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Spike protection? Diodes?

6/16/2005 2:21 PM
Spike protection? Diodes?
Sometimes I read about some spike protection diodes in the output stage of an tube amplifier.  
Where are they placed?  
Does this "protection" will really protect something? And if so, what does it protect?  
And the tone, will be different with those diodes?  
6/16/2005 6:09 PM

They protect the transformer from arcing over and destrroying itself - mainly when it is operated with no speaker.  
Those diodes are out of the signal path and as far as I am concerned have no effect on tone.  
Typically they are placed in reverse bias from plate to ground of the output tubes.
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6/16/2005 6:12 PM

The diodes are a form of output transformer protection. They typically go from the output tube plates to ground. The cathode of the diode (the banded end) goes to the plate. Voltage spikes are developed when the amplifier's load becomes disconnected (bad cable, failed speaker, drunk lead singer) or just normal operation with lots of overdrive. When a voltage spike happends, one plate goes to a very high voltage (that tube is cutoff by a large negative voltage on the grid) and the other plate tries to go negative (the tube can't conduct in that direction). With a good scope you can see these spikes across the speaker and they can be 200V or more peak. On the primary side they will be thousands of volts big which will breakdown the transformer's insulation eventually.  
The bad thing is if the diode shorts out, it could fry you output transformer. Other methods to protect the output transformer are 1) a resistor of maybe 200 ohms across the speaker, 2) a series R-C (snubber) across the speaker (10 ohms 0.1uF) or the transformer primary, 3) an MOV across the primary or secondary.  
As for tone, the jury's still out. One guy I ask said it made an tiny audible difference but it wasn't objectionable.
6/17/2005 12:41 AM

When the diode shorts, it represents almost a dead short across B+. It usually blows the HV fuse almost instantly. It usually doesn't kill the tranny unless ther is no HV fuse.
6/19/2005 9:41 AM

I think I get the idea. I just don't know what those diodes do in order to protect it.  
What should be the ratings of those diodes? 1N4007 will do?  
After some research, I found some schematics that incorporate those diodes, but there are three diodes in series for each side of the primary, it is because the ratings of the diodes, or is because if one shorts, there will be more on that place preventing bad things?  
When using a single diode, the problem is that when a diode shorts a fuse will blow, and the amplifier will not work until the diode is replaced (unless there is a fuse in series with that diode, but if this fuse blows we lose the diode protection). So there should be a better option, like that "MOV", I don't know too much about that, what I (think that I) know about that is that when a voltage becomes larger that the one that is rated for, it starts conducting...  
But, for what voltages should it be rated (planning on the primary side of the OT), and the tone? Once it will act like a "short" when those spikes happen, it should induce some distortion, doesn't it?  
Thanks people!
6/19/2005 9:33 AM
Dai Hirokawa

I think it basically gives abnormally high voltage that shows up on the primary (from the secondary being run open) which can puncture insulation some place to go (ground). I think the rating of 1kVx3 in series = (3kV) is chosen so they can survive any high voltage spiking that occurs. I guess that gives some idea of how the high abnormal voltage can be. Some people having found these often shorted recommend against them. I'm not really sure how useful in practice they are. I've had them, installed them carefully (making sure not to overheat the diodes) and haven't had problems but then again I haven't run the secondary open with the diodes either. Also I've never noticed a tonal change with them except in a EL84-equipped Mesa .22 caliber which sounded deader in the high end, and I'm guessing it was because of the higher gain of the EL84s (greater effect of capacitive coupling from plate to ground?).
6/22/2005 3:09 PM
When the diode shorts, it represents almost a dead short across B+. It usually blows the HV fuse almost instantly.  
Could this situation put in danger to the guitar player?
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