Chinn
68 posts
Jul 02, 2013
9:13 PM
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Last week, I found a terrible looking JT30 in a local music consignment store. Tried out 5 different mics and this one blew the others away completely.
Got the mic home and played quite a bit through it into my Vox DA5 and a Cigar box amp I built which it sounded great through. (I will have a harp appropriate tube amp someday, but this is what I have now).
I opened it up to check out what element it had and what it looked like inside. The previous owner had built this and done an ok job, however some of the wiring came loose just by looking at it.
I refinished the mic externally and it looks great. I also re-wired a portion of the mic to eliminate the cold solder joints.
Now finished and playing, it does very well through my Vox. Sounds great! However, through my home made amp, it has a bad 60hz buzz (just like an unshielded guitar). The buzz is only there when I touch the mic however - no buzz when its on, but sitting on a table. Also buzzes when I touch the Switchcraft Screw On connector....
That problem points to grounding. I have the shielding of the cable grounded to a lug inside the shell. To that same lug, I have the grounding from the element and volume knob. The wiring arrangement is the same now as it was before, but simply with better quality solder joints that won't fail at the first jam I take it to.
I've checked the grounding lug inside, and the connections are good. Any other suggestions of what to do are greatly appreciated.
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Chinn
69 posts
Jul 02, 2013
9:17 PM
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Oh yeah, the mic has a MC151 element in it, which appears to be a 70's or so element based on what I could find on the internet....
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Harpaholic
262 posts
Jul 02, 2013
11:24 PM
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Does it have a cable into the mic shell or a 2501mp screw on connector? You mention both?
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jbone
1292 posts
Jul 03, 2013
4:46 AM
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If you have the 60cycle hum on both amps that could be a ground issue. If it's just from the one amp there is an issue somewhere in that amp.
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Chinn
70 posts
Jul 03, 2013
7:41 AM
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Thanks y'all.
Harpaholic, it does have the connector, but the PO bypassed it and ran the cable straight into the shell (through the connector). Currently, I have the cable shielding soldered to an 18ga wire which is connected to a grounding lug in the shell. The element and pot are also connected to that ground.
Jbone, I thought of that too. I've checked the mic on two different amps and don't get any hum on those, so I think that proves your theory.
Last Edited by Chinn on Jul 03, 2013 7:41 AM
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chromaticblues
1422 posts
Jul 03, 2013
8:23 AM
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Disconnect the wire you installed to the ground lug on the shell.
That's your problem!
As long as you have the shield of the cable to the element ground and the other end to housing of the 1/4 plug. You'll be fine!
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Chinn
71 posts
Jul 03, 2013
8:26 AM
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Chromatic, are you saying I only need to ground the element and pot to the cable, not the shell? I'll give it a try.
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Greg Heumann
2234 posts
Jul 03, 2013
9:17 AM
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The outside of the screw-on connector should be grounding the shield to the shell anyway. What you're experiencing is simply a result of crystal element mics being extremely high impedance, which turns the entire mic/cable into an antenna for EMI that the amp is happy to amplify. These systems are very touchy - body grounding (as you experienced) can change them enough to cause or eliminate the buzz. So can different grounding schemes at the amp end, different kinds of cable, moving away from strong sources of the EMI like fluorescent lights, or anything with a transformer in it (don't worry the amp's transformers are properly grounded to the amp so they aren't the source.)
DO make sure that the switchcraft connector is securely grounded to the shell. The pressed-in fitting originally supplied by Astatic can get loose. Worse, people try to fix it with epoxy which can further insulate the connector from the shell.
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
BlueState - my band
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Last Edited by Greg Heumann on Jul 03, 2013 9:19 AM
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Harpaholic
264 posts
Jul 03, 2013
12:27 PM
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Chinn all of the grounds off the cable and pot are common and can go to the shell.
Are you saying the PO drilled right thru the 2501 and ran the cable thru it?
If the 2501mp is not being used I would guess the problem is the way the pot is wired or grounded.
You have to ground the casing of the pot to the shell and ground lug on the pot to the shell.
I solder a jumper wire from the ground lug on the pot to the casing of the pot and then a second wire off the same lug to the shell.
I run the element ground to the pot then a wire soldered on the inside of 2501mp to the shell.
This way the 2501mp has two grounds.
it's easier for me to do it then explain it
Last Edited by Harpaholic on Jul 03, 2013 1:09 PM
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Chinn
72 posts
Jul 03, 2013
8:44 PM
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Thanks Greg and Harpaholic.
Yes, Harpaholic, that is exactly what they did. I plan to rebuild the connector by putting a new grommet and the soldering the connectors up as designed.
Will report back after the holiday about the results of trying y'all's suggestions.
Last Edited by Chinn on Jul 03, 2013 8:45 PM
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