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Author Topic: Filament current in U67 EF86 tube  (Read 479 times)

RuudNL

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Filament current in U67 EF86 tube
« on: March 16, 2022, 08:08:32 AM »

In a description of the (reissue) U67 power supply (NU67V) I read:
"Redesigned to meet stringent safety standards and accommodate higher filament current of newer premium-grade tubes".
I tried to find the differences in filament current for various EF86 tubes, but all specifications say: "filament current 200 mA".
I am asking this, because an older (recapped) NU67U power supply doesn't put out more than 5.8 V with a 'newer' EF86.
Unfortunately there is nothing to adjust inside the power supply: series resistors are 22 ohm - Zener 15 V - 18 ohm - 18 ohm - 5.1 ohm.
I suppose the best way to get a higher filament voltage, is to lower the value of the 5.1 ohm resistor? (to 2.7 ohm?)

(The power supply doesn't have the internal 'pad'.)
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klaus

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Re: Filament current in U67 EF86 tube
« Reply #1 on: March 16, 2022, 02:02:15 PM »

Ruud,
That series of NU67 is indeed a pain to adjust.
The correct value increment of that last dropping resistor is impossible to hit just right, as values in increments smaller than whole ohms are rare.

I would choose a 3Ω resistor in position R8, and a shut resistor after it to ground. Find the right value of the shunt by first installing a trimmer, then replace the trimmer with a fixed 1/2W resistor of equivalent value.

P.S.: An audio pad was only used in NU67 exported to the U.S., as per request form the North American importer.
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Klaus Heyne
German Masterworks®
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RuudNL

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Re: Filament current in U67 EF86 tube
« Reply #2 on: March 16, 2022, 03:36:25 PM »

Thanks. I will try to do this. The microphone works, but 5.8V is way too low IMHO.
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klaus

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Re: Filament current in U67 EF86 tube
« Reply #3 on: March 16, 2022, 04:08:32 PM »

Neumann specs the mic's heater to be operable from 5.8VDC upwards.
But I agree, neither is the polarization voltage in its sweet spot at that low voltage nor is the noise floor.

I aim for somewhere between 6.1VDC and 6.2VDC, measured at the mic, not the power supply.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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