NOSValves Posted July 2, 2011 Share Posted July 2, 2011 Oops, you beat me to it. Congratulations on the amp rebuild. It looks very nice! Incidentally, it's not hard to change the circuit so that you can use the much easier to find 6U8 or 6GH8A tubes instead of the now rare and expensive 7199. It should take moving a few wires on the 9-pin socket. So if you ever find yourself without 7199 tubes, don't panic. There are better options. I'd rephrase that a bit... There are cheaper options that will work. But not neccessarily better []. The difference is small but the 7199 was designed ground up to do exactly what it does in that amplifier it was really never used for any other purpose which is the reason they are so hard to find and expensive. The 6U8, 6GH8 were originally used and primarily designed for use in TV's and Radio's. They work but have have to weed through the noisy ones to find a good set. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 "You want *negative* feedback going from the OPT secondary (16 ohm tap) to the input of the amp." Wait a minute. Right now I have the OPT output setup for 8 ohm (what my speakers are) and that is also whats being fed back to the input of the amp. Do I need to change this? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted July 3, 2011 Share Posted July 3, 2011 "You want *negative* feedback going from the OPT secondary (16 ohm tap) to the input of the amp." Wait a minute. Right now I have the OPT output setup for 8 ohm (what my speakers are) and that is also whats being fed back to the input of the amp. Do I need to change this? Not an absolute yes or no answer to that one. The feedback can come from the 4, 8 or 16 ohm taps. That is where a function generator and scope will come in handy for fine tuning. We don't even know what the impendance tap the original transformer was setup at. It could of been 60 ohms for all we know. I'd say at this point take a break and enjoy the amp see what your impressions are about the sound and listen for what you might think doesn't sound right. We can tune it later when we have more time. What are you using in front of this amp to control the volume and source selecting? Do you get a reasonable amount of usage out of the available travel of the volume control? Does it get loud with just a slight turn of the volume control or do you have to turn the volume control way up to get room filling sound? Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 3, 2011 Author Share Posted July 3, 2011 On the workbench I'm using an '80s model SS NAD preamp to drive it. The volume control travel is spot on. Not too hot too quick yet gets plenty loud at 3/4 travel. I listened to the amp for an hour or so last night and I can say there is nothing ovbiously wrong there. Finally I gave it a dose of Boston's "More than a feeling" last night at 3/4 volume and it was much louder than typical level for me but no noticable amp distortion through a pair of big high efficiency 2 way speakers (Dukane over Altec bass horns. I'm real happy with it. Craig, thanks so much for you help on this project. The last two weeks have been a refreshing challenge. I've enjoyed peeking inside and tinkering on this old valve circuit. Not to mention ending up with a good sounding amp! I'm reminded once again how much fun it is to learn something! Quote Link to comment Share on other sites More sharing options...
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