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fisher 500c safety resistor keeps blowing

T

tim's cousin

Active Member
Hi All.

Couldn't adjust the bias on one OT. Opened unit up, turned on, saw puffs of smoke from V8 safety resistor (one end had come loose after I touched it) and R133 area. Replaced V8 safety resistor (10 ohm 1% metal), turned on, saw smoke from V9 safety resistor. Replaced R133 and 137 (R137 looked to have been burnt) and V9 safety, turned on, turned off when saw V9 safety was going to blow.

Replaced R133 and 137 both tested normal before installing, after installing they tested 20.2 and 17.4 respectively. Removed R133 and 137 had these same values prior to removal, after removal they tested normal (82 and 22K respectively).

Unit played fine before all this. Any suggestions?
 
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Power take off from R137 and then R133 goes to the Phono tubes. Might want to check the phono tubes for shorts. Is there an IBAM or IBBA onboard to adjust bias?
 
What output tubes are you running? and what is the bias set at? I'm suspecting a tube short, possibly H-K (Heater to Cathode). Pull the output tubes, (mark them for socket 1st) then have them tested for shorts and gas, along with Emissions and gm. Repair the problems in the V8/V9 area. Then with the output tubes out, bring it up on a variac with a DBT (100W), you know the drill.....................note voltages on plates, screens, heaters(on all tubes) check solder joints, components touching, etc. Voltages will be higher with the O.T. out, but all voltages should be proportionately higher. Put phono tubes in even if you don't use phono. If you don't use phono, put in a couple of weak testers. Those will help normalize voltages. Tighten up the output tube socket leaves, loose leaves could also be the problem.
 
What Larry said. Those resistors typically only fail when a tube dies in a unhappy way.

No phono tubes will push the supply more negative, less current flow. Not going to hurt the tubes but they may idle low enough to add some distortion.
 
Thanks guys.

Tubes are Westinghouse, biased at -31.

Can I just put in another OT (Westinghouse) to see if that solves the problem? I've never done most of what Larry wrote after "variac"...
 
Ok. in that case replace both of the tubes that caused problems. but bring it up on a variac and watch the bias on those new tubes and make sure they track with the other pair -31V is fine. See if you can find someone close to test those old tubes. And do everything BEFORE "variac" ;)
 
V9 tube was shot, so that explains the blown resistors.

Problem now is I get no bias reading for V8 at pin 5. R121 reads 240K (should be 330), I'm guessing that's the problem? The other surrounding resistors were fine.

Just thought I'd get confirmation before going forward.
 
R121. In or out of circuit. Test out of circuit. Then do the same with R126 (1K btwn R121 and pin 3 on V8). Swap V-8 and V-9 see if the problem follows the tube or the socket. Tighten the socket leaves for V-8.
 
Thanks Larry.

R121 tested 240 in circuit, R122-124 tested 330 in circuit. R126 (and 3 corresponding others) tested 1K in circuit. Go ahead and replace 121?
 
These are what, the grid to bias supply resistors? Being off value wont cause no current in the tube, but they do need to match.
Im just assuming you mean 330k, not 330 ohms for a grid resistor.

No current flow is more likely an open screen resistor or a dead tube. Hopefully its just the resistor, no screen volts = no current flow.
 
gadget73 said:
Im just assuming you mean 330k, not 330 ohms for a grid resistor.
Click to expand...

Sorry Dave, yes, 300K. I think those are the grid-to-bias resistors; they go to the 1K resistors which attach to the OT.

Maybe it's the tube, if the screen resistors are the 100 ohm ones soldered to pins 4 and 1, I don't have them. I'll check against another OT and post tomorrow.
 
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measure the 100 ohm resistors and make sure they actually measure somewhere near 100 ohms. That will tell you without having to swap tubes.
 
Changed the tube, everything works/measures fine now. Should I be worried that the V8 and V9 tubes went bad at almost the same time?
 
Probably not. But you're running 50+ yr old tubes and bets are they aren't matched (by any stretch of the imagination). it would be best to install an IBAM or IBBA to bias the tubes to a slightly lower level (if you are running as originally designed) and balance the loads on the tubes. The old "set the voltage on pin 6" bias method doesn't take into account that all the tubes are not electrically identical. Each is slightly different so that when at nominal voltages as in the 500c they all put out differing levels of load. As a result you can get a voltage hog, while the other starves, causing distortion, noisy crossover at the zero level of the sine wave, and more stress on the tubes. Balancing the tubes so they produce the same loads is better in the end, as they all contribute equally. The pin 6 voltages can differ as much as 2-3 volts with older tubes, but the output load is equalized.
 
Sorry for calling you Dave, Gadget!

Thanks Larry and Gadget. I have the board to adjust the bias and will set them a bit lower.

What is the advantage to installing screen resistors? I don't have them.
 
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Screen resistors will mitigate or eliminate arcing in tubes. Dave did an article some time ago (10+ years) noting that newer tubes were arcing at a subtantially greater rate than domestic tubes. The answer was a resistor of at least 100ohms. See the article here...........http://www.tronola.com/html/maximize_tube_life.html There is a post in the "Common parts for FISHERs" thread in the stickes that addresses this (post #40). Use the improved procedure at the bottom of the post. https://audiokarma.org/forums/index...-parts-needed-for-fishers.327561/post-6480170 This works well for domestic tubes as well as for Russian or god forbid Chinese tubes.
 
ah, thought you had them installed, but not in stock. Sorry for the misunderstanding.

but yeah, they're useful to have. one screen arc and it can destroy the output tube.
 
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