I have a 1966 gibson ga5t and i love it but recently when i turn the amp on the light will come on and a few minutes later it will shut off on its own. Does anyone know what this could be. My best guess would be to change the tubes but i want to get some diffrent thoughts before doing anything and if its the tubes can i do it on my own

I'd be happy to try to help as long as you post pics of your amp

Seriously, I'm surprise no one stepped up. Come on GGnA! What's up with that?

Yes, it does sound like your power tubes are saying goodbye. How old are the tubes? Changing them is pretty strait forward but we need to find some things out first. Have you googled this amp for tube biasing info? I doubt this amp is cathode biased so if you don't get any other help there is a blog in my profile on how I bias my amp that may give you some tips. Go gather some more info on your amp (type of power tubes, specs, plate voltage, etc) and report back.
Last edited by 311ZOSOVHJH at Jul 26, 2011,
hey, i apreciate the help

i got some pics of the tubes and the insides that i could get at but the size is to big for the fourm . i also read your guide to biasing and i think i will take it to a tech in my area i found a replacment set of tubes on ebay, from what i can tell there are two power tubes one preamp and a rectifier (i am very new to the world of tube amps). i tried to do my research and found this website that kind of explains whats inside the amp.
http://dustyoldamp.wordpress.com/2008/08/05/its-all-about-the-glass/ the tubes in the picture are the same ones i have in mine

when i bought it at guitar center they said the where probobly the origonal tubes

i took this from another website cant find the plate voltage (dont really know what that is ethier)

These featured a 10" Gibson Ultrasonic speaker. The tube compliment for the GA-5 in this series is:
The GA-5T used:
1-6EU7 (pre amp),
1-6C4 (phase inverter),
2-6BQ5 (in a push-pull power amp design)
solid state rectification via 2 diodes.

Control panel layout is: 2 inputs, volume, treble, bass, speed and frequency (tremolo), and a rotating on/off knob with a small pilot light above it (the fuse holder is on the back panel).

It is notable that this design used a box type chassis that is attached to the cabinet by screws through the top like the 60's
Fender amps. Also, the tubes hang down from the chassis.
if they are the original tubes they should probably be replaced.

have you ever had the amp serviced? how long have you had it?

it's a beautiful amp that's for sure.

Prs se Holcomb is the answer
Last edited by AcousticMirror at Jul 26, 2011,
I have never had an amp serviced and i have had this amp since march yea its a very great amp gets real loud 2

The third response down in this link mentions that it is cathode bias.


If that is true, then what that means to you is that you don't have to bias the amp. You should be able to pop 2 new tubes in there and go. It looks like EL84s are the common derivative of what you would look for at a store/on-line. Ask for a matched pair.

What that means is that you do not need to take it to a tech. Now, based on how old this amp is etc, it would still be a good idea to have it looked over real good and possibly consider getting its capacitors replaced.

I found this link (allamps) above along with these others that seemed to be a good read:

Last edited by 311ZOSOVHJH at Jul 26, 2011,