Super scout info needed pls

[Cable Guy]

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.

 

[Shockwave]

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.

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.

 

[nomadradio]

arcing inside the tube i suppose.

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

 

[Shockwave]

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

73

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.

 

[Cable Guy]

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?

Thanks guys, I'm going to get him on the phone, try to get more info from him and some better details. I looked over the driver and final chokes and they looked fine, I will discharge any caps and check continuity. The 4 watts in/4 watts out issue seemed to me like the relay isn't switching, but it is. The red hot plates were his description, as I have only observed filament heating. I will be back later with more details. Thanks for helping me, I know just enough about tube amps to not horribly un-alive myself, while successfully repairing and rebuilding a dozen or so units.

 

[nomadradio]

that cannot be the only failure

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

 

[Cable Guy]

I just got a little more info from him, as they never tell all at first. He said the spark could be from a final tube clip that was loose and popped off mid key, and he also said the fuse popped. He put the clip back on the tube and put foil to the fuse, to see if it worked again...Macgyver be darned, smh. Foiling the fuse gave it access to whatever current it needed to really get stupid. I will check some things and report back later. Thanks again guys.

 

[Cable Guy]

That sounds more like we may have lost plate voltage on the finals and drivers.

That's what I am thinking. I need to check the transformer, rectifying diodes and caps.

 

[nomadradio]

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

73

 

[Cable Guy]

I don't know for sure, but I think these tubes are goners. There is what appears to be a burnt link in every final tube. The trans is good, the cap multiplier checked out and the diodes are rectifying, and then there is this.

 

[Cable Guy]

Per the datasheet, that link connects the cathode to pin 3. Every one of the finals have a blown cathode to pin3 link, so this is a lost cause. Thank you Shockwave and Nomadradio for your insight. I can't say for sure what caused this condition, but I feel the foiled fuse, and a short somewhere, could have been the nail in the coffin.

 

[nomadradio]

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

 

[Cable Guy]

Can you describe some failures that would take the cathode link out?

 

[nomadradio]

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

 

[Cable Guy]

That's a good point, nomad. When you explain that, I can kinda visualize it. When I consider how they all failed that way and popped the line fuse in an instant, I'm kinda thinking something is shorted. I really wished I had a schemo, but since it isn't as populated as other, more robust amps, it should be easier to see what is going on with a fair bit of understanding.

I informed the owner about the blown tubes and he wants to replace them. He knows a guy that has them for $20 each. That sounds like a good price to me if they are in decent shape. Still, I don't want to see the new tubes blow up because something was overlooked because I really think there's something shorted.