replacing valves

I think my JCM900 head needs some new valves (sound keeps fading in and out) but the problem is I don't know which ones. Is there anyway of telling without having to replace the full set?

You can swap preamp tubes around without having to adjust anything. I'm not familiar with Maeshalls at all, but if you 'borrow' a tube from one channel to swap for a tube in the other channel, you can find out if the problem moves around with your tube-swapping.

If nothing changes, that tends to indicate the power tubes, which are more likely to wear out, actually. Preamp tubes should last a long time, but power tubes take a beating, especially if you play the amp cranked.

And remember that power amp tubes (valves) should not just be replaced. You'll need to have the amp re-biased for the new tubes. Failure to do so may cause damage to the amp. This is not a do it yourself thing, take it to your tech for re-biasing and re-tubing. Pre-amp tubes can just be replaced, they don't need any kind of adjustment (biasing).

Originally Posted by: JFRICKAnd remember that power amp tubes (valves) should not just be replaced. You'll need to have the amp re-biased for the new tubes. Failure to do so may cause damage to the amp. This is not a do it yourself thing, take it to your tech for re-biasing and re-tubing. Pre-amp tubes can just be replaced, they don't need any kind of adjustment (biasing).

Isn't it possible to get color matched groove tubes, so you can change the tubes yourself. my fender delux came with tis type of tube and recomends that if I order the right tubes I won't have trouble with the bias.

Originally Posted by: LordathestringsYou can swap preamp tubes around without having to adjust anything. I'm not familiar with Maeshalls at all, but if you 'borrow' a tube from one channel to swap for a tube in the other channel, you can find out if the problem moves around with your tube-swapping.

If nothing changes, that tends to indicate the power tubes, which are more likely to wear out, actually. Preamp tubes should last a long time, but power tubes take a beating, especially if you play the amp cranked.

How do you like to set up your guitar and amp?

Once a long time ago there was a city.
In this city A musician could actually remove the tubes from his amp and take them to the nearest Walgreens, eckerd, or other such establishments.
At these places one could check his tubes for problems, and even by replacement tubes rite there. This is a true story. One could get a power tube for around $6.00. However those tubes were not pre tested, or color coded, but hey who knew. :cool:

Originally Posted by: rightturnonlyIsn't it possible to get color matched groove tubes, so you can change the tubes yourself. my fender delux came with tis type of tube and recomends that if I order the right tubes I won't have trouble with the bias.

You always want to get matched tubes anyway...and don't pay that any attention, always get your amp re-biased when getting new power tubes, no matter what the manufacturer says....

I went out and bought a full set of tubes in the end. I got some power valves that are perfectly matched apparently, but I don't know if the amp was biased before. Should I take it in?

definately take it in. If you bought the amp new then most chances are it was biased but not perfectly. Most manufacters bias amps cold to save money and avoid problems. So it will needs re-biasing even if it was biased properly to start with because not every batch of tubes can be the same even if the are the same type/manufacture. Take it to a tech

how do you bias an amp? i cant even see how to get into the thing! :D

if you cant even get into it, I definately would not advise you bias that your self. Take it in.

There are a few pieces of equipment involved with biasing...It has to do with measuring voltage, then doing some calculating, adjusting, then re-measuring and calculating. Biasing is not a do it yourself thing unless you have a serious background in electrical engineering.

You can easily get shocked by a tube amp just by taking the back cover off and removing a tube. The tubes (valves) hold a charge that can send you across the room, or even worst, kill you. Even after the power has been turned off and the amp has been unplugged. To properly discharge the valves, always turn the power switch off with the standby still on....or turn the standby switch on with the power switch off...

Originally Posted by: JFRICK.... The tubes (valves) hold a charge that can send you across the room, or even worst, kill you. Even after the power has been turned off and the amp has been unplugged. To properly discharge the valves, always turn the power switch off with the standby still on....or turn the standby switch on with the power switch off...

Sorry, mate. You just lost any credibility you might have had on this topic. Tubes do NOT hold charge.

Never!

Period.

End of story.



The filter capacitors in the power supply - now there's a whole 'nother story!


Having said that; I would seriously advise most of our members to have their amps set up by a professional. Poking about in a tube amp can get you dead very easily. It ain't worth it.

Originally Posted by: LordathestringsSorry, mate. You just lost any credibility you might have had on this topic. Tubes do NOT hold charge.

Never!

Period.

End of story.



The filter capacitors in the power supply - now there's a whole 'nother story!


Having said that; I would seriously advise most of our members to have their amps set up by a professional. Poking about in a tube amp can get you dead very easily. It ain't worth it.

My bad...You are correct in that the tubes themselves don't hold a charge, it is the filter capacitors. I was just trying to make the point not to go probing around in a tube amp as they can still shock you even with no power supply hooked up....

I never claimed to be an expert, so credibility on the subject doesn't mean anything to me.

Originally Posted by: JFRICK.... I never claimed to be an expert, so credibility on the subject doesn't mean anything to me.

:D LOL ..........