Silvertone 1484 - Channel 2 Hum

[scstill]

Seems that many Silvertone 1484's have a problem with hum on Channel 2. It varies as volume is adjusted. The hum is gone with guitar plugged in to Ch2. Channel 1 is very quiet with and without guitar. Do you think this was as originally produced or something that could have happened with age?

1) Thinking this is a grounding issue (hum gone with guitar) I tried a shunted jack and this eliminates hum. So am considering this simple fix on one of the two inputs. Would reduce the gain by 6db when plugged into non-shunted input. Any thoughts on shunted jack? Any way to add shunted jack(s) and keep both inputs the same levels as originally design?

But I wonder if there is another circuit issue...

2) Other posts have said its a design issue having the PT close to Channel 2. Shielding the PT does nothing. Changing tubes does nothing. only ground the jack tip helps.

3) Other posts have said since it varies with volume its in front of the pot. Replacing the vol pot does nothing. Tried removing reverb and tremelo tubes with no change in hum. There are other circuit differences between Ch1 and Ch2 in front on the volume. Namely Ch1 has R5, and Ch2 has R9. Do you know why these differences are present? and whether one of these differences could have an affect on the hum??

4) other thoughts not mentioned here on the hum?

[sluckey]

Wire the jacks just like Fender does, one high and one low. The high jack will be just as loud as your orig. 1484.

[PRR]

The NFB around the tube means it won't be JUST like Fender when no plug is inserted. The Silvertone appears to go to way-low gain (but doesn't really).

Since a million Fenders were acceptable, I would steal the Fender plan here.

[sluckey]

The NFB around the tube means it won't be JUST like Fender when no plug is inserted.

The NFB is on CH1 only. CH2 is the noisy channel and it doesn't have the NFB. Noisy CH 2 is a common complaint.

[scstill]

The NFB around the tube means it won't be JUST like Fender when no plug is inserted.

The NFB is on CH1 only. CH2 is the noisy channel and it doesn't have the NFB. Noisy CH 2 is a common complaint.

Is "NFB" is Neg Feedback created in CH1 by the 1m R5? If that is what keeps CH1 quiet, why did Silvertone omit it in Ch2? Would adding NFB to Ch2 be advisable or could it affect reverb/trem circuit?

[AmberB]

I might have to try the 1 meg feedback resistor in the channel that still buzzes in the Plush amp...

[sluckey]

Is "NFB" is Neg Feedback created in CH1 by the 1m R5? If that is what keeps CH1 quiet, why did Silvertone omit it in Ch2? Would adding NFB to Ch2 be advisable or could it affect reverb/trem circuit?

R5 is the NFB. I quit trying to reason with why Sears did what they did in any of their consumer electronics products, but I do know it was largely driven by the $$$. Try the NFB on CH2. You may like it. Not gonna hurt anything.

Your hum issue goes away as soon as you plug in a guitar. You already know what the cure for your symptoms are. Sears was too cheap to use good input jacks. Heck, they didn't even use a speaker jack. That's why that amp sold for $149 brand new. 

[scstill]

So I was playing this kinda loud today and discovered a static noise coming from the speaker cab.
Isolated to one of the speakers (both orig 60s C12Q).
No sound coming from the speaker except the static. Cone does not vibrate. It measures resistive 6.9ohms.
I did solder in a new ofc zip speaker cable, proper polarity (4ohms to the amp).
Could this be another source of the hum?

Not alot of experience with vintage speakers, but is the a recone situation? cone looks perfect.
Is there anything I might try first?
Do you like weber for recone? Or another recommendation?
Or a modern Italian Jensen?

[PRR]

...The NFB is on CH1 only....

Missed that. Well, that's dumb. Ah, I see CH2 drives a opto trem so CH1 needs less gain to be comparable (or cleaner).

Still, even more, the Fender input plan seems the most obvious thing to try.

[SoundmasterG]

Seems that many Silvertone 1484's have a problem with hum on Channel 2.

2) Other posts have said its a design issue having the PT close to Channel 2. Shielding the PT does nothing. Changing tubes does nothing. only ground the jack tip helps.

If you use Fender grounded jacks, and shielded cable, and you use some mu-metal in the right place to shield the jacks from the PT, then the hum does go away almost completely. However mu-metal is very expensive and that would be complete overkill for one of these. That said I did try it on the one I modded for my brother years ago to see if there was a difference. I increased the gain on Ch2 by a fair amount and on Ch1 even more and so it hummed more than the standard 1484 does. An engineer friend had some mu-metal so we tried it to see if it made a difference and it made a huge difference, but I wasn't going to pay for the mu-metal. Way too expensive.


Greg

[DummyLoad]

might be worthwhile...

https://www.ebay.com/itm/Ultraperm-80-Metal-Shielding-Sheet-8-x-5-3-MuMetal-Mu-Metal-Sheet-Audio-Shield-/152491911889?hash=item238139ead1


--Pete

[scstill]

R5 is the NFB. I quit trying to reason with why Sears did what they did in any of their consumer electronics products, but I do know it was largely driven by the $$$. Try the NFB on CH2. You may like it. Not gonna hurt anything.

I tried the NFB on Ch2 and it made the hum/noise worse, so I abandoned it.
I added a single shunt to ground jack in Ch2 and the hum is gone.
The shunted input is normal, the unshunted input is -6db.

[sluckey]

Now that wasn't so hard, was it? Easy peasy.