Erik Mandaville Posted November 20, 2002 Share Posted November 20, 2002 For those of you who have Moondogs, have another look at the great picture jazman shared of the wiring in the Moondogs -- a wiring tour de' force at that! Most of us are using the Moondogs either factory wired, or built according to the directions that came with the kit. If you are familiar with the wiring layout, the filter capacitors used to smooth the DC in the 6SN7 heater supply are two, paralleled 10,000 mfd. caps. These are the bright blue capacitors near the large, dual-section electrolytic attached tot he side of the chassis. This is an interesting modification you all might want to consider. I call it a modification, because the schematic shows just a single 10k, 35 volt (or thereabouts) electrolytic. If you are experiencing any extra hum beyond what 'silence' may be obtained in the null-area provided by the hum balance potentiometer, what you may be hearing -- especially with very efficient speakers like Lowthers or Klipsch -- is a need for slightly great filtering in the 6SN7 rectifier circuit. This would be a rather unobtrusive hum heard only when the amps are idling, with no input signal, and within a close proximity to the speakers. I in fact tried a very similar thing with some $.25 2000 mfd. caps, with some result, all of which was not desireable. There was a definite quieting of the already very low-level but audible hum (not buzz, that's another problem), but at the expense of perhaps some life to the music. But the caps were dirt cheap, and may have been very poor quality, old, or both. The guy who wired jazman's amps knew what he was doing, as can be seen in the very nice wire dress and layout of parts. Adding more capacitance could be a wise move in this situation, and is one I'm going to try again this weekend. I'm going to have a look at a broken preamp for a friend, as well as begin a restoration of an old Sears Silvertone FM radio, and then take some time to work on the Moondogs. I am also going to try to by-pass the 6SN7 Bridge rectifier. No, not to use AC on the filament supply, but employ a hum-cancelling circuit that can be made from two power resistors, instead. I've been looking over the schematics of some other amp designs (all SETs like the Moondogs)that use a similar hum-reducing circuit. Anyway, I thought I would just point this out. If you are getting more hum than you want, or find yourself wondering if that low-level hum should or shouldn't be there, you might try adding another 10K capacitor in parallel with the first to see what happens. BUT!!! Be sure the power supply caps have discharged before you mess around with anything. They can pack one hell of a jolt if still energized. The builder of jaman's amps also included a neat witch to bypass the power resistor across the B+ filter capacitor. Welborne runs the 2A3 plates pretty high already, so much so, that I reduced the B+ by using a different rectifer, and a larger resistor. If you do this, it's good to balance out the voltages in the input and driver stages, as well, since all of those come off the main B+ rail. Oh, one other thing here. I was looking at on an old Dynaco PAS preamp schematic a while back, and noticed the use of a low-value (.01mfd, I think) high voltage capacitor (use a ceramic 1k-volt from Radio Shack)across the power switch. This works great, and I use them on all the preamps I've built, as well. If you get any 'pop'or snap when flipping the switch off, the capacitor will 'snub' that resulting arc, and preventing from traveling down-stream to your speakers. I did the same thing on the Moondogs. A volume control has also been a nice addition! Anyway, I just wanted to share some of this with fellow Welborne Moondog owners. Erik Quote Link to comment Share on other sites More sharing options...
Mike Lindsey Posted November 21, 2002 Share Posted November 21, 2002 Nice post! You wouldn't happen to have the pictures of his wiring would you? Mike Quote Link to comment Share on other sites More sharing options...
edster00 Posted November 21, 2002 Share Posted November 21, 2002 Mike, This link is to the first thread about jazman's amps, and shows the wiring, but it is not the most recent photo that Erik is referring to I don't believe. click here Quote Link to comment Share on other sites More sharing options...
Mike Lindsey Posted November 21, 2002 Share Posted November 21, 2002 I see no link... Mike Quote Link to comment Share on other sites More sharing options...
edster00 Posted November 21, 2002 Share Posted November 21, 2002 Sorry, check again Quote Link to comment Share on other sites More sharing options...
Mike Lindsey Posted November 21, 2002 Share Posted November 21, 2002 Thanks, Ed! I love going back and reading some of them old posts. It was only 10 months ago, but it's interesting to see how all of our systems have changed since then. It's a disease, I tell ya. BTW, here is a closeup shot of those Moondogs... Mike Quote Link to comment Share on other sites More sharing options...
leok Posted November 21, 2002 Share Posted November 21, 2002 Thanks for the description and photo. I'm sure I'll be refering to them when I build my set. leok Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.