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Post by night0wl on Apr 13, 2006 23:56:03 GMT -5
I did it! ...... I managed to swap my Marshall AVT50 for a JCM800 100w combo!!  It is the same amp I posted questions about in another post regarding input jack and one tube not working. The guy was wanting an amp he could play in his livingroom and I was keen on the JCM so I offered a clean swap and he went for it. The valve is still not working and it doesn't matter which socket I plug it into, it still plays up. It also has a sort of 'burn' on the glass about half way up the valve. I am going to replace all the power valves and see how it goes. I also have access to the input jacks now so I will replace that aswell. When all four valves are working the amp sounds awesome! Cheers Shane |
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Post by sumgai on Apr 14, 2006 15:32:41 GMT -5
Shane, "Plays up"? Is that like "acts up"? If so, then yes, by all means, replace at least a pair of those tubes. But it would be better all around if you can do all four at once.  The burn mark you mention, that doesn't happen to be on a tube that is not the same make as the other three, does it? Sometimes those marks are present from the factory, it's a remainder of what they did to evacuate the envelope. Some, but not all, tubes show that as a large splotch on the side of the tube, others may show it at the top, or even in both places. A few tubes don't use that method, so they may show nothing - that's not a matter of being cheap or otherwise, it's just a different method, that's all. sumgai |
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Post by night0wl on Apr 14, 2006 17:44:09 GMT -5
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Post by sumgai on Apr 14, 2006 23:31:35 GMT -5
Shane No problem with the links, they all worked just fine. Those marks look pretty standard, I didn't see anything different from one tube to the next. What you're seeing is the remnants of what's called "the getter". The getter is a gas that's pumped into a tube at normal air composition and pressure. As the air is evacuated, the getter suddenly finds that it's no longer stable, and creates a reaction that tries to consume all the rest of the remaining gasses. It works, in a limited fashion, but by no means does it give us a perfect vacuum. But hey, it's quick and cheap!  What you say about the tube exhibiting the problem no matter what socket it's plugged into is very indicative of the tube losing it's vacuum at a very slow rate. Usually, when this happens, the tube starts glowing red from the extra oxygen that has leaked in, but at this point, the seal around one of the pins is all that's been harmed so far, and the tube is being jarred back into operation by moving it, then as you said, it eventually burns through the freshly made connection once again. BTW, if this symptom were to show up on a socket, no matter what tube was inserted, then I'd say the socket pins needed to be re-tensioned, but fortunately for you, that's not the case. A new set of tubes, and you should be good to go. BTW, good deal on your trade, you got the better end of that one, IMO. sumgai |
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Post by night0wl on Apr 15, 2006 5:04:32 GMT -5
Thanks for the info Sumgai, I will stick a set of new tubes in it and see how it goes Cheers Shane |
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Post by UnklMickey on Apr 17, 2006 14:50:27 GMT -5
hey Shane,
i thought i'd share another experience i've had with octal based tubes, er uh, valves.
the pins that exit from the glass envelope are soldered to the hollow pins of the base.
if one of those solder connections is poor, the valve will sometimes work briefly after being jostled about.
so, if the problem is with a valve, rather than a socket, look at the botttom of the pins, on the valve that's giving you trouble. you may see a solder failure.
unk
BTW, Sumgai's got that right, great deal on that trade! little bit of work required on your part, but much more value after you're done.
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Post by night0wl on Apr 17, 2006 15:30:17 GMT -5
Thanks unk, I will check that out! The tube has been working for a few days without fail but the amp has not been moved so there hasn't been anything to bump it out of its current position. I assume, if the problem is a solder failure, the tube still needs replacing or is it possible to fix it?
Shane
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Post by UnklMickey on Apr 17, 2006 16:05:24 GMT -5
back-in-the-day, i actually repaired solder joints on two or three valves.
i would never do it for a customer, but i was only 15 at the time, money was scarce, and savaging anything for my own use was important.
a really hot iron is important here so you can get the hollow pin hot enough from iron contact at the end.
after you get the solder to re-flow inside the pin, carve any excess from the tip that is larger in diameter than the pin. that will prevent the "blob" from spreading out the socket contacts when the valve is inserted in the socket. if you heated the pin from the side, you'll have to scrape ALL the solder from the side of the pin.
if you make a mess of the soldering, don't put it in a socket, and risk deforming the socket contacts. then you'll need new tube(s) and new socket(s).
unk
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Post by night0wl on Apr 17, 2006 17:47:49 GMT -5
Thanks again unk, I think I'll just replace 'em Cheers Shane |
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Post by UnklMickey on Apr 17, 2006 17:53:21 GMT -5
definitely a prudent plan.
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Post by sumgai on Apr 18, 2006 2:12:28 GMT -5
Thanks again unk, I think I'll just replace 'em
Cheers
Shane
definitely a prudent plan.This gets my Squeal of Approval! ;D sumgai |
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Post by night0wl on May 2, 2006 17:04:59 GMT -5
Ok....I am ordering valves and an input socket today, but I also have another question about this low frequency hum.
When I first turn the amp on the hum isn't there. After a couple of minutes the hum appears regardless of volume settings. The hum gets worse if I increase master volume and isn't too noticable on lower master volumes (around 2-3) but it is still there. It happens when I plug into either input. Could it possibly be preamp or power valves? Should I replace ALL valves and not just the EL34's? I couldn't see any disfiguring or leaks in the filter caps but I guess that doesn't mean much?
Hopefully it isn't a major problem because I have been using it at the last couple of gigs and it sounds awesome!
Thanks
Shane
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Post by sumgai on May 2, 2006 23:25:47 GMT -5
nite,
Time to replace your power supply filter caps. Don't argue, just do it. If you aren't sure how, just ask, there are several of us here that can guide you through the procedure. Or you could just take it to your favorite tech, your call on that one. But the humming that only comes in after the tubes have warmed up, and can be controlled by a volume control, that's the first and foremost indicator of filter caps going bad. Or one of them has already gone bad.
Sorry for the bad news, but it would have had to be done sometime in the near future anyway, so you're still ahead of the game in this trade! ;D
sumgai
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Post by night0wl on May 3, 2006 0:06:37 GMT -5
Thanks sumgai, I will have to start looking for prices of the filter caps.
Cheers
Shane
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Post by night0wl on May 3, 2006 16:07:51 GMT -5
Another quick question....Would it be OK to keep using the amp at gigs until I have it repaired or should I not use it? Is there any danger of damaging the amp from using it while it is humming?
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Post by UnklMickey on May 4, 2006 12:30:25 GMT -5
i wouldn't risk it.
if the problem is just a cap in the power supply, you probably won't damage the amp when it hums.
but are you 100% certain that's the source of the hum?
no, 90% ain't good enough.
that hum might be a symptom, associated with a more destructive problem.
besides, if it does go belly-up in the middle of a gig, what do you do then?
if you carry a spare amp just-in-case, use that instead of the JCM.
keep that "dodgy" valve out of the amp. you can use just 2 good ones, (one on each leg of the output transformer). or use 4 GOOD ones.
replace all the caps in the power supply as Sumgai suggested.
then test the amp until you are confident that the problem is resolved.
then, and only then, would it be prudent to gig with it.
unk
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Post by sumgai on May 5, 2006 12:15:47 GMT -5
Shane, What The Unkster said! Why invite trouble, and potentially more expensive repairs?  sumgai |
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Post by night0wl on May 5, 2006 17:10:49 GMT -5
Cool thanks guys. I found a tech to do the job for me but it might be a short while before I can get it to him to fix. I am using a Mesa .22+ in the meantime which also sounds great so I am in no real hurry. I just love playing that D@MN Marshall!!!  Thanks again Shane |
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