Jack:I own a 77DX and was concerned that a grid current of about 100ma
is needed to develop the legal limit output.A change of tubes resulted
in the same result. I posted a query on Amps about other peoples
experience with this amp. Several people responded that the amp is
designed to show an Ig on the meter that is twice the actual Ig. Some
thought this was an attempt to keep operators from damaging the tubes
from excessive grid current. I do not know what the reason for such a
design is but I think it could be just to give a greater deflection on
the grid multimeter which has a sensitivity of 500ma FS. The manual does
not say anything about this, just to tune for Ig in the range 100-150ma.
The Eimac data sheet on the 8877 shows 74ma for 1000ma plate current and
2075 watts out key down in class AB2 linear cathode driven service.
73, Dan, N5AR
Jack Brabham wrote:
>
> 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 a
> hour or so at a time.
>
> So now I have a untested tube with a largely new power supply (new
> p.w.dahl transformer, new K2AW bridge, new 34 uf @ 5500vdc filter cap, lots
> of re-wiring).
>
> All of the voltages look good, HV, bias, and filament. The bias
> switching works normally.
>
> I get excessive grid current that I can't adjust around. Can't take
> more than 20 watts of drive without exceeding 150 mils on the grid no matter
> how I load it. Otherwise the relationship between drive power, plate
> current, and output looks normal (all proportionately low). All testing is
> into a 1KW Bird Termaline.
>
> Now I'm having a hard time figuring out if there is still something
> wrong with the amp/PS or if this a bad/weird tube ( I do not have a spare).
> I was told that a yu209 was just a "select" 8877 and was a drop in
> replacement.
>
> One other oddity, during testing I inadvertently applied too much drive
> which resulted in the grid current meter audibly hitting the stops.
> However the grid overcurrent circuit failed to trip. This circuit had been
> working before.
>
> I would appreciate hearing any suggestions, speculations, or ideas on
> this problem.
>
> 73 de Jack - W4ASM
>
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