blown tube???

K

kormaniac

New member
Does anyone know what happens to an amp when a tube is on the fritz or has blown? I have a marshall 50W head using a 4x12 cabinet...and the sound just doesn't seem right...I checked all the speakers...they are ok, and I think there are more than one tubes in the head, however, I have yet to open it up. I just wanted to check here first if anyone knows anything about bad tubes. I'm wondering if there is a problem with them...would it be completely obvious? i.e. would the amp not even work then? Could it still work with only one of the tubes not functioning well? Thanks...
It just seems that there is too much treble...or...slight...'buzz-crackling' in high powered distortion mode.

K
 
kormaniac said:
I have a marshall 50W head using a 4x12 cabinet...and the sound just doesn't seem right...I checked all the speakers...they are ok, and I think there are more than one tubes in the head, however, I have yet to open it up.
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If it's an all tube Marshall, it'll have at least 5 tubes.

I just wanted to check here first if anyone knows anything about bad tubes. I'm wondering if there is a problem with them...would it be completely obvious? i.e. would the amp not even work then? Could it still work with only one of the tubes not functioning well?
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It's possible to run a tube amp for a long time with worn tubes or caps. Usually by the time you notice a definate problem the tubes are pretty much toast. It's one of those things that happens very gradually so you don't notice it until it's really bad.

It just seems that there is too much treble...or...slight...'buzz-crackling' in high powered distortion mode.
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Looks like possibly a microphonic preamp tube. Usually the best thing to do is replace the tubes (since that is cheap and easy compared to replacing caps, etc) and if there is no improvement then you know it's a problem with other components. This is the first thing an amp tech would do.

What model Marshall and how old? If it's more than 10 years old it might be bad caps.
 
marshall amp

Its a 50W JCM 900 Hi Gain Dual Reverb amp. Its probably at least 6 or 7 years old.
I will replace the tubes and hopefully that will help. Is it something that is easy to
do? or would you suggest bringing it to a dealer?
Thanks for your comments!
 
An amp that age needs every tube replaced. Its not hard at all, do a little reading and you can do it yourself. Otherwise, have someone do it. Your two power tubes will need to be matched for proper operation. I would say buy a pair of Mesa. You can tap on the tubes with a pencil with the volume turned up and you will hear sound through your speakers from a bad tube, especially a bad preamp tube. A shorted one will look like it has lightning (which it is) or fire inside it, and those need replaced immediatly, and will usually blow a fuse and/or burn your house down.
They are socketed, just wiggle em out, wiggle the new ones in.
The preamp tubes dont have to be in any order or matched, but always crank the amp up on dirty channel when you put em in and listen. Then mix em around. Listen. Mix em around again. Listen. THe 1st tube in your path needs to be the quietest one since any noise it creates will be amplified by the others many many times. You can ususally tell which one it is once is the quietest once you go through them. This is something most techs dont even do and MIGHT be worth the 5 minutes it takes to do it. Make sure you turn the amp back off each time.
And last on my list, always always always make sure you give it at least a one minute warm up before going out of standby mode. Its there for a very good reason. Your tubes will last three times as long if you follow this simple rule, live by it religiously. Slipping once and turning it on within 5-10 seonds, before they are completely heated, ruins every other opportunity to do so.
Bust the old ones outside on the concrete cause it makes a cool sound when the vacuum is released. :) Record it as an effect for your next album.
Peace.
 
tc electronics G major

Does anyone have one of these units and if so could you please tell me about it. I have read the reviews at harmony central but am looking for a little more info.
Thanx
 
tube dude

I'm glad you got this post tube dude! You seem to be the perfect fit!!
Thanks for the guidelines...I will change them asap and the checking for the quietest one is a great idea! Thanks a lot...and if I run into more trouble...I'm coming looking for you!!! Thanks again.

K
 
Dragonworks is smoking crack again.
The answer from the PM you sent me:
Hmmm, looks as if you have a 100 watt head with only two tubes in therefore making it 50 watts. I cant say without looking, though, but it sure sounds that way. EL34 are power tubes.
If the head has reverb, it will probably be a long square looking box. Those four things with springs are probably covers for the preamp tubes. You push them down, and twist them and they will pop off with a spring load. The preamp tubes will be inside. I think they serve to keep the tube snug and to keep RFI out. There is nothing readily accessable on the am that you cant pop off and pop back in safely. Just dont take anything off thats not socketed or whatever. There are capacitors inside that can burn your fingers off with a rather large amount of voltage even when its been unplugged for 6 months, so stay out of the actual guts of the thing.
Have a digital camera? Send me a picture and I'll tell ya what I see. Sounds like you are missing two power tubes, and that will change the overall tone of the amp somewhat. Some amps are designed to run that way (like Mesa) some are not. If its not, you may be harming the amp.
Peace!
Peace.
 
Little confused there dragonworks? :D

One thing to remember about tube amps is there are extremely high voltages present inside the amp chassis and at the sockets even after the amp is off/unplugged from the wall. Just don't go poking around in there and you'll be fine.

If you can find a copy of "The Tube Amp Book" by Aspen Pittman it's chock full of useful information on the care and feeding of tube amps. It also has schematics, lists of tubes for just about every amp ever made and some history/pics. This should be required reading for all tube amp owners.
 
tubedude said:

Bust the old ones outside on the concrete cause it makes a cool sound when the vacuum is released. :) Record it as an effect for your next album.
Peace.
Click to expand...

*smacks forehead*

Damnit! I've been using lightbulbs!
 
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