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Post subject: White Noise (No, not a new Velvet Underground song...)
Posted: Wed Jan 27, 2010 5:18 pm
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Hello guys,

I have a 1975 SF Twin Reverb which just came back from changing tubes, capacitors and one powertube socket...

Anyways, tonight I had a bandrehearsal and I played it and it sounds just fine... You might ask now: "now where the katooties is that white noise that swiss kid made us click on that topic for" well here it comes...

I wanted to brighten up my sound so I 'flipped-up' the bright switch (meaning, activating the bright switch) and to my surprise, there it was, the full load of White Noise... I was really disappointed because I prefer the bright sound to the duller 'normal' sound...

what kind of tubes do I have in my amp you ask? JJ/Tesla 6L6 Powertubes and Tung-Sol 12AX7 preamp tubes...

can you help? Does one of them 'Hum-Debuggers' help by any chance?

thanks in advance

JM5000


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Posted: Wed Jan 27, 2010 11:51 pm
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Hi John, the bright switch puts a 120pf cap into the signal path between the volume pot and the input grid of the second gain stage pre amp tube. If the switch is dirty or the cap has issues, it's just being amplified again. The switches can be cleaned with contact cleaner and the cap can be changed to a silver mica type for clearer performance. But did this also happen with the old tubes? Years ago, low noise 7025's were specified for the V1 & V2 positions. I've found that just the cleaning knocks down a bunch of the noise. The Hum-X devices on the AC power line, filter the ground noise from the wall outlet. I'm not real familiar with the other outboard filters, but a friend of mine did use a stomp box type with pretty good results. But I don't see that being necesssary with a Twin Reverb. Art

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Posted: Thu Jan 28, 2010 1:53 pm
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thank you so much for your reply, I hope the capacitor isn't shite... How easy is it to put contact cleaner on? Do I need to remove the chassis for that? Do I need an expert to do so?

I honestly don't remember if the amp had any white noise for I haven't played it in two months, but I don't think it was there... Do you think there's a connextion between the 12AX7s not being 7025s and the bright switch? If so it shouldn't be a problem to replace 'em, right?

JM500000000000000000000


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Posted: Sat Jan 30, 2010 12:41 pm
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JM5000, cleaning the switch does require the chassis to be removed and the filter caps discharged. The cleaning is just a matter of spraying a small amount into the top opening of the switch and operating the switch a number of times. This is really something that would be part of a regular maintenance cleaning of all pots, jacks, switches, and tube sockets. The NOS 7025's would certainly keep the amp quiet, but they're kind of pricey. I'd go with a thorough cleaning first. Art

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