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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
TubeHead Johnny said:
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.
Thermal runaway, as someone else called it, is a better description of what is going on. The plate glows bright red, a loud hum comes from one speaker, and the mains fuse lets go.
The AC filament voltage is fine. Moving one filament lead down the ladder is a good idea anyway. I understand what you're saying there. I'll be doing that one. Yes, I guess even 35 volts high on the HT is within 10%, barely, so I'll let that one go.
I can install some 10 ohm cathode resistors, but how do I know what the current should be? My schematic only gives voltages.
The rectifier is a silver box with four tabs that says Siemens on the side of it, held against the side of the chassis with a pair of screws. I'll be getting that out of there. What about the pair of diodes on the HT? Should those be replaced also?
I haven't had a chance to check out the coupling caps yet. It sounds like I should replace those just out of principle. I don't understand something though. If some DC is leaking through from the previous stage, why does the bias voltage on the power tubes read perfectly normal and all four exactly the same? Shouldn't some DC leaking through affect the bias voltage? Not enough negative bias voltage is what I expected to see when I first opened this chassis and did some voltage checks. But bias voltage measures normal.
The AC filament voltage is fine. Moving one filament lead down the ladder is a good idea anyway. I understand what you're saying there. I'll be doing that one. Yes, I guess even 35 volts high on the HT is within 10%, barely, so I'll let that one go.
I can install some 10 ohm cathode resistors, but how do I know what the current should be? My schematic only gives voltages.
The rectifier is a silver box with four tabs that says Siemens on the side of it, held against the side of the chassis with a pair of screws. I'll be getting that out of there. What about the pair of diodes on the HT? Should those be replaced also?
I haven't had a chance to check out the coupling caps yet. It sounds like I should replace those just out of principle. I don't understand something though. If some DC is leaking through from the previous stage, why does the bias voltage on the power tubes read perfectly normal and all four exactly the same? Shouldn't some DC leaking through affect the bias voltage? Not enough negative bias voltage is what I expected to see when I first opened this chassis and did some voltage checks. But bias voltage measures normal.
I have seen instances where leakage in old coupling caps increased as the chassis heat soaked over time (an hour or so) in cases where the cap did not appear immediately to be bad.
I would replace the coupling caps to the output tubes to start.
Check all of the grid circuit components associated with that tube and make sure they have not drifted in value.
You have a Siemens selenium rectifier. Replace with modern silicon bridge, add a small resistance in series with the output of the rectifier to trim to something close to the recommended values, probably trim to the high side to reflect your line voltage relative to the design value. (Probable 117V or so)
I believe the high voltage rectifiers in your receiver are already silicon, at the voltage required selenium would be very ungainly.
I would replace the coupling caps to the output tubes to start.
Check all of the grid circuit components associated with that tube and make sure they have not drifted in value.
You have a Siemens selenium rectifier. Replace with modern silicon bridge, add a small resistance in series with the output of the rectifier to trim to something close to the recommended values, probably trim to the high side to reflect your line voltage relative to the design value. (Probable 117V or so)
I believe the high voltage rectifiers in your receiver are already silicon, at the voltage required selenium would be very ungainly.
Pageboy,
Are you using vintage 7868s or current production EH tubes in your 400? Are your 7868s a closely matched quad?
In any event, replace all 4 coupling caps. with 100 nF./630 WVDC ECQ-P(U)s.
Yes, the OEM B+ diodes are Silicon. However, they are noisy. Replace the B+ diodes with a pair of 1000 PIV/3 A. low noise UF5408s.
Are you using vintage 7868s or current production EH tubes in your 400? Are your 7868s a closely matched quad?
In any event, replace all 4 coupling caps. with 100 nF./630 WVDC ECQ-P(U)s.
Yes, the OEM B+ diodes are Silicon. However, they are noisy. Replace the B+ diodes with a pair of 1000 PIV/3 A. low noise UF5408s.
I have old power tubes. I'm running what came with the unit, plus I found a few extras in a tube caddy buried in a corner of an old organ repair shop. I don't know whether they've been used or not, but they seem to be in reasonable shape. I'm sure that what's in the amp now is not even close to being matched, especially since they all show evidence of running too hot - not just the one that burns up. I've been out of the loop on tubes for a few years, and I didn't realize that EH was making new ones.
How much bias current should I be looking for on the power tubes if I install some 10 ohm cathode resistors?
Our weather here is calling for rain tomorrow. Maybe I'll get some time to check into this thing again. I appreciate all the replies. I've learned a good bit right here.
How much bias current should I be looking for on the power tubes if I install some 10 ohm cathode resistors?
Our weather here is calling for rain tomorrow. Maybe I'll get some time to check into this thing again. I appreciate all the replies. I've learned a good bit right here.
You have mail. 
Fisher used truly fixed bias. Without modification, adding 10 Ohm cathode resistors buys little. That fixed bias mandates that the 4 7868 O/P tubes be a CLOSELY matched quad. Good matching of old production tubes is, at best, problematic.
If you make the decision to switch to current production ElectroHarmonix 7868s, there are 2 pitfalls you must be aware of.
Old production tubes have Novar bases. Current production tubes have Magnoval bases. The pin circles are identical. However, the Magnoval pin diameter is slightly larger. The sockets get stretched and old production tubes can no longer be employed.
The 2nd issue is the 330 KOhm grid leak resistor value Fisher employed. That is outside the published limit of 300 KOhms for the 7868. The tubes of yore tolerated that liberty. Current production stuff has been known to run away, when overly large grid leak resistors are in place. The "best" solution for the problem is to change from 12AX7s to 12DW7s in the driver/splitter position. The 12DW7 is asymmetrical, with 1 section like a 'X7 and the 2nd section like a 'U7. The 'U7 section makes a much better "concertina" phase splitter than a 'X7 section. Some parts values change, but topology doesn't. Switching to the 12DW7 in the splitter driver positions allows the 7868 grid leak resistors to be reduced to an in spec. 270 KOhms.
Fisher used truly fixed bias.
Without modification, adding 10 Ohm cathode resistors buys little. That fixed bias mandates that the 4 7868 O/P tubes be a CLOSELY matched quad. Good matching of old production tubes is, at best, problematic.If you make the decision to switch to current production ElectroHarmonix 7868s, there are 2 pitfalls you must be aware of.
Old production tubes have Novar bases. Current production tubes have Magnoval bases. The pin circles are identical. However, the Magnoval pin diameter is slightly larger. The sockets get stretched and old production tubes can no longer be employed.
The 2nd issue is the 330 KOhm grid leak resistor value Fisher employed. That is outside the published limit of 300 KOhms for the 7868. The tubes of yore tolerated that liberty. Current production stuff has been known to run away, when overly large grid leak resistors are in place. The "best" solution for the problem is to change from 12AX7s to 12DW7s in the driver/splitter position. The 12DW7 is asymmetrical, with 1 section like a 'X7 and the 2nd section like a 'U7. The 'U7 section makes a much better "concertina" phase splitter than a 'X7 section. Some parts values change, but topology doesn't. Switching to the 12DW7 in the splitter driver positions allows the 7868 grid leak resistors to be reduced to an in spec. 270 KOhms.
The purpose of the 10 ohm resistors wasn't to change the bias operation. Someone mentioned a few posts up that I could use them to measure the bias current, as that was more important than the voltage. But I don't know what current level to look for them to be.
Nuisance that they can't make the repro tubes exactly the same as the originals. Thanks for the info on those.
Nuisance that they can't make the repro tubes exactly the same as the originals. Thanks for the info on those.
Being able to measure the "idle" current is of value only if it can be adjusted. Fisher's true fixed bias scheme doesn't provide any adjustments.
Current production ElectroHarmonix 7868s fully comport with all the published electrical spec's, but they don't allow for liberties being taken. I agree Novar bases would have been nice. Unfortunately, Novar bases are "unobtainium". OTOH, Magnoval bases are available, as they are used in other current production types, including the 6d22s damper diode. New Sensor (the folks behind the EH label) did both the guitar and HIFI people a huge favor in returning the 7868 to production. An awful lot of quality equipment was in danger of becoming inoperable. Thankfully, that is no longer the situation.
Hi,
I was thinking the 10 ohm (1/4 watt) resistor was to be added as a safety feature. If a tube goes south, then the resistor would pop like a fuse and save the OPT. At least that was the thought and it allowed me to leave the room with a bit of comfort while the amp was running.
Regards,
ck
I was thinking the 10 ohm (1/4 watt) resistor was to be added as a safety feature. If a tube goes south, then the resistor would pop like a fuse and save the OPT. At least that was the thought and it allowed me to leave the room with a bit of comfort while the amp was running.
Regards,
ck
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