I have the same experience with 4CX250B's.
Sometimes the phase-sequence sensor for the 3-phase blowers in the
Standard Radio CT1000's 1 kW autotune PA's failed to sense backwards rotation
and the consequent very low cooling airflow. The net result of half an hour or
so of "soak test"
usually were 4 very black tubes, that promptly were exchanged for new,
and the old ones run through a dishwasher and used for amateur radio projects...
These tubes had the same output as their "non blackened" brethren as far I can
remember.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Monday, November 22, 2004 12:55 AM
Subject: Re: [Amps] Shaded-pole Tube Blower
> Joe Isabella wrote:
> >Seems reasonable to have some kind of interlock for this -- a couple
> >dollars worth of parts could easily save several hundred in new tubes.
> >A friend of mine switched to a pair of 3CPX800s because the squirrel
> >cage fan came loose from the motor shaft in his Alpha. Tubes pretty
> >much melted.
>
> Another experience, for what it's worth...
>
> I once wired the blower capacitor on the 2x4CX400A 2m amp so that the
> motor ran at about half speed, and backwards. (The motor was going
> round, lots of rushing-air noises, right?)
>
> By the time I eventually noticed there was practically no air coming out
> the exhaust, and maybe the tuning was drifting just a little at the 1kW
> output level, the poor 4CX400As were practically black from overheating.
> But when the motor was wired correctly, they came back like new.
>
>
> --
> 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.co.uk/g3sek
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