I have a vintage acoustic 370 amp that uses a 3900uF electrolytic cap to block transistor DC power from the speaker. I've wondered about the case that the cap fails short circuit and I'd like to protect the speakers.
Could I put a fuse in line with the amp feeding the speaker to protect from this? I know I could but more a question of practicality. It's woofers this thing's feeding. I'd just hate to see full B+ on the speakers in the event of a failure.
Could I put a fuse in line with the amp feeding the speaker to protect from this? I know I could but more a question of practicality. It's woofers this thing's feeding. I'd just hate to see full B+ on the speakers in the event of a failure.
That output cap is likely specified to handle full B+ without any issues. You should be able to read its spec on the wrapping.
No need for fuse.
No need for fuse.
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I do not believe the capacitor could easily fail in a short-circuit state. The electrolyte could slowly cook (high DC conditions) and evaporate due to high temperature, the capacitance would drop and eventually the capacitor would become an open circuit and do nothing. Sometimes the cap is of questionable quality and the electrolyte simply ages with time. In your case, the DC would still be blocked, but the AC (sound) would become very thin (no bass frequencies)... and theoretically at the very end, you wouldn't hear anything any more from the speakers.
Capacitor issue aside, if you would like to protect your speakers from accidental overload then consult this chart:
https://barryrudolph.com/greg/fuse.html
https://barryrudolph.com/greg/fuse.html
Sadly fuses do not protect speakers from DC.
Any value you pick large enough not to nuisance blow all the time, will also let ample DC to cook any coil.
If you are worried, add a real relay based DC protector, lots of schematics around, even in this very Forum.
Any value you pick large enough not to nuisance blow all the time, will also let ample DC to cook any coil.
If you are worried, add a real relay based DC protector, lots of schematics around, even in this very Forum.
Must install in correct orientation, otherwise it will not work.
https://www.ebay.com/itm/371946597182
https://www.ebay.com/itm/371946597182
I own several of these speaker cap amps, some in organs I service, and in 40 years none of them ever shorted the output cap. Power down to 1/50th of rating due to high ESR in the rail cap, yes. Too many highs or not enough highs due to high ESR interstage coupler or bypass caps, yes. Random key sustain times, yes. Inop "percussion" (hammond attack) yes. None shorted, of the >400 e-caps I've replaced.
If the DC block on output cap worries you, change it when life exceeds 5 times the service life rating. In the 1960's not even sprague rated their caps for service life rating. I have an atomlytic datasheet from those days. OEM sixties era electrolytic caps lasted about 8000-10000 hours in my ST70+PAS2. 4-5 years at about 2000 hours use a year. So if cap is not modern, replace with an industrial cap rated >3000 hours service life, from panasonic, nichicon, rubicon, vishay, kemet. For high value screw terminal caps that cost $$$ I install snap-on caps glued to a insulator board, with insulator board screwed to the chassis with stand-offs.
Instead of some snake oil fuse. 60's experience proved the output transistor will always blow to protect the speaker fuse. Why from the 70's on nobody installed speaker fuses. If you don't believe it cut the top off a TO3 and look at the gage of the bond wires.
Good to hear. I was thinking my speakers would be safer to a failure behind a cap vs running off of the center of NPN and PNP output transistors, but I'd read somewhere that it may be a concern with the cap failing shorted. I probably should just not worry about it. There are enough things to worry about.
I've not seen the TO3 internals but I had heard of transistors referred to as 3 legged fuses in the past. So that makes perfect sense. I did put a fuse on the B+ feeding the OT. Not that it'll necessarily stop damage when they do fail, but makes me feel a little better. Maybe a false sense of feeling better... lol.
I've not seen the TO3 internals but I had heard of transistors referred to as 3 legged fuses in the past. So that makes perfect sense. I did put a fuse on the B+ feeding the OT. Not that it'll necessarily stop damage when they do fail, but makes me feel a little better. Maybe a false sense of feeling better... lol.