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can someone please explain how this works? Why don't these clip the negative portion of the output ac signal? Also, is the PIV 3000, and how does this protect the amp?
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The only way the diode conducts is when the anode is more positive than the cathode. That means that with the anode grounded, the cathode would have to go to a negative voltage to be less positive than the anode at ground. The plates of the power tubes never go negative, there is nothing in the tubes or the circuits that would drag them negative. Remember that the plates start at 450v or whatever. A signal with 300v peaks would still leave at least 150v on the plate, positive to ground.
So those diodes are never forward biased unless a really big spike occurs. That is why they don't ever clip the signal.
The diode has a reverse voltage rating of 1000 volts. exceed that and it breaks down and conducts in reverse. Get a peak over 1000 volts, and the diode will conduct it to ground. Probably destroys the diode and shorts it out. And that blows the fuse in the amp. But isn't a blown fuse and a shorted 3 cent diode better than a fried output transformer?Education is what you're left with after you have forgotten what you have learned. -
Thanks Enzo,
Where do these "spikes" come from? The B+, the OT, or the speaker? Also, if those diodes are testing bad what does that mean? Finally, usually 3 1n4007 are used reverse-biased... that's 3000PIV right?Comment
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Yep.
WHen you operate a tube amp without a speaker, tremendous voltage spikes can reflect back into the transformer. LArge enough to destroy the transformer by arcing over inside.
How are they testing bad? They are diodes. If they are shorted, then they are shorted. You have no way to test them for reverse breakdown. If you did, it would destroy them anyway. With powr off of course, you can test them for forward junction drop as with any diode.
Power off, and see if they are shorted.Education is what you're left with after you have forgotten what you have learned.Comment
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Ok yes one of 2 tubes set of diodes is testing less than half what the v-drop should be. Can you tell me what this means? Did these protection diodes short because something else is wrong? Should I replace that tube? Is the OT compromised?Comment
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Just as an aside, I have seen a "soft" failure of a diode, in a power supply. The line fuse would blow, so it was "bad," but it checked "good" using the diode check function on a high-quality dvm. But this is not the same as hitting it with 400V. The only clue was, out of the circuit, the bad one had a low (3 meg) reverse-bias resistance. Not exactly shorted, but failed.
MPMComment
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At each position ther are three in series, right. And one side measures lower than the other? Did you check each diode individially?
How much do 6 of these diodes cost? Not much. Not even at Radio Shack inflated prices. There appears to be some doubt about these diodes in your amp. REPLACE THEM. WHy screw around trying to decide if we should try to save a 3 cent part? Put new ones in there and know your protection is OK.Education is what you're left with after you have forgotten what you have learned.Comment
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Enzo,
No there's no issue w/ replacing them, in fact I did and everything is fine. Yes one set of the 3 diodes forward voltage drop was a low .7A while the other set was around 1.6A so there must've been a spike. The paint on the power tube in that socket looks darkened/burnt - from the tube redplating I suppose.
What I'm trying to get at here is what exactly causes these diodes to go bad? I'm simply thinking "ok these diodes protect the amp from large power spikes, but what causes these spikes and if a spike has occured is there something wrong w/ the amp?" Is it a tube? Could the OT be damaged? etc...??Comment
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AS I mentioned, the diodes usually give their life saving the OT. Such spikes almost always come from a loss of load on the OT secondary. This can be from making a loud signal with no speaker, or from an intermittant speaker cord or jack. It can happen in an instant. Tube failures offer no danger to the diodes. After all, if a tube shorts, all that current goes through the tube, not the diodes. However a reverse spike could damage a tube.
If three diodes in series measure .7v, that soumds like one or two of them were shorted. That is why I mentioned checking them individually - just for science. If one is bad, I replace all three. Who knows what stress they faced.
If the amp works OK< then I doubt anything is damaged. it could also have just been a random component failure - it happens.Education is what you're left with after you have forgotten what you have learned.Comment
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It's the usually bit I'd be worried about. There's absolutely no guarantee of the diodes working at all. A better solution is to put a permanently wired resistor across the OT secondaries. Something like 500R 5W. That ensures the voltages spikes never appear.Originally posted by Enzo View PostComment
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Really? How does that work if you don't mind me asking? Also, how would that NOT affect the load impedance?Comment
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Think of it more as protection for the idiot who doesn't understand an unloaded tube amp is a problem. With 250 to 500 ohms on the output jack, there's always something there even with a intermittant open cable/jack. The affect of paralleling a 4 to 16 ohm speaker with a 500 ohm resistor is variable, but on the order of a quarter ohm.
Iirc, Rick Ericsson recommended using the UF4007's for this, since they're much faster than 1N4007s.Comment
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ok cool, yeah makes sense. the 500ohm paralleled w/ the 3.7ohm resistance of a 4ohm speaker is barely changed... 1/4ohm or so as you say. thanks everyone for the input.Comment
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To be sure the beast is dead and to remember the old forum (today Im feeling nostalgic), there is a thread which as quotes and requotes about an older thread:
http://archive.ampage.org/threads/6/...ic_load-1.htmlComment
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Boy is this ever relevant for me right now....i just lost my 18's OT ! I don't know if it was a tube or what, but it began humming and one side glowed brighter (either tube was brighter when put in that side) and the OT read open on one side. So i'm interested in doing this. The classic 30 appears to have this setup. If i just follow that and place the diodes at the plates as in the c30 schematic, is this the correct way?
By the way, is it likely a tube that blew my OT?Comment
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