>
>Hi all;
>
>Trying to save some time. Anyone have experience with R6 (25ohm/25w)
>resistor in the HV overcurrent circuit arcing?
>
>When I 1st opened this amp up, R6, R36(3.3K/2w) and R16 (180k/1/2w) were
>all arced and open.
25w ww resistors are not well suited for glitch service because they have
a low length to diameter ratio. It would be better to use two 10-ohm 10w
resistors in series.
> Found a short against the transformer from the PC
>board where R16 was soldered in. Took care of that problem and amp
>fires up great now with no immediate arcing. The Plate Current seemed
>fine, not excessive. On tuning up though (power out just fine) R6 has
>fried again.
>
Gold-sputtering flashovers could be responsible for frying R6. To test
for loose gold in an 8874, one needs a high-pot tester. Apply positive
5kv through c. 50M ohms and measure leakage current. Currents over 10uA
indicate a problem. Next apply negative 5kv and measure leakage
current. If the current decreases substantially, gold-sputtering is
indicated. If the leakage current remains the same, you may have a leaky
seal. Gold-sputtering does not necessarily mean that a tube is kaput
unless the cathode coating has been dislodged. There is a simple way of
moving loose gold into the base of the tube. To do this, one holds the
tube with the anode cap up and taps vertically on the cap with the flat
side of a 1-oz ball peen hammer. Measure the negative leakage current
while tapping. When the leakage current stops decreasing, you are
finished. note- the tube must be kept anode-cap up or the gold that was
settled in the base will recontaminate the cathode and anode insulator --
whereupon the tapping operation must be repeated. . (a colour photo of
the insides of a gold-sputtered 8874 that was removed from an Alpha 374
is shown in "Parasitics Revisited" in the September and October, 1990
*QST*).
>
good luck, Peter
- Rich..., 805.386.3734, www.vcnet.com/measures.
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