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[reverbperv]

Hey everyone. I'm new to this forum. Thought I could find some help here. I bought a GT66 on ebay for $100. It was described as humming. I figured I would try to fix it. I got it, and it sounded like an air conditioner unit. It does pass audio. I replaced the tube and it still has the same problem. I switched the pad from 0 to -10dB and minimal audio was passed,  but mostly the -10db reduced noise. It looks like someone touched the capsule, because the resonator disc has prints on it. Maybe that is the cause, but I'm leaning more towards the pad network. Any schematics or suggestions? I bought this mic not because I thought it was a hot piece, but because it was a good deal if I could fix it for a small fee. I know that it is a decent mic, by no means an ELA M250 or a C12, but a man's got budgets sometimes, and I really want to get into mic repair and design, so I thought this was a good piece to start with. Also I've been dehydrating the capsule with silica packets to see if maybe it's a moisture issue, but so far no change.

[fiddler59]

I would check the electrolitic caps in the power supply box first !!!

David B

[MagnetoSound]

fiddler59 wrote on Wed, 27 August 2008 17:37

I would check the electrolitic caps in the power supply box first !!!

Yup. There are basically two types of hum that you might encounter in a tube mic.

The first is ripple on the B+ line (or heater supply) that might sound like an air conditioner (or electric motor) running in the same room - which is caused by defective (or loose) filter caps in the power supply.

The second is the pickup of AC fields in the mic or cabling, which is usually the result of a broken or disconnected shield wire in the mic cable. If the hum gets worse when you touch the mic, suspect the shield.

(That said, the shield is often the return path for the supply currents, in which case you will receive a belt off the mic if you touch anything which is grounded at the same time, and the mic will not power up, if the shield is broken.)

If the mic comes on and the hum is steady, check the PSU filter caps.


Dan

[reverbperv]

Thank you gentlemen! I will try the filter caps this weekend. It does power up, so I don't think it's a ground issue. Again, thanks and I'll reply after I do the switch.