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In the Forum: Analog Playback
In the Thread: 7788 tube mic pre design
Post Subject: 7788 ringingPosted by hagtech on: 1/13/2008
I am having a LOT of microphonic trouble with the 7788s in a high gain stage. I run a 40mV input signal into a 7788 triode at 15mA CCS. Plenty of gain, nicel output. But dang, if I bump the tube or board or chassis, the grids inside ring like a bell. More of a shimmer. I can measure one having a very noticeable 5kHz tone. It's a shimmer with a strong 5k resonance, like a tuning fork. This is not a ping that damps out quick. It lasts for 10 seconds or more. One sample is worse than the other. On a scope it looks like I have a 5kHz carrier tone riding on top of the signal. I checked the usual suspects (fluorescent lighting, etc.), but it came down to the tube itself. No RF oscillations.
I'm not sure I can use this tube in such an application (like a phonostage). Romy, how do you keep yours quiet?
jh
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