Glowing Tubes Not During Christmas!

[Holden]

Minor correction to Harvey's statement that NO tube plate should glow - not true! I quote from "the Radio Manual" by
George Sterling, 1928, page 283: "with transmitter in operation handling traffic and all adjustments properly made,
the tube plates should not heat beyond a cherry red."  This is for a pair of UV-204A tubes. Page 313: "note plate
temperature and if not over a cherry red increase plate potential to 1000 volts." This is in reference to a set of
211 tubes, which are fairly popular among the audio set. Older books are replete with such observations of plate
color. Numerous old tubes are designed to have some color when operating normally, however the 6L6 isn't among them!
In normal audio operation, power output tubes will often idle at near their maximum power dissipation. This requires
careful attention to bias, screen, and plate voltages, and plate current. A 6L6 with all these values within mfr
ratings should not glow. In considering changing out tubes which do glow but shouldn't, verify that ALL power supply
voltages are correct before doing so lest the new ones go the way of the bad ones! A tube which glows but shouldn't
in spite of all voltages OK is probably drawing too much current, and is NG.
Howie WB2AWQ

"Harvey A. Kader" wrote:

> Hi Bob.
>
> Just a note to let you know that NO tube should have ANY glow(red or orange) on its plates. Such a glow
> indicates TOO MUCH current running through the plates(for example 80mA versus 40mA). Glowing tubes will self
> destruct sooner or later - the higher the excess current, and the more intense the glow, the faster the tube
> will destroy itself.
>
> Thermal runaway is an example of a condition which produces a very glowing tube.
>
> As far as Chinese tubes are concerned, the small signal tubes are great(I have had zero problems with them), and
> the quality of the output tubes is increasing( slowly).
>
> I hope that this helps.
>
> Regards.
>
> Harvey A. Kader

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"