I have an old Kenwood KA-3500 tube amp, and if I let the thing sit for a while without using it, the left channel wouldn’t work the next time I used it, unless I turned the balance knob all the way to the left and then cranked the volume all the way up. That would fix it for awhile, and then I’d have to do it all over again a couple of more times before it would stop doing it. Well, I didn’t use the thing for about a month or so now, and the left channel’s gone out and cranking the knob over while turning the volume up hasn’t fixed the problem, and I’ve left it that way for about a week now.
This can only mean that something’s crapped out in it, I think. Any ideas on how to tell what’s causing it, and how best to fix it? (It might also be the previous owner’s way of paying me back. I got it after he passed away, and he didn’t like cats, and my cat has enjoyed curling up on it, so he might be paying me back for getting the cat and allowing her to curl up on it.)
Assuming you’re letting the thing warm up before you fiddle with it, I’d check (in order of expense to the operator: )
Is your speaker connection good? Those wires get frayed and jostled and yanked on all the time.
Sometimes the pots (knobs) go bad. Can you switch the R and L and see if the problem repeats itself on the other channel? You might also switch the speaker wires on each side and see if the problem comes back on the other side.
New tubes, dude. Too bad they don’t have those testers in the grocery stores anymore. Anyone else remember these?
In addition to the good advice above, I’d also check the electrolytic capicitors (if present). These tend to dry out over time and become flaky or even completely dead, especially if the equipment hasn’t been used in some time. Sometimes caps left idle will come back some when driven at high levels, since this reforms the oxide layers - which also degrade over time due to chemical action of the electrolyte.
Very, very carefully, do the following–if you are not comfy around electronics, DON’T EVEN START:
Take the amp out of the case so you can see the guts.
Plug guitar into input, speakers to outputs.
Turn it on. Warning, there is some high voltage running around in there!
Start tapping stuff with a stick, plastic screwdriver, etc. Do not use anything with metal on it! Be very gentle–you’re not trying to break it, you’re trying to find the flaky part.
When done, unpulg the amp first, let it cool down, then put everything back together.
Disclaimer: if you touch something in there with your finger or anything, don’t come running to me–I warned ya!
Yes, of course I remember the tube testers. My job as a young man was to take all the tubes out of all the TVs in the house and take them down to the electronics store and test them. Then buy replacements for the bad ones and put everything back together. That’s why I would try the above steps. If you’ve only assembled digital equipment, I would recommend against troubleshooting it yourself–get a tech!
I have had the “privilege” of repairing some old equipment on occasion, both at home and at work. The biggest culprit? Nope, not tubes. It’s those old capacitors. As stated by Q.E.D., they tend to dry up over time. The capacitors becomes “leaky” (shunt resistance becomes too low) and/or they simply loses capacitance (open circuit). On rare occasions a capacitor may short out.
Just a few months ago I was repairing an old flash controller for a strobe light at work. (They use strobes to take high-speed photos of projectiles during impact tests.) The circuit was acting flaky, and I traced it to a power supply problem. (Note to self: when repairing a piece of equipment, check power supply voltages first!) The power supply voltage was much lower than indicated on the schematic. Bad transformer? Nope. Bad diodes? Nope. It was the storage capacitor on the output. I replaced it with a new, $0.50 capacitor and it worked fine.
If you don’t know what you’re doing, don’t pull the guts out of the amp.
An aquaintance of mine once unplugged his tube amp, opened it up, and accidentally tapped a capacitor that was still holding a significant charge. He basically burned a hole in his thumb.
A funky capacitor’s probably not going to respond to poking and tapping. That’s more a tactic for finding bad connections than bad parts. But, while I agree that you’re probably looking at a bad cap, you could try doing a complete left-right swap of the tubes. If the problem switches to the other channel, then it’s a bad tube.
If it stays the same, it’s a bad passive component - ie: a capacitor, resistor, or output transformer. Caps and resistors are easy enough to cope with, but if a transformer’s going, you’ll probably wind up chucking the amp or paying a major chunk of change to someone who can re-wind it or has a replacement.
Well, I did a little searching, and discovered that they’re dirt cheap on eBay, like $20 cheap, which is probably less than what a Russian built replacement tube would cost me, so I think that when I get the money up, I’ll buy one of those, and use my current one for spare parts.