SV: [AMPS] PA-77 question

[Phil Clements]

>Hello!
>Watch out for that filament inrush current. HAS DESTROYED MANY A 8877. You
>limit it with a resistor in series with the primary of the filament
>transformer and short it out after some seconds.

€ Although Eimac does provide a warning about limiting inrush current to
some filament-type tubes like the 3-500Z, Eimac gives no warning about
limiting current to heater-type tubes such as the 8877.


You can kill several birds here with one (cheap) stone.
If your line voltage is around 125 v, your filament voltage is probably over
5.0 volts @ the tube pins, (unless you are using a filament choke) as most
older vintage filament transformers were wound for 115 v input.

I use a power resistor, 50 or 100 watts, at the proper ohmage to obtain
4.9 volts or so at the tube pins, placed in series with the transformer
primary. This does the following for you:

1. Keeps in-rush current in check. (if that is really necessary with an 8877)

2. Makes the tube last longer by running the filament slightly below 5.0v

3. Economically attractive...no moving parts, no shorting relays etc.

Henry uses this in their commercial 3CX3000A7 amps, et al, also.

(((73)))
Phil, K5PC



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