Schottky !N5820 diode failure

[Bryon]

After many years of duty, my SEX amp has developed a problem.  Two of the schottky diodes have blown apart - only the leads attached to the transformer are still intact.  Both were connected to terminal 4 of the PT-3 transformer.  The fuse doesn't appear to have blown.

Can I just replace the diodes? or is there something
 which could have caused the failure, other than a faulty diode, which I should be aware of?

Secondly, does anyone know where I can source replacements?  I've checked Digikey and Mouser but neither has the IN5820 [they have a bunch of IN5821 and IN5822 which are also 3A - could I substitute either of these?]

thanks

Bryon

[JC]

I think my first inclination would be to take a resistance reading across C1 and C2 on the terminal strip that the diodes connected to.  IIRC, that's where the 10,000 uF cap is mounted.  Try this with the tubes removed for starters.

As far as to the specific diodes for replacement, I will leave that up to Doc or Paul. 

[Paul Joppa]

The 1N5820, 21, and 22 are rated 20v, 30v, and 40v maximum reverse voltage, respectively. The forward voltage is only slightly different - 50mV at each step. This will reduce the available heater voltage by 0.1v or 0.2v if you use the available diodes. That is probably insignificant.

I am trying to think how they may have failed, and the only thing that occurs to me right now is excess heat. They are close to the transformer, which does get pretty warm, and they rely on dissipating heat through the leads. I think the ones that go to transformer terminal 4 have shorter leads? So I would replace them but space them further from the terminals with longer leads than before. (Also, make sure that the location, and the feet on the base, allow some air to enter.)

You might check your power line voltage to see if that might be on the high side, leading to slightly higher temperatures.

[Bryon]

Thanks Paul

I just ordered the IN5820 diodes from digikey - I couldn't find them through their search engine as they identified them with a bunch of suffixes but when I went to Schottky diodes, the IN5820's showed up.

I'll have to make sure the Sex amp gets more air flow.

thanks again for the superlative service

Bryon