>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).
--verily
>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.
a familiar story. As I recall, the 2n2222 is rated at 0.5A.
>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.
--A familiar story, Steve. One of our recognized amplifier "experts"
purports that such occurences can be due to intermittent gas in the 8877.
In my un-expert opinion, gas is unlikely to be intermittent.
If you have access to a current-limited HV breakdown tester, you can
determine what is wrong with the 8877.
With the tube upright and removed, apply plus 8kV to the anode. The
other polarity connects to the grid. . Measure the leakage current.
Shut down. Reverse the polarity. Re-measure the leakage current at
minus 8kV. If the leakage current is identical, the tube is gassy. If
the leakage current decreased substantively, gold sputtering is
indicated. [for a discussion of gold sputtering, see *Parasitics
Revisited* 9/90 &10-90 QST]
If you do not have access to a HV breakdown tester, I am equipped to do
this test. I will perform this test at no charge, and return the 8877
via Priority Mail the same day I receive it. If possible, I recommend
shipping the 8877 in an Eimac 8877 carton. Placing the 8877 carton
inside a larger box is not a bad idea. Bubble pack or bagged packing
peanuts makes a satisfactory filler. Loose packing peanuts do not.
Please note the s/n of the questionable 8877 before you ship it.
Rich---
R. L. Measures, 805-386-3734, AG6K
--
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