Help troubleshoot sound from vintage-radio-turned-guitar-amplifier?

Freeverb

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Hi - I build guitar amplifiers into old vintage radios, and I'm having this weird problem with this one. The amplifier board works perfectly with a different speaker, but with this one I get an inconsistent crackle. The only piece of old circuitry in the loop (other than the old speaker) is the switch pot from the old radio, and I have cut all the other leads to the circuit board, so nothing else from the original radio circuit should still be in the system. (Also, I encountered this problem even with the switch pot cut out of the circuit.). Also, the output transformer on the speaker has been disconnected - I simply soldered the speaker leads directly to the voice coil leads and cut the transformer out of the circuit.

What's especially weird is that I've had the amp sound correct with this speaker a couple of times, but then this comes back, and I can't figure out why. Any suggestions? Am I missing something super obvious? FWIW, I'm not an electrical engineer - just a tinkerer.

https://photos.app.goo.gl/3LhzXR5h3LM4c6Xe6
 

Peegoo

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Have you tested the problematic speaker? The voice coil
may be intermittent due to a rub, etc. Set your meter to test for DC Ohms and test across the speaker's tabs. It should read between 6 and 8 Ohms or so.

With the leads still connected, gently compress the cone a little bit into the basket. Gently. Watch the meter as you do this and see if the reading jumps. If it does, the voice coil is rubbing or dropped.
 

peteb

Friend of Leo's
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but with this one I get an inconsistent crackle

a bad connection in the signal path will cause a crackle.

an intermittent crackle could be an intermittent bad connection, in the signal path, possibly heat related.

do you have an alternate speaker to try?
 

Freeverb

TDPRI Member
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40
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Have you tested the problematic speaker? The voice coil
may be intermittent due to a rub, etc. Set your meter to test for DC Ohms and test across the speaker's tabs. It should read between 6 and 8 Ohms or so.

With the leads still connected, gently compress the cone a little bit into the basket. Gently. Watch the meter as you do this and see if the reading jumps. If it does, the voice coil is rubbing or dropped.
I'll check this. My meter read something like 3.2 ohms static, but I didn't move the cone at all. I'll give that a shot and see what happens.
 

Freeverb

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a bad connection in the signal path will cause a crackle.

an intermittent crackle could be an intermittent bad connection, in the signal path, possibly heat related.

do you have an alternate speaker to try?
Yeah, I tried a different speaker and it works fine. What's weird is that this one worked fine twice (once w/o the old switch pot, and once with the old switch pot). Maybe it really is the fault of the switch pot. I'll take that out of the circuit again and see if I can get this to work consistently without it.
 

Freeverb

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So here's the deal - the speaker basket was grounded to the negative lead of the output transformer, so effectively the entire speaker basket and radio housing seem to be acting as an antenna (or something) for the whole amplifier circuit. I didn't realize that the common speaker lead was conductively linked to the speaker basket, nor do I quite understand why (though I have seen some mention online about old tube amps having the low side of the output transformer grounded to the speaker basket, which seems to be the case here, but I don't quite understand why).

So anyway, it looks like I can fix this by cutting the negative lead from the speaker away from the speaker basket. I hope, anyway.
 




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