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Author Topic: U67 Tube 'Polarity' ?  (Read 458 times)

RuudNL

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U67 Tube 'Polarity' ?
« on: November 13, 2021, 09:30:33 AM »

I have been looking at the schematic of the U67, but on the schematics I have seen, it is not marked which pin (4 or 5) of the EF86 is connected to ground and which pin has the (negative) filament voltage.
Of course for a indirect heated tube this is of no importance, but I would like to experiment with a solid-state equivalent of the EF86.
So, more information is welcome!

(Reasons for this experiment: no ageing, no microphonics, low(er) noise, long life.
I have used the Phaedrus VF14 and AC701 solid state replacements with success in the past.)
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klaus

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Re: U67 filament 'polarity'
« Reply #1 on: November 13, 2021, 03:19:20 PM »

Pin #4 = ground
Pin #5 = heater (-)

Quote
Reasons for this experiment: no ageing, no microphonics, low(er) noise, long life.
I have used the Phaedrus VF14 and AC701 solid state replacements with success in the past

Depends how you define succcess.
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Klaus Heyne
German Masterworks®
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RuudNL

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Re: U67 Tube Filament 'Polarity'
« Reply #2 on: November 13, 2021, 04:41:55 PM »

I found this Neumann schematic.
On the schematic it says:
H1 = pin 5 = Ground
H2 = pin 4 = Heater (-)

So, you suggest the schematic is wrong?
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klaus

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Re: U67 Tube Filament 'Polarity'
« Reply #3 on: November 13, 2021, 04:58:19 PM »

What I wrote is correct for the actual wiring inside the mic. The U67 schematic's heater pin assignment is different from the circuit you will find on the mic.

This is not unlike some M49 schematics, where there too, F1 and F2 on the schematics are reversed from how the tube is actually wired in.
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Klaus Heyne
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RuudNL

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Re: U67 Tube Filament 'Polarity'
« Reply #4 on: November 14, 2021, 04:32:18 AM »

Many thanks Klaus!
(Inconsistency between schematic and 'the real thing' is about the last thing I would expect from Neumann!)
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klaus

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Re: U67 Tube Filament 'Polarity'
« Reply #5 on: November 14, 2021, 12:52:17 PM »

... and I am not sure whether polarity matters on a DC heater supply?
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Klaus Heyne
German Masterworks®
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RuudNL

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Re: U67 Tube 'Polarity' ?
« Reply #6 on: December 01, 2021, 03:58:04 AM »

No, for an indirectly heated tube this does not matter!
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