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Post subject: SF Deluxe Reverb hum & buzz
Posted: Fri Oct 12, 2018 5:52 pm
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So my apprentice called up with a stubborn problem that was a loud buzz when ch2 volume and treble was turned to moderate levels. There was also a variable hum . He had recapped and retubed it last week hoping to fix it.
We examined it over a video call and I observed the wire dress between the volume and treble controls was away from the chassis. I explained how critical it was to prevent hum by pushing the wires against the chassis. We hung up and he did the changes, then called back with a surprising fix.
The wire I saw made little difference, but he pushed on the wire on v3 from pin 1 to pin 6. It freaked out with the phantom symptom he had been chasing. He manipulated it away from a nearby sheet metal screw and made the symptom go away. Touching the screw made it buzz and close made it hum. Replacing the wire makes no improvement.
Now this is a plate circuit and shouldn't be sensitive to picking up anything. Is it inducing something into the chassis that is picked up elsewhere?
Fortunately he can dress the wire to eliminate the symptom, but it was weird enuf that I thought I would ask if anyone has run into this or what causes it.
Maybe yours does it too?


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Post subject: Re: SF Deluxe Reverb hum & buzz
Posted: Fri Oct 12, 2018 6:13 pm
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Tim, you sure it's not the jumper between 2 & 7, on V3? I've had cold solder issues with this jumper before. Symptoms are cracking noises to hefty hum. All sensitive to touch.

If he already replaced the jumper --- the socket maybe bad.

Good luck! :)


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Post subject: Re: SF Deluxe Reverb hum & buzz
Posted: Sat Oct 13, 2018 3:14 am
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BMW2002Ti wrote:
Tim, you sure it's not the jumper between 2 & 7, on V3? I've had cold solder issues with this jumper before. Symptoms are cracking noises to hefty hum. All sensitive to touch.


+1

I've seen this phenomenon before.

I recommend a *very comprehensive* inspection of the V3 socket. Replace if necessary.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: SF Deluxe Reverb hum & buzz
Posted: Sat Oct 13, 2018 8:08 am
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It was 1&6 with the blue wire of the reverb tranny on 6.
Its down the road now to an impatient customer. If it comes back, I think a socket is suspect.


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Post subject: Re: SF Deluxe Reverb hum & buzz
Posted: Tue Oct 16, 2018 9:04 am
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In further discussion with the tech, he described initially replacing a burned looking 470 ohm 1/2 watt resistor on the cathodes of v3. When I pressed him on the value, he said it looked original and he could still read the colors a bit.
It seems I've also replaced a burnt 470 on those cathodes before. Now for normal operation, an 820 would be used but some versions also use a 2.2k with a bypass cap.
The 470 ohm would run the tube hot and unbalanced. The extra current going through that sensitive plate wire may be the source of the hum and squeal he was experiencing. I'm not sure how, maybe power supply overload, but it may only occur with this 470 on the cathodes.
But this brings up other questions. What were they trying to accomplish by overdriving the reverb with square waves? Does this improve the sound of the reverb? And is Rivera's name on the print?


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Post subject: Re: SF Deluxe Reverb hum & buzz
Posted: Tue Oct 16, 2018 11:05 am
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The V3 reverb driver also functions as a third gain stage in the vibrato channel and I can only surmise that somebody thought that increasing the conductance there would result in more compression and saturation when the amp was driven hard.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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