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MindTheGap
1249 posts
Mar 05, 2016
9:03 AM
So I was eulogising about my tube amp, after resting it for a while. Then now I notice it has a slight hum. Not much in itself, I have to put my ear quite close to hear it properly. But when mic'd up and in a quiet room definitely a hum. Someone mentioned it to me.

Should it have any hum at all? Trouble is I don't know if it had a hum before or if it's new. Other (SS) amps don't have a quiescent hum. The hum volume doesn't change with the amp volume setting.

(Superbee this is to give you a break from the b radical!)

Last Edited by MindTheGap on Mar 05, 2016 9:03 AM
SuperBee
3484 posts
Mar 05, 2016
1:04 PM
If the hum doesn't change with the volume setting, it's not a big deal...no, that's not quite right...if the hum does change with volume setting, that's a problem which should be addressed. If it's a static level, it may or may not be a problem.
Many amps do have some noise and of itself it doesn't necessarily indicate a problem with the amp.
It's a shame you don't know if it's a new thing. That would tell us something had changed. If it was my amp, at this point I'd just remain aware of it and consider whether it's getting louder. If it interferes with your ability to enjoy using the amp, by all means track it down. If it gets louder, ditto. If you're curiosity overcomes your capacity to ignore it, you will have to investigate. But otherwise I would just remain aware of it and not worry...much.
You could try it in different rooms, different power outlets, try it with cable plugged in an cable, disconnected...you know, all the variables.
It could be a poor earth somewhere, or a component breaking down...although I think it's often the case they will not be so steady. Often the amp will start making a noise after its warmed up, or make a noise on start up which does away when it warms up.
SuperBee
3485 posts
Mar 05, 2016
1:46 PM
But sometimes it's a question of so-called 'lead dress' and moving some of the internal leads can help.
Specifically, iirc your amp is the Joyo 5 watt with the 6z4 rectifier tube...I'm trying to recall if it's the older model with the eyelet/turret board rather than pcb and I believe it is...so that's a very champish amp..I think it has a separate power switch...
I don't recall if they run AC heaters, but I think that's likely. The heaters can get a bit of noise into the circuit.
What I mean is that even if everything is doing what it should, you have fairly solid AC current running around the amp to the filaments, and DC also going to the tubes and all the power of the circuit board, then you have the AC signal running through everything and a couple of big transformers and a speaker so there is plenty of opportunity for a component to pick up some noise along the way.
That said, your signal path seems ok as the noise is not getting in prior to the volume control.
The noise could be almost anything in the amp, a poor earth of solder connection could be a likely culprit. So if you seriously wanted to eliminate it, that could be a place to start. If you knew whether it was a development or a design issue you could eliminate somethings. But it could be the circuit design. Some of these amps buzz due to the design of the grounding. I always have to check when I'm building stuff...should I use isolated jacks and run a ground lead to the 'star' grounding point, or just ground the Jack directly to the chassis. I think the latter is more likely to develop problem over time as corrosion may set in or the part work loose.
In short, I think maybe prudent to check the earths
MindTheGap
1250 posts
Mar 06, 2016
12:33 AM
Ta. In that case I'll take the advice of keeping a note of it an see if it gets worse.

I guess refining my question it's: if there is ANY hum, does that indicate a fault? I'm taking from what you said that's not the case. I may just try putting in the alternative preamp tubes and see - Just because that is something I can do.

EDIT: I put in another preamp valve and it made no difference.

Last Edited by MindTheGap on Mar 06, 2016 10:01 AM


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