Bad hum in old Gibson, Help!

[Thin Twin]

I am brand new to this forum. I was inspired to find this forum because of an old Gibsonette GA-8 I have been dragging around for 20 years. When I got it I plugged it in and it was very scratchy and had a hum in it. I pulled the back and saw crumbling resistors and a cracked cap. Well, fast forward to last week. I finally got the old thing out and decided to fix it. After some digging on line I found the proper schematic, ordered the parts and just fired it up about 30 mins. ago. The scratchy sound is gone, but it has a nasty humm. I am pretty sure it is 60hz. Also, when it is plugged in to ac power and the power switch is OFF you get a small shock from the faceplate. While playing, you can feel the same thing in the guitar.
What do I need to do to correct this? Could it be the transformers? if so which one? I really want to get the thing working again and play it. I have a silvertone that sounds super clean and an old 1950's Wards that sounds pretty good, not as good as the silvertone, but I want to put the Gibson back in action.
Any Help?
Thanks in advance,
Jon

[J Martin]

Thats the one with the 6BM8 tubes? I built that circuit a few times when I needed something to fit in a small box.

One experience I had a few years ago with a Gibson amp using muti-section filter caps may be of interest. I replaced two two section caps with individual axial caps. I grounded them the same way the two section caps were grounded. When I was done there was a backround hum I couldn't trace. On a hunch, I regrounded the caps the way I would as if I was building an amp, that is with the grounds placed at the part of the circuit the caps served. Then, no hum at all.

Also make sure the power sections of the 6BM8 tubes are in reasonable balance. I hope you installed a grounded mains cord.

[Thin Twin]

J Martin,
Thanks for the response! This unit uses the 6V6GT tubes and a 12AX7. I haven't touched the power cord on this amp, when you say a ground mains cord, do you mean a 3 prong power caord? Thanks again for the help! It is a lot of fun working on these old amps, I think I'm hooked!
Jon

[J Martin]

Yes, I meant a three prong cord. The black wire goes to the fuse then the power switch then the power transformer. The white goes directly to the other power transformer primary lead. The green is preferably connected to its own screw on the chassis, but a transformer lug will do if you don't want to make any new holes. Use a crimped on serrated lug, #6 or #8 depending on the screw rather than soldering it if you can.

[Dave Curtis, dB AudioTech]

Yes, I meant a three prong cord. The black wire goes to the fuse then the power switch then the power transformer. The white goes directly to the other power transformer primary lead. The green is preferably connected to its own screw on the chassis, but a transformer lug will do if you don't want to make any new holes. Use a crimped on serrated lug, #6 or #8 depending on the screw rather than soldering it if you can.

IMHO, a soldered connection beats a bolted one hands down; but not everyone can solder a connection to the chassis and do a good job of it...

It's very rare when I don't solder the ground to the chassis.

-DC

[J Martin]

I have seen a lot of those chassis soldered connections fall right off, especially in old Fenders. All of the modern amps have bolted grounds as far as I can see. Other than the gounding wire needing its own connection, I don't know what the regulatory agencies say about a soldered ground. Probably info at UL or NEMA. I admit, I've been too lazy to look up the exact wording.

[R.G.]

The regulatory agencies are very clear - don't solder the ground wire to the chassis. They spell it out - use a dedicated bolt to hold the safety ground wire to the chassis and use toothed washers so the high contact pressure between tooth, ring terminal and chassis make gas-tight high-pressure connections.

Solder connections are fine for high conductivity - except that solder has plastic creep failure under vibration. It's not a good mechanical material. Mechanical swaging is much more reliable over the long run for retaining a connection.

[Dave Curtis, dB AudioTech]

The regulatory agencies are very clear - don't solder the ground wire to the chassis. They spell it out - use a dedicated bolt to hold the safety ground wire to the chassis and use toothed washers so the high contact pressure between tooth, ring terminal and chassis make gas-tight high-pressure connections.

Solder connections are fine for high conductivity - except that solder has plastic creep failure under vibration. It's not a good mechanical material. Mechanical swaging is much more reliable over the long run for retaining a connection.

Correct. I no longer solder to the chassis as a sole means of grounding.

-DC

[Amp Kat]

I see a lot of amps come in with bad and loose ground connections because they didn't use that lock washer and I go back and add one. That lock washer digs into the metal and if done right isn't coming undone. Seen lots of SVT's back off transformer nuts and ground connections too but ,those lock washers work everytime.

[Dave Curtis, dB AudioTech]

I see a lot of amps come in with bad and loose ground connections because they didn't use that lock washer and I go back and add one. That lock washer digs into the metal and if done right isn't coming undone. Seen lots of SVT's back off transformer nuts and ground connections too but ,those lock washers work everytime.

I know you're probably talking about Magnavox SVTs, but I worked on a Crampeg SVT III Pro that had lost it's (torroid) tranny nut. It acted like a wrecking ball inside that chassis. I got it working and guarantee that nut will not loosen again.

[Amp Kat]

I know you're probably talking about Magnavox SVTs, but I worked on a Crampeg SVT III Pro that had lost it's (torroid) tranny nut. It acted like a wrecking ball inside that chassis. I got it working and guarantee that nut will not loosen again.

Also had an SVT Classic that did it also and was bouncing around in there taking out crap as it went until it fried the main board. Couldn't figure out what had done that damage until I accidently saw the missing nut on the tranny and found the nut lodged in a corner. So yeah I know the feeling.