SS Diode-audible mechanical or electrical tick noise ?

[Tonewood]

I'm stuck & received no response form repair fourm

Just finished my first scratch built amp, it's a 5f1 style with solid state rect. Worked great for two days. Went in third day and added a bright cap. The amp started ticking at a varying rate of a couple times/sec to many times/sec, almost a buzz. Removed the bright cap. Still ticked. (I think the problem is unrelated to the cap change)

Tick volume from the speaker goes up and down with amp volume. Turns out I can hear the tick inside the chassis & it's about as loud as a ticking wrist watch. I believe it is coming from the 1N4007 diodes. I used a hard plastic rod with my ear at one end and the test points at the other. The sound was loud and clear from all four diodes and the snubbers riding on the diodes. I clipped off the snubbers and the click was still there. I put in new diodes and snubbers and it still ticks. This is the first time I have used ss diodes-

Is it possible that ss diodes make an audible mechanical or electrical noise (not amplified) am I way off?

Thanks

[R.G.]

It's possible I guess. However, I've never seen it and I've used the 1N4000 series for over three decades. Your description sure makes it seem like something is having an intermittent arc. I'd venture to say that any 1N4007 that ticks audibly is either being overstressed or defective.

Since you removed the snubbers and replaced the diodes. I'd guess that it's something else and just happens to come from near the diodes. What else is near the diodes?

Does anything you do seem to cause the ticking rate to vary, or is it just getting steadily faster?
Does mechanically damping the diodes by putting the eraser end of a wooden pencil on them damp out the tick?
There is a test which can be very revealing. It requires some setup to do safely. Wait til it's dark outside. This happens once per 24 hours, but if you happen to be having a solar eclipse, that would work, I guess. 8-) Open the amp up and put it on the bench where you can power it up. Carefully note what's nearby you and things you should not touch - like the insides of the amp. Then turn the room lights out. Verify that you hear the tick, and look very carefully for small arcs of light inside the amp. DO NOT ATTEMPT TO MOVE AROUND OR TOUCH THE AMP WITH THE LIGHT OUT for obvious reasons. If it's arcing, you can see the light unless the arc is entirely inside a component, like the power transformer.

[Tonewood]

Thanks R.G.-
The diodes probably aren't defective since I have replace all of them (although all from the same batch). Not sure how I would be over stressing them, the transformer is a 515v w/ct. So I think it is something I am doing wrong.

The only thing near the diodes is the octal base they are mounted on & the hookup wires.

The ticking varies both faster and slower. I haven't found a repeatable way of changing the speed. Damping the diodes with the chopstick didn't stop the ticks- I will double check this this afternoon since you brought it up & I never dampened them all at once. I will also look closer for a arc tonight.

The diodes are connected to the bottom of an octal socket very close to unused metal parts of the socket. Can they arc to the ungrounded unused lugs that they are VERY close to?

Thanks

[R.G.]

Thanks R.G.-
the transformer is a 515v w/ct.

The reverse voltage on a diode in a FWCT (two diodes with CT grounded) setup is two times the transformer peak voltage. If that's 515Vct, meaning 257.5V on each side of the CT, then the peak DC voltage out is 364Vdc, and each diode will see 728V peak as a reverse voltage when it's off. If the transformer is 515V on each side of the CT, then the DC rectified voltage is over 720V (neglecting losses) and the diodes will be seeing in excess of 1400V reverse, and yep, they can be failing even if they're good. I'm guessing you have the first case, 257.5-0-257.5.

The diodes are connected to the bottom of an octal socket very close to unused metal parts of the socket. Can they arc to the ungrounded unused lugs that they are VERY close to?

Yep. Sharp metal edges cause electric field concentrations which make arcing easier by ionizing the air next to the edges.

[Tonewood]

Yes the transformer is 257.5-0-257.5 or so.

I went to the shop last night to watch the sparks fly & chopstick/damp the diodes and I couldn't get it to fail! This is the first time it hasn't failed since the problem started.

I will raise the diodes further from the socket to avoid the "sharp metal edge" problem and leave it at that. If I end up finding another cause, I will post it here.

Thanks again for all the great info! It will help me plan my next build.

Scott