Silvertone 1484 60Hz hum

[bazmcc]

Hi all, done some searching on forums about an issue I'm having, but can't find anything that relates exactly to my issue.

Recently done a 3-prong power cable conversion on a Silvertone 1484. I'm pretty confident the wiring etc is all good, but when I turn the head on, I get a 60Hz hum from the head itself. This outputs from the loudspeaker once it's warmed up too.

- The hum is present with nothing plugged in and with channel 1 & 2 volumes fully down, and with reverb off.
- Plugging in guitar to any jack does nothing to the noise.
- Volume knob on channel 1 does nothing to the hum.
- Volume knob on channel 2 increases the hum, as does the bass and treble obviously.
- 12AX7 tubes V1 and V2 were noisy when tapped with pencil (V1 in particular). Have reseated/swapped existing tubes positions without any reducing hum, but less noisy now when tapped with pencil. Don't have spare tubes on hand to try right now.
- Tapping on either 1/4" jack for channel 2 with a pencil gives noise.

Therefore I'm thinking issue is somewhere in channel 2 wiring/tubes.

I stupidly didn't check the noise of the amp before I started the 3-prong conversion, so don't have anything to reference it against to see if the new power cable conversion has caused it, but as I say, I'm pretty confident it's been done properly. It's a fairly straightforward wiring job which I read up on a lot before undertaking.

Any advice would be appreciated before I have to take the amp to a tech.

Thanks!

[The Dude]

If you pull V1, is there still hum?

[bazmcc]

[ATTACH=CONFIG]55928[/ATTACH]

If you pull V1, is there still hum?

Yes, still there with V1 pulled.

[bazmcc]

With tube V3 (6CG7) pulled, hum still present, but much lower level. Not sure if that's tube that acts as phase inverter or one that drives the reverb.
Loud hum returns if I put other 6CG7 tube in V3 position.

With no tube in V3, hum doesn't increase with volume control on channel 2 as it does when tube is situated.

EDIT - just found out V3 is the phase inverter tube, so guessing that's not solving the issue, just masking the cause of it somewhat. Tried swapping 6CG7 tubes anyway, no difference to the hum.

[The Dude]

It's an old amp. Have the filter caps been changed?

[bazmcc]

I'd say no to be honest. It's a friend's amp and is pretty new to him, but doesn't look like it's been touched in a while, especially since the 3-prong conversion hadn't been done either. I'm thinking it maybe something along the lines of old components and needs serviced, but my friend thinks the hum is louder than before I swapped out the power cable, so I'm just trying to make sure it's nothing I've done! Lol!

Basically I've cut out the old 2-prong, cut out the death cap, and rewired it following modern standards.
Hot > fuse > switch > PT.
Neutral > PT.
Ground > chassis.
Cut the convenience outlet and ground switch out of the circuitry entirely.
Used 9-10ft 16awg power cable.

Think I can rule out dirty power as I'm plugging it into his newly-wired studio sockets. Don't have a power conditioner or ground lift on-hand right now to try, but thought that might be worth trying tomorrow.

Just really trying to make sure I've covered all bases I can manage myself, especially regarding the power cord conversion, before I advise handing it over to a tech.

[SoulFetish]

The last one we had in, I did a 3 wire power cord install. Double check something. The polarity cap shares a ground connection with another node, if I remember correctly. It's easy to inadvertently clip both leads because it's a tight fit on the grounded lead of that cap. So you might want to check that, and make sure your other grounds are good.

Also, just so there's no confusion, this amp isn't laid out in a typical linear fashion with "V1" all the way to the right and counting off in order going left. The far right tube is the 6CG7 reverb driver, and V1 is the 3rd one in from the right. Here is a labeled schematic which makes it easier to follow.
Silvertone1484.pdf

So Starting from input, I would eliminate the possibility of bad tubes by substituting known good ones. In fact you should be able to pull tubes 4 & 5 and still be able to have both channels work and pass dry signal. If the noise goes away, you are likely to find your problem in the 3 reverb stages here.

[bazmcc]

Thanks for all the advice so far.

Yeah, that’s the schematic I’ve been following, but I appreciate the checking that! I did almost make that mistake when starting to trouble shoot!
By polarity cap - you mean the one I removed? There was another wire grounded to the same point of the chassis that I left intact. I’ll double check for sure but I remember not being sure about it so made sure to leave it until I knew more about it.

Removed tubes 4 & 5 but hum persists.
Only tube I haven’t swapped so far is tube 6.
I don’t have any spare tubes on hand right now, so all swapping has just been with tubes in other positions, but only swap/removal that’s made any difference is removing tube V3.

I tried a ground lift this morning (no change) and when I get time later today I plan to double check the ground wire you mentioned and also my ground wire from my new power cable. I currently have it going to the chassis via the nut for the metal bracket that holds the wooden board in place across the middle of the head. Ground is secured to the small bolt on the chassis end of this bracket. No other bolts were easy to get at, hoping this would be sufficient grounding?

[Sweetfinger]

A couple possibilities. One is filter caps. The other is the spring popped off the clips inside the reverb tank.

[bazmcc]

A couple possibilities. One is filter caps. The other is the spring popped off the clips inside the reverb tank.

Thanks, will check this out. Think I've eliminated possibility of reverb issue by removing tubes, but will check it anyway

[soundmasterg]

Does it still have the "Planet" brand filter caps in it? How about he can cap? Doe sit look original, or at least is it a 20-10-5 can cap? If so those are original and should be replaced. Regardless if they are original or not, if the positive end of the electrolytic caps are bulging or bubbling, then they should be replaced also.

These amps hum more than a typical Fender because they put the Power Transformer in the middle of the chassis for balance reasons. That is almost the worst place to put it from a hum standpoint. The amps have somewhat low gain in the preamp so it is ok for the hum level, but if you up the gain in the preamp at all then the hum gets really out of control.

My money is on replacing the electrolytics.

Greg

[Spencer.sdk]

I had this same issue. It was heater-cathode leakage. The 2nd preamp, reverb recovery, and phase inverter (of course) are all after the volume control... So any heater to cathode leakage in those tubes will be audible with the volume down. Try pulling the phase inverter out. If the amp is quiet, it's not the output section-- it is the preamp and driver, and it is most likely heater leakage, especially if you've confirmed that it's 60Hz.

You have two options... Figure out which preamp tubes are leaky and replace them. And/or... Elevate the heater supply to reverse bias the leak and stop it.

I created a 40V divider off the B+ and connected it to the heater winding center tap (disconnecting the center tap from ground). Problem solved.

[Jon Snell]

If you remove the death cap and temporarily lift the ground wire that you fitted, what is the result?