I recently bought a Harmony h400a tube amp. It runs off of 3 tubes. I got the amp off ebay and they said the hum is due to a blown tube. Well I bought the amp and a new set of tubes for it.

When I got the amp today. I turned it on with the old tubes in and there was a loud loud hum. No matter what the volume was at. The hum remained constant. I changed out the tubes and then turned it on again. All 3 tubes lit up. The hum was still there. I plugged my guitar in and you couldn't even hear the noise from it being plugged in. The hum was constant still and loud. I turned up the volume and at near max, you can BARELY hear the guitar. It is very faint and really quiet and pretty much buried by the hum.

I am really really freaking angry as of now buying this on the lines of it just being the tube and when he said "the amp turns on and hums" I took it as the regular guitar hum. Not a huge loud hum.

I am not a guitar amp professional. Is there anything I can do to fix this problem? Is it a fuse?

I pretty much but a broken amp thinking that it would be fixable due to the seller. I would like to fix this beautiful amp and start playing. What can I do?
Does it need to be biased? That is your best bet at this point, if its not that its likely something you're not going to like.
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Unless its electronic drums.

it's a 5W with tube rectifier, don't think it would be biasable, at least not easily.

What tubes did you put in? Looks like it runs off some funky ones
They are the same model and everything of the previous Harmony tubes that were in there. It is not the tubes. I am sure of that.
Assumming the replacement tubes are not duds, and the speaker is hooked up (wires aren't corroded, good connection to the speaker tabs) you should probably just take it to a tech, unless you're comfortable working inside it and have some experience. Can't say what's wrong, but unless it had work done to it in the past few years, it needs new filter capacitors, possibly some more expensive repairs as well.
The only place near me that does repairs is outrageous. They charge 80 dollars to just look at the damned thing and then charge you for the parts and labor afterwards. Even if you know what is wrong. They charge 80 dollars for them to find the problem.

I bought the amp for 90 and tubes for 20, the seller said the tube was just blown. I want to find this guys house.

What would I have to do to try to fix it myself?
Well, you can always file a dispute for item not as described and get your money back, or talk to the seller and see if he'll pay for the repairs.

My local techs charge that as well, they do a bench fee ($80 for an hour, $50 for a half) but the lowest increment they charge is half-hours, no free estimates. It's not uncommon but it's not exactly useful.

If you open the amp up and see the capacitors aren't cylindrical anymore, have bubbles/growths/bumps somewhere, it means they're bad, and don't play the amp until you replace them. They can be bad without being disfigured as well.
As far as troubleshooting yourself, you'd need a digital multimeter and a soldering iron at the very least. I don't think you can find much help online due to liability issues, but basically you're going to need to drain and remove the old filter capacitors, and install new ones. I don't know what specs you'll need, you should try to find a schematic online. Playing with these can be fatal, so if you've never done it before or don't have much experience it may not be the best route. Electrolytic capacitors aren't exactly cheap either, and there could be other issues as well. Maybe there's a video somewhere, but you basically need two insulated alligator clips along with a 10k 1/2W or higher resistor soldered together, then you connect the clips to the + lead and chassis, wait, then measure. (there are different ways to do this). These are polarized, so you must install the new ones in the same direction. Always make sure they are fully drained before touching them, and they can recharge as well.

There's probably a tutorial somewhere online, or a video lesson, but it's not something that should be taken lightly. It's also not some magic voodoo that you should be afraid of, just be aware that you can die and need to be thorough and careful. Hopefully someone like dlrocket or another tech can chime in and give you some more meaningful help if you decide to go this route.

That being said, I'd tell the seller you want a refund and if he doesn't honor it file a paypal dispute, you should win it easily. Shipping might be a bitch, but you'll spend the same amount of money just buying the replacement caps
Can't get a refund. He never stated that the amp was in working condition. But told me that the tubes would fix the problem.

What do I do?
If that last post by pak wasn't enough of 'what to do' then I would recommend calling up some places and asking if they have any old fireworks from a recent holiday, proceed to opening up the amp and placing the fireworks inside, fuse touching the tubes. Rock out and let hilarity ensue.

For only $110, I would just give it up. Chances are it's not worth the money to invest in. Best of luck though.
Okay. The guy said no. I filed a complaint. I'll see how far this gets.

I figured I can keep the tubes because i want this amp. But I will wait till I find another.

With shipping, I figure about I will get 70 dollars back. But someone off craigslist might buy it for 70 with the original tubes. He knows of the problem so all is well
I've been fiddling with mine for a while and am looking for another to resurrect from the grave. Email me if you want to sell it.
wow dude. that sucks. but, i hate to be the doomsayer or anything, but i buy older amps quite a bit. the issue you have def sounds like a cap problem, try and refrain from using it too much as i hear it is not good to play it with bad filter caps.

worst still is those older amps may have funky caps, i know my kay and gretsch both had strange ones that had to be bought from weber, else they would have had to change some onboard values and making the amp less original sounding.

almost all the older amps i have bought have needed a cap job, it almost goes without saying for anything from the 50's and 60's as capacitors didn't last as long back then anyway (maybe 10 to 15 years of use at most i hear).

if you can get a good tech, and a nice cap job that will be one great little amp. i hope this doesn't discourage you from the vintage market, we need to step up to help keep these relics in circulation as they are becoming dying breeds and being harvested for parts or being junked.

please show the amp a little lovin, it'll pay ya back.

edit: btw, does it still have original speaker?
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Last edited by gumbilicious at May 24, 2010,
Sounds like a dodgey cable to me. Have you tried another one? Try jiggling it in the socket a bit too.
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