Sounds like a gassy tube. I had a similar problem with my 87A. I replaced
one of the original tubes because the amp was periodically failing with a
filament current fault. Then the amp started getting random faults, mostly
related to power supply voltages. Eventually the faults became more
frequent, and one of the 20A fuses kept blowing. Even the 1A fuse in the
step-start faulted once.
After tearing the amp apart and thoroughly testing the power supplies and
other components, I realized it had to be a tube arc and replaced the new
tube with the old one. Sure enough, that fixed the problem. The filament
current problem went away, too. I'm pretty sure that fault was the result of
poor pin/socket contact that occurred when I removed and reinstalled the old
tube to troubleshoot another problem. Eimac replaced the bad tube under
warranty. So, my amp is still operating with the original set of tubes
installed at the factory in 1995.
Evidently, if the arc is big enough it can blow the fuse before the fault
detection circuits in the 87A have time to react. I suspect the same is true
of the 89. I don't think it's a good idea to rely on the "fuses are cheaper
than tubes" credo. If the arc can cause the fuse to blow, it may cause other
damage. If I were you, I'd see if I could borrow a spare 3CX800 tube and use
it to confirm that the problem is being caused by a bad tube, and if so,
which one. Then I'd replace the bad tube.
73, Dick WC1M
> -----Original Message-----
> From: Robert Carroll [mailto:w2wg@comcast.net]
> Sent: Sunday, August 05, 2007 4:35 PM
> To: amps@contesting.com
> Subject: [Amps] Alpha 89 Fuse Blowing
>
> Seeing there have been several recent discussions of amps blowing
> fuses, I'd
> like to ask for opinions on my Alpha 89. For about two years now the
> amp
> will be trouble free for a couple of months and then while sitting idle
> in
> standby, blow what has been every time but once the upper of the 20A
> fuses.
> I replace the fuse and all is well for a month up to a few months and
> it
> blows another fuse. Most of these have blown when I am out of the
> shack,
> but occasionally I will be watching while it occurs-no pop no snap, but
> the
> amp suddenly is off-no sign of the red fault light. The last time this
> happened, I had keyed the amp with very little drive and when I did so
> I saw
> the red fault light turn red as everything coasted to a halt. This was
> the
> first time it had failed during transmit, and again a fuse replacement
> revived it. The tubes in this amp probably have 300 hours or so on them
> and
> deliver 2KW without complaint. It is easy to suspect a tube flashover,
> but
> replacing a fuse every few months is a lot cheaper than replacing the
> tubes,
> especially given their relatively low hours. I have snooped around
> looking
> for spit marks and have found none. I have vacuumed the amp with no
> change.
> This amp has been a little squirrelly from the day it arrived from the
> factory, in contrast to an Alpha 86 which has not missed a beat other
> than
> having the fan changed-still on the original tubes. I suppose I could
> swap
> tubes at some point to see if the trouble follows the tubes, but I like
> to
> keep the mitts off something which is working. If any of you have
> suggestions as to possible causes or methods of tracking down such an
> infrequent problem I would appreciate hearing them.
>
>
>
> 73
>
> Bob W2WG
>
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