erik2A3

With the above advice about measuring filament voltages, and since this forum is read by MANY people, it's extremely important to include such information as the orientation of the amplifier. The two pins immediately to the left of the alignment pin change depending on whether one is looking at the amp from above when it is right side up (and from the front or back) or from the bottom when upside down (and how the amp is positioned relative to ones's viewpint). Measuring the voltage is of course appropriate, but someone might not know, and may not think to ask. The subsequent voltage readings could hence be misleading, and, in this specific case, could expose the unaware user to B+ voltages still stored in the power supply. As mentioned above, the manual clearly recommends that storage charges be removed prior to any work being done.

I'm going to paraphrase from the actual Moondog assembly manual: It is assumed that the building (and thus troubleshooting and repair work) of these amplifiers is being undertaken by someone with previous experience in tube amplifier construction, and ideally by someone also capable of reading schematics. A reminder is given about the presence of lethal voltages, and that if the builder has ANY uncertainty or doubt about any aspect of the work invovlved to, as is shown, GET HELP. Moreover, the manual also advises the builder that, should problems arise, to BE SURE that all storage charges be bled off prior to doing any work. The assumptions here including proper use of soldering equipment, knowledge of multi-meter use for taking readings of resistance, AC, and DC current, and proper and safe methods of releasing high voltage storage charges from power supply filter capacitors. Having seen the insides of these amplifiers in published images in the past, they are not layed out and built exactly as shown in the construction manual. There is not necessarily anything wrong with that (mine are different too and involve far less wiring for the same connections). The point is that it will be difficult for a novice to clearly match one to the other in this case. Building crossovers is vastly different from effective and safe troubleshooting and repair of equipment that can kill someone if the wrong thing is done accidentally; and the potential for such accidents increases exponentially with reduced experience and knowledge. The Moondog manual does not give specific instructions on the use of soldering or multi- meter. Nevertheless, it's always good to have a copy of the manual of one's equipment -- if nothing else to pass on to the tech. Who may want it as a reference. In this case, for someone who knew what he or she were doing, repairs would be straight forward and would not require a schematic. This problem is rather simple to resovle. Erik

The hum null pot has nothing to do with the 6SN7s. The rectifier referred to above is not the GZ37 B+ rectifier. There is a solid state diode full wave bridge rectifier in the 6SN7 filament supply, plus a DC ripple filter capacitor of 10,000 - 20,000uf (if another had been added in parallel). There could be something wrong with the AC portion of that (before the rectifier) or something afterwards in the rectifier and filter section -- or connections to the 6SN7s themselves. That the tubes light up in the other monoblock obviously indicate the problem is not the valves. Even overly loose tube pins can cause this problem. They can be r-tensioned, however. Touch the wrong one by accident, and you can have problems. Do not take chances where potentially lethal voltages are present unless you have some experience. If you have not done this work before and know how to check voltages or for remaining storage charges, it would be best to send it someone who does. In fact, you can make problems far worse if you don't know what to do. I have worked on many amplifiers where well-intentioned attempts at repair resulted in greatly compounding both problems and repair costs. Check with Craig (if he is taking on additional work right now) or other professional technician. erik

There is something going on with your driver stage filament supply, Neo. The 6SN7 filaments use DC (consisting of a dropping resistor, rectifier, and filter capacitor). The filaments of both tubes are then connected in parallel. This amp actually only needs a single 6SN7, since only half of each tube is used. I would also recommend not powering the amp up again until it's checked out. If you are not familiar with working on high voltage circuits, or know what to look for, I would suggest NOT working on it yourself, including testing voltages. If you're not used to doing this, it's very easy to accidentally touch a high voltage connection, such as a plate or B+ filter capacitor, and get severely shocked. Honestly, don't do this work unless you know where (and how) to begin to troubleshoot the problem. Even messing around inside the amp while it's unplugged can be dangerous, since there may be a remaining storage charge in the power supply. erik

Gregg: That's the shielded cable that goes from the RCA jacks to the volume controls. I agree, it looks nice. Will always gave pretty thorough attention to things like that.

Jay: Great! I mentioned some of the changes that were made. Erik

I think if it's been reliable and you're happy with how it looks and sounds, that is what matters. "What I think" about it is a pretty open-ended sort of question and could mean many different things. If there is something specific that you're interested in, I'll do my best to offer an opinion. I didn't really know my way around them until I figured the circuit out by making a map based on the physical wiring inside the amplifier (the one above). I always knew the general topology, but the specifics didn't become clear until I went to the trouble to figure it out -- which was confirmed when I located a schematic. It's really a pretty simple design, which to me is likely related to its very open and clear presentation. These restorations don't really change the essential elements of the circuit, other than for replacing a transformer, likely two, so they are matched for equal frequency response. The parts connected to the input are still there, just in a different form. The parts connected to outputs are still there, also in a different form in terms of what jacks are discarded and others added. The cathode bias resistor bypass capacitors are still there, just renewed electrolytic types (which may be one of the things I decided to change on the one I'm giving away. The interstage coupling capacitors are still there, but have changed in terms of both type and value. Using comparatively low-value capacitors on these vintage amps was not uncommon when they were being designed, but the original in this amp had a low end corner frequency that is a bit higher than more modern designs. Based in part on the input impedance of the output stage, the value of the coupling capacitor has been increased a bit. Within reason, coupling capacitor values can be altered a bit, depending on how someone chooses to voice the amplifier. The power supply filter capacitors have also been replaced with new, high voltage electrolytics. This specific amplifier most likely uses a ground plane scheme, which may work better in some systems with what's connected to it than others. If it's working well for you, as I said above, that's what matters. Hopefully I've touched on some of the things you may be interested in. Erik

Hi, Jay Seems fine to me, but what matters more is what you think about it. Erik

Bruce True enough, it 's not inexpensive. I had been wondering if Welborne sources the power transformer from Triode Electonics. I have to say that a bit more power for our La scalas in this big area made a difference. The Baldwin does not have they same low end control of the OTL amplifier, but it is not too bad either, just kind of different. I need to get a good sub for the LS, though. I think/ hope The Baldwin above is going to be a very nice amp for Dee's Heresy/sub system. It's a great combination, and has been a source of inspiration for all the work I did on it. I could have had it running in about 30 minutes, but there is so much more that went into its revival beyond making it 'just work.'. For a SET lover like me, the 6l6 in push pull is like a 2a3 or 45 on steroids; all the air and presence at 10 X the power. And of course just an expression of my own response to it. I was actually gping to turn the amp into a PP 2a3 I had been working on, but am glad I didn't. Replacing the riveted octal sockets wasn't exactly fun, but grinding the rivets down with a rotary tool and then punching them out sure made it easier. My dad was able to translate some of the color coding on the mica caps that are used in a rather uncommon to me ( to him it wasn't) plate-shunt position, which are probsbly there to filter RF. I wish my paragraph breaks would actually show up when I post! Erik

Bruce The welborne labs upgrade board for the St70 is apparently very good. I Haven't heard it myself, but I believe Allen Kimmel, who also influenced aspects of the Moondogs, had a hand in the dynaco upgrade (for which a larger power transformer is also available -- you know how hot the stock trannie runs....). The dual rectifiers for the Baldwin are the 5U4 type.

Dee Thanks, that saves me some time. It's very close to being done now. Erik

....I'll share a picture one of these days. I'm on the Mac and all my images are on the other computer. Erik

Hey, Gregg: Thanks for taking this in the right light, and for the encouragement. Still working on this thing, but waiting for a couple of more parts, I have found connections that were crimped but not soldered that date from when the amp was first put together. So proceed with some discrimination when rebuilding and rewiring something like this. Discovering how hot this amp actually get inside -- there is a power supply bleeder circuit with large power resistors that get pretty warm, plus the output stage cathode resistors, make me think it might be, to one's liking of course, to not use a bottom plate. I have the large rubber feet mounted on the flange, by way of a larger fender washer between the flange an the top of the foot. And then a cap nut to finish it above. I don't think there is anyone that is able to finish beatup chassis and transformer bells like Will (his powder coater needs to be credited too, without a doubt!), but this amp really came to life with the work done inside. Good grief it's a nice sounding amp, too. There is something kind of SET-like about it, but with an enormous scale and sort of jump factor that makes listening really fun. The improved grounding scheme makes music jump out of silence, with incredible dynamics. The exterior view is the same, except for my addition of the amber pilot light that was installed where the power switch used to be, and the switch moved to the side, with the fuse also relocated.

Despite the majority of information offered to the contrary -- such as comb filtering issues, etc., -- I stacked a pair of Heresies, the top cabinet upside down as mentioned above, and loved the way it sounded. If I remember, this was awhile back, I also experimented with disconnecting one of the tweeters, for a sort of variation on the MTM D'applitio (apologies for the likely incorrect spelling of his name.......I was just reading an article by him yesterday) array. Sometimes all the testing and intellectualizing of what is ultimately an emotional experience and reaction does not equate with tested response. I've heard components with pristine specifications and ruler flat response that either were painful to listen to or simply just not very interesting or involving. Sometimes little peaks here and there, which some designer actually strive for, can bring some excitement and realism to the presentation. Have fun with it and experiment Erik

Let me quickly add while on this subject that those of you with Baldwin amps that don't have some of the changes I made on the one I have here does NOT mean that yours is not up to snuff. If it's working and sounding good to you that's all that matters. I am extremely picky, pickier than most, about any extra noise from electronics when paired with high efficiency speakers, and will continue to search and experiment to get at the root of an issue. I did the same thing with my Transcendent amps, and mentioned some things that helped there, too. I spent two weeks on these modifications, and could never afford to pay someone for that amount of labor and reworking -- on top of that, I love the challenge! My wife often says I wouldn't be happy if everything with my electronics was working exactly as it should. I recently also bought an as-new PrimaLuna preamp, but was unhappy with the value of potentiometer used, as well as part of how it was grounded (which caused noise, amd which I was able to completely remedy). That's just me. I'll get something and almost immediately try to re-enginner aspects I think might be improved -- which doesn't always happen. Erik

Forgot to mention: The two vertical cathode resistors for the output stage have been replaced with modern aluminum types mounted directly to the chassis, but by way of 1/2 aluminum stand-offs. The original two, one of which seemed to have led a stressful, possibly slightly overheated life, cracked when I very slightly moved the solder terminal lug when installing a new length of wire to it. It was also 10 ohms below its stated value, which isn't drastic. It was the damage that was unacceptable. An important aspect of grounding, which some here may know (though I've known a number of techs who haven't) is that it CAN help to ground the center tap of the power transformer directly to the common (ground) connection of the first filter capacitor -- and then from there, not to daisy chain filter caps, but connect them according to their stage in power distribution of the rest of the circuit, individually to an uninsulated ground buss. I made the ground buss using #14 wire, toward one side of the chassis near the input jacks. All power, input, and output stages were connected individually for each channel to this buss, and from there, on the Baldwin, a single wire is run to the chassis on the side of the amp with heavy wire, and connected at the point with a ring terminal and star washers. The new owner of this amp had purchased and sent to me another quad of Winged C 6L6s, which sound spectacular. Erik

Speaking of Baldwins......... I've spent the past two weeks completely rewiring one I'm sending to an old friend. It's essential exterior architecture remains the same, but the plumbing needed a thorough going over, including replacing a few of its antique carbon comp resistors that had drifted, installation of brand new octal sockets for its dual rectifiers and compliment of four 6L6 outputs, new cathode bypass and coupling capacitors -- and lastly and without a doubt most importantly, a single star ground configuration. Power supply, input/driver/phase inverter stages, and output stages have all been separated, with the stereo channel signal grounds separated via a 10 ohm resistor in a very unique position (not the common Dynaco RCA ground tab to ground connection, but derived from that). The Baldwin was designed to use the chassis as a ground plane, with common ground connections at the nearest convenient place on the chassis. While this works, I was impressed with how much of an improvement was gained in terms of the drastic drop in background/ground loop noise through 104dB speakers afforded by star grounding (which always seems to help). It's been a long while since I did anything at all with this amp, but it is playing music in a way that's pretty amazing. There are several amps I could live with on a daily basis, and this is without a doubt one of them. The La Scalas are in a very large listening space -- the entire first floor of our house, and as much as I like the SEOTL I've mentioned many times in the past, it simply was completely unable to fill that space well without running out of steam. That amp is only a bit over a single watt per channel, and does just fine in our upstairs combined HT/listening room. For a larger space, the horsepower of an amp like the Baldwin makes a really big difference. I'd forgotten what a great amp this is, and even better with these latest updates and modifications. I had also experimented with bypassing the cathode resistor of the first stage, which among other things offers a bit of gain, but found the balance in frequency response tilted rather than even from top to bottom. Bypassing is one of those things easy to try, and easy to do away with if needed. While mentioning this, I should add that I also removed the old (and on this amp severely deteriorated 50k input potentiometers, and replaced them with a fixed value 50K (actually two 100K ohms in parallel) metal film resistor from grid to ground. it's also kind of an eye opener in terms of how new metal film resistors can sort of 'wake up' an amp, though I would say too much of that can be, in a subjective sense, maybe not such a good thing. Input stage cathode resistors also needed to be replaced. All filament wiring was also replaced with new fresh, undamaged (just from age) wire. Wonderful amp, and I'm on the look out for another for a similarly done complete restoration. All the above work was done minus a schematic, but I finally was able to get my hands on one which enabled me to check operating voltages, etc. Happy, happy! erik

I think we all share that same affliction. Good luck with your cap experiments!

Did I ever say electronics wasn't a science? As I said, I work on tube amps every day, and am reasonbly well- versed as far as how tubes work, and what role each component within the glass envelope performs, how they interact, and so forth. As far as Craig's opinion on the possible ill effects of the process under scrutiny, I do not see any scientific fact revealed, only his belief, with which as I also said I am inclined to concur, and nothing more.

I suspect, too, if you reread that last couple of sentences that Craig wrote, you'll likely discover is that what he has done is propose a hypothesis, and concludes with the reflection that he has no difinitive proof to back up his claim. Thus, he may be accurate about this and he may not be. For the record, I tend toward his take on the matter. Then again, I'm familar with super-cooling of electronics (though am not aware if that is still in practice, particularly with regard to computers) in liquid nitrogen, as a means of preventing overheating. So, again, to each his or her own. I have had people -- audiophiles- laugh out loud about how crazy they thought it was that I was still using devices as archaic as vacuum tubes in my preamp and amplifiers, let alone feezing the dang things like so many popsicles before I pop them in the components to listen to music.

I know Every aspect of valve internal structure to which Craig referred, and have been building and repairing vacuum tube equipment for as long. He knows that. I also nowhere in what I had written indicated that I disagree with him. No need to go back and reread what he wrote; I both know what he wrote and moreover completely comprehend what he is talking about. My stance remains the same: if people believe in what they think is right for them, in light of whatever risk that may or may not involve, that is their business to do as they please. I would rather not purchase such a device, but respect every right of others -- of whom there are many -- who do believe in the practice, either for valves or for the wire they use to string their components together.

Ah, the old bypass trick. Wow, I haven't seen those images in a long while. Do keep in mind that what we are looking at is a visually-based graph, not a human and highly subjective interpretation of the sound. Measured response is potentially useful and intersting, but when it comes down to simply listening and making a choice of preference of one thing over another (whether a cap type, the best choice at a buffet, or the latest hit at the box office), what is 'best' on an individual level may not be what everyone else or the critics like. I've bypassed many capacitors and yanked the cap out as often as I've left it in depending on the circuit in which it was used. One is free to experiment, and this is among the easier of experiments to perform.

If some people invest in tubes that have been buried in the Antarctic for three years or the Sahara for ten, and they are happy about doing it and belive it makes a difference, so be it! It's their business. It's really no different from the single ended triode vs push-pull debate. Everyone knows that single ended amps are better, particulalrly if the coupling caps are yellow. I find that caps in blue, red, or black shrink wrap sound muddy in the midrange, where red ones always sound arguably better in the low end. Wink, smile, lol, etc

Pete I loved the sound of motor run caps in those networks. People use those things as input to filter capacitors and cathode resistor bypass caps, too. Doubtless the new caps you are looking at are well made (they certainly should be), but hold on to Bob's GE caps too! They are arguably the closest in sound to the original network -- if that's what one prefers. People have made some pretty drastic/traumatic changes to the original design of the Heritage speakers. Nothing wrong with that, just an observation. The type A is a very simple, yet very effective design. I've NOT been surprised at how many extremely high-end speaker designers prefer the low order networks (as in 6dB/octave) for their passive designs. Then there are those on the other side of the fence........... there is always the other side of the story.

wow -- I haven't seen those amps in years, and there they are, perched side-by-side on top of the speaker. Some changes I made in that amp sounded (to me) pretty fabulous, and surely quicker on it's feet than another 2A3 amp I have. Those things are the closest I've heard to not having an output transformer at all, and they just seemed to sound much bigger, somehow, than their few watts might suggest. And there are some VRDs in that hollow state metropolis too. Very cool picture to say the least. You take nice care of your equipment! Erik