G7 with background noise

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moondog

Member
Joined
Jan 21, 2009
Messages
6
Hi!
I'm new on this forum.
I've built a tube mic with the Gyraf G7 schematic. I use an Peluso CEK12 capsule and the Lundahl trafo and a Mullard EF86. The mic works. No buzz or low freq disturbance of any kind. But the S/N ratio is weird. I get a lot of background noise. Sounds like a distant waterfall in the background. My voice comes through loud and clear but the noise in the background is really audiable.
Lot of volume out of the mic. I don't have to turn the gain up very far on my pre-amp to get a good strong signal.
The B+ is 160V from the PSU without the mic and drops to 115 whith the mic connected. Could there be something there...? A bad cap or connection. A bad tube? (even though it seems to work fine) Where to look?
I'm very greatful for any help I can get.
/Anders
 
the first thing i would imagine is the tube. EF86's are infamous for this. Did you have it tested? In these circuits you need a low noise tube
 
Ok, I have another EF86. I will try it out tomorrow. If the noise changes in any way, then it must be the tube...
 
I changed the Mullard for a RTF tube and it made all the difference. Now there is much lower noise and it sounds good to. Thanks for the tip!!!
 
You might want to reads the metas and search for G7 threads there are other sources of noise maybe you can make it have even less noise.  Noise in a tube microphone is not just from the tube.
 
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