Erik Mandaville Posted January 25, 2003 Share Posted January 25, 2003 Caps off to Leok! I completed one side of the cathode bypass early this morning. This gets to be kind of a long story, but in brief, the change required removal of the volume control I installed previously on the the Moondogs. Gain went down slightly after the cap was taken out, and things were just much better with the volume pot out. The signal now sees a fixed input of 100k (referenced to ground), which is in front of a 1K grid stopper to the 6SN7 grid. With the volume control (100k log taper), the output impedance to the input stage was probably around 60k, or so, which assisted in making the amp. less sensitive. For sake of comparison, the volume control on the other amp. is still in place. The difference with the cathode bypassed vs unbypassed is considerable -- so far for the better for me. There is a 'jump factor' and clarity that is not nearly so prominant in the amplifier still using the bypass capacitor. If I can use certain art terminology to describe the difference: presentation without bypassing has excellent contrasts in value -- meaning dynamic changes are more pronounced. High frequency characteristics are, gosh, just really wonderful -- like highlights of sunlit crystal in a Vermeer painting. Classical guitar was especially note-worthy. I listened to performancs throughout the day by Julian Bream, Pepe Romero, and an absolute knockout (all of you should get this CD if you don't have it!): Kurt Rodermar's guitar transcription of Bach's Goldberg Variations (Sony Classical). This last CD is incredibly good played through the La Scalas. The piano version of this great work is good but the guitar version is to me more engaging. Since piano is obviously capable of lower octaves than standard 6 string guitar, Rodermar had guitars custom built to be able to hit the lower notes. Please get this if you don't have it! Driving my Lowther horns, the modified cathode circuit maybe sounds just a bit thinner, but hardly so. Through the La Scalas, I don't notice any lack of bass response the the capacitor taken out. In fact, gain has gone up considerably due to the removal of the volume control; so much so, that I'm going to have to take the 250K pot out and replace it with either 100 or 50 k ohms. Alright, I'll close my mouth on this now. If you've got Moondogs, you might want to give this a try. I appreciate the work Leo did on this, and his sharing it with all of us! How does the idea of an Ultrapath Moondog sound?! Erik Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 25, 2003 Author Share Posted January 25, 2003 The capacitor I'm talking about is C5 on the Moondog schematic. It is called a 'bypass' capacitor because it is installed across the cathode resistor, which is referenced to ground. For anyone else who may read and be at least slightly interested, Jean-Francois L., who designs and build the recently profiled Pantheon preamp, encouraged this modification heartily. Thanks to him, as well! This is such a simple modification, which won't hurt the amp, but may reduce bass response for some, depending on listening tastes and preferences. By the way, I type really fast, and sometimes repeat words...so I apologize for combining the the wheat and chaf together! Erik Quote Link to comment Share on other sites More sharing options...
leok Posted January 25, 2003 Share Posted January 25, 2003 Erik, I like your first line. Glad the simpler circuit works for you too. I must be pretty dense .. I still can't figure out what you mean by ultrapath wrt the Moondogs. Is it transformer instead of cap coupling? Interesting about the guitar transcritpions of the Goldberg Variations. I'm just beginning to warm up to that collection by Bach (the version I have is the newer Glen Gould whom I've always found difficult). Maybe Kurt Rodermar's interpretation will help me out. I'll give it a try. Meanwhile the Moondogs keep getting nicer. picked up a pair of 1947 RCAs from Tube World. They're in now .. very nice. After I installed them my wife and I spent Fri. evening lounging on the couch listening to the local Classical FM station. Music is so alive with that amp. leok Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 26, 2003 Author Share Posted January 26, 2003 Thanks for you feedback, Leo. Well, I've been studying the schematic for Jack Eliano's DR amp., which I've had since it was published in Vacuum Tube Valley sometime ago. I even started to slowly purchase parts to build it, but ran out of funds for four expensive transformers -- which is basically the primary cost for the project, of course. The secondaries for the DRD OPTs are, according to Jack E's design, also wound with silver wire, and I didn't even bother to ask the price of those. I was going to try it with copper, which would have been cheaper, but probably not as 'detailed'. So I started thinking of ways to possibly apply some of the circuit design into the Moondog, which as you know is a fairly conventional SET circuit. And a good one it is, too. I think Ron W. mentioned somewhere that he used two 6SN7s in the input and driver stages because that's what the chassis plate (used for other amps, as well)was already set up for. This is getting off the point, though. In general, the Ultrapath approach (I'm sure you already know this!)returns the signal path from the OPT to cathode v.s. the more usually-seen power supply to cathode. On the Moondog, it looks as though the cathode bias is incorporated into the hum/null pot circuit. Since a large poly cap is used in that position rather than an electrolytic, it might be simple as removing the grounded end of that capacitor and connecting it the juncture of T2 and R8. I was looking at this late last night when I was tired, and I'm going to study it some more in a bit -- the whole thing was sort of a fleeting idea that I thought might be possible. On top of this, the DRD amp uses rectified AC on the 300B filmts., and that would be a very quick thing to incorporate into the the Moondog, as well. This would involve the addition of a small filament transformer, which could be installed inside the amp on the side wall -- with primaries wired in parallel with those of the main power transformer. A simple full-wave bridge and subsequent filter capacitor (say a 4,700mfd. or two paralleled)and a dropping resistor are all that would be needed to complete the job -- and almost all the parts can be bought at Radio Shack. As you can see, the idea is not tightend up yet, but just a thought right now. I tell you, there are times I wish I had a scope where I could see, as well as hear what was going on! My pop in AZ. has three of those things, and maybe I can get him to sell me one! Anyway, thanks again for your focused work on that C5, Leo. The change for me has been a very positive one. Time to get the other side finished, so I can listen to music the rest of the day! I have the Glenn Gould Goldberg CD, too. The guitar version By Kurt R. is very, very different! Regards, Erik Quote Link to comment Share on other sites More sharing options...
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