My personal suspicion is that the series R is an ill-conceived attempt at a
"glitch resistor" that will act like a big fuse if something fails or arcs,
protecting the tubes.
AS for your one tube glows, the other doesn't, I had something very similar
(but not so striking) in my SB-220. One tube anode would glow more
brightly than the other under drive. I put it down to mismatched
tubes. Then, a couple of months ago, the filament on that tube went out
altogether - once again, I had melted the solder in one or both filament
pins. So I resoldered the tube with silver solder I got years ago from
Rich, AG6K. Then I took emery paper and carefully polished the filament
pins and the corresponding socket parts. As a final touch, I swapped the
spring clips that hold the pins in the socket between the filament pins and
two of the grid pins. Result - the two tubes now seem to be taking about
equal power, judging by their anode color.
My hypothesis is that resistance between pin and socket electrode at the
filament pins was both reducing the amount of filament voltage on that one
tube and causing overheating at the tube pin, which eventually melted the
solder and also detempered the spring clips. Because there wasn't enough
filament voltage, the tube was taking less than its share of the load.
Anyhow, it worked for me.
73, Pete N4ZR
At 12:12 AM 3/20/2007, Brian Moran wrote:
>I have an Amp Supply LK500zc, which I purchased (used) about 3 years ago,
>used briefly, then put on the shelf until recently... when I powered it
>up, there was a nice small snap (not a big one, a small one, like a small
>bug zapper) on keydown, and I just turned it off and put it aside (this
>during a RTTY contest).
>
> This design is similar to many of the dual 3-500z designs out there --
> parallel filament, 3000 no load V, etc.
>
> Digging into it, I find that in addition to the parasitic suppressor on
> each tube, there's a series R of 1 ohm, 3 watts to the top of the plate
> choke -- on one side (the rear side), this "R" is open, but arcs nicely
> when keyed.
>
> Not sure why the R's are there, so I took them out, they look like
> they'd get brittle sitting near those nice hot tubes getting
> alternatively heated and cooled with way more than 3w of heat. Some nice
> teflon-coated 18g wires the same length as the removed resistors are now
> installed.
>
> Getting everything back together, I notice that one tube (the 'front'
> one) glows orange under keydown conditions, while the back one doesn't.
> Ah! Bad tube! I label them, swap them, and try again. (BTW, get about
> 900w, 650ma at 2800v, into a dummy load with 90w drive. Seems a bit low to
> me).
>
> The FRONT one again (remember, back tube in front now) glows more than
> the rear one. AhA! I'm getting old(er), I must have mis-remembered. Swap
> the tubes again (for sure), try again, and the FRONT one again glows more
> orange under keydown...
>
> So, the FRONT tube always glows 'hotter' than the rear one. Anyone
> know WHY this might be? The RF compartment is not different from the
> front to the rear -- no extra metal near the envelope of the tube...
>
> One thing I haven't checked is the cathode-side of the circuit, but
> according to the schematic, the filaments are directly tied together.
>
> Anyone have any experience or ideas, or have seen this before?
>
> I did a search on the Amp-Archives for 'unbalanced' or 'uneven' glow,
> and did come across a spirited exchange about the series-R's in an
> SB-220... but not really something like the situation is now...
>
> Thanks,
> Brian N9ADG
>
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