6N7's in phase inverter service

[rsumperl]

Good evening everyone,
Came across some 6N7's in a box of tubes I bought on "The Bay". I've seen some schematics use them for output tubes and some using them in phase inverter service. Anybody have any input on their use as a phase inverter?

Thanks
Ray

 

[claudiomas]

nice tube for Schmidt,Long Tailed Pair phase splitter, similar to 6H7C (6n7s) foton

 

[Johan Potgieter]

They are rather similar to the European ECC33, often used as audio amplifiers and phase inverters in the day. When using as long-tailed pair, just observe the spec. of maximum heater-cathode voltage of 100V; elevate heaters somewhat.

(It is often cited as similar to the 6SN7, but not really; a higher µ of 33 compared to 20 for the 6SN7.)

 

[artosalo]

I have used soviet 6H7C (=6N7) as a DC-connected LTP in one of my amplifiers from the past and it have worked perfectly.
The gain is some 4...6 dB higher than 6SN7 as a LTP.
Attached the schematic.

 

[Ketje]

Yes, that's a LTP.What Claudiomas shows in that datasheet is not !
Artosalo, you should put a protection diode between grid and cathode (1N4148 conducting if grid turns positive).At startup there is more then 400V there :-(
Mona

 

[artosalo]

I agree. In theory the diode should be there, but no 6N7S have failed during 6...7 year of regular use (some 10 pcs.)
6N7 is designed to B2 operation. Its max. g1 current specs is 22 mA peak.
Maybe its grid is exceptionally robust.
On the other hand, that amplifier has main power and anode power switches and I have suggested to switch anodes on after one minute of warm-up.
This procedure eliminates high voltage at the grid.

 

[Johan Potgieter]

Regarding a protection diode grid-cathode, one is not so much worried about the possible grid current. In the circuit shown the maximum current could only be 1,6mA and then only for an instant, before the cathodes warm up enough to allow emission to quickly bring the operating voltages to normal.

It is also the grid-(cold) cathode potential. There is a reason why some close spaced tubes have a cold Va.max. of some 500V. Not because of bad experiences by me, but for the negligible extra cost and complexity I would 'chicken out' and add the diode.

 

[Ketje]

With no current the supply goes higher then in service.Cold cathodes mean no current= no voltage drop on the anode and cathode resistors result supply voltage between grid and cathode.
If you don't like semiconductors in tube amps a neon bulb does the tric too (NE-2).
Mona

 

[artosalo]

Once again, I agree with you. In theory the diode should be there,
But how real this problem is ? Have you ever discovered damaged tubes because of missing diode is such circuit ?

 

[Ketje]

No, in the old days when we switched from tube to selenium rectifiers there was allways a neon across.It lites up a moment at turn on.
Mona