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Author
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Topic: Static from old Silvertone amp
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Bill Leff
Member From: Santa Cruz, CA, USA
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posted 15 December 2003 10:32 AM
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I bought a 60's Silvertone 1481 amp off eBay awhile back. Its a little 1 6v6 amp (like a Fender Champ) with some kind of 8" alnico speaker, puts out about 5 watts or so. I love the sound of this little guy (great with guitar and lap steel) but the amp produces a lot of ambient static noise through the speaker. It doesn't matter if an instrument is plugged in or not, same volume of noise. The noise increases when the tone knob is turned up (ie more treble). It may also get worse after the amp has been running awhile (not completely sure on this last observation). I've swapped tubes but this makes no difference (tubes are 6v6, 12AX7 and 6x4 rectifier). What is the likely culprit of this and what do I need to do to fix it? Thanks
Bill
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Ken Fox
Member From: Ray City, GA USA
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posted 15 December 2003 11:31 AM
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Likely culprit is carbon composition plate load resistors on the preamp tube(s). They can produce an intermittent snap, crackle and pop sound. The tone knob on your amp is after the preamp tube, so the noise is back at the preamp tube area. Also a dirty socket can cause trouble. Wiggle the tube a bit and see if there is static produced from the movement. Below is a link to your schematic:
http://www1.korksoft.com/~schem/bargainbin/silvertone_1481.pdf V1 (the preamp tube) in the amp has two plate load resistors each at 100K ohms. I would try to replace them first. 1/2 watt should be fine. |
Bill Leff
Member From: Santa Cruz, CA, USA
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posted 15 December 2003 12:02 PM
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Thanks Ken.Wiggling the tubes doesn't change anything. I wouldn't describe the static as "intermittent", it's just there continually. The content of the white noise changes a little (it's not a clean "whoosh" that stays exactly the same) but it's always there. Does it still sound like it's the plate resistors? I'll start by putting some cleaner on the tube pins to clean up the sockets to see if that helps. Thanks Bill |
Ken Fox
Member From: Ray City, GA USA
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posted 15 December 2003 12:35 PM
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Plate loads are usually intermittent. That sounds more like a tube. However you have substituted tubes already. You might still replace the plate loads and the two .01 coupling caps and even the .003 cap at the tone control. Fortunately, there are few parts in the amps preamp stage, it should be an easy fix.
An easy test for plate loads is to take a jumper to ground and then to the tubes plates. That turns off the tube and will help ID which resistor and stage the noise is coming from. The power is current limited by the 100K resistor during the test. Caution, be sure you are on the right end of the resistor, or you will short out the power supply.
As old as this amp is, I would also look at replacing the filter caps, unless they look recent. There is a 5uf filter for the preamp plate load B+ that could be giving a bit of trouble as well.
Cleaning the tube sockets is the first thing I would do. I use denatured alcohol and just dip the tube in the alcohol (I put some in the lid). Insert it in and out several times.
[This message was edited by Ken Fox on 15 December 2003 at 12:37 PM.] |
Jay Fagerlie
Member From: Lotus, California, USA
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posted 15 December 2003 01:16 PM
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Just to add to Kens post....
The 'right' side is the junction of the tube and the 100K resistor. Pin 1 and/or pin 6 for a 12A( )7 type tube. Touch the other side and it's 4th of July!(I hope I'm not steppin' on your toes Ken, better sure and safe that not and smoked!) |
Ken Fox
Member From: Ray City, GA USA
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posted 15 December 2003 02:14 PM
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By all means, a good call. You cannot be too safe!!!! Thanks. |
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