>I have been fixing/restoring an Alpha PA-77 for the last few months. ... ....
>
>.... failure of the 8877 filament.
Did you measure any leakage between the heater and the cathode in the
kaput tube?
>........
>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.
Typical grid-cath. potential in an 8877 is 55Vrms at full suds. 150mA of
grid current represents approx. 8.2w of grid dissipation.
>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.
>
To find out if something weird is going on with the tube, you need a high
pot. that will produce c. 8kV. Roughly 2/3 of the kaput 8877s I have
tested have gold sputtering damage. To find it you have it, you either
need a hack saw or a high pot. tester.
>One other oddity, during testing I inadvertently applied too much drive
>which resulted in the grid current meter audibly hitting the stops.
Have you compared the full scale reading of the grid current meter
against a DMM connected in series with it? Do the current meters have 3a
back to back protective diodes across the movements? // How much drive
was too much?
>However the grid overcurrent circuit failed to trip. This circuit had
>been working before.
I would measure the resistance between the C and the E in the transistor
that interrupts the grid current when grid current is excessive. If the
transistor is C/E shorted, chances are that you had a glitch in grid
current which caused it to short. (8877s have enough cathode emission to
produce over 40a of peak grid current)
>
- later, Jack .
- Rich..., 805.386.3734, www.vcnet.com/measures.
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