Silvertone Amp in Case

[J Luth]

My first time looking at one of these, just looking quickly for a friend. He may return it.

Guitar and amp were purchased from GC about a week or 2 ago, looks to be a 1448.

It was delivered and the owner tried it. He states that it came on and worked fine for a few minutes then shut off. He claims he did not try it again.

I brought it up on the Variac and a current limiter and when it warmed up, it has a bad hum. Seeing as how it still had the original caps, I replaced the three section electrolytic with 3 individual caps and it still does it. All other resistors and caps are within spec and I can’t find any shorts. I have also tried a new set of tubes.
Hum can be heard in the video. I tapped the jack to see if there was input.

Hum.mp4

[Enzo]

The very first thing we do with an amp like that is reverse the wall plug in the outlet.

This amp is what we call a widowmaker, it is connected DIRECTLY to the mains wiring. And we call that a "hot chassis".

To safely use this amp, you need to add an isolation transformer. The amp is dangerous.

[Jazz P Bass]

Does it 'hum' with the volume control turned full off?
The 12AU6 is very much a 'hummer' at full volume.

[J Luth]

Enzo, I'll give the reversing the plug a try although I may have tried that but not sure. I know about the hot chassis issue just have not worked on any.
First thing is to try and find the cause of the hum. He probably wont use it much if at all, just wants it to work if possible.

Jazz P Bass, Yes it does hum with the volume off. Turning the volume does increase the hum slightly but not a lot. The amount of hum makes the amp unusable. I have 2 sets of tubes and the hum is the same with both sets.

[Jon Snell]

Firstly I would replace R2 & R4 and C2 then remove C4 check if the hum has gone. If it has, replace C4 with a new 10n capacitor and test again.
If still there with C4 out, check the ground for C5.
Before you let it out of your sight, fit an isolation transformer rated at 50Watts to cope with the loading. A torroid transformer may fit better as it is not so large. Connect the earth lead to the chassis.

[J M Fahey]

Meanwhile, half for safety, half for hum reduction, grab your trusty neon bulb screwdriver and touch chassis with amp ON, try reversing mains plug.
Use the position where bulb does NOT light or does it minimally.

In any case, the proper solution is a real mains isolating transformer.

Even if "hardly used", do it anyway, who knows who will be using it in the future.

[Jazz P Bass]

Does it hum with the 12AU6 removed?

[J Luth]

Jon, I'll see if I have the parts and try.

J M Thanks

Jazz, Yes it hums with 12AU6 pulled.

[J Luth]

Appear to have found it. Ground for the replacement caps. My mistake. Live and learn.

[eschertron]

Appear to have found it. Ground for the replacement caps. My mistake. Live and learn.

Live is the important thing (when working on high-voltage gear!). Otherwise, the learning's not much use

[SoulFetish]

These "Widowmaker" models aren't worth the trouble to make them safe, IMO. An isolation transformer alone will not make this amp any safer than a vintage amp with a 2-prong cord. You still need to bond the chassis and any exposed conductor, (ie. chassis/faceplate/etc.) to the Earth conductor on a proper 3-conductor power cord.
There are better Silvertone amps