Silvertone Hum

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Silvertone Hum

Postby Jaybird803 » Thu Jan 11, 2007 3:46 pm

Silvertone hum

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I have been going through my silvertone model 1451 amp trying to solve a mysterious hum problem. I recapped the amp and most of the hum was gone except when i turn offthe amp there is a noise through the speaker that sounds like a big fat rubber band being plunked. However this does not happen when I am plugged in at my work bench. KLast night I took the amp to practice and when I plugged in and turned on, the humm was so loud that I could not use the amp. When I got home, I plugged it in, and no hum. Still the twang though. What could be going on here? I love the sound of this little guy, 3-5 watts, 12AU6,50C5, and a 35W4. volume control and an input. I put a Weber sig8 in it to replace thr 5" speaker it came with.
Any ideas?
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Postby songdog » Thu Jan 11, 2007 4:13 pm

Are you using any effects with the amp?

Does the amp hum at the same loudness with and without the mic connected? Have you checked/ruled out the mic cable(s).

Does the amp have a 3 prong cord or 2 prong cord?

Have you tried replacing the tubes?
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Postby Jaybird803 » Thu Jan 11, 2007 4:29 pm

Hum happens with or without mic cord, Tubes have been checked and are good. Hum is the same at any volume setting. There is a 3 prong plug and I am not using any effects. As the amp says to me Hmmmmmm>
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Postby t bone bruce » Thu Jan 11, 2007 7:01 pm

Those power amp tubes will hum somewhat anyway. They use line voltage to power them. I hope you're running with a 1:1 isolation transformer, because if anything goes wrong in the amp, you'll get 110v right on the lips.
It could be the quality of the power supply you're plugged into, if it happens in one place but not another. As for the "twang", I don't know what causes that.
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Postby jeffl » Thu Jan 11, 2007 8:07 pm

There haven't been any flourescent lights on the power circuits that you're plugging the amp into,have there....? Flourescent lights or improperly grounded outlets can cause hums.
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Postby Jaybird803 » Thu Jan 11, 2007 8:14 pm

Besides the OT, there is another small transformer that I assume would be an isolation Tran. Will try to post a pic if I can.
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Postby ricochet » Thu Jan 11, 2007 8:16 pm

Yeah, the hum's likely coming from the power circuit you're plugged into at practice if it's not happening at home. Or from something emitting a strong electromagnetic field in the environment, like fluorescent lights, an electric motor, a CRT or power transformer. Look around.
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Postby Jaybird803 » Thu Jan 11, 2007 8:49 pm

I took a pic. just for fun. The transformer on the chassis has 2 red wires running to pins 3 &4 on the 12Au6, 1 red wire going to pin #3 on the 50C5, and 1 red wire going to pin#1 on the 35W4. I took a pic also. OT came all taped up.
http://demont.net/harmony/myharmonies/J ... hassis.jpg
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Postby thebluesbox » Fri Jan 12, 2007 6:19 am

the small transformer is the output trans. Your safe with that if you have the 3 prong grounded plug., the hum might be tubes or bad cap or ground.
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Postby t bone bruce » Sat Jan 13, 2007 9:22 am

One transformer is the OPT- the taped up one, as you said, and the other is to provide 6 volts for the preamp tube. If you've got a 3 pin plug on there where is the earth wire connected?
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Postby Jaybird803 » Sat Jan 13, 2007 11:29 pm

If yoou look close at the pic you will see a green wire next to the fuse holder. That is the ground wire, and it is connected to the chassis. I have decided to do a mod to the amp that I found on a web site. It will make the amp safer, and hopefully eliminate the hum.
Here is a classic old micro-power amp that I restored to safe and toneful service. Sold originally by Sears, the Silvertone 1430 harkens back to a time when line isolation transformers were considered an expensive luxury. The tone of such single-ended amps is still revered by many harmonica and guitar players who frequently risk electrocution by using one in its original hot-chassis form. Good tone isn't very useful to dead musicians, so among other modifications I added a line isolation transformer to this amp.

The Amp Before Modification. I drew the following schematic from this amp before its modification:



Notice that the user's sweaty hands are separated from one side of the 120 VAC line merely by a 68K-ohm resistor and a parallel 0.05-microfarad capacitor. A short in either of these components could be fatal to a grounded user. The only component transformer-isolated from the power line was the heater of the 12AU6 preamp tube. This was necessary to avoid exceeding the 12AU6's specified maximum average heater-to-cathode potential of 100 V. (Either orientation of the non-polarized plug is possible--and users could experiment to find the lowest-hum orientation. One orientation would give a heater-to-cathode potential of 120 V without this small transformer).

Aside from the 12AU6 preamp tube, the only other active device in the model 1430's audio signal path is a 50C5 pentode used as a single-ended (class A) power amplifier. The output transformer was physically attached to the original 6-inch speaker, old radio-style. On disconnecting the secondary and testing the amp into a 4-ohm resistive dummy load, I rated this amp at about 1.25 W maximum power before soft-clipping. The speaker itself was in poor condition due mainly to a rotted surround, but it still "worked." There was quite audible continuous background hum, but I could make most of that go away by clipping extra filter capacitors in parallel with the old filter capacitors. However, low-level signals were audibly modulated at 60 Hz, which made the decay envelope of notes sound bad. (Adjusting the trimmer capacitor at the second grid of the 12AU6 had no effect. If anyone out there knows what this trimmer was supposed to do, please email me and I'll post it here so we can all learn something



The bottom part of this schematic represents the "isolation module," which includes two separate power transformers: a 50 V-A 1:1 line isolation transformer for the high voltage circuit and a 25 V, 2A filament transformer for the heater circuit. I used a voltage-multiplying rectifier and wired the heaters in series-parallel, with 47.5 VDC across the heater network under load. I simplified the audio circuit, basing it on the Harmony H-303A, which uses the same pentodes as the Silvertone 1430 but has an isolated power supply. I replaced all three tube sockets with ceramic types and included a shield on the preamp tube. I detached the output transformer from the original 6-inch speaker and re-installed it inside the chassis where the 12AU6 heater isolation transformer used to be (it is the same physical size). I replaced the speaker with a new 6" 4-ohm unit from Jensen's "Mod" series.

The amp now has very little background hum, and a bright responsive tone with beautiful clean ring-outs. Here is a link to more photos of the modified Silvertone 1430.

The model 1430 is the same amp as my 1451.
I will try to post the schematics.
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Postby Jaybird803 » Sat Jan 13, 2007 11:38 pm

Image
[/img]http://demont.net/harmony/myharmonies/Jaybird/1430plain.jpg

Schematics for project
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Postby Jaybird803 » Sat Jan 13, 2007 11:39 pm

Image

Origional unmodded schematic
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Postby Jaybird803 » Sat Nov 17, 2007 7:28 pm

After struggling several times with this amp, trying to install an isolation transformer for the whole circuit, with no success, I gutted the thing. Next I rebuilt it with a power transformer, 6x4 rectifier, el84 power tube, and a 12ax7 preamp. The design is pretty much a Kalamazoo model 1 with a Champ 8 ohm triode OT. I now have a safe, quiet, and very tonefull little harp amp in a classic silvertone cabinet. The amp is not much larger than a lunchbox, but the 5 watts out of the 8" Arkady speaker says WOW. I hope to post some pics of the project.
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Postby jeffl » Sat Nov 17, 2007 8:36 pm

Cool Jaybird! Nothing like one of those "What the hell is that...?" amps.
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