JJkizak Posted June 30, 2011 Share Posted June 30, 2011 It's really funny to me how people are rebuilding older tube amps that were average in the old days and expect them to be state of the art when they get done. No way. JJK Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted June 30, 2011 Author Share Posted June 30, 2011 It's really funny to me how people are rebuilding older tube amps that were average in the old days and expect them to be state of the art when they get done. No way. JJK No, you've missing the point. If this amp is state of the art when I'm done it's a total failure. To be more specific: If this amp sounds like a Denon AVR through my Belles it will be a total failure. Been there, done that. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 30, 2011 Share Posted June 30, 2011 It's really funny to me how people are rebuilding older tube amps that were average in the old days and expect them to be state of the art when they get done. No way. JJK Problem with State of the Art is it sounds like dung and rarely lasts..... Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 30, 2011 Share Posted June 30, 2011 Just to update where we are. The board is done with all new caps and resistors. Im thinking with the larger transformers blocking the old cap hole it might be best to go with both of the big caps under chassis as shown below. Are there noise concerns with the location of the caps? Supposed to have the transformers here from AES in the next day or two and will hopefully get it all together. That will work just fine. Pick you up some super glue. Lightly sand the area where on the cans and the chassis make contact then glue them in place. It even works better if you can snug them right together then after they stick to the chassis run a bead of glue between the two cans to hold them together. Use medium grit sand paper to give the glue a rough surface to grab ahold! This amp is going to be state of the art LOL!!!!!! Quote Link to comment Share on other sites More sharing options...
JJkizak Posted June 30, 2011 Share Posted June 30, 2011 Ok, I'll cut you some slack. I wish I kept my Fisher 202B so I could sell it for big bucks. But I gave it to my buddy and he is still using it. JJK Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 1, 2011 Author Share Posted July 1, 2011 Hey Craig, got the Hammonds and I notice on the primary side there are two wires for the screen grids. This is being handled by the power supply transformer presently. What to do with these two "SG" wires on the new Hammonds? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted July 1, 2011 Share Posted July 1, 2011 Those are for Ultra Linear connection of the output tubes. For now tape them off and we may use them later. We could even make it switchable on the fly from straight Pentode to Ultra Linear or Ultra Linear/triode.... or Triode/Pentode. But get it working in stock pentode configuration first. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 1, 2011 Author Share Posted July 1, 2011 OK, great. I hope to work on this tonight after the little guy is in bed. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 1, 2011 Author Share Posted July 1, 2011 I've made some progress. I have the output transformers in and the primaries wired. I have the big power caps in and mostly wired. The best news is that I know what's left to do! Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 I believe I have it wired barring some obvious mistake. Check it out. Guess I'll plug it up and see what happens! Edit: I tried it for a few seconds. While both tube sets looked good on warmup, amp played clear for a second or two but then I got a feedback like tone and shut it down. I'm guessing this is where those 8 ohm load resistors come in to play.[:$] Quote Link to comment Share on other sites More sharing options...
NOSValves Posted July 2, 2011 Share Posted July 2, 2011 Most likely a grounding issue or a problem with the negative feedback circuit. How did you confirm you were using the proper sides of the transformer windings to connect to each tube in the PP pair? It matters which plate lead goes to each tube in the pair. I really wish you would of done one section at a time. Doing the whole thing at once for the rusty techinician leaves us with a "almost anything could be wrong" equation to try too diagnose. Hopefully its something simple. You're going have to either tackle this on your own for the rest of the weekend or wait until next week for guidance. I have a ton of company for the holiday weekend here. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 Thanks for the help Craig. I simply wired the new transformers as the old. On primary sides looking at the amp schematic and the tranny schematic the primary wire colors are the same and located the same on the schematic. It was a match. red wire center tap, blue wires soldered to where old blue wires were, etc. We can work on it next week, that will be fine. I appreciate all your help with it. In the meantime I could open the feedback circuit and see if that corrects the problem? I'll do some investigation and see what I can find. It certainly could be a ground problem because I went from a case ground on the old sectoral cap to a new ground location. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 OK, I snipped the feedback circuit and that stopped the loud runaway feedback. Now I have what I believe to be a much lower 60hz tone in the background. So certainly some troubleshooting to do. I'll hunt around on it some this weekend. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 Progress has been make. I found where I had crossed two pink wires on the 50uf x2 cap. swapped those and the 60hz like tone gone. Now I have an amp that seems to be working properly, but without the feedback circuit. I'm tempted to swap the blue/brown wires on one of the transformer primarys to see if I indeed have that backwards and reconnect the feedback circuit on that one. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted July 2, 2011 Share Posted July 2, 2011 Well the plate leads from the transformers to the output tubes look strange to me. They look to me to be installed opposite channel to channel. But who knows which channel is correct.... this would create a huge oscillation with the feedback attached and disappear when disconnected. It's possible that they are correct and the front end of the amplifier is wired from the factory reverse polarity channel to channel but its not very likely................. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted July 2, 2011 Share Posted July 2, 2011 The easy way to confirm is to trace the output tube back to the 7199 phase inverter. The output tube with the brown transformer plate lead should trace back through the new coupling caps on the tag board and end up at Pin 1 of the 7199, The tube with the blue plate lead from the output transformer should trace back and connect to Pin 8 of the 7199 via the coupling cap. This should be identical in both channels. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 Eureka! I swapped the blue & brown tranny wires on each side and reconnected the feedback circuit and all is well. The amp is quiet with no hiss or hum and when the audio signal hits it the music comes alive.I can already tell this is a giant improvement over the stock stuff! Quote Link to comment Share on other sites More sharing options...
rongon Posted July 2, 2011 Share Posted July 2, 2011 Progress has been make. I found where I had crossed two pink wires on the 50uf x2 cap. swapped those and the 60hz like tone gone. Now I have an amp that seems to be working properly, but without the feedback circuit. I'm tempted to swap the blue/brown wires on one of the transformer primarys to see if I indeed have that backwards and reconnect the feedback circuit on that one. Hi there, I hope I can be of some help. That howl you heard when you powered up the amp was probably reversed polarity to the output transformer ("OPT") primary (blue and brown leads). You want *negative* feedback going from the OPT secondary (16 ohm tap) to the input of the amp. Negative feedback ("NFB") will lower the gain of the amp, because it's derived from the output fed back in opposite phase to the input. If you switch polarity, you have positive phase signal injected into the input, adding the output signal to the input and increasing the gain of the amp. This will result in oscillation ("howl") as the output is fed to the input and the output feeds the input some more, and the output feeds the input some more... So yes, try swapping the blue and brown leads between the two output tube anodes (pin 3 on the 6V6's). The color coding on the old OPT's might not correspond to the same polarity as the color coding on your new Hammonds. We want to get a negative polarity version of the output going to the point where it's injected into the input. Negative signal will detract from the gain at the input, but decreases distortion and frequency response at the output, which is usually considered a good thing. I think you're almost there. I'm dyin' to hear what you think when you wire your amp up in triode. But I agree that can wait. Careful with those big power supply capacitors! They pack a wallop. (Please don't ask me how I know...) Ron G Quote Link to comment Share on other sites More sharing options...
rongon 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. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted July 2, 2011 Author Share Posted July 2, 2011 Hey Ron, thanks for the help with the amp and the info on the 7199 tubes. I didn't know that. As far as the amp goes, it has been a real joy working on it. Couldn't have done it without Craigs help. Until we started this project I did not realize how important the transformers are. I have recapped a few old amps before but I have never replaced the original "boney" trannys before. Craig has opened my eyes for sure! 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.