Intermittent Buzz Decware Zen SE84Bged

[quietglow]

Hi all, I'm hoping someone can give me some suggestions for solving a problem with my Decware SE84B.

I've had it for some time and recently it picked up an intermittent buzz -- it sounds more crackly than 60hz ground buzz, but similar. It's loud enough to be heard through quieter music. Duration is usually a minute or so, though sometimes longer. It seems to have no connection to whether the amp is warmed up or not. When the sound isn't there, its dead silent.

If I pull the input cables while it's happening, there's no change (so it's def the amp). If both output tubes are removed, it doesn't happen. If the input tube is pulled, it does happen. I've replaced both output tubes, which made no difference.

What I am looking for is suggestions as to where you'd start trying to diagnose the problem. The amp's typology is pretty dang simple, and the parts used in it are super robust. Seems like it's too young for things like capacitor failure etc.

 

[Jhm]

My SE84B developed an increasingly frequent crackling in one channel after 4/5 years of regular use. It was tube-independent, as in your case. And then one day that side went silent. Turns out the 47K carbon film resistor on the plate of one of the driver tube triodes had died, and I had been listening to its early failure signs. A new matched pair of resistors solved the problem. I'd check the sound of each channel separately to begin with, in case it's a single channel problem.

 

[quietglow]

Interesting. Mine has seemed to be involving both channels, so theoretically that should very much limit the number of things it could be. I should have mentioned this in the first post but I thought at first that switching the bias switch would quiet the buzz temporarily. Maybe it did, but it doesn't anymore. Could a failing switch cause that sort of trouble?

 

[Jhm]

> Could a failing switch cause that sort of trouble?

Yes, I think it can. If the contacts are oxidized, they can probably be cleaned. I seem to remember I did just that once or twice, back in the day. I eventually disconnected the switch because I never used the 2.7K bias position. I noticed a definite improvement in clarity but since I changed the bias resistors at the same time, it's difficult to tell. I do suspect even a fully functional switch degrades the sound to some degree.

 

[quietglow]

I eventually disconnected the switch because I never used the 2.7K bias position.

This is a good point -- I don't either. Rather than mess around with cleaning or replacing it, I think I'll just remove it. Thanks for the suggestion!

 

[quietglow]

Alas, that didn't solve the problem. The switch is out of the circuit, and the sound returned (after just long enough that I'd decided it was cured, of course).

To me, it sounds like something is failing slowly -- the buzz fluctuates in volume (though not in tone). I suppose it's time to start testing stuff.

 

[Jhm]

A prime candidate for failure in this amp is the 1K, 10W resistor in the power supply, replaced in later versions by a 50W resistor. Power dissipation is about 10W here, so increasing the power rating was obvious (and the over-spec'd resistor probably sounds better, too). The resistor in my SE84B did look seriously stressed-out. Something inside the resistor had melted, as the picture shows, and I found a drop of it on one of the cables nearby. I replaced it with 3 parallel 3K, 12W Mills resistors.

An externally hosted image should be here but it was not working when we last tested it.