[Amps] 8877 - Which Bias voltage ?

[Phil Clements]

> I am about to activate an RFPP-1000 amp which uses
> 1 - 8877 @ 2750v. (exactly what '5PRO recently described.)

I have converted several of these.
If you remove the choke, add 30 mfd or so filter capacitor,
and step-start, you will be able to set the taps on the HV
transformer to 4 kv under load. You will then need a 12 volt
zener for proper bias. I use the 50 watt units for simplicity.
The 8877 seems to shift into "overdrive" @ 3800-3900 volts
on the anode. Less drive required, and c. 2200 watts output,
but you will need the 12 volt zener to keep ZSAC below 180 ma.
Be sure to set the heater voltage @ the tube pins to 5 volts or a little
less....some of these units came tapped @ 104 volts on the filament
primary when the unit was used on a 208 volt service.
I hope this helps.
(((73)))
Phil, K5PC


>
> My last remaining conversion change is to chose the Cathode
> bias voltage to use. Here's what has been published:
>
> K8RA-8877:   10Volt/1watt zener &  2N3055
>                           " for 8.2v cathode voltage ".
>                          Text indicates " bias for 18ma idle current."
>                          ( That can't be for the tube idle plate
> current, can it ??)
>
> W8ZR-8877:  12 volt/50watt 1N2812
>                          150ma.  idle plate current
>
> Ameritron-AL-1500:     7.5 Zener
>
> RA's-8877 in QST:   8.2 volt 50watt Zener for SSB
>                                     22  volt 50watt Zener for CW
> (Ref: Radio Handbook W6SAI / 1996 ARRL Handbook.)
>
> So my questions are:
> ================
> -What bias voltage is needed for SSB ? for CW ?
>
> -Advantages / Diadvanages of using a 50w Zener vs.
>   a 2N3055 & a 5 or 10watt Zener ?(Other than $$$).
>
> Charlie - N1RR
>
>
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