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Old 21st July 2002, 12:54 AM   #1
mig-ru is offline mig-ru  Russian Federation
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Default problems after rebuild

alrite, i have rebuilt my soundcraftsmen 220 stereo amp. ive got a problem now. when i turn it on the 6X5 rectifier starts glowing bright yellow, not the normal orange and i hear a loud 60hz hum out of the speakers, the cd player that i hooked up could be barely heard with distortion. no to setup what's going on: prior to the rebuild the 6X5 rectifier sparked a lot, traced the problem down to the failed power electrolytic(40/30/30MFD multisection, only the 40MFD hooked up to the 6X5). ive been told on rec.audio.tubes that my problem is most likely due to a damaged 6X5. i was also suggested to unplug all tubes and plug in the amp to see if my power transformer will heat up and or have smoke come out, i did this and the transformer stayed lukewarm so i sense no problem there. any ideas/suggestions appreciated. thx in advance.
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Old 21st July 2002, 12:57 AM   #2
mig-ru is offline mig-ru  Russian Federation
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also, forgot to mention, aftere replacing the multisection cap with 3 new electrolytics (40, 20, and 20MFD), the rectifier now doesnt spark, but does glow the bright yellow.
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Old 21st July 2002, 01:22 AM   #3
Duo is offline Duo  Canada
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Is it the rectifier filament that glows too brightly or is it the plates themselves glowing?

BTW, why did you replace the 40, 30, 30 cap with 40, 20 and 20?
That would introduce some hum into the amp most likely...
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Old 21st July 2002, 02:05 AM   #4
mig-ru is offline mig-ru  Russian Federation
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the filament is glowing. on the caps, i was told that 20MFD will be sufficient, and wont change things drastically. the hum is very loud, so i dont think its that.
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Old 21st July 2002, 05:26 AM   #5
Duo is offline Duo  Canada
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Okay, it sounds like the rectifier tube is maybe shorted out. If you have an ohm meter, check for shorts between the plate and filament pins on the tube... Also, check to see if the rectifier filament supply winding on the trannie is shorted to the plate supply winding. I suppose you can get away with smaller caps, but you'd be much better off and safer with something even bigger than the original 30, 30 setup. Anyway, I hope you get your amp working properly, but it sounds to me like the rectifier is definately shorted so try to look into that and try a replacement if you can...

EDIT: The chances of a transformer getting fried by a sparking rectifier are actually very high in this case. A friend of mine collects antique juke boxes. One of the amps in a jukebox had fried caps and the rectifier shorted out and melted the transformer. To tell if the transformer is fried, it's usually easiest to check for low resistance on the plate supply wires and between the heater and plate wires like I described earlier...
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Old 21st July 2002, 05:58 AM   #6
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Dmitri,
Your 6X5 is connected directly to the 40 mF section before a line goes off to power your output transformers and power tubes, but it is still connected to the rest of the sections of the filter cap but usually with large resisters that reduce the voltage before sending it off to your driver tubes or the power grid of the power tubes. The reason that big multi cap is called a filter cap is that it finishes smoothing out the 60 cycle waves of the AC current into the flat DC current. Those 60 cycle waves cause 60 cycle hum and if your filter caps are not large enough you will not get that hum out. Keep the multy cap you have but get some smaller electrolitics you can solder in there under the chassis to get your capasatance back up to where it needs to be. Those old amps tended to use the minimum capasatance possible because that was where a lot of expense went. To go 40/40/40 would be much better than 40/20/20. Fortunatly caps are cheap these days, just make sure you get high voltage ones.
As far as the glow of your 6X5 I am not going to make guesses. But a new 6X5 might be a good idea. Smoking amps are no fun.
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Old 21st July 2002, 06:02 AM   #7
Colt45 is offline Colt45  Serbia
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There is a possibility you hooked the 6X5 to 12v istead of 6v.

Provided, that your PT even has a 12v winding.


I guess shorts are possible too. do you have a continuity checker/ohmmeter ?

check all the pins. the only pins that should have continuity are pins 2 and 7 (the filament). the plates and cathode should not be touching each other, nor the filament.

If you dont have a continuity checker, you can build a really simpler one... hook up a battery to a small lamp. leave one side of the connection open. the wire from the battery and the other wire from the light form your test lead. if there is continuity, the bulb will light..
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Old 21st July 2002, 09:01 PM   #8
mig-ru is offline mig-ru  Russian Federation
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went through the power transformer with a multimeter, the heater is 6v, HV both are 268v. also last night i was seeing what would happen with just the 6X5 in the amplifier, and after a little time a wire melted inside the tube and the tube went from the irregular bright yellow glow back to the normal orange glow. it seems that the wire lead back to the cathode pin. the cathode pin is connected to the 40MFD electrolytic and to the other tubes (two 12AX7 and two EL84) through a 2.7kilohm resistor. what could be the problem there? thx for the help.
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Old 21st July 2002, 10:37 PM   #9
SHiFTY is offline SHiFTY  New Zealand
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Dude- buy a new rectifier. That one sounds like it has "issues".
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Old 22nd July 2002, 04:38 AM   #10
Duo is offline Duo  Canada
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colt45: The heater and the cathode are the same thing in a 6X5...
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