When a good 8877 is properly tuned (slightly heavy loading)
it will put around 2300 watts into the antenna with 55-65 ma
of TRUE grid current. I have had several Alpha 77DX/77SX's
and all of them read HIGH on the grid meter. I think Dick had
in mind that it would help to save tubes. NEVER let the grid current
on an 8877 exceed 75 ma under any circumstances. I set all my
grid-trip circuits to this value.
GL!
Phil, K5PC
>
>Alek and all who replied,
>
>Thanks for the helpful ideas.
>
>I opened the grid return line above the bias switch (Q5) and inserted a VOM.
>When the VOM indicates 100 ma. of grid current, the panel meter reads 250
>ma. If I tune using the VOM indication for grid current the amp seems as I
>would expect. Makes about a KW on low power with about 50 watts of drive
>and 100 ma of grid current
>
>So I feel fairly comfortable that the panel meter is off by about 250%.
>The meter is a multi-meter and works normally for the other readings, so I
>think it must be basically ok. It reads about 32 ohms resistance, but I
>don't know what it should read..
>
>All the resistors in the circuit are within reason for 28 yr old carbon Rs,
>the 3.3 ohm reads 4.0 ohms, the 680 ohm reads 711. The circuit is... grid
>current is returned to ground thru the 4 ohm. The 680 comes off of the high
>side of the 4 ohm and goes to the 0 to 1 ma meter, reading voltage drop
>across the 4 ohm.
>
>Oddly enough, when I run all these values thru good old Ohms law, the thing
>seems to working as designed
>
>Look like to me that the 680 should have been more like 1600 ohms, maybe I'm
>missing something.
>
>Problem with this is it implies that the amp was designed wrong and has been
>used that way for 28 years. Pretty far stretch, too far for me.
>
>I'm not familiar with 8877s, this is my first one. I'm wondering if the
>"ideal" grid current is 80 to 150 as indicated in the manual or is it the 40
>to 60 you actually get when tune up per the book (considering the meter
>error).
>
>So I'm still working on it. Ideas?
>
>73 de Jack - W4ASM
>
>
>
>.
> ----- Original Message -----
> From: Alek Petkovic
> To: Jack Brabham
> Sent: Saturday, September 11, 1999 1:28 AM
> Subject: Re: [AMPS] PA-77 question
>
>
> Jack, (Any relation to the famous Australian formula 1 driver?)
> Sounds to me like your grid current metering circuit is playing up. I
>think your grid current meter is telling you lies. Check your meter shunt
>etc.
>
> 73, Alek.
> VK6APK
>
>
>
>
> At 10:15 PM 09/10/1999 -0400, you wrote:
>
> I have been fixing/restoring an Alpha PA-77 for the last few months.
>It had apparently been out of service for some time and has had a string of
>failures. Fix one thing, something else breaks. It was also covered inside
>with tobacco residue, for which I found a great cleaning technique.
>
> The next last event was failure of the 8877 filament. I picked up a
>yu209 from a reputable hamfest source. After baking the filament for 24
>hours I tried to load it up and all the magic smoke escaped from power
>transformer. This was the first time the amp had been "on" for more than
--
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