Hi, I've just finished my first tube condenser mic based on Gyraf's G7. The capsule is a K47 from Maiku and I've modified Gyraf's circuit with a different transformer and some basic discrete switching logic to provide three pickup patterns (figure eight, omni and cardiod). Everything is on self etched PCBs using my own board layouts, including the power supply. The actual tube is an EF86EH - it says "Electro-Harmonix" on the glass and "Made in Russia 19 07". B+ is 200 volts and heater voltage is at 6V, so fully within specs.
When testing it on the bench, everything worked fine and it sounds great.
I was actually surprised how low its self noise was, even compared to other condensers I have (commercial and DIY). Until this morning, it was actually the quietest mic I own with regards to noise - even after hours of continuous testing! I felt lucky
Last night I wanted to see how warm it gets after hours of use and left it powered over night. This morning (about 10 hours later) it was still working fine, not even lukewarm on the outside but the output noise had increased by about 20 dB since yesterday. On the other hand, the signal level hadn't changed., except that everything was covered in a very noticeable hiss.
Since then I've replaced the tube for another russian EF86 (different brand with cyrillic letters) and this one is fine again with the same output levels and low noise.
I have very limited experience with tubes, but may I assume that the first one was a dud or should I be concerned that leaving the mic powered for hours can harm the tube?
When testing it on the bench, everything worked fine and it sounds great.
I was actually surprised how low its self noise was, even compared to other condensers I have (commercial and DIY). Until this morning, it was actually the quietest mic I own with regards to noise - even after hours of continuous testing! I felt lucky
Last night I wanted to see how warm it gets after hours of use and left it powered over night. This morning (about 10 hours later) it was still working fine, not even lukewarm on the outside but the output noise had increased by about 20 dB since yesterday. On the other hand, the signal level hadn't changed., except that everything was covered in a very noticeable hiss.
Since then I've replaced the tube for another russian EF86 (different brand with cyrillic letters) and this one is fine again with the same output levels and low noise.
I have very limited experience with tubes, but may I assume that the first one was a dud or should I be concerned that leaving the mic powered for hours can harm the tube?
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