> 1. Replaced the capacitors with new capacitors and bleeder resistors. 2.
> Replaced the existing rectifier board with a new board. This board
> includes the replacement of the zener with 8 - 1N4005's. 3. Installed the
> Parasitic Suppressor Kit.
I've never seen his kit. I can't believe all the people making money
selling things that aren't needed!
> Here is my problem. About a few months ago I was operating normally and
> the H.V. power supply failed. There was no "flash" or "Bang" ... just
> quit. I have had the "flash" and "bang" when I first fired up the amp
> when I bought it and operated it without any of the mods being installed.
> That's one on the main reasons why I initially installed the Parasitic
> suppressor kit.
The flash and bang with virtual certainty was not caused by a
parasitic. You've been duped by the sales pitches to sell parts. The
most common cause of a flash and "big bang" is gas or barnacles
(whiskers) inside the tube.
The tube getters itself during an arc, and then when you replace
the parts taken out from the arc and power the amp back up, in
many cases it doesn't bang again for a while.
> I opened up the amp and found that one of the 8-1N4005 bias diodes had
> failed. It was quite obvious ... charred and black circuit board at the
> diode location. The H.V. fuse was still intact. I called Harbach and he
> couldn't think of anything obvious that would make the diode fail and he
> concluded that possibly the diode may have been faulty.
A number 30 wire 1/2 inch long is a silly thing for a fuse. The
response time of any metallic fuse is very long compared to failure
time of any semiconductor.
The best thing would be to add a current limiting resistance, like a
ten ohm HV pulse rated resistor (RCD 175P series is good). You
can continue to use the fuse, but at least add the resistor.
> I replaced the diodes and operated normally for a few days and the same
> thing happened. A bias diode had failed again.
You have a bad tube(s). Maybe a seal is leaking. You had one in
the beginning, you have one now. You will continue to have one.
> It should be noted that I always operate the amp in CW/Tune (1kW)
> position. If I operate the unit at SSB (2kW) the H.V. Fuse will open with
> a "flash and a bang" ... yes ... even after the mods.
Yep. That's because the voltage breakdown of a tube or tubes is compromised.
Look at it this way. Suppose you did have a "parasitic". Why would
the parasitic make the tube arc? The emission is limited by the
filament area and power of the tubes to maybe ten amperes or so
per tube maximum. The HV breakdown of a good tube should be
over 11 kV, since in normal operation peak anode voltage
approaches twice the DC supply voltage and when underloaded it
can be even higher.
When you get a big bang like that, it is because the HV is faulted
to "ground" through a low resistance non-current limited path. If
that low resistance path is through a normal tube, current can't ever
reach more than 20 amperes. Diodes will take twenty amperes for
a reasonable period of time.
The big bang occurs because you are dumping hundreds of
amperes of current from the supply through a bunch of
components. Unless you have a dielectric failure that creates a
path through those diodes, you have a gassy tube or tubes.
The SB-220 sets itself up for problems by not directly grounding
the grids. If the grids were directly grounded, any fault path through
the tube would be less likely to involve the filament because the
grid would be a "shield". You can still have excessive current out
the cathode if the tube is gassy, but at least you have some
reasonable chance of the current being limited.
You've made all the mods, now all you need to do is put in a good
set of tubes. The mods will make you "feel good" about the
amplifier, and the tubes will fix the problem.
Be sure the new tubes high-pot to at least 11 kV, and be sure you
occasionally get the anodes red during operation to keep the tubes
gettered. And by all means, add some current limiting in the HV
rail.
> I haven't HI-POTTED the tubes yet but they look clear (no blackening).
>
> I'm sorry I'm so long winded here but this is my first posting to this
> group after monitoring it for a while.
>
> Does anybody have any idea why the bias diodes would fail?
>
> Thanks,
>
> John (VE7JDB)
>
>
>
>
>
>
> --
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> Problems: owner-amps@contesting.com
>
73, Tom W8JI
w8ji@contesting.com
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