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Help me fix a crackling guitar amp.
October 24, 2010 8:00 AM   Subscribe

Help me diagnose my (probably user-induced) intermittently crackling tube guitar amp!

Okay, here are the data points:

-- Brand new Blackheart BH15H head, worked fine out of the box

-- I installed the Bitmo Triple Bypass mod myself (which involves soldering in a three-way switch and couple of capacitors and resistors).

-- After the mod the amp works as expected -- overall tone much improved -- but produces a 'bacon frying' crackling and noisy volume pot when I first turn it on. After about 15 mins to half an hour, the noise goes away and it works silently.

-- I have checked all my solder joints and they seem solid

-- I have swapped out both the tubes and it makes no difference

-- I have cleaned the volume pot, no difference

At this point I'm suspecting either (1) a dry joint that I can't find which is closing once the innards of the amp warm up or (2) some DC across the pots from a leaky cap, but can't understand why this would clear up once the amp gets warm.

Any suggestions?
posted by unSane to Technology (8 answers total) 1 user marked this as a favorite
 
It sounds like you have a dirty potentiometer. It is probably poorly lubricated inside which causes the crackling when you adjust it, but after it warms up, it works ok.

You might try spraying into the pot with an aerosol can of cleaner made for the purpose of cleaning and lubricating electronic components, or you may have to replace the pot.
posted by gnossos at 8:18 AM on October 24, 2010


Well, normally I'd say you have a cracked tube, although it could also be that one of the tube sockets is loose.
posted by wierdo at 8:33 AM on October 24, 2010


Because the time frame is 15-30 minutes, I'm betting it has something to do with the warmup time of the tubes. Not a fault of the tubes themselves, but that your modification messed with some kind of compensating circuit or noise reduction circuit meant to silence weird noises that might naturally occur before the tubes are fully warmed up.

(What I'm envisioning is that the designers put in something that compensates for the long warm up time of tubes, and you bypassed it.)

Or, that one of the components you installed are faulty or not audio-grade.

One option would be to get your soldering iron and just reflow all the joints in the thing. Might take a while.
posted by gjc at 8:36 AM on October 24, 2010


-- I already cleaned out the pot completely. I'd rather not replace it until I've satisfied myself it's not something else as it's an awkward repair.

-- The tubes are fine and I have cleaned out the sockets but I will double check

-- The tubes are warmed up long, long before the noise subsides. The audio circuit is very simple and the only bits I bypassed were tone caps, to allow other tone caps to be substituted.
posted by unSane at 9:18 AM on October 24, 2010


Welcome to the wonderful world of tube amp noise! It can be completely infuriating. I have built several amps, so I will try and do my best to help you out.

First and foremost, have you tried replacing both the preamp and poweramp tubes?

Next on the list is the chopstick test, but a disclaimer first:

*I AM NOT RESPONSIBLE FOR ANY DAMAGES YOU DO TO YOURSELF OR OTHERS WHILE PERFORMING TUBE AMP MODIFICATION. YOU ARE WORKING WITH HIGH ENOUGH VOLTAGES TO KILL.*

Anyways, what you want is a long, non-conducting object. Many people use chopsticks but make sure it DOES NOT conduct. Open up your chassis so that is exposed, plug it in and turn it on, taking special care not to touch ANYTHING. Then, take a chopstick in one hand while holding your other hand behind your back and just starting poking things with the chopstick. Capacitors, resistors, tube sockets, whatever. If the noise changes or disappears when you touch a component, that means you either need to resolder the joints of that component or replace it. (The reason you hold your hand behind your back is so that if you do get electrocuted, the current will not cross your heart and will most likely only be painful, not fatal.)

Also, do you have access to an oscilloscope? Scopes can make finding noise sources relatively easy.
posted by mungaman at 10:48 AM on October 24, 2010


from mr. pineapple, our resident amp-builder:

"I agree with OP's first assessment. What he calls a 'dry joint', I call a 'cold joint', but he just needs to go back and re-do the mod and insure that each joint is completely solid by reflowing the solder. That's what I would do."

(mr. p called out "cold joint" before he had even heard the whole question.)
posted by pineapple at 11:05 AM on October 24, 2010


I took a real close look at the joints and a couple looked questionable so I reflowed them. Good news: the crackling was gone. Bad news: the volume pot didn't work any more. On the multimeter it looked like an internal short in the pot so I took it apart and cleaned it and managed to get everything working properly, but in reassembling the pot I damaged it (not really designed to be serviceable, I guess). So now I have a quiet amp with no volume control, and a new pot on order!
posted by unSane at 8:56 PM on October 24, 2010


Final resolution: it was the pot. Now replaced and everything is working fine.
posted by unSane at 8:22 PM on December 13, 2010


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