Super scout info needed pls

Cable Guy said:

Hi all,

I got a 12 tube, 4x8, super scout. I think it's a sos brand, not sure. Looks pretty good under the hood, but doesn't work. Putting 4 watts in gets 4 watts out, regardless of hi/low switch. Both relays make contact when keyed.

Was told it was working fine, until a spark was observed near the tubes, at which point the output dropped to nil and the plates cherried up. The operator was probably driving the snot out of it at the time tbh. :sigh: I can't see evidence of a spark under the hood, but could have been arcing inside the tube i suppose.

So does anyone know what voltage the hv trans puts out? I cant see the capacitance-voltage multiplier values to figure it, and I have a gut feeling the hv transformer is shot. Any other info would be appreciated as well. I'm in no hurry with this thing. Thanks for looking.

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The good news is, you can't turn tube anodes cherry red with a bad power transformer. My first question would be, what tubes turned red, drivers or finals? If only the drivers turn red, check the plate choke feeding DC into the caps of the eight finals. The flash that was seen, may have been that choke burning open. The choke is the part that is fed from the bottom, with one wire through a grommet, at the center of the eight finals.

Proper plate voltage would be someplace around 900 volts DC, plus or minus about 75 volts. Judging by your description, that is very likely to be intact and lethal.

Cable Guy said:

arcing inside the tube i suppose.

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It's tube tester time. The final tubes have negative DC bias voltage on the control grids. The four driver tubes have no bias at all, the grids are grounded.

It's a balls-to-the-wall design, best I can tell. That brand never got popular around here, so the pics are all I'm going on. Driver tubes with no bias will tend to overheat if the radio's carrier is turned down. Even faster on sideband.

The flash most likely came from inside one of the tubes.

73

nomadradio said:

The flash most likely came from inside one of the tubes.

73

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I'm thinking an arc inside one of eight finals, cannot be responsible for reducing the output power all the way down to four watts. That sounds more like we may have lost plate voltage on the finals and are looking at the feed through RF from the drivers. There is also the possibility that something like a pader capacitor could be shorted on the output however, I would expect that to bring the output all the way down to zero.

While it certainly is a possibility that the flash was the result of an internal tube arc, that cannot be the only failure causing these symptoms.

Shockwave said:

that cannot be the only failure

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Probably just the first domino to fall. How many more of them flopped over is the question. The design includes absolutely no parts to limit fault currents or to minimize collateral damage when a tube suffers catastrophic failure.

Sounds more like a tube shorted and then clobbered the high-voltage supply. Never underestimate the odds of fratricide, one bad tube clobbering other tubes wired in parallel with it.

A tube tester will only be the starting gate. The question is how many potholes lie between there and the finish line.

73

And maybe consider a "glitch" resistor in the HV line once it's working again.

73

We call this the "blown fuse" syndrome.

The 8950 tube seems to be susceptible to this sort of failure.

This one was.



Learned to eyeball that thin link from pin 3 to the cathode at the center of the tube before even bothering to put it into the tube tester.

73

My experience points to excessive RF circulating currents.

Maybe. Seems like that 'fuse' link should handle the tube's DC current. Running the amplifier peaked incorrectly is what I blamed it on most times we saw this.

Big hazard there is with amplifiers that parallel four or six tubes. The Plate Tune control is in fact a "fine" tune. The actual circuit capacitance is determined by the equivalent capacitor to ground the plate cap of each tube added together. When new tubes are installed, that capacitance may be different from the previous set. Sometimes it's different by more than the capacitance value of the Plate Tune control. This throws the true resonant peak adjustment one whole turn of the knob beyond it's tuning range. When this happens, the coil for that circuit gets adjusted to bring the front-panel control back into range so it can be set to a true resonant peak. Until that's done, tube stresses will be unnecessarily high, and power reduced both.

My favorite example is a customer from almost 40 years ago. Ordered new tubes for his Varmint XL1000. Had six 6LF6 tubes in the final stage. He didn't want to wait until our shop was open next, and put them in to try out. Said he got half the power the older tubes gave. Kept tuning, and the power dropped, tuned some more and it kept dropping. When it got down to 100 Watts he turned it off and brought to us. He had been popping that 'fuse' cathode link in one tube, then the next. Only one final was intact when we saw it. I did show him how to tell when a pi-network coil has to be adjusted to protect the tubes. He wasn't happy about having to buy another set of six tubes, but couldn't blame anyone else.

73