Post by Admin on Jun 26, 2013 5:37:53 GMT -5
Troubleshooting your tube amp
Some tips on fixing problems with old tube guitar amps.
Tubes cause at least 50% of tube amp problems. If you have a tube amp and you haven't tried changing all the tubes, then do that first. Even problems that you wouldn't associate with tubes could be the tubes. If changing all the tubes fixed the problem, then go back and try to figure out which tube it was and replace that one. If it was a power tube (usually the bigger ones, and there are usually an even number of them), then you probably have to have the amp biased. It's always best to change all the power tubes at once, not replace one at a time.
When gigging with a tube amp, always bring at the very least one of every tube in the amp and some proper sized fuses for the amp. It's even better to just bring a backup amp. We that use tube amps must take the bad with the good. We pay a higher price for the amps and maintenance, and to a lesser extent pay for expected breakdowns all for the payoff of awesome tone (jeez do I sound like a Finance major yet?)! You have to take the bad with the good and assume that your amp is going to break sometime. They all do.
These tips are for blackface/silverface Fenders. But the logic in some of these tips can be used on other amps as well.
I highly recommend getting Gerald Weber's books. There is quite a lot of self promotion in there, and a few inaccuracies, but they are good. This guy knows his stuff and is very generous with his knowledge in these books (but he does get what, $25 each for them?). I'm not going to go through biasing on the web page, but Gerald does in the books. Get the first one first (hip tube amps). I've found that a lot of the things Gerald has found in fixing problems mirrors my experience exactly!
Remember, tube amps have voltages that will shock the daylights out of you, even when unplugged. Don't go inside it if you don't know how to drain filter caps!
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Troubleshooting at the gig (or troubleshooting without opening up the amp)
Troubleshooting at the gig assumes you have no time to open up the amp and look inside or have much time to do anything at all! It also assumes that if these suggestions don't fix the amp, that you need more time and probably need it opened up. This is about the extent of troubleshooting you can do without opening the amp.
1. The amp blows fuses
This is a common problem. Usually it's a shorted rectifier or power tube. Take all the tubes out of the amp, replace the fuse and turn on both switches. If the fuse blows then you probably have a shorted filter cap or in extreme circumstances, a blown power transformer. This is not going to be fixable at the gig. If the light stays on when you take all the tubes out, shut the amp down and install the rectifier tube (if it has one), then the power tubes, then the preamp tubes one by one turning the amp on every time you install a new tube. When the fuse blows, you've found the bad tube. Sometimes the fuse will blow if you simply flip both switches on at the same time (which you should never do! Shame on you if you do this! Let the tubes warm up about 30 seconds before you turn on the second switch!). If you find that the fuse blows after replacing one of the power tubes, and still blows when you install a known good power tube, you could possibly have a blown output transformer.
2. There is a crackling sound that happens all the time or sporadically
This is usually bad preamp tubes. Unplug tubes one at a time going from right to left looking from the back while the amp is making noise. The one that stops the noise is the one you want to replace. Replace it with a known good one and see what happens. If you still have noise, you very likely have a bad component somewhere inside the amp.
3. Controls on amp are very scratchy and make too much noise to adjust on stage
Turn off the amp and turn the noisy controls vigorously through their whole range. This clears the garbage out of their path temporarily. This will probably get you through the gig. You should have the pots cleaned with pot cleaner though. Or possibly have the pot replaced. If this doesn't fix the pot, then you may have other problems.
4. Reverb doesn't work
Make sure the reverb send and return cables are plugged in tight, and the red one is on the right. Replace the third tube from the right looking from the back (second on a Princeton Reverb). This is the reverb driver tube (12AT7). Replace the fourth tube from the right looking from the back (third on a Princeton Reverb). This is the reverb recovery tube (12AX7). If all this doesn't fix it, try plugging into a different reverb pan from that backup amp you should have brought!
5. Tremolo doesn't work
Replace the fifth tube from the right (forth on a Princeton, second on a Vibro-Champ, third on a non reverb amp) looking from the back (12AX7). If that doesn't do it you should check to make sure that your footswitch is working properly. The tremolo will not work on blackface/silverface Fenders without the footswitches plugged in on almost all the amps.
6. One channel works but the other doesn't (not applicable to Princeton, Champ)
If the vibrato channel works but the normal one doesn't, replace the first tube on the right looking from the back (12AX7). This is the first and second gain stages for the normal channel. This is the only tube associated solely with the normal channel. If the normal channel works but the reverb channel doesn't work, replace the second and fourth tube from the right looking from the back (12AX7's).
7. Both channels don't work but there is still hiss coming out of the speaker
Change the sixth tube from the right looking from the back (12AT7). This is the phase inverter tube and is the only one associated with signal on both channels. It is also the tube that is nearest to the power tubes on most amps.
8. Amp makes sound but is very low level and distorts easily
Make sure the speaker is plugged into the right jack. Plugging into the ext. speaker jack will make the amp very quiet and it will distort at a low level. This is bad for the amp. It's a good way to damage it in short order. Also, make sure all the tubes are plugged in securely. If this still doesn't do it, start replacing tubes starting at the right and going left (looking from the back).
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Troubleshooting at home
Once again, don't do this unless you know how to discharge filter caps! You can shock the daylights out of yourself even if the amp is unplugged.
First I will go through some general maintenance type tips. All of these require removal of the chassis. You'll need a soldering iron and meter too.
1. Clean the pots
Get some electrical parts cleaner that is made for cleaning pots. It will have a lubricant in it which is what pots need. A lot of people recommend Caig Deoxit, but I just use some stuff I get at the local warehouse. Spray a little cleaner in from the top where the terminals are on the pot. Turn it through it's range a few times to get the cleaner everywhere. If the pot still makes noise after cleaning and turning it a few times, you could have problems elsewhere in the amp.
2. Tighten the pots
Many of the pots need to have good contact with the chassis because their ground is part of the circuit. Tweeds especially. Check the resistance from the body of the pot to the chassis. If you get anything more than an ohm or two, then you need to either tighten the pot or take it out and turn the star washer underneath. I've found pots that have a resistance from the body to ground of higher than 1k! This will cause problems if something is grounded to the back of the pot! Spraying the chassis where the pot makes contact would be a good idea too.
3. Check for bad solder joints
These things show up on Fenders all the time and cause some very strange problems. The circuit board usually isn't the problem. All the components were bent and run completely through the eyelets and got a good joint usually. The ones to look for are the grounds going to the brass grounding strip under the pots. These will frequently lift up a little. Or they may not even look bad till you look really close. You'll need a fairly hot iron to reheat these. It's not a bad idea to remove the old solder and put in new. Another place to look for bad joints is the tube sockets. These are sometimes hard to find. The preamp tubes are especially suspect. If you see what looks like a loose wire or even a little bit of a gap where the wire goes through, remove the old solder and put in some new. The pots with components grounded to the back of the pot are worth looking at too.
4. Check for broken component leads
This happens quite often. If you poke around in the amp with a non conductive probe (like a plastic pen or marker), you'll sometimes find that the lead has broken off a component. It happens right where the lead goes into the body of the component. This happens to resistors, and to Mallory filter caps most often. If you look right up near the body of the component, it sometimes looks like the lead is misaligned. If you poke it, the lead may move. Those old carbon comp resistors can crack right through the middle too. Look underneath at the filter caps as well, these are not immune. But these should probably be changed anyway!
5. Remove solder in places where there is too much
Some people seem to think that globbing a bunch of solder on a joint is the way to keep it together. This is not true. I spend a lot of time sucking big globs of solder out of amps that someone worked on. Just add enough solder that it covers the joint and holds securely. Solder is for making a connection, not physically holding things together.
6. Fix things that look obviously hacked
Some people have no respect. I see amps with wires all over hell duct taped together and wire nutted. Put in clean and straight wires here. No wire nuts or other crimp type devices inside the amp! It's usually much easier just to replace the whole wire than try to splice it with something that could eventually fail anyway. A lot of times you'll see components soldered on to what looks like the leads of an older component that was cut out. If components look like they have their leads soldered to old leads that were cut, fix it. Just get a new cap or resistor and take out all the old garbage. Your amp will thank you! Besides, doesn't it feel good to get the amp back in shape?
7. Tighten everything down
Transformers come loose, as do jacks and everything else. Make sure everything is tight, but don't crank on things too hard. We are dealing with self threading sheet metal screws, they can't be tightened too hard. Speakers always come loose after time. You should check all this stuff periodically too.
8. Change the filter caps
Just do it. Even if they are not bad or causing problems, just change them. If the amp is anywhere near about 20 years old or older, the caps need changing. Don't forget the cathode bypass caps and the bias supply filter cap!
9. Retension the tube sockets
It is important that you drain filter caps before you do this. Don't try to retension sockets with the chassis still in the amp. The plate pin of the power tube will shock the crap out of you. Don't do it!
This really only works on the power tube sockets. If you look down in there you'll see that there is a curved part of the pin and a flat part (not always, some of the newer amps have different kinds of sockets). Take a little tiny screwdriver or a paper clip and bend the flat part inward so they contact the pin better. Don't bend these too much, because if you do, the pin of the tube will come in and get caught on the pin that you bent in too much. It can sometimes bend the pin of the socket down and your tube won't go into the socket. This pretty much ruins the socket. Be careful! The preamp tube sockets aren't really easily worked on. This is something that I do if I suspect that a preamp tube isn't making good contact. First, try putting the tube in a pin straightener if you've got one. That may do the trick. But what I do is bend the pins of the preamp tube inward VERY SLIGHTLY. I want to emphasize VERY. If you go at it with a hamfist, you will crack and ruin the tube. As a matter of fact, the tube may be weak anyway and it will crack even if you are careful. Make sure you bend all the pins in about the same amount. If you bend them too much, then the pins of the tube can get hung up on the socket and you'll bend them even more and possibly break something. If you've done this correctly, you'll notice that the tube goes in a little tighter than before, which should make better contact than before. If you can't do this or it doesn't seem to fix anything, just get a new socket. They are cheap.
10. Check for leaking coupling/blocking caps
In older amps, especially Fender tweeds in my experience, the blocking/coupling caps can start leaking DC. The job of these caps is to keep high DC voltage from getting to places where it will cause damage or make the circuit not work (grids of tubes or pots come to mind). Usually one side of a blocking cap will have DC on it, and the other will have none. This is not always the case though, so be sure to check the schematic (you check these by lifting the side that has the lower voltage on it). If you find much more than .1 volts DC on the side that there isn't supposed to be DC, then the cap is a candidate for changing. In tweed amps, the red or yellow Astrons are almost always leaky. In blackface, there are some reddish brown ceramic caps that are candidates for changing. The ones in the tremolo circuit, and the .02 in the vibrato channel are bad more often than not. Also, the 500pf and 250pf's are worth looking at.
--------------------------------------------------------------------------------
Amp hums very loud
This could be many things, and there are many different kinds of hum! I'll go through a few things here.
1. Amp buzzes loud when you take your hands off the strings
This is going to sound incredibly simplistic, but it happens! Usually, switching the ground switch on the back of the amp fixes this. Not many people are aware of this, and I get asked this question more than you'd think ("Why is my amp buzzing?"). If your amp doesn't have a ground switch and has a 2 prong cord, take the plug out of the wall and turn it around. This changes the ground refrence in the chassis which is the same effect as the ground switch.
2. Silverface amp/blackface Bassman hums loudly
Silverface amps, and the blackface Bassman, had a bias balance circuit that would make the amp hum very loud if it wasn't set right. You can turn the bias pot a LITTLE either way and see if it reduces hum. If no difference is heard, stop turning it and put it back to about where it was. These should be converted to the standard blackface bias circuit anyway. Some later silverface amps had a "hum balance" control too. This is a pot on the filament supply that balances it. You can turn that one with no damage. It's best to adjust that pot with the amp turned up loud but with nothing connected to the input. Adjust it untill you hear the least amount of hum.
3. Change the tubes
The filament in a tube can cause hum. When tubes get old, sometimes they hum. Try unplugging tubes going from right to left looking from the back. When you pull out a tube that reduces or eliminates hum, try a new one in that position. The rectifier tube could be causing hum as well. Sometimes in a 4 power tube amp, one power tube will fail and this will cause the amp to hum.
4. Change the filter caps
I know I know, I'm sounding like a broken record here. But when filter caps lose their filtering capability, they cause the amp to hum. Their whole job is to turn AC into DC the best they can, which is partly to reduce hum! The bias supply cap is also suspect.
5. Check for bad grounds and bad solder joints
One more thing to check that I've already mentioned. Make sure that all solder joints look solid. Poke around inside the amp with a pencil or piece of wood to make sure that solder connections are good.
6. Check your input jacks
The input jacks on amps are usually shorted. This means that when you unplug your guitar cord, the jack gets shorted to ground. If these shorts don't work, the amp may not work right (in the case of a JCM800 Marshall, if the short isn't working on your low gain input, your high gain input won't work!), or cause it to hum. These can't be retensioned. Just get new ones!
Some tips on fixing problems with old tube guitar amps.
Tubes cause at least 50% of tube amp problems. If you have a tube amp and you haven't tried changing all the tubes, then do that first. Even problems that you wouldn't associate with tubes could be the tubes. If changing all the tubes fixed the problem, then go back and try to figure out which tube it was and replace that one. If it was a power tube (usually the bigger ones, and there are usually an even number of them), then you probably have to have the amp biased. It's always best to change all the power tubes at once, not replace one at a time.
When gigging with a tube amp, always bring at the very least one of every tube in the amp and some proper sized fuses for the amp. It's even better to just bring a backup amp. We that use tube amps must take the bad with the good. We pay a higher price for the amps and maintenance, and to a lesser extent pay for expected breakdowns all for the payoff of awesome tone (jeez do I sound like a Finance major yet?)! You have to take the bad with the good and assume that your amp is going to break sometime. They all do.
These tips are for blackface/silverface Fenders. But the logic in some of these tips can be used on other amps as well.
I highly recommend getting Gerald Weber's books. There is quite a lot of self promotion in there, and a few inaccuracies, but they are good. This guy knows his stuff and is very generous with his knowledge in these books (but he does get what, $25 each for them?). I'm not going to go through biasing on the web page, but Gerald does in the books. Get the first one first (hip tube amps). I've found that a lot of the things Gerald has found in fixing problems mirrors my experience exactly!
Remember, tube amps have voltages that will shock the daylights out of you, even when unplugged. Don't go inside it if you don't know how to drain filter caps!
--------------------------------------------------------------------------------
Troubleshooting at the gig (or troubleshooting without opening up the amp)
Troubleshooting at the gig assumes you have no time to open up the amp and look inside or have much time to do anything at all! It also assumes that if these suggestions don't fix the amp, that you need more time and probably need it opened up. This is about the extent of troubleshooting you can do without opening the amp.
1. The amp blows fuses
This is a common problem. Usually it's a shorted rectifier or power tube. Take all the tubes out of the amp, replace the fuse and turn on both switches. If the fuse blows then you probably have a shorted filter cap or in extreme circumstances, a blown power transformer. This is not going to be fixable at the gig. If the light stays on when you take all the tubes out, shut the amp down and install the rectifier tube (if it has one), then the power tubes, then the preamp tubes one by one turning the amp on every time you install a new tube. When the fuse blows, you've found the bad tube. Sometimes the fuse will blow if you simply flip both switches on at the same time (which you should never do! Shame on you if you do this! Let the tubes warm up about 30 seconds before you turn on the second switch!). If you find that the fuse blows after replacing one of the power tubes, and still blows when you install a known good power tube, you could possibly have a blown output transformer.
2. There is a crackling sound that happens all the time or sporadically
This is usually bad preamp tubes. Unplug tubes one at a time going from right to left looking from the back while the amp is making noise. The one that stops the noise is the one you want to replace. Replace it with a known good one and see what happens. If you still have noise, you very likely have a bad component somewhere inside the amp.
3. Controls on amp are very scratchy and make too much noise to adjust on stage
Turn off the amp and turn the noisy controls vigorously through their whole range. This clears the garbage out of their path temporarily. This will probably get you through the gig. You should have the pots cleaned with pot cleaner though. Or possibly have the pot replaced. If this doesn't fix the pot, then you may have other problems.
4. Reverb doesn't work
Make sure the reverb send and return cables are plugged in tight, and the red one is on the right. Replace the third tube from the right looking from the back (second on a Princeton Reverb). This is the reverb driver tube (12AT7). Replace the fourth tube from the right looking from the back (third on a Princeton Reverb). This is the reverb recovery tube (12AX7). If all this doesn't fix it, try plugging into a different reverb pan from that backup amp you should have brought!
5. Tremolo doesn't work
Replace the fifth tube from the right (forth on a Princeton, second on a Vibro-Champ, third on a non reverb amp) looking from the back (12AX7). If that doesn't do it you should check to make sure that your footswitch is working properly. The tremolo will not work on blackface/silverface Fenders without the footswitches plugged in on almost all the amps.
6. One channel works but the other doesn't (not applicable to Princeton, Champ)
If the vibrato channel works but the normal one doesn't, replace the first tube on the right looking from the back (12AX7). This is the first and second gain stages for the normal channel. This is the only tube associated solely with the normal channel. If the normal channel works but the reverb channel doesn't work, replace the second and fourth tube from the right looking from the back (12AX7's).
7. Both channels don't work but there is still hiss coming out of the speaker
Change the sixth tube from the right looking from the back (12AT7). This is the phase inverter tube and is the only one associated with signal on both channels. It is also the tube that is nearest to the power tubes on most amps.
8. Amp makes sound but is very low level and distorts easily
Make sure the speaker is plugged into the right jack. Plugging into the ext. speaker jack will make the amp very quiet and it will distort at a low level. This is bad for the amp. It's a good way to damage it in short order. Also, make sure all the tubes are plugged in securely. If this still doesn't do it, start replacing tubes starting at the right and going left (looking from the back).
--------------------------------------------------------------------------------
Troubleshooting at home
Once again, don't do this unless you know how to discharge filter caps! You can shock the daylights out of yourself even if the amp is unplugged.
First I will go through some general maintenance type tips. All of these require removal of the chassis. You'll need a soldering iron and meter too.
1. Clean the pots
Get some electrical parts cleaner that is made for cleaning pots. It will have a lubricant in it which is what pots need. A lot of people recommend Caig Deoxit, but I just use some stuff I get at the local warehouse. Spray a little cleaner in from the top where the terminals are on the pot. Turn it through it's range a few times to get the cleaner everywhere. If the pot still makes noise after cleaning and turning it a few times, you could have problems elsewhere in the amp.
2. Tighten the pots
Many of the pots need to have good contact with the chassis because their ground is part of the circuit. Tweeds especially. Check the resistance from the body of the pot to the chassis. If you get anything more than an ohm or two, then you need to either tighten the pot or take it out and turn the star washer underneath. I've found pots that have a resistance from the body to ground of higher than 1k! This will cause problems if something is grounded to the back of the pot! Spraying the chassis where the pot makes contact would be a good idea too.
3. Check for bad solder joints
These things show up on Fenders all the time and cause some very strange problems. The circuit board usually isn't the problem. All the components were bent and run completely through the eyelets and got a good joint usually. The ones to look for are the grounds going to the brass grounding strip under the pots. These will frequently lift up a little. Or they may not even look bad till you look really close. You'll need a fairly hot iron to reheat these. It's not a bad idea to remove the old solder and put in new. Another place to look for bad joints is the tube sockets. These are sometimes hard to find. The preamp tubes are especially suspect. If you see what looks like a loose wire or even a little bit of a gap where the wire goes through, remove the old solder and put in some new. The pots with components grounded to the back of the pot are worth looking at too.
4. Check for broken component leads
This happens quite often. If you poke around in the amp with a non conductive probe (like a plastic pen or marker), you'll sometimes find that the lead has broken off a component. It happens right where the lead goes into the body of the component. This happens to resistors, and to Mallory filter caps most often. If you look right up near the body of the component, it sometimes looks like the lead is misaligned. If you poke it, the lead may move. Those old carbon comp resistors can crack right through the middle too. Look underneath at the filter caps as well, these are not immune. But these should probably be changed anyway!
5. Remove solder in places where there is too much
Some people seem to think that globbing a bunch of solder on a joint is the way to keep it together. This is not true. I spend a lot of time sucking big globs of solder out of amps that someone worked on. Just add enough solder that it covers the joint and holds securely. Solder is for making a connection, not physically holding things together.
6. Fix things that look obviously hacked
Some people have no respect. I see amps with wires all over hell duct taped together and wire nutted. Put in clean and straight wires here. No wire nuts or other crimp type devices inside the amp! It's usually much easier just to replace the whole wire than try to splice it with something that could eventually fail anyway. A lot of times you'll see components soldered on to what looks like the leads of an older component that was cut out. If components look like they have their leads soldered to old leads that were cut, fix it. Just get a new cap or resistor and take out all the old garbage. Your amp will thank you! Besides, doesn't it feel good to get the amp back in shape?
7. Tighten everything down
Transformers come loose, as do jacks and everything else. Make sure everything is tight, but don't crank on things too hard. We are dealing with self threading sheet metal screws, they can't be tightened too hard. Speakers always come loose after time. You should check all this stuff periodically too.
8. Change the filter caps
Just do it. Even if they are not bad or causing problems, just change them. If the amp is anywhere near about 20 years old or older, the caps need changing. Don't forget the cathode bypass caps and the bias supply filter cap!
9. Retension the tube sockets
It is important that you drain filter caps before you do this. Don't try to retension sockets with the chassis still in the amp. The plate pin of the power tube will shock the crap out of you. Don't do it!
This really only works on the power tube sockets. If you look down in there you'll see that there is a curved part of the pin and a flat part (not always, some of the newer amps have different kinds of sockets). Take a little tiny screwdriver or a paper clip and bend the flat part inward so they contact the pin better. Don't bend these too much, because if you do, the pin of the tube will come in and get caught on the pin that you bent in too much. It can sometimes bend the pin of the socket down and your tube won't go into the socket. This pretty much ruins the socket. Be careful! The preamp tube sockets aren't really easily worked on. This is something that I do if I suspect that a preamp tube isn't making good contact. First, try putting the tube in a pin straightener if you've got one. That may do the trick. But what I do is bend the pins of the preamp tube inward VERY SLIGHTLY. I want to emphasize VERY. If you go at it with a hamfist, you will crack and ruin the tube. As a matter of fact, the tube may be weak anyway and it will crack even if you are careful. Make sure you bend all the pins in about the same amount. If you bend them too much, then the pins of the tube can get hung up on the socket and you'll bend them even more and possibly break something. If you've done this correctly, you'll notice that the tube goes in a little tighter than before, which should make better contact than before. If you can't do this or it doesn't seem to fix anything, just get a new socket. They are cheap.
10. Check for leaking coupling/blocking caps
In older amps, especially Fender tweeds in my experience, the blocking/coupling caps can start leaking DC. The job of these caps is to keep high DC voltage from getting to places where it will cause damage or make the circuit not work (grids of tubes or pots come to mind). Usually one side of a blocking cap will have DC on it, and the other will have none. This is not always the case though, so be sure to check the schematic (you check these by lifting the side that has the lower voltage on it). If you find much more than .1 volts DC on the side that there isn't supposed to be DC, then the cap is a candidate for changing. In tweed amps, the red or yellow Astrons are almost always leaky. In blackface, there are some reddish brown ceramic caps that are candidates for changing. The ones in the tremolo circuit, and the .02 in the vibrato channel are bad more often than not. Also, the 500pf and 250pf's are worth looking at.
--------------------------------------------------------------------------------
Amp hums very loud
This could be many things, and there are many different kinds of hum! I'll go through a few things here.
1. Amp buzzes loud when you take your hands off the strings
This is going to sound incredibly simplistic, but it happens! Usually, switching the ground switch on the back of the amp fixes this. Not many people are aware of this, and I get asked this question more than you'd think ("Why is my amp buzzing?"). If your amp doesn't have a ground switch and has a 2 prong cord, take the plug out of the wall and turn it around. This changes the ground refrence in the chassis which is the same effect as the ground switch.
2. Silverface amp/blackface Bassman hums loudly
Silverface amps, and the blackface Bassman, had a bias balance circuit that would make the amp hum very loud if it wasn't set right. You can turn the bias pot a LITTLE either way and see if it reduces hum. If no difference is heard, stop turning it and put it back to about where it was. These should be converted to the standard blackface bias circuit anyway. Some later silverface amps had a "hum balance" control too. This is a pot on the filament supply that balances it. You can turn that one with no damage. It's best to adjust that pot with the amp turned up loud but with nothing connected to the input. Adjust it untill you hear the least amount of hum.
3. Change the tubes
The filament in a tube can cause hum. When tubes get old, sometimes they hum. Try unplugging tubes going from right to left looking from the back. When you pull out a tube that reduces or eliminates hum, try a new one in that position. The rectifier tube could be causing hum as well. Sometimes in a 4 power tube amp, one power tube will fail and this will cause the amp to hum.
4. Change the filter caps
I know I know, I'm sounding like a broken record here. But when filter caps lose their filtering capability, they cause the amp to hum. Their whole job is to turn AC into DC the best they can, which is partly to reduce hum! The bias supply cap is also suspect.
5. Check for bad grounds and bad solder joints
One more thing to check that I've already mentioned. Make sure that all solder joints look solid. Poke around inside the amp with a pencil or piece of wood to make sure that solder connections are good.
6. Check your input jacks
The input jacks on amps are usually shorted. This means that when you unplug your guitar cord, the jack gets shorted to ground. If these shorts don't work, the amp may not work right (in the case of a JCM800 Marshall, if the short isn't working on your low gain input, your high gain input won't work!), or cause it to hum. These can't be retensioned. Just get new ones!
