OT: Darkened NOS(?) RCA 6AH6 tubes?

[marta_kson]

As this group has a deep understanding and waste knowledge of hollow state devices, I would like to ask for an opinion about the state of those tubes pictured in the link below.

They are RCA 6AH6 that appears on the market from several sources, claiming they are NOS/NIB. Are there any possibilities the blackening on the inside of the glass have been deposited as part of manufacture? On one tube in particular there are clear lines visible in the black material, as it has been rubbed off by the mica wafer when it was inserted. If so, they was clearly dark from the factory.

Nevertheless, it's a foreign matter present inside the tube. Are there any reason to belive they are of inferior quality?
Unfortunately I have no tube tester, but listening tests on a receiver with those as RF and first mixer gives an impression they offer more noise and less amplification. I don't trust my own judgement there, as knowing what is in the sockets definitely puts on some bias...

Please give me an opinion and maybe an explanation of the darkening other than deposited cathode material. It will be greatly appreciated. Thanks in advance for any advices.

  <http://www.martakson.se/scratch/rca_6ah6_dark_tubes.jpg>
 

[Dekatron42]

I don't know anything about the specific tubes that you ask about but I know that the EF91 has a blueish looking band, you can read more about it here: http://www.r-type.org/addtext/add102.htm

/Martin

[Instrument Resources of America]

First some clarification. "IF" the darkened coating is EVENLY distributed around the inside of the glass, and totally surrounds all of the internal structure of the tube, it is quite possibly a deliberate smoke coating deposited there at the time of manufacture to make the tube less susceptible to outside light. Outside light does affect the operation of almost all tubes, to a lesser or greater degree. This smoke coating was quite often seen, and used to reduce this effect.   Ira.

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[Charles MacDonald]

On 16-01-16 07:04 AM, 'marta_kson' via neonixie-l wrote:

> They are RCA 6AH6 that appears on the market from several sources,
> claiming they are NOS/NIB. Are there any possibilities the blackening on
> the inside of the glass have been deposited as part of manufacture? On
> one tube in particular there are clear lines visible in the black
> material, as it has been rubbed off by the mica wafer when it was
> inserted. If so, they was clearly dark from the factory.

a quite a few tubes had a carbon coating on the inside of the glass to
control static and suppress the blue glow from the electron beams.

a 12BY7 which is a similar tubes generally has such a coating, so I
would not be surprised to see it here.

--
Charles MacDonald Stittsville Ontario
cm...@zeusprune.ca Just Beyond the Fringe
No Microsoft Products were used in sending this e-mail.

[marta_kson]

Great Thanks for all the information! You are all quite right and this was something quite new to me.

When I look carefully with a magnifying glass there are definitely lines in the coating where the mica wafer has scratched it when the tubes internal parts was inserted in the glass envelope. The bottom edge of the coating is very sharp and not aligned with the mica. There are no longer any doubt what so ever about the coating has been added as an improvement at the factory. Knowing the facts those coated ones are the tubes to prefer.

I have now also tested in the receiver with a signal generator and they are at least as good as my other 6AH6 without coating.