Erik Mandaville

Someone unknowingly gave you a nudge in the right direction, but a little more work is needed. Wish I could help. bump!

"Even the big Moth Audio 2A3 I heard were lacking but they were being played through some monster folded horn furniture pieces with Lowther drivers. Impressive looking system but my Belles sound far better even with the "liability" of needing a crossover network." Excellent point. The Moth in folded horn Lowthers is pretty impressive. I also use PP El-34s on our Klipschorns. But........an argument so weary and beaten to death here. Moondogs are nice 2A3 amps, too. I particularly prefer the absence of coupling capacitors in the Moth, which is just my cup of tea, but that combined with a Loudspeaker that doesn't have the drawbacks of passive inductors and capacitors to split frequencies between multiple drivers can, as you say -- in the right implementation -- make for some very good sounding tunes. Many people find amps such as the Moth and Moondogs on speakers like Klipschorns, Belles, and Lascalas wonderful. That's why they continue to use them. Some don't. Some think using antique Klipsch Heritage horn speakers is absurd...laughable (I've heard that before). Some don't. Erik

If he wanted to install the inductors, how would he do it? There really wasn't an explanation given for the crossover he has. Erik edit: And what would the values of the inductors be? ....hopefully you'll get some help.

But what's the answer to kg4guy's question? Erik

If the 'noise' is the hiss some owners are reporting, the chance that it's related to the AC house supply is very slim IMO.

"Hmmm? Sounds like a good controversy I missed." I wouldn't say that. It was just a discussion about some common modifications some have made to this particular amplifier. Just a slight alteration of words to reflect accuracy regarding the position of a filter choke, Mark. Seems there is always controversy regarding SET amps on this forum, but this one is okay I think -- at least so far. Have a good day, Erik

""Between" was a poor choice of words. I didn't want to post the schematic... we've been through all that before." Well, that's really what it looks like at first. An easy interpretation to make, Bruce. This very thing has come up before about the Moondog. Erik

There is much less chance of possible arcing with a 5AR4, which makes a good substitute for the GZ37. It's not quite as nice looking (depending on what's in one's eye as far as beauty goes...), but appropriate. Others use the 5Y3 in 2A3 amps, such as George Wright's. It's a great tube, too, but has a directly heated cathode for an instant high voltage application. I think Sovtek makes an indirectly heated 5Y3, however. Erik

Bruce: "The choke is in the power supply, between the first two electroytics, coming off the GZ37..." It does look that way at a first glance. The filter choke is actually between the 10uf film input to filter cap and the first of the two 100uf electrolytics. In effect, the 10uf bumps up B+ immediately after the rectifier, and from there through the choke and a large voltage dropping resistor. As far as textbook values go for the GZ37, 10uf is actully more than twice the suggested max value for an input to filter capacitor. There are a couple of people I know who have experienced premature rectifier failure as a result. Others with the same amp haven't had any problems at all. One could get by better, IMO, with a smaller value -- such as 1uf - 4uf, and then reduce the value of the large resistor mentioned above to compensate. What happens is that the big input capacitor yanks up the B+ up too high, and the resistor says 'no' and knocks it back down again. Going the other way around, IME, is possibly more appropriate, mainly in that it's easier on the rectifier. The opinions of others may not see it the same way; I just prefer to stick to the book and extend tube life if possible. Erik

"Where in the Moondog circuit do they use the 15H choke? just curious." Read again the quote you copied regarding its comparison to the grid choke. The information you're looking for is already there. It's really not all that large in terms of value of inductance. In that respect, the grid choke is nearly 5,000 times greater, despite its physically smaller size. You are reading that correctly -- nearly 5,000H. Erik

Oh: The MQ grid choke will fit fine on either side of the 2A3 socket (underside of chassis), which would be a great spot for it if you decided to adopt one for each amp. Erik

Bruce: Of course -- please pardon the safety issue commentary. I do things like that all day long, and it just happens! Soldering in live chassis has been mentioned here before, and that's something that scares the hell out of me. Getting zapped is not my idea of fun, and I always want to mention the seriousness of working with active components like this. Problems can happen when we forget to be afraid! Maybe try those little mods one at a time to see what each does, and you never know, maybe all of them combined make for a really nice sound for you. An equal chance, too, that you won't like any of them. I mention these in part because the first owner of you amp and I were throwing ideas around about the Ultrapath years ago, and were both experimenting with it. He makes an incredibly nice turntable, by the way. I have a new idea for the Klipsch Heritage crossover autoformer, but I need to check some things out about it first! Erik

Yeah. They were here -- I improved a couple of connections, and that was all. We talked about the output iron a few times, actually. You have MQ trannies, which, as far as the secondary output connections are a little more straightforward than those from EP. Electra-Print requires combinations of the leads on the secondary windings for the desired output impedance. FYI: The Wright amps I liked so much also had MQ output transformers. Not all that large in size, huge in sound. On removing the bypass capacitor on the front end: It's the small red electrolytic toward the left of the image you posted. If you want to take it out, just two clips with your cutters and it's gone. I would take time to remove it properly, and so another can be put in it's place if you prefer it. I've used very expensive Black Gates, Elna Cerafines, and very cheap counterparts of the same value in that same position and they sound identical to me. The grid chokes I don't mind paying for. Cheaper isn't always better, but sometimes it is..........yawn. Have fun -- check your filter caps with a voltmeter to confirm that they are discharged before doing anything in there. Be sure the amp is unplugged, etc., etc. (I know you will do that). Just to be safe. Erik edit: if you want to try to take out that bypass cap (the red one), be sure you don't cut the leads of the cathode resistor that's part of the same connection. Just the leads for the cap. But: Look carefully: you should be able to just remove the extra leads from the capacitor over to the 6SN7 socket, and leave the capacitor in place in case you want to put it back in the circuit. If you want to do this, I can show you the steps. It might be a mod you like.

After that you can try the famous.....drum roll.....'ULTRAPATH CONNECTION" Which is much older than the DRDs. Yank out the Solen across the 2A3 cathode bias resistor, put in another with a higher voltage rating (little more capacitance wouldn't be bad), and connect one end to the top of the bias resistor, and instead of grounding the other side, connect to B+. Ultrapath. And! you can try removing the cathode resistor bypass cap on the input stage -- the little 100uf job. Won't do any harm in the least, but will possibly drop your gain, particularly at lower frequencies -- but this is in theory. FR can sometimes be a casualty, but there are LOTS of people who do this and find it more musical, smooth, etc. I'm mentioning these as easy modifications where it really is possible to hear the results of what you do -- I did, anyway, and prefer the bypass cap in the circuit.

Bruce: MQ part# BCP16Ni The Chokes are much, much smaller than the parafeed plate load choke or the 15H filter choke in the Moondog, and really don't take up very much room. For me, they are easily worthy of a couple of extra carefully drilled holes, or you can also find a place along the inner side of the wooden chassis. Two small wood screws will do the trick there. My Moondogs are so non-stock looking inside right now, that I honestly can't remember what the original looked liked in terms of available space. If you want to post or email an under the hood shot, I can give you an idea of where you could mount them on the chassis. Perhaps better yet (if you decided you wanted to try them), test them to see what you think by attaching to the wooden chassis first, and if you find them unsuitable, I'll by them from you at your cost. *hint: for soldering to the terminal lugs on the choke, you might need to very carefully remove some residual varnish, and then just slip the lead through so it makes contact on the lug at the point where it enters on one side and exits on the other. Don't wrap the wire around a bunch of times. It might even be worth soldering with the choke inverted. If excess solder is applied, it will hit the floor instead falling in on the windings. In other words, the chokes don't have leads already attached to them -- you have to do that. 20AWG is fine. Erik

Parallel feed outputs can be done with the 45, too. the MQ grid-choke in the Moondog was really a neat modification. I'll get another pair one of these months. They are not nearly as cheap as the resistors they replace, but are a very nice part of the circuit, IMO. An easy mod, but for the Moondog would need to be installed on the underside of the top plate with two screw holes (plus machine screws/nuts), or against the inner wooden chassis frame. A very nice little part. No schematic needed, just take out the resistor between grid and ground on the output tube and install the choke in its place. I had one that 'opened' internally (extremely rare), and MQ/Pantheon replaced it at a significantly reduced cost. Good customer service. Erik

"Especially the ones that have never heard them!" ...or heard them under the right circumstances, which with that kind of amp can be important. Even something as seemingly unexpected as a crossover type (as in order of slope) can understandably turn someone off. The DRD 45 or SE OTL (Transcendent Sound) in conjunction with super high order (PASSIVE) networks would not, IMO, be the best combination. That's an extreme example, but I suspect some have listened to these low powered amps with associated equipment that was not able to take advantage of some of the positive aspects of such low power. Then again, I'm sure there are those who simply don't like them, regardless of crossover type or speaker efficiency. Fair enough. I had a pair of 2A3 amplifiers here to work on, where one of the monoblocks was configured for a different output impedance from it's twin, while the other one on inspection looked as if someone had jammed the 2A3 in backwards (there are four pins, which can be confused). This event sadly turend into another SET mud-slinging contest. These same amps, once repaired and essentially completely rewired (including matching output impedances), with new power supply filter capacitors, became my favorites over not Weborne DRDs, but the Moondogs. That's JUST my personal preference and taste. But, the Moondogs modified as far as input/driver and hot-rodded for 5 volt filaments (fully regulated/filtered DC) sound actually kind of similar to Parafeed, which if you haven't tried that, you might really, really like. check out Lessards' Horus schematic. It's a straightforward design, and the choke on the grid of the 2A3, which is a cousin to the Heritage crossover autoformer, is a really interesting component. If the Moth amp weren't directly coupled, I would use on in each channel, but you could add them to your Moondogs easily enough. They're installed in place of the grid to ground resistor on the 2A3. The 5687 is an awesome input driver stage tube, too. I once mentioned the paralleling of the 6SN7 to LeoK and the Ultrapth from the DRD, and both of those are very good sounding on the Moondog. He took that a step further to shunt/parallel feed output. Parts in this case are NOT cheap, by the way. 'Course you might like to leave your Moondogs completely stock, and they are good amps that way -- they just don't need two 6SN7s. The grid choke would be an easy retrofit, though. Check the MQ website for the schematic and parts if you get interested sometime. It's a good build from scratch project...........................and I have tests to grade. Erik

Absolutely. Trying something personally is the only way to really know for sure -- and even then sometimes things change. People have sold their big-size, small-power triode amps in the past and found they missed them later. The opposite of that is true, too. There are some here who can't stand the sound of single-ended triodes, but that's because they just don't know any better. 'Spose I should install a 'wink' here, although it might be kind of interesting not to! I also have very efficient speakers, and enjoy both low and high power depending on what I'm using them for; and my high power, less than 50 watts, is in the form of both antique and very modern technology. You may not find a significant difference between 2A3 and 300B output levels, though, but rather some differing characteristics about the tubes, themselves. What 2A3 amps were you looking at? Erik

With that kind of efficiency, 8 watts really should be fine -- as you said you found it satisfactory. What matters is what you like. Try with higher power, too, and choose what you prefer. For HT, you might need more....and again you might not. Have fun, Erik

That's the new K-horn network? Erik

Fini: Check your grounding scheme, as well, making sure that you don't have power supply grounds and preamp/amp circuitry grounds combined together. Depending on how you;re grounding this circuit, the preamp section and power supply section should have their own ground nodes, which are then each connected to a single star ground on the chassis, usually near the input. Also, if you have a center tapped power transformer, it's helpful for the center tap ground position to be the same as that used for the first input to filter capacitor -- which may be film-foil type or an electrolytic. Erik

Fini: If there is leakage caused by an AC filament supply, it wouldn't be 120 cycle hum. If this is a preamp, it might be a good idea to use DC filaments. What Mike said about the center tapped filament transformer can help, as can the 100ohm resistors on each leg of the heater supply with the other ends grounded. That still may not be enough, though. In a preamp, because of the lower signal levels, you might like to try DC, which, if you were able to do this, wouldn't be hard for you. Those step up transformers look really nice b the way. For DC filaments, Radio Shack will have the full-wave bridge rectifier that would be needed. The ripple filter capacitor across the neg/pos DC output can be in the range of 10,000uf, but you may need a dropping resistor to bring the voltage back to what you want. Adding the capacitor will likely bump the voltage up a little bit. You can install the resistor after the first filter cap in series with the pos. end of the next electrolytic and from there run the leads to the heater pins on the tube sockets. You can also make the rectifier with 4 individual diodes, which might be a little cheaper. If you want some asistance, let me know. A power resistor would be needed in the above description. If the hum is in fact 120 cycles, you might try adding extra capacitance in the power supply. Good luck; that looks like a nice little project. Oh, I wanted to mention, too, that for RCA jacks and speaker posts, you might find the larger Weller soldering gun helpful. Erik

Jim: Single-ended 2A3s and 300Bs can be great with Klipschorns. I've had many different types of single-ended amps, including the SE OTLs and DRD 45s that were a marvelous match. We mainly use the K-horns in our HT right now, and find a more powerful amp suitable for movies. They Dyna 70 is a great amplifier for music too, though. That Pink Floyd SACD I mentioned, though, was the one time I heard the Moth clip. Things just got grumbly and wooly sounding when we pushed it beyond its limits. And SET was downright bad with the comparatively inefficient monitors I built. I couldn't turn the volume up very much at all before they started sounded very unpleasant. Granted the room in that case was pretty big, and the situation probably would have been better in a den or study type of setting. Erik PS: Transcendent Sound SE OTLs wired for monoblocks provided a sound I remember well. Just amazing, and had outstanding bass response for a 4 watt amp. OTLs are cool amplifiers, although the SE OTL got roasting hot for its small output. I don't care about that; it sounded great. Erik

Jim: I have to say that I think his comments are totally on the mark (that really wasn't a pun, by the way...). There are some things they do incredibly well, but I would agree that organ music is absolutely not one of them. Well, at least the kind of organ music with those really, really low, dive-to-the-center-of-the-Earth notes. It's just not the right application for them. The SET brighter-tilt suggestion may also be part of this, I can't deny that, because as much as I enjoy and often use SET amps, they simply do not IME have the same kind of bass response ability of a more powerful amplifier. My listening habits are actually very similar to what he outlined above, at least in the past few years. I prefer to listen closer to the speakers, and at only moderate volume levels most of the time, but this is when we're using the Lowther/Moth system. The surround sound with Klipschorns, Heresies, Dyna ST 70 and Teac amps, is a totally different animal, and one that also includes a subwoofer. it's capable of what to me is enormous and very clear sound, and is great for organ music, heavy rock/metal, etc. Pink Floyd SACD was kind of a true 'WOW' experience for me, but I don't listen like that very often. It sure was fun, though! The single driver Lowthers, which may be the case for the Fostex too, is that they have this sort of 'real' and/or very sharply and clearly defined quality with the right kind of music. Something can be said for the lack of a crossover, and I think that character trait is at least partly responsible for the very sort of open sound of these speakers. They just can't hit those really deep, chest vibrating notes really well. And I would say you're a fine rep for the SET bunch! You like your 300B amp tons, and can always get something more powerful sometime if you'd like to try it. I would freely admit that more power has some advantages, and is one of the reasons we have a Dyna 70 around here. We listen to many different kinds of music, and I like the way each system 'works' whithin a certain comfort zone. But yeah, SET and small single-rangers probably isn't the best for the complexity of large orchestra and organ. One speaker with a whizzer can only do so much, ya know.. Erik

Jim: I think there is something to be said for that, but I suppose it's also a matter of listening habits, speaker placement, and that sort of thing. I also find the Lowthers a little more directional than the two way monitors I made, but I found that a little less toe-in reduces that effect. Some people even remove the phase plugs, despite what's said about the subsequent (apparently) ragged response. I borrowed some new phase plugs from Lowther America that I liked very much, but they tended to focus high frequencies even more. Still, from where I sit, the sound was very good. I'm going to purchase them one day soon. There is also the climbing impedance issue associated with the Lowthers, which some repair with a swamping resistor across the speaker terminals. I've done that in the past with them; I'm not using one now. Others complained of what has become known as the "Lowther Shout" in the region of a few kHz. It's not been a problem with me, but I will say that the drivers took literally several hundred hours to break in (mechanical break-in in terms of driver surround) and sound more balanced. But yes, over the past decade of using this kind of speaker, I have talked with a fair number of people who found what you have but still liked this kind of loudspeaker better than multi-driver types. I do find the Lowthers more sharp or bright than some other speakers we have, but for me that's a characteristic that helps make certain instruments more real to how I know them to sound in real life. Violins and acoustic drums, for example, can have an amazing sort of brash or bright quality that Lowthers reproduce particularly well. Our Klipsch Heritage speakers are good in that way, too, which is why I liked them since we first listened to a pair of Heresies in the 80s but couldn't afford at the time. Always a trade off somewhere, it seems. Erik