Brian
Joined: Feb 25, 2006
Posts: 19342
Des Moines, Iowa, USA
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Posted on Mar 05 2018 11:19 AM
I don't think I turned on my 1966 Dual Showman in something like 12 years. Well I did this weekend. I had forgotten how the standby switch worked, so I'm pretty sure I did it wrong, and hit it with full juice without warming up.
So now when I turn any knob whether it's volume or tone, the amp seems to cut out and it makes a crackling sound. Once you stop turning it seems to behave normally and seems to be as loud as I remember it. Any ideas?
Either I damaged it or the caps were just too old/unused anyway.
I'm going to sell it, just want to give the buyer a heads up on what likely needs repairing.
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Gilette
Joined: May 04, 2014
Posts: 734
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Posted on Mar 05 2018 11:42 AM
When electrolitic capacitors aren't used for that long they're losing their properties. Normally we 'reform' caps that old before we use them. It means you charge and discharge them a few times with a limited current to form a healthy aluminium oxide layer on the capacitor plates. Sometimes it takes a few days to get them back in shape. What happend with your amp is a bit more drastic; I've seen it work with an old record player but consensus is that you try to avoid hitting deformed caps with full voltage and current. When you're lucky nothing happened and it can be revived to former glory. Or you could face a recap. Best is to have a tech look at it.
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 05 2018 11:56 AM
Agree with Gilette - the amp's been sitting idle for a long time and probably needs a recap. Take it to a tech.
— He who dies with the most tubes... wins
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Chippertheripper
Joined: Mar 11, 2011
Posts: 819
Semass
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Posted on Mar 05 2018 12:08 PM
it just sounds like dirty pots, based on the description.
You did get lucky I think hitting everything with full voltage, but oxidized pots cause what you describe. work them. spray some contact cleaner in there, the good non residue kind. try it again.
if you get to a point where the knobs move without causing any crackle, and you still hear eggs frying or some sizzle here and there, then you have a cap problem. this has been my experience, anyway.
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skeeter
Joined: Feb 27, 2006
Posts: 2065
Virginia, USA
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Posted on Mar 05 2018 01:12 PM
^^ Agree with Chipper, try the contact cleaner spray on the pots first.
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Atomic Mosquitos
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Sonichris
Joined: Mar 06, 2006
Posts: 1907
Wear gloves - I'm in the Rockies
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Posted on Mar 05 2018 03:54 PM
Has the amp ever had new electrolytic caps? If not, then it's definitely time to have them replaced. maybe the new owner will want to do it, or have it done. I wouldn't do it just to sell the amp, personally.
If it has had caps while you've owned it, they "probably" are OK, and you did no real harm by turning it on without engaging the standby. There are a lot of myths about the standby switch, and I personally think its a convenience for in between sets, more than a requirement. Others will disagree. As long as the tubes have 30-60 seconds to warm up before playing, I can see no reason why the standby must be engaged during warmup. The switch doesn't cut power, only sound.
The crackling sounds like dirty pots, as others have said. you may not even have to use cleaner, just turn them back and forth a bunch and see if that takes care of it. It's just oxidation on the contacts, and sometimes the wiper will clean itself off.
Since you are selling, it, Let the prospective owner know if/when the caps were or weren't done.
Or, sell it to me, and I'll do all that stuff! 
— "You can't tell where you're going if you don't know where you've been"
Last edited: Mar 05, 2018 15:55:48
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Brian
Joined: Feb 25, 2006
Posts: 19342
Des Moines, Iowa, USA
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Posted on Mar 05 2018 04:03 PM
Thanks for the responses, I appreciate it. I didn't think it was the pots because every single one of them did it. But it could be that. Not sure I have any pot cleaner any more, but maybe I can get some... or just try turning them a bunch too.
When I got the amp I had all the caps replaced. So they are old (10-15 years), but not 50.
— Site dude - S3 Agent #202
Need help with the site? SG101 FAQ - Send me a private message - Email me
"It starts... when it begins" -- Ralf Kilauea
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 05 2018 04:10 PM
Other causes of intermittent crackling could be:
1) an intermittent/cracked solder/ corroded ground return connection in the preamp,
2) one or more cracked solder joint(s),
3) loose tube socket pin clamp(s),
4) coupling cap(s) leaking DC onto one of the pots.
5) worn out output tubes
6) dodgy plate/load resistors
— He who dies with the most tubes... wins
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