50plus strange noise on attack

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50plus strange noise on attack

Postby scrapgold on Thu Jan 09, 2014 5:13 am

New member here...
I've been trying to pinpoint a quirk in my 50+ for some time now. Techs have been little help, mostly due to the strange circuit of these amps. Hopefully someone here has some experience!

After the amp warms up thoroughly, there is an odd rattle/static/scratch type sound on the attack of chords. This subsides after the chord has been ringing for a second or so...not noticeable with single notes or leads. Ive played through two other cabinets and tested with different preamp tubes, though not with a different speaker cable as I dont think that could be the problem.
The vast majority of the components in the amp are factory.

Is this just the sound of the active preamp? Is this a symptom of leaky caps? Is this a symptom of something nightmarish? Does anyone know what the heck I'm talking about?

Any help is greatly appreciated!
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Re: 50plus strange noise on attack

Postby Papa Dog on Thu Jan 09, 2014 11:39 pm

does it do this on both channels?
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Re: 50plus strange noise on attack

Postby scrapgold on Fri Jan 10, 2014 12:09 am

Yes. A tech/builder friend of mine is working with me on this...we are gonna check out the coupling caps next. Any other suggestions?
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Re: 50plus strange noise on attack

Postby Papa Dog on Fri Jan 10, 2014 2:00 am

ooops...I forgot...Welcome to the Vintage Amps Forum and the Sound City section.

First check the tube. Try swapping in a known good one if you have one on hand or new if you do not. Of course sometimes even new tubes can be DOA.
If it isn't the tube then I would check the cathode bypass cap and cathode resistor. Maybe go ahead and change them out, as the cathode bypass is electrolytic and could be dried out.
If that doesnt fix it I would check the V1 plate resistors, the grid stoppers and grid leaks, and go from there.

My bet would be that either the cathode bypass cap is bad, or one of the redsistors on the front end has shorted or a solder joint has gone bad, or some combination of the above. But it doesnt happen until the amp is warmed up?

Might be something else down the chain, but that is what put me onto the tube as a suspect.
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Re: 50plus strange noise on attack

Postby scrapgold on Fri Jan 10, 2014 4:48 am

Perfect! I'll keep posted about the progress. Thanks for the advice!
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Re: 50plus strange noise on attack

Postby GregEdge on Fri Jan 10, 2014 4:13 pm

I had this same problem with my 50+. Mine was resolved when I went through my amp piece by piece- I could never replicate the sound with tapping. But I finally found the resister that was making all the fuss, after tireless effort. The carbon in the resistor was on its way out, and would be rattled around when the strings were attacked, especially the lower strings, resulting in the fuzz noise you were describing. The resistor would sit normally when doing some soft-medium strumming. I can't remember which resister it was, but I'm almost positive is was around the preamp. Hopefully this helps.
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Re: 50plus strange noise on attack

Postby Unit_1 on Fri Jan 10, 2014 11:14 pm

before i started working on amps i had a friend replace the input jacks on my 68 sfdr after cat pee mixed with beer did a really good job of oxidizing the jacks and removed the plating on them

all was good. years later it started doing what you're getting, and when i took it out to have a look, now that i DO work on amps, i found a cold solder, and a crimped but *unsoldered* wire right next to a big ugly glob of solder

they were pot metal jacks so i replaced them with the long neck switchcraft jacks and i must say i did a magnificent job on soldering them up in a jig before gently slipping them into the very tiny crowded area where they live now...

replaced the absolutely trashed shielded input wires while i was at it. shielding was frayed and barely any of it was left to contact the ground

problem gone
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Re: 50plus strange noise on attack

Postby Papa Dog on Sat Jan 11, 2014 12:16 am

Good answer Unit_1.
The 50+ has Cliff jacks, and they very well could have become coroded or dirty.
The QC on some sound city amps seems to be where they did "cost control", so it is always a good thing to look for sloppy or non-existant solding on the joints in those amps.
Fer instance, my personal 200+ had a cap in the tone circuit which came from the factory in place but not soldered at all.
A bit of schmutz getting in ther would have made fer a horrible sound and might have been kinda hard to track down.
I found it because I was re-flowing ALL the solder joints on that amp, just to be sure.
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