Rickresource Rickenbacker Forum

Hi,
I have a 1968 Sunn 2000S amp. It sounds great, but as a played note is decaying, there is a fizzy sound. I've eliminated the speaker being the problem, as this occurs through different cabinets. I just had the electrolytic capacitors replaced, and the problem remains. The tubes check out fine. Any ideas?

I'd check it for CO2 contaminaton. Maybe there's an Alka-Seltzer tablet stuck in there somewheres?

Could be a speaker going if it's not CO2 contamination.

I have read somewhere (probably here: http://www.angela.com/catalog/guitar-amp-parts/INPUT_JACKS.html) that some extraneous noises can arise from the shorting tab on the input jacks, so that's something to check.

Thanks for the help guys. It's a pretty old amp, so any beer that has been spilled has long since lost it's carbonation!
I've checked it with multiple speaker configs, so it's not that. It's odd - the note sounds great, but as the note dies out it gets replaced by this buzzy/fizzy sound. Please don't tell me to check for a beehive.

It is most likely an oscillation, and is probably caused by a bad signal cap letting DC on to the next stage, which throws off the bias points, and sends a stage or two into cutoff, which is the buzz you are hearing. The brown signal caps (film caps) and the green Cornell-Dublier signal caps (paper caps) that were used in these when new aren't all that great, so pretty much any new film cap would sound better. In my 200S I rebuilt, I used a mixture of Sprague orange drop PS polyester series, and Solen caps. Since I use it for bass, I'm looking for a nice clean and round tone. If I was using it for guitar I may have tried some Mallory 150 caps or Sozos or something along those lines. I'd suggest to replace the signal caps in the amp with new film caps, or have someone do it for you if you don't know what you're doing. It would be cheaper to just replace all the film caps than to have someone troubleshoot to find the bad one.

Wow, Greg. You're a fountain of great info!

Listening to you, electronics almost makes sense!

Thanks for all the info Greg. I just had the caps replaced (9 total) and the problem persists. It's aggrevating.

Maybe a bad solder joint or resistor?

Were the power tube sockets cleaned and retensioned?

Tony, the caps that you had replaced were the electrolytic caps right? A couple bypass caps on the cathodes of the preamp tubes, and 5 power supply caps are pretty much what those consist of.

The signal caps are something else entirely, and are made of film, or paper. There are a total of 26 caps in the Sunn 2000S, and most of them are the signal caps. If one of them is bad, it can leak a DC voltage to a stage after the cap, which can throw the bias point off of that following stage, and cause it to amplify part of it's cycle in cutoff, which causes an oscillation sometimes, which can sound like a fizzing on top of the note, or cause sustain loss, or any number of things. It is probably one (or more) of these signal caps which hasn't been replaced that is bad.

You could have someone test the caps in the amp to see which one(s) are bad, but you would have to pay for labor. Since it takes awhile to unsolder and test and resolder these caps, it is cheaper to just pay to have all the signal caps replaced. The price of the caps are no more than a couple dollars for each cap. This will almost certainly solve your problem unless a resistor has drifted somewhere and is causing the stage to which it is connected to have a bias drift, in which case you would have the oscillation still. Resistors are cheap too.

It could also be a bad solder joint, most likely one of the grounds. Reflowing the solder in the joint usually fixes this, although you can also check the resistance on the grounds to make sure they are as close to 0 ohms as can be.

Thanks for the compliments Paul! I work very hard to learn all I can about tube electronics.

Greg,
Thanks for all the info. It's a shame that the tech didn't do what you've outlined when the amp was on the bench. Maybe he figured that replacing the electrolytic caps would solve the problem. Well, I'm going to have him look at it again.

Fortunately for us, you hard work pays off Greg. Thanks for your thoughts, I learn a great deal from them.

Thanks for the compliments Peter. One day I'll build and sell my own line of amps but still gathering knowledge at the moment.

Tony, glad to help. Let us know what the end result ends up being.