Gibson GA 8T Discoverer Tremelo Help

[CHall1525]

Hi guys just found the forum and need some help.

just got a GA8T as a project, needed to rewire the speaker and footswitch and got new tubes.

Today is the first day to try and run the amp and when turned on there is an increasing hum that seems to come on as the powere tubes warm up. It is not affected by the volume, bass or treble knobs, but if i turn the tremelo on it effects the output both depth and frequency.

Im a complete newb when it comes to amps and have been looking over the schematic i have for the amp, and everthing seems to be intact.

the only thing i have added to the amp are the tubes (2x 6eu7s, 2xel84, 1x ez81) also got a new bulb for the lamp i have either a 14v or 6v either one didnt change the sound.

Thanks in advance for any help.

Here are some pics of the guts




and the schematic i have
http://www.gibson.com/Files/schemati...von%20trem.pdf

[Chuck H]

Your schematic attachement doesn't work. It hardly matters since it's a redily available schem:http://www.webphix.com/schematic%20h...namps/ga8t.pdf

The next thing I would check would be the power tube bias. Turn the trem all the way down and measure the voltage on the power tube cathodes.

The amp looks fairly unmolested (good AND bad). You should probably replace the filter caps and any other electrolytic caps in the amp before going any further. It's straight forward work and it needs to be done anyway. Then take voltage readings at the tube socket pins and post them. It'll be working pretty quick.

FWIW I think those old Gibsons sound pretty cool. They're not that loud. An output driven line out can be added to boost volume for louder gigs and such. Really simple circuit.

[CHall1525]

http://www.gibson.com/Files/schemati...von%20trem.pdf
its actually a little different than that schematic you showed. its the same as the ephiphone ea 35t
see if that link works

one question, any way to shut this thing up while i work on it? i know im not supposed to unhook the speaker so i guess i just deal with it?

Thanks for the quick response.

[olddawg]

Dummy load. The easiest thing to do is to use a couple of 10 ohm 5 watt resistors in parallel for your purpose. They are available locally and cheap. If you work on amps much you will need to buy or build a proper load bank though.

[Chuck H]

I tried to post this morning but the site crapped out on me. I gave up after two attempts and went to work. Anyway... This is the box I built. It does 4, 8 and 16 ohm loads, is good to better than 150W and has a low level output for listening to whats going on at low volume. Cheap to make too. I used those aluminum finned resistors (the 50W are a good deal at Mouser) mounted on an aluminum chassis. The 5W rheostat could just as well be a cheap "wire wound" 3W Alpha pot. And the switch is is a plain jane on/off/on toggle and not some expensive thing like many amp impedance switches.

[52 Bill]

Chuck, thanks for posting the dummy load schematic.

To me it sounds like the amp needs new filter caps. It will be hard to find multisection caps like the originals, but you can use individual caps instead.