[Amps] 8877 - No plate current

[Jim Garland]

Chris,

Bruce might be correct that the filament is bad, but I"ve never known a 
filament to open up in an 8877, except for some tubes back in the 1970s 
which had defective filament welds.

Another possibility is that you don't have any plate voltage from your 
HV supply. You can verify this by applying a small amount of drive power 
to see if any grid current is drawn. Be careful to apply only a watt or 
two of drive power for a very short time, because operating the tube 
without any plate voltage can easily burn out the grid.

Another possibility is that the bias circuit is not providing operating 
bias, and the tube is cut off with its standby bias. You can verify this 
by seeing if the tube draws any plate current when driven. I'm guessing 
that is not the problem. At this point, I"d put my money on a lack of 
HV, with a bad tube being my second choice.

73,

JIm W8ZR


On 10/10/2017 11:19 AM, w8hw at comcast.net wrote:
> Hello Chris,
>
> I have a feeling you do not mean output SWR, I think you input (from 
> exciter) is high. If that is what you do mean, then my first guess is 
> the fillimate is out. Fillimate most often burn out at the moment of 
> turn on.
>
> To prove this is the problem can be done two ways. 1. you can ohm out 
> the fillimate. 2. Check to see if the tube gets warm (warning... 
> because of high voltage do not touch the tube plate for this test) 
> using IR gun. You can buy them for around $10 and they are good to 
> have around for many other things. Also check to see if you fillimate 
> voltage.
>
> Feel free to contact me direct if you need more information. My email 
> is in QRZ.com.
>
> 73, Bruce, W8HW
> =================================
>
> -----Original Message----- From: Chris Allingham
> Sent: Tuesday, October 10, 2017 12:54
> To: amps at contesting.com
> Subject: [Amps] 8877 - No plate current
>
> I have a homebrew 8877 amplifier based on the QST articles from the 
> mid-80s
> and the ARRL Handbook.  I did not build the amp.  I believe I am the 3rd
> owner.  I have owned the amp since 2013.
>
> I haven't used the amp in several months.  I powered it on recently, and
> now there doesn't seem to be any plate current and the output SWR is high
> on all bands.  The amp was working fine the last time I used it.
>
> Any suggestions on what the problem might be?
>
> Thanks for your assistance!
>
>
> 73,
>
> Chris VE3FU
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