erik2A3

Hi Blackbird, As I mentioned, the fact that I pointed this out is, again, absolutely not a reflection of the skill and or knowledge or competence of the person who refreshed some of the caps in your amps. It’s important for me to emphasize that. If there is one design I know inside and out, it would be these Moondogs, and I just had to point out this easily-made and understandable misinterpretation. Making the corrections will be good for the health of the amps in the long run. You have a great example of the species to enjoy - hold on to them!

If we lived nearby, I'd pop over and do the work for you in 10 minutes - FREE!!

Hi Blackbird, Apologies to begin with here, but the person who worked on your Moondogs (and from the looks of it did a good job), is NOT correct about resistor values. R13 is actually composed of a parallel pair, which on the parts list will be shown as resistor A and resistor B. However, you will also see that they have the same resistor reference number as related to the schematic. Parallel resistors will double the power-handling ability of that particular part of the circuit, but the value of resistance will be reduced -- just as two 8ohm speakers connected in parallel will reflect a new nominal impedance of 4 ohms. Connected in series, those same speakers would double the impedance load to 16 ohms. Resistors are the same. So. Had R13 been in another position in the circuit, such as the heater supply, the value would have been much more critical. As you mentioned above, the amps sound good to you. However, it would be wise, in my view, to correct this situation - and it's an easy fix: the present 1.8kohm power resistor can stay exactly where it is (no need to remove it). Get an additional 1.8k ohm resistor of the same kind and power rating, place it on top of or along side the one that's already there, join corresponding leads, crimp, and solder. Clip excess lead, and that's it. So glad you were able to get a pair of these fine amplifiers! Oh, I meant to say it would be very easy to conclude the resistor values were incorrect if one were not familiar with the particular amp circuit in question. So, this is absolutely NOT saying anything against the tech! I poked around a bit and found the following regarding Moondog parts layout. I was going to dig out my old Moondog manual, so this link saved lots of time. R14 was shown twice, so is a typo. http://i-vol.com/welbourne_labs_moondog_2A3/moondog_info.htm IMPORTANT: I see I completely forgot to mention that the same is true for R10! This is much more critical to the circuit since R10 forms the cathode/filament bias resistor. There is heavy current going through this area, and it's important to make sure, Blackbird, that the resistors used have BOTH the correct value (2K) and power rating. I am nearly finished repairing a pair of Welborne Laurel mono blocks for a friend, and used a 50 watt chassis-mount resistor in this place for better heat dissipation. So once more for R10: IF THE TECH USED A SINGLE 2K POWER RESISTOR FOR R10, HAVE HIM ADD ANOTHER ONE IN PARALLEL WITH IT FOR A FINAL VALUE OF Ik.

Jeff, I regrettably sold those amps a long time ago. I'm fairly sure you are trying to obtain some. mechanical decoupling with pyramid washers? In turn, I would like to suggest you try splitting. power supply duty and AC output signal between a large plate choke and small, parallel-feed output transformer -- connected in parallel (aka "shunt") feed fashion; and, instead of the usual grid leak resistor. in the finals stage, use a very high impedance grid choke instead.

Hi Jeff, I absolutely built the Moth Si2A3. I didn't have all the parts for it. It was actually my (absolutely incredible!) wife who contacted Craig U., and talked to him about the possibility of our buying the chassis, OPTs, power transformer, and other passive parts in order to build it myself. Moth never sold their products like this, but it's how we were able to acquire what at the time would have been difficult financially. And he agreed, sending everything in the beautiful wooden flight case with the Moth Audio logo branded right on the top of the box. I was in the process of doing some rewiring of the regulated DC filament supply for the 2A3s in order to use AC current and hum null pot, when I had to have major back surgery. Shortly after that, after my mom passed away in Tucson, my wife and I decided to move to Arizona to be closer to my dad. The Moth was a very quiet amp with DC heaters, and I also used it as my main headphone amplifier. And yes, we love the sound of it as much as its looks. I've used with both our 15ohm Lowther PM2A drivers in a large, back-loaded horn, as well as the Klipschorns we had at the time. I will say that direct coupling between its paralleled input/driver stage (twin triode) and 2A3 sounded very detailed, with sharp leading edges; and that is a presentation I really like. You wrote above that " Adding resistance in series to a power supply, Eric, is the exact opposite of what I strive for. ( See Dr. Halijak 's letter, page 8, of my KT88 build thread )" I totally agree with that. As you know, series resistance like that is often found between filter cap sections; and it was through the building of Dr. J. Lessard's parallel-feed Horus 2A3 mono blocks over ten years ago that I also gained an appreciation for a choke in the position instead of a resistor. In fact, that's one of the modifications I'm doing to the pair of Welborne Laurel IIs I have here right now. Simple can most definitely be a good thing; and direct coupling in a 2-stage amp such as this is surely simple. The Moth is not a straightforward amp to wire, however, since there are three levels of wiring that are all dependent on one another. The finished product was well worth the work though! I'm sure I posted images of it here quite a few years ago. Very glad to hear your amp is up and running and passed the smoke test! erik

Hi Jeff, You observed the following with regard to the (purely subjective) benefits of two stage amps and direct coupling: "Once this is understood, why would anyone waste their time, money, and life ..........using approaches that are second best in topology" It's not a waste of time, money, and life if someone prefers something else, such as a three-stage amp with a parallel-feed final (or muesli mixed with unfiltered extra virgin olive oil instead of milk or yogurt!). And what about well designed OTLs? The school of thought that the otherwise essential lump of metal hanging on a valve amp's output stage is evil and must be banished forever! Or other audiophiles who think I'm an absolute fool for listening to horns with outrageous efficiency, driven by amplifiers weighing nearly 50 pounds that groan (or hum) and struggle to pump out one single watt! I guess my belief is that there are no absolutes. The Moth amp might have been judged best overall in that case, but that doesn't mean that any of the others might not have come out on top under different circumstances, or with other listeners. And you and I and thousands of others know that interstage and direct coupling were around long before Moth Audio, as are numerous other topologies. And that's part of the journey, isn't it? And hey, how did that new amp turn out? Are you listening to it now? Why are your impressions?

Bruce, I found the same thing -- activity definitely seems to be less; and what I do find seems to focus a bit more on what was rather than what is. My wife and I were just talking about how the years have gone by so quickly. But, as we acknowledged, both EP and MQ are outstanding. Electra Print was also responsible for the DRD (Direct Reactance Drive) designs that Welborne sold towards the end. Those amps were also unbelievably quiet, and the quality of the kits even better, IMO, than the earlier Laurels and Moondogs. Electra-Print first published their DRD 300B circuit in Vacuum Tube Valley Magazine. My point is that knowledge of single-ended design using these historic triodes was/is significant, and that fact could only be an advantage to their wide range of products -- from complete amplifiers and circuit designs, to an extensive line of both push-pull and single-ended transformers, power transformers, plate chokes, grid chokes, etc., etc. Welborne's change from MagneQuest to Electra-Print was most definitely NOT a compromise in terms of either build or sound quality. And just for the record, when Vacuum Tube Valley did a comparison of some of the more well-known single-ended amps from that time period, it was Moth Audio's direct-coupled stereo 2A3 amplifier that came out on top. Moth Audio sold me their last chassis for the design, as well as all the parts needed to build that amplifier from the schematic. While the amp does not use an Electra-Print power transformer, it does use EP output transformers.

Right, however, it's still obviously important. I have a ton of work to do on another person't Laurel IIs, but I will take my Moondog OPTs apart if you want. It's busy work for me, and I can do it fairly quickly and post every single shot here on the forum. I would just rather try to get this other gentleman's amps back to him as soon as I can. I put aside three other build projects so I could fix these for him. I will post a shot of the giant MQ trannies compared to my Moondogs. Honestly, it could be used for a boat anchor! I suppose I feel now that it's important to me, not just you, to confirm the identity of your transformers. Sheesh, that was a long time ago. I offered to look them over. I've never charged anything for work, including an amplifier someone asked me to fix that the designer himself wouldn't work on -- after it had caught on fire. That was a good bit of work, but it was done for a friend, and so very much worth the effort. A good guy.

So, Bruce.... Have a look at Blackbirds pictured just above. The leads for the secondary impedance winding taps are just to the left of the filter choke. All of the color codes seen there are used by Electra-Print. They are the same codes that are in the manual I have for my Moondogs. MagneQuest has always had a brown lead as one of those (Electra-Print Does not). Checking your link, I can make out that brown lead in the picture you posted. If you like, I'll tear open my Moondogs and take a picture of mine, although the entire inside is different. I used very little of the construction manual, and built primarily from the schematic so I could make direct connections. The transformers I have on my amps, right now, are from Electra-Print. Take off your OPTs if you want. Undo the B+ connection in red, and the blue one to the 2a3 plate, and then the others, including common and positive on the speaker jacks. Expose them so they are all clearly visible. Then go to the MagneQuest site and refer to output connection combinations for that transformer, and you should find a match. Pretty elementary detective work (wink)

Bruce, You might recall we've visited the subject of your OPTs a number of times. So, yes, we could probably go back to that old link you provided earlier and have a look at the color coding on the secondary winding of the transformer. If you'd like to compare them to my Electra-Print, we can do that too. Tell you what: I'll have a look at the inside shot you provided earlier in this thread, and tell you what to look for. Or, have a look at the picture of these recently repaired and re-capped Moondogs. Do you know where the secondary winding combinations are?

There is a plausible reason...too tired right now to explain! Glad you're amps are up and running again and that you're happy with them! The net values of resistance are actually very similar. It's not related so much to the output iron brand. I happen to be working on a pair of Laurel IIs right now, and am just totally impressed by the massive size of the MagneQuest OPTs on the amps. They are enormous and very Impressive. It's so great to see these great amplifiers from W. Labs during it's prime.

Would you PLEASE consider moving to Tucson so I could stop by and pick them up!!?? Sigh.....they look very, very nice indeed!

Definitely someone local is preferred, so that’s good. Just for the record, the output transformer secondary winding color coding actually suggests Electra-Print as the maker of the OPTs. I would point out that Electra-Print is arguably on par with MQ, and preference for one or the other being simply a matter of taste. I’ve worked on both and heard both is THIS specific circuit, although I can say with confidence that A-B-ing these amplifiers can be difficult unless all aspects of it are equal: same tubes, same resistors, caps, chokes, etc., and the only difference is the output transformer. Whether MQ or E-P, I would say it’s a win-win either way. I’m sure they’ll be barking happily again very soon.

Right and good! 👍 I would have been happy to help with this totally free of charge in terms of labor (you would buy necessary parts post-inspection). I am in Arizona, so packing for safe shipment back and forth for two mono blocks would not be cheap. Surely there should be a guitar amp tech not too far from you that could handle this very easily. One aspect about the construction of this amp, is that the solder terminal strips are setup for reference during building. Consequently, there are quite a number of connections that are indirect -- and subsequently increases length of leads, and so on. As a result, someone not familiar with the circuit might take a bit longer to sort things out; which may add to cost if whomever takes this on charges by the hour. Hopefully there will be a reasonable flat-rate inspection fee, to which of course would be added (the truly needed) parts and labor. The Moondog is such a great amp, and has really become something of a classic. It was built on the same chassis as their Laurel 300B mono blocks, and actually does not really need two 6SN7 input/driver tubes. This is because only half of each section of the two tubes is used, and he did that partly as a way of finding something to do with the extra octal socket hole that was required by the Laurel 300Bs. It might also be a good idea to confirm that the output transformer secondaries are configured for the nominal impedance of your loudspeakers. Those leads are color-coded and clearly visible in the good pictures your took of the wiring.

Cool! Amazing to see a response to something I posted fifteen years ago! Erik M. and I are one and the same. ...miss Dee...

Blackbird, there is no reason to leave these particular amps on constantly. All you are doing is aging your tubes significantly and prematurely. It is possible to install a standby switch on the B+ plate/anode supply so that ONLY the filaments receive current. I am extremely familiar with this circuit and also own a pair. I also know that, as designed, there is an aspect of the power supply that could use some slight alteration: As Marvel mentioned, the 10uf cap is the input to DC ripple filter, and is the first capacitor immediately after the rectifier. If you are using the GZ37 rectifier, that particular tube should ideally not see more than 4uf in this type of power supply. While it most likely would not present a problem in cases where the amps are used for a few hours and powered off, leaving them on for extended periods as you say you have is simply not a good idea - particularly given the fact that this circuit does not have a high voltage standby switch. If there is one thing that can damage components it's heat, and your amplifiers definitely show signs of that. Given what you have, very helpfully said about them, I would stop using them (including switching them on and off to listen and look for problems) immediately and take or ship them to a good tech. The 47uf Solen is the final stage cathode resistor bypass capacitor. There could very likely be more than just one problem with them, and to just put in expensive new capacitors does not address causes beyond age alone. More importantly, DO NOT poke around inside yourself unless you know exactly what you are doing. That big black missile-like thing is the main high voltage filter capacitor (which these days sometimes sell for nearly $2,000 used — rather ludicrous IMO, but not my money). Such capacitors can hold a serious charge for surprisingly long periods of time, and can deliver a painful shock. These are really nice amps, and I will also never sell mine. It is worth the wait to have them properly checked out and repaired as needed. Damaged amps can also damage speakers, so I would really consider taking them out of service now until they are repaired and tested. But take heart in the fact that they can most definitely be fixed and made to perform as new.

Yes! Looks very promising, too!

Are you pretty much decided against shipping them, Applebee’s?

GLWS! If I lived closer, I would be very interested - as I was when the Oris 150s first came out years ago. The images of the complete system appear to be using Lowther drivers with the long phase plug. Is that correct? I have both Lowther DX4s and PM5As that I would LOVE to try in them. Very, very nice!

Thanks for your comments garyrc. We had and totally enjoyed Klipschorns for several years, actually. Both speakers were in good corners, and the dimensions of the room they were in were perfect. Looking back now, I'm sure that was the best sound we've ever had - spectacular imaging, clarity, excellent low end response. We in fact are using a very nice pair of La Scalas now, mostly because the house we are now in just doesn't have good corners for K-horns, but yes, I am aware of the AK 6 (and, unfortunately, its price of admission). I admit that part of my interest in this has to do with just wanting to give the more modern speaker models a try. I am aware of Audyssey, thanks for mentioning that, and the link! So your wife is into that same kind of music! That's pretty cool. Mine has an amazingly good ear, and has been my strongest critic, both positive and no so positive over these past couple of decades of building and experimenting. I thought up a new crossover design for our Las -- to get rid of as many parts as I could (I don't care for crossovers, and our other main speaker system, Lowther back loaded horn with an efficiency of about 106dB, doesn't need or comploy frequency dividing networks of any kind. And so the one I'm just finishing up now is straightforward: single series cap on tweeter, band pass with one inductor (I wound myself) and one cap for hi and low pass sections, and an iron core choke on the woofer. That's it. So! Thanks again for responding!

+1! Many designers I've followed say the same. While they will often use measurement in an academic sense, fine tuning and voicing is always done by ear.

I've been looking at both of these speakers as possible candidates for use in a Classic Rock and Heavy Metal dedicated system. While I listen primarily to classical and acoustic jazz, my wife has mentioned that she would like to listen at louder volumes, with speakers that can reach further down than what we have now, without the use of dual subwoofers (for which we just don't have space). I would really like to know if there are those of you who have been able to listen to both the KG 5.5s and RP 280Fs, and what your opinions are of them -- similarities, differences, etc. ALL of my experience with Klipsch has been with larger Heritage models (Klipschorns and La Scalas, as well as multiple pairs of Heresies), and am really interested in giving one of the more contemporary models a try! And yes, I admit it! The copper metallic woofers on black front baffles just look amazing to me. Of course I know of the upgrade options for the KGs. Any thoughts, observations, impressions, etc. would be very much appreciated! Thanks in advance, erik

Davesans, Beautiful! The wood looks amazing, and I really like how you arranged tubes, power tranny and switch. I always enjoy seeing work like this. Thanks for sharing.

Very nice! Three channel as you are describing is probably the best sound I've had. Starting with PWK's little mini-box that I built fro the 3rd derived center channel, I eventually bought a second-foot Lexicon DC-1 DSP that, for me, was incredible and totally transformative! I still have the Lexicon. Even as a two-channel preamp, or 2.1, it's a thoroughly transparent line stage.

Thanks wuzzer. Would you say the titanium replacement from Bob and crossover rebuild made a significant difference? I see you have RS-7s. (was there an 'RS' series? Did you mean RF-7s?) I wanted to ask you about the KG4s vs RF-7s.