Spike protection diodes in PA

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
musicheals
Newbie
Newbie
Posts: 45
Joined: Thu 05/13/21 12:08 am

Spike protection diodes in PA

Post by musicheals »

I read in the trainwreck pages about the Spike protection diodes on the anodes of the power tubes.
I'm don't understand the principle of this mod. :|
It would be interesting, if someone ever tried this. Does this really make sense and more important: does it influence the tone?
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Spike protection diodes in PA

Post by TriodeLuvr »

I believe the idea behind this is that the diodes limit the voltage excursion at the transformer if something goes wrong, such as the load (speaker) opening. Positive-going voltage at each anode is somewhat limited by the internal resistance of the output tubes. However, negative-going voltage cuts off the tube's conduction and thereafter encounters no resistance. So, if the magnetic field of the transformer changes in a way that might create an excessively high negative voltage at the anode, the diode at the respective anode will conduct and prevent transformer failure. That's the theory as I understand it, anyway.

As long as the diodes have a sufficient voltage rating and are in good condition, they shouldn't affect tone. I believe the diodes Fender uses for this purpose are rated 3KV.

Jack
1 x

musicheals
Newbie
Newbie
Posts: 45
Joined: Thu 05/13/21 12:08 am

Re: Spike protection diodes in PA

Post by musicheals »

Thanks a lot Jack.
So I think it is a good protection and very usable.

Thanks a lot
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Spike protection diodes in PA

Post by TriodeLuvr »

Protecting output transformers against every possible circuit failure is a complex undertaking. The PDF linked below discusses this topic in detail. I haven't read the entire paper, and I would caution against taking it on faith (like most things online). LOL, it says I shouldn't have installed a B+ fuse at the CT of the output transformer. That tells me the piece might be more theoretical than therapeutic. People tasked with this sort of thing often imagine risks and failures that are non-existent or extremely rare.

I don't doubt that many types of failures can damage an output transformer. However, after years of servicing hi-fi gear in my 20s and 30s, only one type was ever seen in numbers of any significance. Specifically, leaking coupling caps can create a gradual climb in output tube bias current that goes unnoticed until it's too late. This usually destroys the tube(s) and the output transformer. It's been my experience that coupling caps leak due to age, not necessarily hours of use. So, the best solution to this problem is probably just to replace all the coupling caps at the output tube grids if they're more than 15 years old. It's ironic that in an instrument amp that sees intermittent use, the tubes themselves can last much longer than this. For tubes, the criteria is usually hours of use, not age.

The second area of concern is the issue of accidentally disconnecting the load. This can potentially result in a shorted output transformer. I have only seen this failure mode a couple times, and whether it was the result of over-voltage caused by user carelessness or just a random internal failure is impossible to know. Perhaps this is more common in instrument amps than in hi-fi, but commercial guitar amplifiers have not traditionally incorporated any electronic measures to guard against it.

It would be interesting to talk to someone who services guitar amps for a living to get their take on all this.

Jack

https://dalmura.com.au/static/Output%20 ... ection.pdf
0 x

musicheals
Newbie
Newbie
Posts: 45
Joined: Thu 05/13/21 12:08 am

Re: Spike protection diodes in PA

Post by musicheals »

Thanks a lot Jack!
Interesting to hear, that even coupling caps go bad with age. I think it depends a lot on temperature and moisture in the air....
0 x

Post Reply