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Serious Micro-Phonics and Hum from Citation V

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Posted on July 7, 2012 at 01:06:10
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
Inmates,

have been testing a Citation V on and off over the last month or so. I re-built it with McShane kits. All new tubes, new input jacks. All new resistors replaced with the resistor kit, choke, In- rush limiter, New McShane power supply kit all new caps......

This amp seems to be running fine and has seriously strong output and bass response. I can detect no "power leak" or weakness sonically of any kind. .... but this amp is very seriously micro-phonic.

It's so micro-phonic that you can hear the grids in the tubes pinging with the heating and cooling. If you gently tap the chassis or even walk across the floor it transmits and amplifies it like a microphone.

Every tube is micro phonic. If you tap the input tubes, or driver tubes, or output tubes, they will all thud and ping very loud. Tap any of the transformers, it's like a mic being broadcast.

I changed out every tube with another new quad and every 12by7a and 6cg7, still the exact same result. I somehow doubt every tube is micro phonic, and that every replacement tube I switched out is also micro phonic.

The Amp also exhibits a fairly decent hum, however, I think the hum is just the minor transformer hum that is being picked up and transmitted like every thing else. So I want to say it's the same problem as the micro phonics.

Someone suggested this can happen as the result of lose or bad tube sockets. I checked all of them (and de-oxed them) they are tight and secure. Again I doubt all 8 sockets are bad.. I want to say this has to be something bone-headed and simple. Something I did on the restore....

Could this be a bad ground? The re-build calls for a ground bus to be made and all the PS caps to be run to that ground on the mounting bracket, which I did do..

I am terrible at trouble shooting and I am not sure where to start.. Any suggestions welcome as usual..

thanks in advance!

Jeff

 

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RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 10:17:41
Triode_Kingdom
Audiophile

Posts: 3026
Location: Texas
Joined: September 24, 2006
In my experience with generic tube amplifiers (not specific to the Cit V), those symptoms point to two possibilities. First, you may have lost the NFB connection. That will increase the gain tremendously and also increase hum. Second, the amplifier may be oscillating at a frequency above the audio range. That almost always creates microphonics and hum, and can also burn out tweeter voice coils.

--------------------------
Buy Chinese. Bury freedom.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 11:08:19
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
"Buy Chinese. Bury freedom" - agreed, keep buying Chinese, support Authoritarianism..

The NFB possibility,if the NFB was not connected wouldn't I hear that tremendous big boomy "wide" -sound as well? I have heard this amp with it disconnected, and it has a very noticeable "big" wide bass-y sound, I do not currently detect that. If the NFB was the problem, would I hear this? I will go through that loop to be sure..

Now your second idea is intriguing.."oscillating above the audio range" You said it can ruin a tweeter, would there be anything I could hear or detect, that would indicate it's killing my tweeter? I do not own a scope..

What would cause this oscillation and is there any way I can test for it?

I should mention I am not a "super tech" so a really involved explanation might go over my head :)

thanks for your reply,

Jeff

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 12:18:20
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
I also rebuilt a Citation V with the same kits and I also get a good amount of hum I could never track down. The amp is collecting dust due to it:(. I dig it out every six months to a year and try to figure out the issue but I never have any luck.

If you get any progress please post or drop me a line and what you did.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 12:41:38
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
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.


 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 13:32:42
Triode_Kingdom
Audiophile

Posts: 3026
Location: Texas
Joined: September 24, 2006
The tweeter comment was only intended as a general warning about using the amp in its present condition. Based on what you've said, I wouldn't power it on again without dummy loads and a 'scope. Anyway, if the NFB connections appear good, I think you'll need a more experienced hand to go in and troubleshoot. Not to state the obvious, but have you discussed this with Jim McShane? If he's not able to help over the phone, it would seem you only have a few alternatives. One would be to pack it up and send it to Jim for a look. The other would be to locate a local shop or vacuum tube afficionado who will look at it, preferably someone willing to consult with Jim while investigating the problem. Maybe if you put out a general distress call here on the tube forums, someone can recommend a resource in your area.

--------------------------
Buy Chinese. Bury freedom.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 14:33:19
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
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

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 14:35:58
Jim McShane
Dealer

Posts: 5169
Location: Chicago
Joined: January 13, 2003
For a moment let's assume you haven't made a wiring error, and it is likely that if there had been a hyper-microphonic tube in there that it's not there now.

And I can be 100% sure the kits are not at fault, there are hundreds of them out there working great.

Of late there have been some cases reported of problems related to output trafo issues. There have been a few cases of oscillation as well as various other issues. But the likelihood of it happening in BOTH channels is fairly low. Does the amp make the noise from both channels or just one? I don't mean which one you are tapping, I mean does the noise come out of one or both speakers? And is this the amp that had the broken internal connection that you and Mike Samra sorted out?

Next - we'll now assume a wiring error could be at fault. It is not difficult to hook up the output trafo wrong - especially since after 50 years the wire colors have changed quite a bit and they are difficult to sort out.

Check the heater voltages on the tubes too - especially the 12BY7As.

Let us know what you find! We'll continue from that point when you reply.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 14:39:02
Jim McShane
Dealer

Posts: 5169
Location: Chicago
Joined: January 13, 2003
Danny,

I really think that if you sent the amp to my guy Don Sachs it would be fixed. Hum is a BEAR to track down sometimes, and that's even for guys who do it all the time.

It's a shame to have that great amp sitting idle!! So if you want to contact Don you can look him up on his site. The URL is below.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 14:42:41
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
Yes it may be in fact what I have to do. I do have a dialog with Jim, he's always willing to help. However I usually try to see if it's something simple by pining the forum first before contacting him, he's quite a busy guy and I don't like to hound him if it's something minor I can figure out. But yeah it looks like this could be involved and if we need to scope this amp out then I will probably do as you suggest.. Hate to admit defeat :)

thanks,

Jeff

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 15:00:22
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
Jim!

man hate to drag you into this, yes the micro phonic sound will transmit through both channels, the hum is a tad higher in one channel than the other.I am suspicious about it being a tube issue. The tubes I am using are the new ones you sent..

and yes this is the same amp that Samra and I fixed the trafo on.

I can check the heater voltages on the 12by's tonight..

thanks,

Jeff

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 15:15:07
Jim McShane
Dealer

Posts: 5169
Location: Chicago
Joined: January 13, 2003
I suspect some trafo rewinding is in your future I'm afraid. But the unit is worth it!!

But maybe it's something else, so I'll keep my fingers crossed!!

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:19:47
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
wow, that would be very serious, what would cause this? I thought that the windings are very "inert" and if they are wound and stable through their life they would not "wear" out..

J

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:23:43
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
Jim, is that due to them needing some wax like potting to dampen the wire like is done with guitar pickups? I'm guessing done with tar instead.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:28:46
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
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.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:33:22
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
Hi Jim, Don's a great guy and has helped me a few times in the past but I haven't figured it out. I'd be embarrassed to send it off in it's current state because the once nice job isn't so nice looking now after trying many different things to solve the issue. That and it's a pretty heavy thing to ship:).

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:43:30
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
Man, I hope you find it and don't have to incur the high price of a rewind. That would be heart breaking.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:46:02
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
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

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:49:52
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
Danny,

I am not sure what I'll do if that's the case. I'd probably sell it for parts, it's all new.. It wouldn't be hardly worth it to re-wind the transformers.. I have another one I could upgrade and just call it a learning experience..

J

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 20:51:49
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001
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.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 21:15:48
mr9iron
Audiophile

Posts: 214
Joined: March 13, 2002
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

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 7, 2012 at 21:34:18
DannyR
Audiophile

Posts: 339
Joined: January 21, 2001



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.

 

I'm Interested, posted on July 8, 2012 at 10:28:02
Triode_Kingdom
Audiophile

Posts: 3026
Location: Texas
Joined: September 24, 2006
I hope this works out OK for you. If not, and you do indeed decide to sell this for parts, I'd like a crack at it. Please keep me in mind.


--------------------------
Buy Chinese. Bury freedom.

 

"Also my hum disappears if I short the inputs", posted on July 8, 2012 at 11:34:46
JJ Triode
Audiophile

Posts: 655
Location: Northwest
Joined: December 2, 2004
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.

 

RE: Serious Micro-Phonics and Hum from Citation V, posted on July 8, 2012 at 12:19:02
Jim McShane
Dealer

Posts: 5169
Location: Chicago
Joined: January 13, 2003



Danny,

The insulation on the wires and the wires themselves can both fail. The windings are subjected to repeated heating/cooling cycles, they move ever so slightly as a result of expansion/contraction from heat, and from magnetostriction; and if there are any solder joints inside them they can work harden. See the pic of a Cit II trafo I posted as an example of an internal wire failure. This pic is of a power trafo but the failure can occur in an OPT too.

Keep in mind too that magnet wire now is FAR better, especially the insulation.

BTW, getting Cit V trafos rewound is significantly less than getting Cit II trafos done since the V's aren't potted. I wouldn't rule out keeping the amp - but Triode Kingdom's offer is nice to see - this particular amp's life is FAR from finished!

Oh - the potting material used in a Cit II is called "permafil" it's a mixture of tar and ground slate.

 

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