How can you tell if a tube is bad?

beandip

beandip

Frito's Better Half
I pulled my GA-30 out of the closet for the first time in awhile, and I plug it in, plug the guitar in, give the tubes time to heat up, start getting hum, I'm just thinking "Ok, it's a really old amp, cant be perfect". So I start to strum and I got nothin. Tried all the other inputs (there's 3 for instruments and 1 for a microphone) and I still got nothing. I'm not sure if it's the tubes, since all the bigger ones lit up (2 brighter than the other, but the other did light up) or if all the inputs went bad (which I seriously doubt, I mean, all of 'em?). So, if you guys could help me out, that'd be great. Also, what are the big tubes called and what are the little tubes called? Sorry for my ignorance on tube amps, but I just got this one not too long ago and havnt learned much about tube amps yet :smack:

edited to say that I had this amp repaired by a local guy who owns an amp repair company called Scrapyard Amps. He replaced alot of the caps, the big tubes, and installed a 3-prong plug.
 
Re: How can you tell if a tube is bad?

The best test usually is a known good substitute for the tube you think is bad....Providing you didn't take out a resistor on the output tube...This could ruin the known good tube also?
 
Re: How can you tell if a tube is bad?

I havnt touched sh*t on this amp. I'm too afraid I'll ruin it forever. I just wanna know how come I cant get a sound out of the damn thing. it's making me really mad.
 
Re: How can you tell if a tube is bad?

1) The big tubes are the "power tubes"....they make the signal that drives the speakers.

2) The small tubes are the "pre-amp" tubes. As the name would imply they are at the front end of the signal path and do the inital amplification of the signal. But the little ones can do other things as well like driving the reverb pan and making up the gain on the other side of the reverb pan. (most of the signal is lost going through the springs so it needs a bump after the output.) They can also act as a phase inverter before the power stage of a class AB amp (most tube amps fall into this catagory.) Since in a class AB amp one big tube amplifies the positive portion of the wave and the other the negative portion you need for a phase inverter.

3)The little orange bit that's glowing inside is the heater. A tube needs to get hot to do it's thing. If you can see the heaters working that's a good thing. The fact that they don't all look about the same sounds like it could be a problem.

4) Are you sure you don't have the amp in stand-by mode? I doubt you do but...well you know.

5) Is the speaker hooked up? You said you got hum but is the amp itself humming or is the hummm comming out of the speaker?

6) If you lightly tap a tube with a pencil or similar object and you hear ping ping ping as you tap coming out of the speaker the tube has become "microphonic" with means just what it sounds like it means. The tubes is acting like a kind of microphone and amplifying external vibrations. It can still "work" but it's not good especially if it's a combo amp.

7) If you see a blue/purple haze inside a tube that means the vacum is giving up allowing a bit of air inside...it looks cool but is VERY bad...outright failure is right around the corner and it could damage other parts of the amp when it goes.


That's about the extent of my knowledge. I'd say take that puppy back to an expert and have him/her give a look-see.

Good luck.
 
Re: How can you tell if a tube is bad?

alright, the hum is comming from the speakers, and two tubes have that haze you were talking about. One you cant see inside of until the amp is turned on and the tubes warm up, then you can see the heater, but that's it (this dosnt have a purple color to the haze, it's more like a dark grey). The other one is crystal clear, and when it warms up there are little bubbles around the edge of the top and side of the purple haze.
 
Re: How can you tell if a tube is bad?

You can't really tell much from the brightness of the heaters (also called filaments). Depending on the internal construction of the tube, some are brighter than others, some you can't see at all from certain angles.

The grey haze your talking about is all the time, even when the amp is off, right? That's just crud inside the tube, probably from being old. The blue/purple haze is a glow on the inside of the glass. It usually on power tubes and it's nothing to worry about. There are web pages about it; if you want to know more, Google.

Assuming the amp worked when you put it away, your problem is likely corrosion related. Since you're getting hum out of the speaker, the power amp stage is probably okay. I'd concentrate on preamp tubes. After unplugging the amp and letting it cool down for a while, remove each tube from its socket (one at a time so you don't mix them up). Wiggle it gently in the socket to try to scrape off any corrosion that's accumulated between the pins and sockets. Or, you could try the same with a little shot of Radio Shack TV tuner cleaning spray (contact cleaner). Let it dry out well before you power it back up. Also, try not to get the spray on the tube envelope - it might take off the printing, making it hard to ID the tube later. Then try the amp again, and report back.

To go any further than this, a schematic would be helpful. There are several models of GA-30. To help find the right one, what kind of tubes are in it, and how many of each? If you can't ID the tubes by number, at least list them by # of pins. (Big 8-pin sockets or little 9-pin?)

Or.... go to Schematic Heaven.com and look on the Gibson page for an amp that resembles yours. Don't let the schematics scare you - just look at the tube placement pictures and find an amp that's got the same tubes in the same layout as yours.

Post back here, or e-mail me. It won't probably won't be hard to get it going again.
 
Re: How can you tell if a tube is bad?

Well, I'd concentrate on that 6SC7 in the phase inverter stage. If one of the 6SJ7 preamp tubes acts up, it wouldn't take out all four inputs. Also, try cleaning the pots with a bit of that contact cleaner, then work them up and down a few times.

The bad news is, these tubes are not as common as the more modern 12AX7's, etc. which will make diagnosis by tube-swapping more difficult.

Try the cleaning and wiggling first.
 
Re: How can you tell if a tube is bad?

alright, I wiggled and I cleaned, and still nothing. But the tubes are getting really hot really fast. When it was working, the tubes didnt get this hot. And the amp wasnt working when I put it in the closet. The damn thing wouldnt even come on. I was playing through it cranked about half way and all of a sudden it died. I didnt have the money to get it repaired again, so I just stuck it in the closet.
 
Re: How can you tell if a tube is bad?

beandip said:
And the amp wasnt working when I put it in the closet. The damn thing wouldnt even come on. I was playing through it cranked about half way and all of a sudden it died.
Click to expand...

Ohhhh.. that's different. Unfortunately, it enlarges the list of possibilities as to what could be wrong. Did it smoke or stink or anything when it died? You might still fix it by tube-swapping, but figuring out which one is bad is tough. You'll end up having to spring for a full set. The 6V6's and the 5Y3 will be easy to get. The 6S*7's more difficult & expensive.

That's a well-respected and somewhat collectible amp. If you can't afford to get it fixed right now, put it back in the closet until you can. Or, learn amp repairs yourself. Ampage.com is one good site, and I highly recommend AX84.com as a place to learn the basics.
 
Re: How can you tell if a tube is bad?

Thanks for the help. It didnt stink or smoke when it died, it just died. the 6S*7's are the original tubes, but the two 6V6's and the 5Y7 arnt. I wish this amp was up and running again, cause it sounded killer. Crank it up halfway, and the overdrive from pushing it along with the distortion from the original and torn speakers gives off an AWSOME sound.
 
Re: How can you tell if a tube is bad?

9 times outta 10, it's the plastic barrel input jack that has broken or cold solder joints.
If you step on the cord, it often causes the input jack to crack it's solder joints onto the PC board. Before you worry about tubes.....and provided you know what you're doing with a soldering iron, unplug the amp, take the chassis out, and re-wet the solder connections to the jack.

That, and making sure the preamp tubes aren't dead, is the first place I'd look. The fact that there's a humming noise is a giveaway......just like when you plug an amp in and leave the cord dangling without a guitar.
 
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Re: How can you tell if a tube is bad?

My amp was humming with no apparent reason. I took the pre.amp tubes off and it was still humming so I gathered it was something in the power section.

I replaced the EH 6l6 with a cheap matched pair of Peavey 5881's (Sovtek 5881 Wtx+).

Almost all of the hum went away! Also, the amp sounds a lot "warmer".

The strange thing is that the Sovtek 5881 Wtx+ are supposed to be exactly the same as the 6l6EH's I took out.....Strange, but I can hear the difference.... :confused:


Marin
 
Re: How can you tell if a tube is bad?

I just checks EVERY solder connection in the chassis, and they're all fine. Reheated them all, now they all shine like silver, but I'm still getting a hum and no sound out of my amp. This is driving me insane!
 
Re: How can you tell if a tube is bad?

Hmm. Tube amp problems are almost always the tubes or input jack. If you can get the tubes, do it. If not, don't turn the amp on at all. It'd suck to blow up another part of the circuit by turning on an amp that has shot tubes. I'm sure Jeff Seal could help you out.
 
Re: How can you tell if a tube is bad?

tried every cable I have, and they all work fine. I guess I'm gonna have to call Scott at Scrapyard Amps and have him take a look at it. It shouldnt be too much. He did a complete revamp of it for 130 (lots of new caps, new switch, fuse, jewel light, tubes, 3 prong plug, transformer or something that looks like one).
 
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