04-23-2016, 10:22 PM (This post was last modified: 04-23-2016, 10:24 PM by cthendrico.)
So I finally bought my dream amp of a gu-12 on eBay. It was reasonably packed but sometime shipment the output transformer fell out of its bracket and pulled its red wire apart, its blue and brown wire away from the circuit board tearing some of the circuit traces with it! (power tube fell out and broke as well) It appears that one of mount bolts on the bracket had at sometime fallen out and it was super glued? to the chassis instead of going to the hardware store and buying a 25 cent bolt and nut. Infuriating. My question now is the situation salvageable? Looks like the output transformer took a ding or two as it fell. So my first question is
1. Does the transformer look salvageable?
and
2. How hard would it be to reattach the wires to circuit board with the traces broken.
I have taken pictures for reference. Please help or let me know if i might be missing something.
More pics...
04-24-2016, 08:19 PM (This post was last modified: 04-24-2016, 08:21 PM by Hangman.)
That is very unfortunate! Sorry to hear about it.
On the bright side. I think it is likely that the transformer is still functional. you can fit it back into the A-frame and use some varnish or epoxy to hold it in place. the red wire can be extended using new wire and heat shrink, and you can solder the blue and brown wires directly to the pins of the power tubes.
its a bit of work, but worth it for sure. Lets us know if you need some specific guidance, i would be happy to go into detail.
Also, document your progress in this thread. These sorts of problems are not totally uncommon and your documentation could help others in your situation
Ok. So i looked all over for the bolt that wasn't holding the one side of the transformer. i thought maybe years ago it had fallen and gotten lodged under the speaker or something or maybe under the reverb tank. Never could find it so I just went to the hardware store and got the 55 cent piece. I actually bought two to hold both the transformer. I didn't think to solder right to the tube terminals. Thats a way better idea. I doubt I will have enough cable so i'll probably have to splice the blue and brown cables as well. No biggie I suppose. Its a exceptionally clean unit. This is my 4th ampeg by the way. I guess while i'm in there i'm going to convert it to a three prong cable as well.
The one thing i'm a little nervous about is biasing the power tubes. I have never done that. any details or know of a different post that walks one through the biasing of these kinds of amps?
Ok this project has started back again. I might need some specifics. For instance when I tried to put the transformer back I just noticed that the bracket and transformer are too wide for the holes in the chassis. It seems like my entire transformer and bracket don't fit and that why one side was glued? I started looking at pictures of other gu 12 and it seems like they all have different transformers then how mine looks? Could this be a hack job I'm inheriting?! Please look at the above pics of my transformer those familiar with these amps and let me know. This is a black lettering Magnavox era of these amps? Because my writing color on my transformers matches up with the schematic glued to my chassis? Am I tripping?
Also if I have to splice the wire once I get the transformer attached my wire on my transformer says that it's 22 gage and rated 500 volts. I know I should match the gage. Does it matter how much voltage the wire can handle? The schematic says 370v? Am I risking problems by putting in 22 gage write rated at 300v? Radioshack doesn't carry anything else
If the transformer doesn't fit the holes, it isn't the right transformer, that doesn't necessarily mean it won't work, but if you want something faithful to the original, Mercury magnetics has a replacement.
the voltage rating of the wire also refers to the breakdown voltage of the insulation. there are plenty of place only that you can find wire, if its a matter of volume, you could probably buy a short length from your local repair shop.
07-17-2016, 03:21 PM (This post was last modified: 07-17-2016, 03:32 PM by cthendrico.)
Ok so I went ahead and soldered the wires directly to the circuit board and replaced the broken power tubes. Initially the amp worked great. The tremolo and reverb sounded wonderful. It was clean and clear and had that great amp overdrive. I played it for like 2 hours that night and it sounded great. The next day at lunch i came home and started playing. Again it sounded wonderful. About a hour into playing it mysteriously lost the reverb, then the tremolo and then completely shut off. I have not been able to turn it on again at all. The pilot light will light up but absolutely no sound comes from the amplifier now. no hum no hiss no nothing. The tubes are not glowing at all. What could that mean? Did the output transformer crap out on me?
Here's a picture of the other side of the board. Everything looks in order. The bigger cap looks like it's partially dried out but still working I believe
That is odd... take your volt meter and check the power supply. check to see if you are getting DC voltage on the plates, also, check the AC filament voltage.
its hard to say what happened, but once we get some measurements taken, things should be more clear.
