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| Re: U47 voltage questions [message #340163 is a reply to message #340142 ] |
Fri, 09 May 2008 19:44   |
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Klaus Heyne
Messages: 1959
Registered: April 2004 |
Platinum Member |
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J.J.:
Just make a simple measurement of the plate voltage of any healthy VF14- first measure the voltage in a stock U47, then measure the SAME tube in your clone with the funky resistor you are using.
If your plate voltage drops below 32 VDC after the mic cavity has heated up to a stable temperature, or if it drops more than a couple of volts between the first five minutes of operation and measured after 12 hours, forget your resistor scheme!
If you have a thermometer, compare the cavity temp in both mics. You should not allow temperatures much higher than in the stock U47.
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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| Re: U47 voltage questions [message #340179 is a reply to message #340163 ] |
Fri, 09 May 2008 21:05 |
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 J.J. Blair
Messages: 9751
Registered: May 2004
Location: by the sea & sand |
Platinum Member |
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Thanks Klaus. Already did the Neumann 47 temp. After an hour of operation, it was 91˚. HUGE difference. I think the T-USA topped out at 160˚.
I'm actually waiting for a wirewound from somebody, but in the meantime, I just used a couple Caddock flat resistors totaling 1750Ω, and an extra regular metal film 30Ω to get the right voltage to the tube. I'm in a session now, so I can;t mess with it more for the day, but I'll see how the temperature works out, with the Caddocks heatsinked to the bell.
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