Here are some things to look for if your tube amp is making intermittent crackling type noises. These pesky noises can come and go or be triggered by certain notes. There are a few things that can make an amp “crackle” and I will try to give you guys some things to look at so you can try to diagnose the issue and possibly fix it yourself.
On a vintage amp intermittent crackling can be caused by a few different things. The first and most obvious is the tubes. You should always have a good set of tubes in your amp. This reduces the chance for noise and crackling as well as provides the best sound and reliability. In a vintage Fender amp “tweed disease” can cause this unwanted noise to spoil your tone. There is a video explaining this and the remedy and you can view it here:
Also remember to clean your tube sockets regularly. I like to use Caig DeOxit. And while you’re at it clean the pots and jacks as well.
Here is a video showing how to clean a jack:
Another thing to look at are the plate load resistors. These are usually a 100k value and are the resistors that connect the DC supply to the plates of the preamp tubes located on pins 1 and 6 of a 12AX7 or similar tube in most amps. Same thing applies for older 6SL7 tubes or similar. More often than not the plate load resistors back in the day were a carbon composition type and actually have an inherent hiss type noise all their own, brand new out of the box. As the Plate load resistors age (particularly the carbon composition type) they tend to make more noise and can sound like frying bacon in the background. If you change these with modern carbon composition equivalents the crackling will go away. Or you can use carbon film or metal film resistors and never have to really worry about the crackling or hiss again. The tone and to a certain extent the feel of your amp WILL change. This is the subject of much debate among techs and tweakers and I stand by my statement because I hear a change and I prefer the carbon composition resistors most every time.
Some modern amps can exhibit this sporadic crackling from the lower quality tube sockets. I have seen this more than a few times with brand new amplifiers making all kinds of weird noises. It is a laborious job that requires steady hands and patience but by changing the sockets with higher quality phenolic or better yet ceramic, you will be sure to greatly reduce this variable.
Another source of crackling to look at, especially in combo amps, is the aluminum foil on the amps back panel. This foil can drive you crazy if you don’t know to look for the problem there. Some younger techs are unaware of it and trust me it can be maddening. For example say you have the amp on your bench with the back panel off and the amp plays fine. Sounds great, plays great and no noise. You figure the owner is crazy or it was a fluke and the tube socket cleaning you just did cleared it up. You mount the back panel on the amp and fire it up again. A few minutes later just as you’re ready to turn the amp off and call the owner……its back! This foil is supposed to shield RF and other extraneous noise. I find that all it does is cause headaches and a piece of metal screen you get from a hardware store is much better. Metal screen material was used on vintage amps, particularly Fenders starting in the 1960’s. Anyway the problem is that the foil does not make good contact with the chassis and will cause the crackling that can come and go every minute or every month. To remedy this replace the foil with the metal screen or put tape or thin weather stripping around the open edge of the chassis where it comes in contact with the foil on the back panel.
Loose components can also cause some crackling or even the signal dropping out that sounds like crackling. Some components to look at are pots, jacks, transformers and mechanically connected grounds. Make sure all these components are secure and the mounting nuts are tight. On the Fender blackface and silverface amps there are mechanical ground lugs mounted to the chassis on the power transformer studs with locking nuts. These can come loose form years of vibration making an intermittent connection. The ground lug connections are inside so you have to open up the amp to check them.
One of the last things I’ll mention is to look for bad solder joints. Over the years a solder joint can oxidize and the connection will be either bad or intermittent. I had a solder joint that was intermittent on a 1967 blackface Vibrolux Reverb. It drove me a little crazy until I re-flowed almost every solder joint on all the tube sockets. You could not see it was bad and I could not identify the EXACT joint so I touched up every single one. Problem solved.
I’m sure that there are some other case scenarios for crackling noises but these are just some of the culprits you should look for. Always try different guitars and cables before you start to take your amp apart or tear your hair out. Sometimes it’s the simple, obvious things we look at last and then wonder why we didn’t look at them from the start. I hope this helps some of you with any crackling noise related issues. Please feel free to contact me if you have any questions.
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