> As the new owner of that same amp, I'm pleased to report it is still
> working beautifully. I had to make ONE change that Bob didn't forsee.
> The filament transformer in the Alpha 70V was built with huge windings
> for the 30 amp current needs of the vapor tube. It's "battery cable"
> sized heater leads are testimony to that. The 8877 draws only 10 amps
> for it's filament. Those large windings likely provide little useful
> internal resistance to limit start-up current. (I don't trust it's
> step-start circuit to provide a suitable slow-rise since the HV isn't
> brought on-line until the timer completes it's cycle) It also makes it
> easier for slightly high AC mains voltage to cause the steady-state
> filament voltage to remain too high. Such was the case with Bob's old
> amp. He lost the filament in his first 8877 only 6 months after the
> rebuild. He replaced it with a new one making no modifications to the
> amp. I think he believed the tube was at fault. He passed away a
> short time later. Since I had pre-purchased the amp from him while
> still alive, his daughter saw to it I got the amp. I checked the
> filament voltage and found it exceedingly high so home-brewed a
> resistor using a length of small enameled copper wire wound on a
> ceramic form. This brought the filament voltage on the pins back down
> to just below spec. Anyone contemplating this mod should keep that in
> mind. Unfortunately, the compact PA-70V just doesn't have room for a
> separate filament transformer. It's a combo transformer so one can't
> usefully alter the primary voltage to gain the desired result. It can
> only be treated on the secondary.
This is an Alpha problem common to many of their amps built pre-1985 that
have a common HV-filament-control transformer. The 77DX/SX are included in
this group. These are the ones I have experience with. I never had the
pleasure of working on one of the "Stanley Steamers!"
There are several ways to go about adjusting the filament voltage. Your
method is one. Most surplus radio emporiums have a great selection of power
resistors in fractions of ohms that can do the job. I discovered another
method when doing 160 meter mods to a 77SX. I needed a filament choke to
keep the RF out of the PS compartment anyway, so I just wound the choke with
a wire size suitable to drop the voltage at the tube pins to just under the
suggested value. Works great; two birds with one stone. (I love to do that!)
The reason for the problem with the Alphas is that they were designed for
230 volt mains. Hardly anyone has this anymore. My mains here are a stiff
242 volts. In several areas of the country, folks see up to 247 volts or so.
Guys running the Alphas that have not had this problem addressed are doomed
to a shorter tube life. My experience with 8877's is that if operated
according to the factory specs will last the average ham 25-30 years.
(((73)))
Phil Clements, K5PC
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