Amp Problems :(

Lifeofsobriety · #1 11-05-2007, 10:59 PM

Hey guys I'm new here!

I've been having trouble with my amp lately and I am trying to figure out whats wrong with it.

I have a Hughes and Kettner Switchblade 100W Head. I bought it used and for the first little while things were fine. The other night I turned it on and there was nothing but an extremely loud low pitched hum. When the amp is in standby the hum goes away, and when I turn up the volume the hum gets louder. If the amp is up loud enough, the guitar barely cuts through but is crackled and muffled. The hum is present whether there is anything in the input or not. I'm just wondering if anyone could tell me what the problem could be. This is my first tube amp and im still a noob

Thanks

David McCarroll · #2 11-05-2007, 11:03 PM

try reseating the tubes, try replacing the speaker and power lead - if these don't solve your issue, I think you are off to a decent amp tech. Tube amps have very high voltages floating around in them, and power supply capacitors which can hold a very significant charge for several days after the amp is switched off, so if you don't know EXACTLY what you are doing, you are best not to fiddle with the innards of a tube amp, or any mains powered device for that matter!

Lifeofsobriety · #3 11-06-2007, 01:30 AM

Is there anywhere you could point me to to help with that process. Like i said i'm pretty new to tube amps and don't know how to do a lot of those things.

Thanks

Tank · #4 11-06-2007, 06:40 AM

Well the switchblade has a regulated power supply with a round transformator that is sized for double the voltage the amp needs. So it is unlikely (but not out of the question) that the amps power supply is dead (maybe you got a lemmon).

First try a different cab to see if maybe your cab has a problem (sometimes the described problem comes from failed magnets in the speakers), however it is unlikely all your cabs speakers died at the same time.

Next try to disconnect the amp from any power source, let it sit for at least 30 minutes so all capacitors have emptied. Then try to remove the power tubes and put them back in.

As the switchblade has a digital section it may be very likely that the effects loop has broken, disable all internal effects of the switchblade and see if the problem persists.

All other things that could cause this problem are harder to check and you should always go to a certified H&K repair center and let a tech check your amp. H&K has high pride on the quality of their products and if your amp is younger than 2 years you should not fiddle with it at all, as H&K will fix it for free (very likely).

jemplayer55 · #5 11-06-2007, 12:18 PM

Quote:

Originally Posted by Tank

Well the switchblade has a regulated power supply with a round transformator that is sized for double the voltage the amp needs. So it is unlikely (but not out of the question) that the amps power supply is dead (maybe you got a lemmon).

If it was the power supply transformer he'd have no sound at all! And it's very unlikely from the description to be a speaker problem!

Quote:

Originally Posted by Tank

Next try to disconnect the amp from any power source, let it sit for at least 30 minutes so all capacitors have emptied.

PLEASE.... do not follow this theory. It's a tube amp and the capacitors will hold a charge considerably longer than 30 min. Remember.... like David said tube amps work on high voltages (325 to 475 volts DC and higher) and can kill you if you don't know what you are doing. As far as the caps.... you need to physically discharge them before "playing around" inside an amp unless you're experenced and know exactly what you are doing and have a schematic handy!

You can easily check all of the tubes especially the preamp tubes but anything else is best left to a tech or someone who has experence with tube amps.

You can check out www.ampgarage.com for more info on tube amps. It is a forum of professional and amature amp builders. There may even be someone on there who can offer some real assistance with H/K amps. But like everyone has said...... no experence? Take it to an amp "doctor"! Don't risk hurting yourself or your amp further playing a guessing game based on info from a guitar players forum..... JMHO! Best of luck tracking down the problem and getting it repaired.

mfergel · #6 11-06-2007, 12:37 PM

I didnt' read the full thread, but double check the fuse. I had one for about one week and it did the same thing. Fuse was blown but the amp powered on, so I didn't think to check the fuse.

jb4674 · #7 11-06-2007, 01:33 PM

I would also check the manual and what it says regarding the issue you're having. Chances are, it could be the same problem mfergel was having.

Jimmy

Lifeofsobriety · #8 11-06-2007, 01:37 PM

Thanks, I put up a post on ampgarage. I was wondering if anyone could reccomend a good amp tech. I live in utah and i have been unsuccesful in trying to find one. My amp is less than 2 years old, i bought it from the bay would this affect whether h&k would fix it for free??? I appreciate al your guys help

Salamander In The Sun · #9 11-06-2007, 01:37 PM

Have you ever unplugged your guitar from your amp while it was turned on?

screamndemon69 · #10 11-06-2007, 01:42 PM

My old '87 Mesa did something similar. After a check up at my local repair shop, I swapped out my power tubes (shop recommended two but I replaced all 4) and added a new fuse and voila!! I have had zero issues with my Mesa.

Call all your local shops and see if they have an amp repair dept. If you send it to H&K it will cost you shipping and repair fees but in the end it might be worth it as they should REwarranty the amp.

Good luck.

Lifeofsobriety · #11 11-07-2007, 03:55 AM

Quote:

Originally Posted by Salamander In The Sun

Have you ever unplugged your guitar from your amp while it was turned on?

Yes, I also have a cable with a mute switch and whether it is on or not doesn't change whether or not it is making the noise.

Tank · #12 11-07-2007, 04:29 AM

Of course it is not the transformer itself, but with many tube amps the transformer is not sized correctly, hence a peak that you got from your power outlet (can happen) can more easily damage the components inside. if the transformer is oversized a lot, like in this case, the transformer can mitigate such a power surge.

The voltage has nothing to do with how long the capacitors hold the power stored in them. The recommendation I made came directly from the H&K manual of my TriAmp and is not a guess. Of course if you want to make sure, wait longer. I changed the tubes on my H&K like that and had zippo problems.

Of course if you are unexperienced, you should rather let a certified tech do the job.
H&K will fix it if you have the original receipt of the amp, to make sure it is not been purchased as a "grey import" or has a warranty that is already void. In which case they will also fix it, but you have to pay for it of course.

jemplayer55 · #13 11-08-2007, 11:10 AM

Quote:

Originally Posted by Tank

The voltage has nothing to do with how long the capacitors hold the power stored in them.

Yep, you're right there..... but the capacitors (not tubes or valves to our friends overseas) will hold a charge for much longer than 30 min. unless there is a source to ground, to bleed them off. As far as the voltage is concerned.... it was mentioned because some capacitors inside tube amps store high voltage loads to keep an amp from having high voltage spikes during start up. (you know.... like from a output transformer)

Quote:

Originally Posted by Tank

The recommendation I made came directly from the H&K manual of my TriAmp and is not a guess. Of course if you want to make sure, wait longer. I changed the tubes on my H&K like that and had zippo problems.

Changing tubes to trouble shoot a amp is relatively easy enough anyone who owns a tube amp can perform.... and can be done shortly after an amp has been shut down. But checking the circuitry (including the capacitors) for problems that may have led to premature tube failure should be left to a pro or someone with experience building and servicing their own amps.

But if waiting is a problem for some, have at it...... You may be shocked at what you find!

jemplayer55 · #14 11-08-2007, 11:44 AM

One other thing about just changing tubes..... you may need to re-bias them (power tubes) unless your amp was designed with fixed bias. If not they make a handy little tool called a BiasProbe that's relatively cheap and will plug right into a good volt ohm meter.

Anyway, you'll occasionally hear guitarists say things like "we just bought new tubes, stuck 'em in, and started playing", and "We didn't worry about biasing, and you don't need to now", or "Biasing is a myth". In most cases, you don't have to bias your tubes when you change them. You can just plug a new set in and start playing, especially if you aren't too particular about setting up the amp for the absolute best tone. But with the right tools it's easy enough to check.

However, if the new tubes you have plugged in are different enough from the ones that were in there, with respect to current draw for a particular grid voltage, well..... They may end up biased too hot for your amp. In that case, your new tubes will start to glow cherry red on the plates, either at idle or while playing, and they will soon "fry"! In addition, the tube may short out and take out the output transformer in the process, leading to high dollar repairs. The opposite..... not high enough grid voltage if there is other problems and the sound/tone suffers too!

Tubes of the same type from different manufacturers will usually vary greatly in current draw at a particular grid voltage! Even two different tubes of the same type from the same manufacturer can vary widely in their current draw. For these reasons, it is always best to check the bias after installing a set of tubes.

If you're in an emergency situation, such as a blown tube in the middle of a gig, you can go ahead and stick in your spare tubes, but you should turn the amplifier on and look at the plates of the tubes (the large dark grey metal element) in the dark, both at idle and while playing, just to make sure they aren't glowing red.

Often, when a tube fails, it will take out the screen grid resistor, and any new tube you plug in will glow red, or won't work at all. In this case, you have no choice but to repair the amplifier before using it.

So the moral??? Don't assume the manual will tell you all you need to know! Besides, It's best to educate yourself while taking it to a qualified tech! Most will be glad to explain things.

jb4674 · #15 11-08-2007, 12:31 PM

Call HK and ask them what tubes go in there. To save yourself a headache, if you decide to replace the tubes, simply buy the exact same tubes that the amp has in it and you won't have a problem.

Also, check this list and call the dealer/repair center that's closest to you.

http://www.hughes-and-kettner.com/de...hp?mode=search

Jimmy