube amps. Your speakers are not.

SET lovers tend to be very passionate about their choices, and they're on to something, no doubt. There's an infectious immediacy about the SET sound. But sometimes the passion takes on a metaphysical character that seems to imbue the topology with the ability to transcend the limitations of mundane physics, like adequately powering inefficient speakers with challenging impedance curves.

If you're certain that SET amps are your manna, I'd suggesting thinking about the amps and speakers as a unit, a single purchase, without trying to retrofit amps to your Harbeths. That way you have a much better chance of hearing the magic a SET *system* can make.
Pani, don't agree with you. I have a VAC 300 watt push pull mono blocks and my Absolare 845 mono blocks are better in every way. Less mechanical, more open, and much more natural. The VAC is great don't get me wrong, but sounds a little veiled in comparison.
It hasn't been established that the Harbeth is incompatible, just speculation. Edoit is doing fine with his present 30 watt amplifier so a similar power SET is at the very least worth an audition. Either he'll like it or he won't. Listening is the only way to determine this. It could disappoint him or reveal a honesty and realism he's not heard before. Let the ears decide.
I appreciate your comments but I must agree with Jwm (Jeff). He and I have been good friends for many years and have heard numerous amplifiers in our respective systems the past 22 years. VTL, ARC, CJ, VAC and on and on. His 845 PSET is simply superior sounding and far more natural than all the previous amps mentioned in his home. The VAC IMO was the best of the push pull amplifiers he had. Even the VAC PHI 300.1 mono blocks were out done by his sublime performing Absolare Passion amplifier. Jeff like many others was skeptical about SET capabilities, not any more. Pani your opinion is just as valid as ours, the point is that we all hear as distinct individuals.
Many have expressed that harbeths are incompatible with SET.
Is this a problem of topology (SET vs push-pull) or stated power?
I'm using a el34 push giving out 28 watts and I hardly go have to go past 9 o'clock on the volume knob. To my ears, the 28 watts seem more than enough for me in terms of power with good frequency extension.

If it's topology, it means harbeths with its low sensitivity are generally incompatible with any SET amp, irregardless of it's power output. Given a hypothetical world where both push pull and SET generate the same power output, does push pull drive harbeths better simply because of topology?
As Almarg pointed out, it is not necessarily efficiency, but the impedance curve of the Harbeths that MAY make them incompatible with most SET amps. Most SET amps have a fairly high output impendance compared to other topologies. Push pull amps can also have a highish output impedance, but, typical SET amps tend to be even higher. The high output impedance of most SET amps will interact with the low impedance of the speaker in ways that will cause frequency response to be uneven. The same can be said of OTL amps which also tend to have higher output impedances. The only way to really find out if such interaction renders the speaker/amp combination unacceptable is to try the amp with the speaker (it is even possible, though less likely, that such interaction will be felicitous).

I am somewhat on the fence as to whether the 28 watts will be adequate or not. Clearly, that amount of power with the Harbeths will not allow for playback at extremely high volume in a large room. But, if you play at reasonable levels, this amount of power may be adequate. As Atmasphere pointed out, long before a SET amp has reached its rated power, it will already be straining. I have personally found that SET amps do distort somewhat gracefully--they become "thick" and slightly muddy and stop getting louder, while pushpull amps will distort more harshly (albeit at higher volume levels for the same rated power). It may well be the case that the higher powered SET amp will be pushed into obvious distortion only for brief moments and you might be willing to live with that. Hence, it is really hard to say one way or another if a particular power level is adequate, which is why some people advocate huge amounts of reserve power (I don't because I like the sound of lower powered amps of all kinds).
Hi Charles and Jeff, I have not heard the Absolare. My comment was more general. Just like a 300B, an 845 also has its own signature. In fact companies like Line Magnetic audio have amps where they use 845 output tubes driven by 300B as driver tubes just to compensate for the that certain lack of delicacy. But even with those kind of designs I could not accept the 845 way of presenting music. If I get to hear the Absolare, I will definitely post my thoughts in some or the other thread, thats a promise :-)
Edoit, I second all of Larry's comments, including his emphasis on the word "may," except that I would add "widely varying speaker impedance" to his mention of low speaker impedance.

The fact that the speaker's impedance varies widely, from about 6 ohms at mid-bass and upper treble frequencies to as high as 34 ohms just below 2 kHz, will cause the frequency response of the amp/speaker combination to be highly sensitive to the output impedance of the amplifier that is used. Since SETs tend to have particularly high output impedances relative to most other topologies, and also because output impedances tend to differ significantly among different SET designs, the acceptability of the resulting sonics pretty much comes down to a matter of chance.

An ideal speaker candidate for use with SETs would have high efficiency, high impedance, and relatively little variation of impedance as a function of frequency. Which is not to say that results will always be unacceptable if those criteria are not met, but you would be taking a big chance if you were to buy (without return privileges) before trying the amp in your system.

If you do try out a SET in your system, given the impedance curve of your speakers what you should particularly look out for with respect to potential frequency response issues would be an over-emphasis of frequencies in the 1 to 3 kHz area. And, correspondingly, an under-emphasis of the mid-bass and upper treble.

-- Al
In my experience, push pull has dramatically more shove than SET. 28 p/p EL34 watts isn't equivalent to 25 845 SET watts. In fact, I had 15w Quad p/p amps that were tremendously more powerful than the 25w Audions SETs. As others suggest, impedance much different between topologies.
I enjoy these types of discussions as they highlight the marked differences and conclusions we form. I still own an el34 PP amplifier that's 40 watts ultra linear and 20 watts triode mode. My 8 watt 300b SET literally embarrasses the PP amplifier with my speakers. This is vastly different from your experiences but that's how it goes. We all have varying outcomes. I'm in the minority here and that's okay, I respect all the participants here. Maybe I'm the oddball but to my ears SETs consistently sounds better because they are more natural and realistic. Needless to say, YMMV.Edoit "listen" to your Harbeth with your list of fine candidates.
Lots of good points above especially the posts by Larryi and Keithr. It is easy to fixate on the power rating and debate a 8w vs. 15w vs. 30w amp. The reality is that the numerical rating often does not correlate with how a particular SE amp sounds on a particular speaker. It is much more important to find an amp that is a synergistic match to the particular speaker. With a SE amp, it is important to use a speaker of high efficiency and easy-to-drive impedance characteristic. This is true regardless of the rated power output of the SE amp. In my case, I use a SE 46 amp which only has a little more than 1 watt output, yet it sounds much more powerful than the rating would suggest.....provided you use a compatible speaker. My speakers are around 98 db efficiency and a fairly flat 16 ohm load so they qualify as a nice match. Nevertheless, as good as the 46 amps sound, and they are the best sounding SE amps I have heard, for my tastes and on my speakers I eventually always return to my PP amps and usually find them to be a more satisfying sound overall. My PP amps happen to be more powerful (around 35 watts) and that probably contributes to the unstressed dynamic peaks but the PP amps also have more control, a tighter grip on the music. Interestingly, some people hearing both amps on my system prefer the SE 46 and some prefer the PP KT-77. There is room for both camps.

It can be difficult to predict one's preference just from reading forums like this one. You really have to live with each amp in your system for an an extended period and then decide for yourself.
I think this discussion leaves out an a couple of important factors. The first is the size of the listening room and the second is the listening level preference. The impedance curve is important as well, but to me speaker efficiency is a much lower priority. For a few years now we have been showing our 300B SET at 10 watts with Fritzspeaker Carbon 7s at 6 ohms and 87 dB. Room size was 13 x 18 or so. No problems whatsoever. Couldn't even get the amp to clip. Room overload was the only issue because we could play it loud.
Hi Clio09,
Not sure if you've read the entire thread. Earlier posts have mentioned room
size and listening levels as important variables that must be considered. As
I told you at CES this year, your amplifier was terrific sounding.
I run 85dB (if that!) Infinity RSIIb speakers with a homebrew 200WPC, zero NFB, SET amp and it's the best sound I've ever heard. Crystal clear highs, full lush mids, rock solid thunderous bass. Sounds like a fleawatt SET, with twin superchargers!
Unlike some of the reports here, I do not have a problem with bandwidth at any volumes I care to enjoy, including LOUD.

So, anything is possible.

Action Shot

If it's topology, it means harbeths with its low sensitivity are generally incompatible with any SET amp, irregardless of it's power output. Given a hypothetical world where both push pull and SET generate the same power output, does push pull drive harbeths better simply because of topology?

Yes!! What is not getting the air time it should in this discussion is the fact that to take advantage of an SET, **you must not push it past about 20% of full power**!

You can go past that amount with a push pull amplifier and not have ill effects.

This is all about your amplifier investment dollar- with a speaker of efficiency that low, your investment dollar in an SET will not be served due to the fact above.

The impedance curve is another issue! It is pretty evident that the Harbeth is expecting that the amplifier used with has some negative feedback to linearize the output of the loudspeaker. Your EL34 amplifier has that feedback. If you put a zero feedback amp on that speaker its likely that the range of that high impedance in the midrange will take on some prominence.

To understand more about this phenomena, see this link:

I guess Atma's point also has relevance to my situation, as 20% of 200W is still 40W, which should be able to push my speakers to a little more than 100dB, which is pretty loud.

The bonus is that at 40W, the amps are still in Class A1. Above that they begin to move into Class A2, which would mostly be for big dynamic peaks.

In any case, they work superbly with the RSIIb speakers. However, if you were to buy them commercially (if they were available) you'd be paying a LOT of money, as the parts cost alone was over $11K, with about a year of my time spent in planning and building. The 2.3kV voltage on the output tubes might also cause a little consternation in the family unit...

So I guess the moral of the story is...you can power a low efficiency speaker with an SET very effectively, but it's better if it's a monster-powered SET so you don't operate anywhere near full power for most music, and you'd better be prepared to spend a lot of money doing it.

Would you agree with that, Atmasphere?
Ait, congratulations on what is certainly one of the most spectacular DIY amplifiers ever!

As a point of information that may be relevant to the thread, do you know what its output impedance is, or alternatively, its damping factor?

Also, what load impedance is its output tap optimized for?

-- Al
Allow me to back up Ralph's point on impedance from personal experience.

For years I owned a pair of highly regarded speakers which the manufacturer rated at 90 dB efficient. I auditioned many amps with those, both tube and SS. One day a friend brought over a tube amp he scratch built which he estimated would be rated about 30-40 wpc. Since he'd had good results on a few other speakers we were both surprised when it simply fell on it's face attempting to drive mine.

After some discussion he suggested two reasons for the failed performance. First off, my speakers were designed for time/phase integrity so had first-order crossovers. The crossover board could be viewed by removing a cover on the back of the speakers and that revealed a very complex (number of components) board. My friend joked that all that was needed was the addition of a transformer and they could power themselves.

The second reason was the impedance plot. It dipped slightly below 3 ohms at two frequencies. I don't remember exactly now but they were around 50 Hz and at 10K Hz.

His amp worked with other 90 dB speakers but failed with mine since they could not adequately drive that load.

In fact I owned a pair of Ralph's MA-1s for nearly a year. With non-complex music they sounded wonderful. But with complex music (full orchestra symphonies, big band jazz, etc.) at moderately loud levels, even their 100 watts was not adequate. That was the fault of the mis-match, not Ralph's amps.
Al, they have an output impedance of about 1.2R, which is in large part due to their 13K : 4 output transformers (Monolith Magnetics S-833); they spec them at 12k:4 but my measurements say 13k. The OPTs weigh 62lbs each and have an amorphous double-C core with varilay winding and teflon interwinding insulation. I'm running the output tubes at 2.3kV and 160mA, which is well within the specs of the 833C with forced air cooling (400W max plate dissipation). I had plans to add in some local NFB around the 833 to drop the output impedance further, but listening tests told me it wasn't needed - bass is detailed and well controlled as is.

The Infinity speakers are actually a relatively benign load below 100Hz, ranging between 6R down to 14R at 40Hz and about 10R at 20Hz. They dip to 1.6R at 9kHz, but there's not much energy up there and the top end sounds just fine even with the low damping factor in that region.

The power supply on the amps is massive (about 3/4 of the amp chassis is PS) with 400 joules of energy storage just for the output stage! That helps a lot with transient power demands, as does the additional 35 joules of energy storage for the driver stage; very helpful when grid current is demanded in A2, at full power it consumes 200mA of grid current!

So, the robust power supply combined with a relatively benign low frequency region in the speakers, allows me to get away with a zero NFB SET amp where you might not think it possible. Again, it's all about the match among system components, a speaker that dipped to 1R in the LF region would most likely be a boomy disaster.

PS: do you live anywhere near central NJ, Al? If so, you could stop by for a listen any time. Just let me know...
Ait, that's awesome. When I read you first post, I thought 200WPC SET was a typo and you meant 20WPC! That must be quite an amp. Congrats!
Ait, thanks for the detailed response. Wrm's choice of the word "awesome" is an understatement, if anything :-)

I'll add to your comments, relevant to the thread, that the 1.2 ohm output impedance is of course significantly lower than that of more typical SETs. And it is low enough so that the effects on frequency response resulting from the interaction of that output impedance with widely varying speaker impedance are not likely to produce unintended results when used with speakers designed with the expectation they would be used with either push-pull tube amps using feedback or even solid state amps.

And thanks for the invite. I'm not in NJ, but every once in a while I have occasion to visit northern NJ, and I'll keep your invitation in mind.

Best regards,
-- Al
The first is the size of the listening room and the second is the listening level preference. The impedance curve is important as well, but to me speaker efficiency is a much lower priority.
For me, #1 priority is amp's ability to drive speaker sufficiently. If not, SQ will suffer whether low or high listening levels from my experience.

Listening level is not a factor in my book since I listen to all different levels depending on the situation.
Ait, have measured the bandwidth of these behemoths?
This is an amazingly interesting subject. I really appreciate the responses, technical explainations and detail. Very interesting indeed

Thanks all
For me, #1 priority is amp's ability to drive speaker sufficiently. If not, SQ will suffer whether low or high listening levels from my experience.

Could not agree more.

Small signal bandwidth is 7Hz - 66kHz, -3dB; the OPT is of course the limiting factor and those are the actual test results provided by Monolith Magnetics. When I checked the amp with a .05V signal using TrueRTA on my laptop, which only measures from 10Hz -20kHz, it was less than 3dB down at 10Hz, and still pretty much flat at 20kHz. At full power, saturation occurs at 32Hz; I can honestly say that I don't notice any frequency limitations soundwise even at very loud volumes. I had an old friend visiting for the past weekend and we listened into the wee hours to everything from Sonny Rollins to Black Sabbath, and it all sounded great...when we cranked up Yello the walls and floor were shaking from the subsonics.
That is quite impressive to get that sort of bandwidth out of an SET of that sort of power! Do you know the configuration- by any chance is it a parafeed circuit?
06-30-14: Atmasphere
That is quite impressive to get that sort of bandwidth out of an SET of that sort of power! Do you know the configuration- by any chance is it a parafeed circuit?


I designed and built it, so I'm very familiar with the configuration. It's a series feed two-stage direct-coupled single-ended design; no parafeed. The driver tube (the excellent Russian 6E5P tetrode, triode strapped) has an active gyrator/mu follower load to supply the grid current demanded when the amp goes into A2.

You can see the LTSpice schematic in the first post of this thread:

The Midlife Crisis

I simulated both parafeed and series feed as I was brainstorming this build; both have their advantages and disadvantages as you well know, but I decided to go for broke and make it series feed, which required me to get very expensive custom gapped transformers from Monolith (these are now part of their Summit line). I could have used a less expensive, smaller transformer with parafeed but then I'd need a custom plate choke to handle the voltage (which could swing to nearly 5000V!) and a high-voltage capacitor as well so it ends up not being much of a cost or weight advantage for parafeed. I opted for the simplicity of series feed.

As I said, the OPT is massive to be able to handle such high power; here's a picture of the wound core before strapping and potting, with an ECC83 tube for reference:


Each monoblock weighs about 180lbs, and is built on a Landfall Systems aluminum chassis.
By the way, the actual schematic of the build differs slightly from the one in the first post of that thread (evolution!). The actual amp uses the AOT2N60 instead of the original AOT1N60 for better power dissipation, there are gate to source protection diodes around the 2N60, the resistor to the gate voltage supply is slightly larger, and there is a 6000V gas discharge tube across the OPT primary to protect it in case of overvoltage.
I remember reading that thread. Nice work!