Hello,
I've been slowly restoring an old console PP tube amplifier. I have no idea what make/model it is. Input tubes are 12ax7's, the output tubes are EL84's, rectifier is 5U4.
I replaced all the paper caps with orange drops and I got a major improvement in sound. Now I'm looking at the electrolytics and wondering if I should just leave them alone. There's two big metal can caps, each with multiple caps in them. One's a Sprague (40 mfd 450v, 60 mfd 450v) and the other is Sangamd (two 40 MFD 350v and two 50mfd 25v).
I can feel the Sangamd one vibrate when I put my finger on it, does this mean it's going bad or is that normal?
I've been slowly restoring an old console PP tube amplifier. I have no idea what make/model it is. Input tubes are 12ax7's, the output tubes are EL84's, rectifier is 5U4.
I replaced all the paper caps with orange drops and I got a major improvement in sound. Now I'm looking at the electrolytics and wondering if I should just leave them alone. There's two big metal can caps, each with multiple caps in them. One's a Sprague (40 mfd 450v, 60 mfd 450v) and the other is Sangamd (two 40 MFD 350v and two 50mfd 25v).
I can feel the Sangamd one vibrate when I put my finger on it, does this mean it's going bad or is that normal?
I can feel just the slightest vibration on the transformer, but that cap vibrates like crazy. I can't feel any vibration on the Sprague cap next to it and only a little vibration on the chassis, so I'm pretty convinced that specific capacitor is the source.
Both electrolytic caps are visible. If I do end up replacing one or both, I'd like to hide the replacements in the chassis and leave the old ones there for looks.
I do have an oscilloscope, I'll hook it up when I have a chance and see if anything looks unusual.
Both electrolytic caps are visible. If I do end up replacing one or both, I'd like to hide the replacements in the chassis and leave the old ones there for looks.
I do have an oscilloscope, I'll hook it up when I have a chance and see if anything looks unusual.
You should replace those electrolytics fairly immediately, they are if any equally or even more unreliable than the coupling caps you have already replaced.
The vibration hopefully is from the power transformer, if not it is a pretty good indication of internal stress in the capacitor and is a clear sign that it should be replaced. Is it getting hot?
Suitable replacements can usually be found at antique electronic supply. Depending on cost you might want to consider JJ types instead if you can make them fit physically speaking.
The vibration hopefully is from the power transformer, if not it is a pretty good indication of internal stress in the capacitor and is a clear sign that it should be replaced. Is it getting hot?
Suitable replacements can usually be found at antique electronic supply. Depending on cost you might want to consider JJ types instead if you can make them fit physically speaking.
If the caps were inside I was going to suggest changing them anyway.
If outside I was going to suggest opening them up and fitting donor caps inside the cans.
However, you seem to have another solution which is equally viable.
If the amp. is working there is probably not a lot wrong with the caps. Electrolytics do dry out in time and fail, however, this usually causes them to go short circuit and blow a transformer or a few diodes. If they dry out and go open circuit they usually cause the amp. to hum like crazy unless another cap. is keeping the ripple down in a CLC or CRC circuit.
I would certainly replace them anyway.
If outside I was going to suggest opening them up and fitting donor caps inside the cans.
However, you seem to have another solution which is equally viable.
If the amp. is working there is probably not a lot wrong with the caps. Electrolytics do dry out in time and fail, however, this usually causes them to go short circuit and blow a transformer or a few diodes. If they dry out and go open circuit they usually cause the amp. to hum like crazy unless another cap. is keeping the ripple down in a CLC or CRC circuit.
I would certainly replace them anyway.
Thanks for the advice. I guess I'll go ahead and replace them just to be safe. I also need to add a fuse and replace the power cable. And hopefully I'll figure out how to get rid of that "old electronics stink" it emits once it heats up.
How critical are the values? For some reason a 40uf cap is $20 but a 47uf one is only $2.
How critical are the values? For some reason a 40uf cap is $20 but a 47uf one is only $2.
Depends on whether you have a tube rectifier or not - if not I'd go for the larger cap. Replacement multi-section caps are readily available and IMLE the JJs work pretty well and aren't very expensive.
The stink will go away with use, perhaps a good tube & chassis cleaning is in order as well. None of my tube electronics emit any smells unless something is amiss. (In the cases of restored stuff like my G36 MKIII, and TD-124/II they were stinky for a while, but this waned with use.)
Having the first cap after the rectifier hum or vibrate may also be due to a faulty rectifier, if the device uses an early semiconductor diode bridge. You want to check that one out, and possibly replace it if you intend to use the amp a lot. Early bridge SS rectifiers will frequently have become very lossy and unreliable with age, and will heat up excessively during use. This might not cause the amp to stop playing, so having sound is not an indication that everything is OK. This problem just lowers the B+ to some degree, at least up until the bridge shorts out and kills the mains transformer.
I restore vintage European tube radios as part of my tube habit, and generally I don't switch them on for even a brief test, unless I have replaced all paper and electrolytic capacitors, and given any SS rectifier a very solid looking over. Tube rectifiers are usually fine, of course. The large PSU electrolytics in cans I usually disconnect and leave in place for looks, and simply fit new ones out of sight below the chassis. Might be an alternative if you cannot find cans, which fits your chassis.
Just my $0.02 worth.
Frank.
I restore vintage European tube radios as part of my tube habit, and generally I don't switch them on for even a brief test, unless I have replaced all paper and electrolytic capacitors, and given any SS rectifier a very solid looking over. Tube rectifiers are usually fine, of course. The large PSU electrolytics in cans I usually disconnect and leave in place for looks, and simply fit new ones out of sight below the chassis. Might be an alternative if you cannot find cans, which fits your chassis.
Just my $0.02 worth.
Frank.
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If you probe the first filter cap on the 5U4GB, then the ripple will indicate if the rectifier is ok and providing balanced currents in to the cap.
You can retain the above chassis cans for aesthetics - eg. solder a small tag strip to a can terminal and reconnect wires and the new electro (which will be much smaller) to the tag strip. For downstream caps, you could consider just adding a parallel new electrolytic (again it would be much smaller and could solder directly to original can terminals) - this will bring the cap impedance back to low level to help hum and interstage coupling, and will bypass any ripple current in to the new cap.
Ciao, Tim
You can retain the above chassis cans for aesthetics - eg. solder a small tag strip to a can terminal and reconnect wires and the new electro (which will be much smaller) to the tag strip. For downstream caps, you could consider just adding a parallel new electrolytic (again it would be much smaller and could solder directly to original can terminals) - this will bring the cap impedance back to low level to help hum and interstage coupling, and will bypass any ripple current in to the new cap.
Ciao, Tim
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