[AMPS] Fault in 8877 amp

[Steve Bookout]

Hello all, 

I thought I would post this here to get the collective wisdom of the
group.

I have an 8877 homebrew amp which is causing me some fits.  It is
basically from a design published in Ham Radio magazine in 1983
describing a modular concept 8877 amp.
It uses a hypersil Xformer rated at 4000 Kv at 1.5 amps CCS; plenty
beefy.

Anyway once many years ago, while doing the initial tune up, the power
supply flashed to ground thru some type of failure.  It took out several
components, so I don't know what was bad to start with.  
6 kva shorting directly to ground will get your attention.  BIG pucker
factor here.  It made such a BANG, that I, literally, didn't touch it
for 9 years.  Didn't even move a screw.
Well, I tore it apart checked everything and replace a few bad parts. 
This time I added a 50 ohm/50 watt resistor in the B+ line.  (Ain't
hindsight wonderful).

All has been fine until about 6 weeks ago.  Hit the key on 20 and the
amp went 'FLOOOOOOP'. Loud, but not like before and kinda drawn out.

Anyway, the grid trip circuit tripped and was locked up.  The AC pwr
supply relay arced closed (B+ on).  
I took things apart and found the 2n2222 in the grid trip was shorted.
Both sets of back to back diodes across the meters were shorted, but
meters OK.  Apparently, the fault caused the values of the resisstors in
the grid meter circuit to change.  I couldn't get the trimmer to
calibrate the meter; changed R's and now OK.
The tube had apparently shorted externally from the plate to gnd.  I
found a real small arc point on the bottom of the anode fins next to the
tube body and another on the grid ring.  The ceramic in between looked
like someone had left a cigarette butt on it.  Slightly discolored.

Well, I put it all back together and double checked everything I could
think of.  All OK.  
Was working fine in this last WPX CW contest on 40 for the first 45
minutes.  Then 'FLOOOOOP'.  Same thing. I haven't torn it apart yet due
to disgust, mainly.

I run this amp at .75 amps and keep the grid current at about 40 ma with
the trip set at about 70 ma.

We had not changed a thing for 45 minutes.

The only thing I can't accout for is that when I tune up, if I hold the
carrier for 10-15 seconds, I can watch the grid current slowly creep up
maybe 10-15 mils. 

I'm running an open frame 10 amp relay for the imput/bias and a vac
relay for the output.  Forget the make but, it looks like three white
Lifesaver stacked with terminals between.  Coil is 28 v and at one end
of the stack.

Does this problem sound familiar to anyone?  Any ideas on additional
things to check.
This is getting old.

73 de Steve, NJ4F
(The guy who was real loud on 40 for 43 minutes)

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