Gibson amp

[Big chief]

Just picked up a 1956 Gibson GA 70 amp has had some repairs i replaced the fuse holder turned on a very loud hum no sound from guitar. Replaced Rectifier tube, power tubes, pre amp tubes no change. Any help would be great.

Thanks

[bmccowan]

For that time period, Gibson used rather cheap electrolytic caps inside the chassis. To make matters worse, they are positioned directly above tube sockets, in a small chassis, and get wicked hot. So I would replace all of them with F&T or Nichicon capacitors. Even if it does not solve the problem, it needs to be done anyway. What else you try depends on your experience taking voltage readings, your patience for troubleshooting,etc. Likely causes are that something that is supposed to be grounded is not; or something that should not be grounded is. Be careful and I suggest you read up on this site and others on troubleshooting.

[Big chief]

Thank you it has new caps and some resistors. The problem was the fuse holder was loose where wire was connected
Before I changed the fuse holder the amp had no hum but would cut out due to the loose connection. And sounded great

[sluckey]

So, you had no hum, then you replaced the fuse holder, and now you have very loud hum.

It's very unlikely that replacing the fuse holder has anything to do with the hum issue. But you may have inadvertently/unknowingly introduced a totally unrelated problem while replacing the fuse holder. Show us some pics of your work.

[Big chief]

Photos of the amp as you can see these repairs were made before I got this amp all I gig was replace all the tubes and replace the fuse holder

[John]

Try putting the old tubes back in.   


Never mind, read your OP again. You changed tubes in an effort to get rid of hum.


That pot right by the fuse holder, I assume is the on/off plus volume? Get that yellow wire away from the fuse holder, it looks like it's all but touching the lug. It might be picking up AC hum.

[sluckey]

That pot right by the fuse holder, I assume is the on/off plus volume?

That's not a pot. It's a rotary switch, OFF-STBY-ON.

[Big chief]

The wire at the top of the new fuse holder was to short I added wire so to reach. I used a PVC coated wire with shrink warp to cover the repair joint. Could I have damaged the wire from the transformer or is the transformer going bad
Transformers appears to be original. Maybe I should find Plexi to repair amp if it is beyond my knowledge

Thanks for all your help

[sluckey]

FYI, all the GA-70 schematics I find on the net have five pots, not three pots like your amp. IOW, you may have a difficult time locating the exact schematic. This is typical of Gibson.

[bmccowan]

I also do not find that schematic. But the Fender 5e5 Pro is a nearly identical circuit, as far as I know, and that schematic is everywhere.
In looking over your photos, I would comment that some of the previous work is pretty questionable. Since this is a valuable amp - Country & Western model name? - I would want to redo all of that work. In terms of the hum, I see a lot of bare wires close to other bare wires and close to unintentional grounding points. Something gets moved a 1/4" and you have a problem. For example there is a bare wire with a solder connection near the makeshift plate holding the filter capacitors in place. That plate is no doubt grounded through the bolt to the chassis. Not saying that's the culprit, but its just an example. Also, are those push-on connectors on those filter capacitors? Be careful if you monkey with those caps as they can store quite a bit of juice after the amp is shutdown.
Done right - you will have a great amp there - nice find!

[shooter]

A said, this is a GREAT example of why fender won out 
I've done a few smaller Gibsons, and if you take the time they can be "re-wired" and cleaned up pretty easy.  It is work, pretty much every wire will be un-done and re-routed in a more logical manor but you'll have a great find when done.

the top wire on the fuse holder also looks suspect, I would expect a shiny flow that fills the hole and flows onto the tab

[plexi50]

In the past 6 months i have worked on at least (5) Gibson GA-20 and Ranger amps. They all have one thing in common. The ground for the CT and first filter cap. I have since when replacing PS caps wired them as they were wired when built. The ground,CT and first filter cab are key to a hum free amp. Not to mention i can't see how the caps are wired in the pic. I will get it fixed next week when it arrives and post what the problem was.

[bmccowan]

Having rewired many Gibson amps, I am interested to see how this goes. It's good to know Plexi that you have found consistency in getting the hum level down. I just do this as a hobby, but have managed to find and rebuild - 2-GA-20s (1 octal preamp), a GA-40 LP, a cool GA-25 with both an 8 and 12" speaker, a GA-30, a BR-6, a BR-4, a GA-8, A GA-9 (hardest amp to work on - tiny turret board with jumpers running under the resistors and caps on the board!) and a few GA-5 variants. Every time I finish one, I think, "Ok you damn fool, that's the last Gibby" and I stumble into another that is different.

[bmccowan]

Plexi,
I got distracted thinking about how crazy I was with all the Gibson amps, and forgot to ask my question. Are you saying that in your experience its best to retain the original grounding scheme, rather than try to improve upon it. I think that is what I've done with mine, but can't be sure, as some were done some time ago. And since many amps have previous repairs its sometimes difficult to determine how it was originally done. For the amps you've worked on the first filter cap and CT are always grounded together? And the filament CT - I think that is sometimes floated off the power tube cathode - is that what you are seeing?
Thanks

[plexi50]

Yes and the original grounding scheme is very important. Slucky set me straight on that a few months ago. At the time I just assumed I could take the CT and ground it to the chassis and all it would be fine. But you must have that CT and first filter stage and ground go to the unused pin one of the power tube to have it as it was originally wired. it will work if you ground the CT to the chassis but as Steve set me straight the resistor and the filament has to be grounded as well and all of them being at the same point as it was made originally is the way it should be when caps are replaced. It can mess with your mind just a little bit if you've been working on Marshall Plexi and fenders all these years and then you stumble upon an old Tweed Gibson where everything looks strange and different from what you're usually a custom 2 and the grounding scheme. and the only grounding that Amp really had was the rivets on a painted chassis which was not a good idea to start with so having all the ground wires going to the unused pin of the power tube is crucial. I'm on my phone right now talking this so I'll get on my computer and Overlook what I said tomorrow because sometimes what I say doesn't come out right on the phone. Sluckey is much better in his wording and can explain exactly what you may have done wrong there if you did wire the filter caps differently on the ground
of the first filter cap

[bmccowan]

Thanks Plexi for the insight. Indeed I started years ago on Fenders after finding a non-working Princeton Reverb at a flea market, brought it home, and thought, "I bet I can fix this thing." I caught the bug, learned some stuff, avoided electrocution, and went on to many more amps including Fenders, Valcos, Magnatones, and Gibsons. When I started seeing unused tube socket pins used as convenience connection points I thought, "what is this crap?" But in most cases I replaced filter caps using the same connection points. At times I installed a lug under a PT bolt and attached the CT and the main filter cap ground at that point. To my recollection, that worked ok too. Again, thanks for the info as its likely to help anyone chasing hum on Gibsons. I'll be watching for your experience with Big Chief's amp which obviously had some "creative" previous repairs.