So what happens to a tube power amp with no load?

phishaholik · #1 02-12-2013, 04:24 PM

Last night I pulled my all tube amp out for the first time in months. My wife and kids came home while I was hooking it up and everyone was yelling. I decided to let it warm up for a few minutes while I calmed everyone down.

Unfortunately, I didn't plug the speaker cable all the way in and while they distracted me, my amp was running without a load.

It still sounds wonderful, but there's a buzz now, when I touch one of the knobs it pops and buzzes super loud and it seems to break up and get loud earlier.

What did I do?!

Michael

stingray96191 · #2 02-12-2013, 04:32 PM

transformer overheats and explodes. maybe .

phishaholik · #3 02-12-2013, 04:33 PM

I forgot to mention that it was in standby the whole time.

Michael

JimmyM · #4 02-12-2013, 04:35 PM

You may not have done anything, meaning the problem could have developed while it was sitting. However, it's possible you could have stressed the output transformer and/or tubes and caused a failure somewhere. If it were me, I'd do the chopstick test on the tubes and see if you can find a faulty one (tap each tube with a chopstick or handle of a wooden spoon while it's on and running and listen for problems like crackling and microphonics...do not use anything metal!!!), then if that turns up nothing, take it to a tech.

EDIT: In standby? Output transformer's fine. You can't hurt it if you're in standby. Likely a tube and/or a ground problem. Betting on a bum tube.

geddeeee · #5 02-12-2013, 04:37 PM

If it was in standby, then you are OK. If you had switched out of standby, then problems....
Probably just a dodgy or dirty pot. Try cleaning with electric contact cleaner.

JimmyM · #6 02-12-2013, 04:43 PM

Quote:

Originally Posted by geddeeee

If it was in standby, then you are OK. If you had switched out of standby, then problems....
Probably just a dodgy or dirty pot. Try cleaning with electric contact cleaner.

That could be, too

But use a contact cleaner made for pots like Deoxit or you can ruin them by cleaning out their lubricants.

phishaholik · #7 02-12-2013, 04:52 PM

Thanks guys, starting to feel better. It definitely sounds like a grounding problem with the buzz and popping.

Can I spray the lubricant in under the knob or do I have to remove the pot?

Michael

Mr. Foxen · #8 02-12-2013, 05:04 PM

Need to spray on the pot which will be on the other side of the panel to the knob.

Some amps have a standby switch that isn't one and is just a mute. Can be quite annoying if you are expecting there to not be high voltage in a running amp when you are poking about trying to fix it.

BogeyBass · #9 02-12-2013, 05:07 PM

as mentioned stand by is ok, no juice going to plates in stand by.

crackle and pops could be dirty pots, bad/microphonic preamp tube
or bad plate/screen resistors.

also a easy thing to tap tubes with is a pencil and use the eraser end. almost any tube is slightly microphonic some better than others. its just when the internal plates are getting loose from vibration that is when you hear really bad crackles and sputters.
so tapping them is testing for this. and should be obvious right away.

phishaholik · #10 02-12-2013, 05:34 PM

Ok, I just checked all the power and preamp tubes and none of them are microphonic in the slightest. There's definitely something going on with one of the pots. I tried moving the knob a whole bunch and when I turn it all the way off, there's a very loud pop and the buzz increases. I turn it up a little and it pops again and the buzz decreases. Could this just be a bad pot and the speaker cable was just coincidence?

Michael

B-string · #11 02-12-2013, 05:47 PM

Quote:

Originally Posted by phishaholik

Ok, I just checked all the power and preamp tubes and none of them are microphonic in the slightest. There's definitely something going on with the presence pot. I tried moving the knob a whole bunch and when I turn it all the way off, there's a very loud pop and the buzz increases. I turn it up a little and it pops again and the buzz decreases. Could this just be a bad pot and the speaker cable was just coincidence?

Michael

Try working the pot back and forth quickly, you may be able to clear the corrosion.

phishaholik · #12 02-12-2013, 06:37 PM

That's what I've been doing and it no longer pops and crackles. I'm starting to think I have a corrosion problem since the amp sat for so long. I bet the buzz is coming from a dirty jack or something. I was really worried, but hopefully it just needs to be cleaned a little bit.

Michael

B-string · #13 02-12-2013, 06:53 PM

I think you are correct.

Mr. Foxen · #14 02-12-2013, 07:49 PM

What amp is it? Sounds like it should get a going over.

BassmanPaul · #15 02-13-2013, 10:22 AM

Quote:

Originally Posted by Mr. Foxen

What amp is it? Sounds like it should get a going over.

+1 when a amp hasn't been powered for a while, moisture can get into it and cause problems. On older amps the capacitors may have become leaky enough that they can't "reform" enough to be still viable.

Tech time - some TLC is in order.

Lo-E · #16 02-13-2013, 10:30 AM

Quote:

Originally Posted by phishaholik

That's what I've been doing and it no longer pops and crackles. I'm starting to think I have a corrosion problem since the amp sat for so long. I bet the buzz is coming from a dirty jack or something. I was really worried, but hopefully it just needs to be cleaned a little bit.

Yes. Storage for long periods probably just left you with dirty pots, especially if you live in a humid area. Give that amp a good going over. Clean the pots and sockets - MAKE SURE YOU HAVE DISCONNECTED THE AMP FROM POWER AND DRAINED THE POWER SUPPLY CAPS FIRST!!! - and you should be good to go. If you don't know how to drain the caps, stay out of that amp and bring it to a tech. The DC voltages in tube amps can kill you - even days after they've been unplugged!

phishaholik · #17 02-13-2013, 10:55 AM

Thanks again everyone!

The amp is only a few years old and has very little play time on it. I was gutted when I thought that I messed it up.

I'm positive that it just has some dirty pots and possible a jack or two. The pop from the dirt in the potentiometer really made me think that it was a grounding issue, but after turning it for a while it got much, much better. I'm going to clean the pots and all of the jacks as well. I live near a lake and it is a high humidity area. I just let my baby sit too long.

Michael

JimmyM · #18 02-13-2013, 01:13 PM

Yep, tube amps need to be played semi-regularly for that reason.

B-string · #19 02-13-2013, 01:48 PM

Even when I retired my V4B from gigging I worked the controls and played through it twice a year. It helps to keep everything in good working condition. At the minimum once a year.
Will save you a lot of worry in the future.