?? - Blue glow in 2E26 final, hot 7868 modulator

[Jan Axing]

Joel Govostes wrote:
>
> Hi all.  I'm trying to bring a Lafayette HE45B (6m AM) back to life, and
> have noticed the final amp tube glows blue upon transmitting.  Wonder if
> this is usual, or indicates a dud tube (2E26).  I have tried two different
> tubes; both do the same.

Usually nothing to worry about. Tubes operating with a plate potential
above a certain point (200-250V or so) glows blue, most of the glow
is on the inside of the plate. Some tubes have a blue glow on the envelope
caused by stray electron bombardement. Probably dependent on the envelope
glass quality. If the blue glow is dense and has a hint of green in it,
the tube might be gassy but not neccesarily dud.
>
> In the same rig, the 7868 modulator tube seems to kick off a lot of heat.
> It is not touchable for quite a long time after powering-off the rig, even
> if it has not been transmitting.  Could this be a symptom of a problem as
> well?  (Not sure how such tubes "normally" behave.)  Incidentally, I am
> barely getting any modulation on the carrier, and what is there is very
> distorted, though the tubes check out OK:
>
> (6EA8 speech amp --> 6CN7--->7868 modulator-->RFC--->plate of 2E26 final).
>
> Rcvr works fine, and carrier output appears adequate.

It sounds like the 7868 is out of bias, way out. If there is a coupling
capacitor between the 6CN7 and the 7868, check it for leakage. This
is a common problem in old audio amplifiers. Anyhow, check the 7868 bias.

Jan, SM5GNN

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