I came across the version of the Gibson GA-8T that was produced from '62-'66 (The version with 5 tubes). It was missing the 6CA4 tube and didn't power up. With a new fuse, the power light turns on and the 4 tubes light up.

So, I ordered an Electro-Harmonix 6CA4 from tubedepot.com put it in the amp and flipped the switch. After a few seconds, the tube produced a blinding bright light and I turned the amp off immediately. The tube was burnt out. Since then I purchased some off brand 6CA4 from ebay, tried it again, and had the same results.

My experience with electronics is limited to solid-state practice amps and effect pedals. Any suggestions would be extremely appreciated. Please help! I wanna play/record this baby...

You apparently have a problem with the heater circuit. Some tubes will flash very brightly upon power-up, and it's fairly "normal". Are you sure the tube is burnt out?

I suppose if it's "blinding" light, it would indicate major tube fryage though.

Check the voltage to the heater pins on that socket, to start with.

Have you given the boards and components a good eye balling? Look for burns marks etc. anything obvious that shouldn't be there. Sounds like you may have a duff component somewhere around the tube.

Glowing bright sounds like what happens when you plug a tube into a socket with a much higher filament voltage than it wants, like if you plug a 6V tube into a 12V socket.

I can't find a 5-tube version of this amp. Do you have a schematic? All I can find is the 4-tube one that doesn't include any 6CA4.



The 6CA4 is a rectifier tube that plugs into a mini socket (same as a 12ax7 plugs into). Why would they replace the 5Y3 with a weird and uncommon 6CA4, and what was the other tube that they added?

I'm guessing you're maybe plugging the tube into the wrong socket somewhere. Does the amp have a tube chart on it?

"The second GA-8T was part of the Crest Line range of amps and was in production from 1962 upto 1966 and used two 6EU7's, two 6BQ5 pentodes and a 6CA4 rectifier. This was the same amp as the Epiphone EA-35T Devon"

So I guess they did actually swap the rectifier for a weirdo. You sure you've got the tubes in the right sockets? The 6BQ5 (aka EL84) sockets and the 6CA4 socket are gonna be identical, as are the two preamp sockets. A tube chart laid out correctly is gonna be real important here...

Or, you can open the amp up and look at which socket is the rectifier, which two sockets are the output tubes, and which two are the preamp tubes. That wouldn't be so hard, actually, even for a novice.

Just a couple of points:

Be careful. Check what Voltage the amp is set to run on. Some power transformers let you switch between UK/European/US Voltages. If you feel confident enough, then you could check the valve base voltages as per the schematic, although these will vary when unloaded (probably be a bit higher). As already mentioned, look for burnt or damaged components, and if you suspect a bad resistor then take it out and measure it.

A good amp tech should be able to help also.

FWIW 6CA4/EZ81 is actually a very common rectifier, probably used in nearly as many small amps in the 60's as the 5Y3, just not quite as many GUITAR amps since Fender never used it.

The key thing is, you say it glows bright "after a few seconds", meaning it probably only starts glowing super bright after the cathode starts emitting. Unlike the 5Y3, the 6CA4 has an indirectly heated cathode which means potentially one side or a center tap on it's filament supply could be grounded. If the cathode pin accidentally touched or arc-shorted to one of the heater pins then after a few seconds of warm up the B+ could short across the filament supply and make the heater glow super bright, but you'd think it would also probably blow the fuse.

I'm not SURE this is what's happening, but at least it gives you something specific you can check (assuming you ever look at this board again). If you have an meter, take out the tube and (with the amp long turned off and unplugged) measure the resistance between pins 3 and 4 of the socket. If there's low resistance of like, say, under a K or so, then that's your problem.

Ned

PS,

Before you check that resistance you should check the DC voltage between pin 3 and the amp chassis to make sure it's 0 or pretty close to 0. If it's not, leave the amp off for another day and try again. But it'll probably be 0 to begin with.

I'm in the presence of geniuses! Thank you so much for all the advice. I'll keep you posted on my progress. Thanks again!

good luck...

gibson amps are amazing..

does it have the original speaker?