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In Reply to: RE: Serious Micro-Phonics and Hum from Citation V posted by mr9iron on July 07, 2012 at 01:06:10
Anyway, giving your amp some thought my guess is that the gain is really high on your build. Maybe you got a value of a resistor wrong somewhere. Mine seems to normal gain. Does yours get loud fast compared to other amps you have at the preamps same level?
Did you hook the negative feedback to the 16ohm output? If you did it to the 8 without adjusting the circuit it would increase gain etc.
Follow Ups:
Danny,
yes my V does exhibit this characteristic, it has what I would describe as a "burst" at just past 9:00. at this point the amp really tightens up and punches the bass and lower mids like no other tube amp I have tried.
I think this is a normal characteristic unique to this amp and it's one I like. I had another V that did the same exact thing.. Maybe Jim can confirm if this is normal. I do think the amp sounds it's best at this point just past 9:00
..regarding the problems you and I are experiencing, I want to emphasize I think the amp s behaving normally out side the hum and micro-phonics, granted I do not have a scope but just pure listening at that volume seems to sound "too good" if something major was out of whack.
did you do the ground bus? My other V did not hum very much, just ever so slightly, hardly noticeable. It was an older McShane kit without the added .68 in the feedback and no ground bus on the PS caps.. do you think something could be up with the ground system?
I will def let you know if I find anything. It's too good sounding of an amp to have it let sit with these problems..
Jeff
Hi Jeff, Yeah I did the ground bus but maybe I did it wrong or the wire I used was too thin. It's the core out of a rg6 cable. Also my hum disappears if I short the inputs. It's aggravating not to be able to figure it out. I'm sure you're experiencing the same emotions. I've build and redid other stereo and guitar amps without anything else being this stubborn.
This is a clear indication that the hum does not originate inside your amp, so there is no need to redo the internal wiring, etc.
Maybe you do have excess gain due to too little negative feedback, as others posted. If your feedback circuits are all correct (I believe this amp has several local as well as global FB loops) and you still have more gain than you need, an effective kluge to reduce the hum would be to put a resistive voltage divider just inside the input jack of each channel of the amp.
This won't help the microphonics, but you may get some help there from using tube dampers (I like Herbie's Hal-O products.)
Other than that, you could check if your preamp has excess hum, or try some better-shielded interconnects.
Danny,
yes also the same here, if I short the inputs the hum will disappear.
Jim is right, however, there is no tube amp I have ever listened to or restored, that quite sounds like this little guy, it packs a punch, so if you can send it out, I would. I have talked to Don Sachs before and he is super nice to deal with.. I'd send it if I could.
not sure the gauge would be a problem on the wire, maybe try no ground bus, or grounding direct to the chassis..it wouldn't be too hard to try..
Looks like my ordeal is much more involved...
J
yeah, also I could get a bunch of new wire and just start the wiring process over cutting out everything already installed. I wish there was more room in the amp to make it easier/nicer though.You're right, the amp does sound rather special. If it didn't vibrate my woofers to the touch and hum it would most likely be in service as we speak.
I even tried to wire in a hum canceling pot to the heaters hoping it was it. No dice.
What would be great is some nice pictures of a amp done that's working good. examples to mimic.
Edits: 07/07/12
Danny can I call you Dan-"O" I just remissness that name to the old Hawaii 5-0 of my child hood.. :)
...yeah if I find out anything at all on this I will let you know. I also feel the pain of the cramped quarters in side the chassis of this amp... if you read my old post on this beast you'd understand why, search "a citation V chronicle" and it looks like it's still not over :)
Jeff
Jeff, sure. Dan-O is more then fine. Great show btw. They sure don't make em like it anymore.Looking at your Chronicle posts I see that I even replayed to it and totally forgot. Anyway, your grounding is much much better than mine.
BTW, that use to look a lot better until I moved this, that. re-soldered this that etc etc etc trying to figure out what went wrong.
Edits: 07/07/12 07/07/12 07/07/12 07/07/12
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