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Old 8th May 2008, 06:09 PM   #1
pageboy is offline pageboy  United States
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Default Fisher 400 questions

Noob here, with what may be some basic questions. But I have to start somewhere. I have a Fisher 400 that has a habit of burning out one of the power tubes. It's always the same one. I had a different but identical unit several years ago that I never should have let get away from me that would always do the same thing. Different socket, but the same tale. So this is maybe a common problem with these?

I did some voltage checks last night, and found that the negative bias voltage is right on. What else I found is that the HT on both the plate and the upper grid of the pentode (correct term is slipping my mind right now) is running about 35 volts too high. Heater voltage is fine. First, since all the voltages are the same on all the power tubes, why is it always the same socket that burns out a tube, no matter what tube is in it? I can even see that one glowing brighter than the rest of them. But why?

Second and more importantly, how do I get the voltage down where it belongs? Will bad filter caps let voltage drift too high? From the looks of things in here, they're all original. The only thing I can come up with is to add another stage of dropping resistor and filter cap in front of the whole lineup.

How do I identify if these are selenium rectifiers or not? I've read on here in the past about the reasons to replace them.
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Old 8th May 2008, 06:19 PM   #2
gni is offline gni  United States
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It might burn out since it is the first tube to receive energy
when powered up. . .it is absorbing the power on surge and
taking itself with it. . .Take a good look at the schematic and
compare it to the real thing. . .there have been similar posts
that describe the same problem. . .
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Old 8th May 2008, 07:20 PM   #3
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Sounds like one of the 0.047 uF coupling caps between the phase inverter and the power tube grids is leaky, which in turn drives that grid of the output tube positive, then goes into thermal runaway. To check this, disconnect that cap (the end connected to the output grid) and measure voltage of the disconnected lead to ground. If there is any measurable DC voltage there, that capacitor is bad. Make sure you keep the bias supply wire connected to the grid.
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Old 8th May 2008, 11:56 PM   #4
gni is offline gni  United States
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Ah! a tube guy. . . probably better answer than mine.
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Old 9th May 2008, 12:14 AM   #5
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John provided a good hint. Replace all 4 coupling caps. with 100 nF./630 WVDC Panasonic ECQ-P(U)s.

The concern about a Selenium rectifier is well founded. Selenium rectifiers are ticking TOXIC time bombs that should be replaced, as a matter of routine, whenever they are encountered. 4X UF4007s will get the job done. The forward drop in Silicon is smaller than the drop in Selenium. The resistance in the negative PSU must be tweaked to deal with that fact.

Jim McShane is a good source for the parts I mentioned and other stuff of use in overhauling a vintage tube unit.
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Old 9th May 2008, 03:15 PM   #6
pageboy is offline pageboy  United States
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Thanks for the replies. I'll check out those coupling caps. Sounds like a good possibility.

I'm aware of the reasons to replace the selenium rectifiers. What I don't know is how to tell what kind these are and if they've ever been replaced or not.

Should I not be concerned about the HT running 35 volts high? I thought that sounded excessive.

I'm sorry, I don't know what you mean by PSU. Noob here, still picking up all the acronyms.
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Old 9th May 2008, 03:53 PM   #7
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PSU: Power Supply Unit.

later 400s use silicon rectifiers for bias, which would be fine to leave alone. But the filter caps in the bias supply are questionable in any case... High HT voltage could be due to tubes that have lost some emission and drawing less current. And could be partly due to today's higher line voltage. But +/- 10% is considered normal in tube circuits...

Bias voltage should be set for proper output tube current... voltage alone tells you very little. This can be checked if you add 10 Ohm cathode resistors to each tube.
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Old 9th May 2008, 03:55 PM   #8
kevinkr is offline kevinkr  United States
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PSU - power supply unit.

Now you stated that the tube is "burning out" - is it that literal, i.e. one day the filament no longer lights - or is it that the tube goes into meltdown - plate glowing red, and then blowing the mains fuse?

I interpreted it as a burnt out filament, and if that is the case the fix is a bit different. I have had this problem multiple times with 6L6 family tubes in one of my power supplies, and in this case it was actually filamentary inrush, and this is the first tube right after the power transformer? If this is true there are two things you can try, the first is to lift just one of the transformer secondary connections and extend it with some modest gauge (say 24ga) wire and connect it to the furthest away of the 4 power tubes - this tends to equalize the voltage drop across all the power tube filaments (make sure you identify the same pin at the other end of the row of power tubes as you do not want to short the filament transformer inadvertently) - the other thing you can do is place an NTC thermistor in series with the filament, one having a cold resistance of 1 ohm or so and a hot resistance of 0.2 ohms will reduce the inrush to safe levels.

Depends on what kind of selenium rectifier is fitted, and not all Fishers depending on vintage have selenium filament/bias rectifiers. (I've seen some with silicon in later models.) Some have stacks of fins, and come in variety of colors, gray being about the most common, others made by Siemens are just little silver boxes with 3 or 4 tabs for connections coming out of them - these are generally held against the chassis by clips or in some cases a pair of screws.

Incidentally the problem with seleniums is that when they malfunction they burn, and the gases produced are very toxic. The usual failure mechanism is that the internal resistance increases to the point where the IR (current) heating causes them to burn. Note that they also should NOT go in your trash, if they end up in a landfill as they decompose the selenium can leach into the surrounding soil and possibly contaminate ground water. Recycle with other electronic items.
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Old 9th May 2008, 04:50 PM   #9
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pageboy,

Have you measured the AC filament (power tube) voltage?
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Old 9th May 2008, 06:50 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by TubeHead Johnny
pageboy,

Have you measured the AC filament (power tube) voltage?

I should have suggested this, but unfortunately it does not tell the entire story, it depends to a significant degree on the winding source impedance and the amount of current that flows during the initial cold filament inrush. You can have a winding that is very close to the ideal 6.3V at all times and still experience this problem. This is one reason why it is not a bad idea to size the winding to the load so that at time 0 the current is limited by an appropriate level of internal resistance dictated by the voltage regulation requirements at the stated load.
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