What might cause a buzz in a Blues Junior?

[studio1087]

I've been taking a Pro Junior to our practice space for band practice because it's small and loud enough. I was taking the PJ home after practice.

Tonight I took my Blues Junior with the intention of leaving it there. 15 watts, 12 inch speaker, reverb (you know). I thought it would be a practical amp to leave there and then I wouldn't have to haul an amp back and forth.

The Pro Junior was dead quiet with a Gibson SG.

The Blues Junior is buzzing badly. I have a silent tuner and I'm stepping on the tuner between songs because the BJ buzz is obnoxious. Bzzzzzzzzzz.

The BJ was a Craigslist score 4 years ago and it gets light use. I've never changed the tubes. It's always behaved.

We have good lighting. No fluorescents. Good power. The Pro Junior and my Dr Z Mini Z were dead quiet in the same power strip.

What should I do first.....and next.

I just want it to run quietly.

Tubes??

Help please. Thank you.

 

[Wally]

NObody, eh??? Well, I"ll bump...
That amp has EL-84's in it. I take it that you bought it and have never had it looked at by a tech, right? Imho, an amp needs to be a 'known quantity'. When I buy a used amp, I evaluate everything about it. A young fellow who had bought a Super Champ off of ebay maybe 3-4 years ago brought it in...finally...after it stopped working. I had listened to it when he got it and suggested that he let me give it an exam. I didn't really think it sounded the way it should. IT was 'working' so he declined.
At any rate, he plays the amp until it quit working. The power tubes were shot. I stuck in another set of power tubes and the amp came to life....lots of noise, too...but it worked. I quickly took some measurements and shut it down. The bias voltage was off, and the tubes were drawing a HUGE amount of current. I replaced a bad bias cap, the bias voltage and current draw came into the proper range, and the amp sounds fantastic.....like it didn't when he first got it. The noise was gone, too.
So....yes, you could have bad tubes. Have you put eyes on the tubes...especially the power tubes...while the amp is working? Redplates? IS the bzzzz there with nothing plugged in? Is the bzzzz related to volume adjustment on the amp? How about movement of the other controls in relation to the noise?


Bzzzz, right? Not hmmmmmmm?

 

[dr.chevalier]

Does the BJ have a keeper on those el84s like the PJ? My keeper was real sensitive to certain frequencies and buzzed like crazy. I got a couple years out of it with no buzz after pulling it off. Then the el84's started buzzing. It's about 15 years old so I figured I got my money's worth out of it.

 

[mrboson]

If it's more of a buzz rather than a hum, and it seems like from the description like it is, and it also seems like it goes away when the signal chain is muted (i.e. engaging the tuner), then I suggest that it could be a noisy preamp tube. The first one in particular. The Blues Junior seems pretty sensitive to a noisy tube in that first spot. Try swapping around the order of the first three tubes (the 12AX7's). Positions V2 and V3 are not too sensitive regarding noise, so sometimes finding the quietest one of the three to put in V1 on a Blues Junior solves the buzz (and it costs nothing to try). One way to confirm, plug nothing in, crank the master volume to max and Volume to zero. If the amp is working properly, you should hear nothing but a little hiss. Bring the Volume up. You will start to hear a little noise, but it shouldn't be too bad. You should not be hearing that buzz you are describing. If you are, then the problem is somewhere in the electronics and a tech would need to help you find it.

Then do the same thing with a guitar plugged in. If you get lots of buzz as the Volume turns up, first thing I'd try is replace that tube in V1.

BTW, I wouldn't buy a new set of preamp tubes just to do it. It's the power tubes that need periodic replacing, and at your amp's age probably best to to just plan on a pair of new EL84's anyway. It may not solve the buzz, but it will likely liven up the amp.

If you do in fact find that switching around preamp tubes helps, you could consider buying a couple new ones (good to have spares anyway, right?) and seeing if that helps. I have a drawer full of them from various projects, and I swear, I had to just try a bunch in my Blues Junior V1 until I found one that was dead quiet. Some new Electro-Harmonix I had bought were all very noisy. I ended up finding a Tung-Sol that was quiet.

Finally, the Blues Junior circuit has a really crappy setup from the input jack to the input resistor in front of the first preamp tube, which is why I think it can be so prone to noise. Some modders fix that by bypassing the trace on the circuit board from the input jack and hooking up a shielded wire from the input jack to the input resistor for V1. That made a huge difference on my old BJr.

But I would try the preamp tube swap thing and get it as good as you can. If it is still too noisy, well, the BJr is often considered a noisy amp. A tech might be able to fiddle with it and get it is quiet as it can be, but can't guarantee a silent amp.

 

[Vizcaster]

1. a trip to the tech would not be a waste of money,

2. some things that occurred to me are simple things like tubes or mysterious things like the dress (fancy amp tech word for being neat) of the wires and ribbons that connect the tube sub-board to the main PCB. There's a chapter in the instructions for the BillM mods (you need to buy some mods from him to get those valuable instructions) about how to check and rearrange the shape of those ribbon wire connectors to avoid oscillations and noises. Also the stock Blues Jr. does not provide for adjustment of power tube bias, but one of the popular mods is adding a trim pot for that purpose.

 

[waparker4]

I am confused, if the problem is the amp, then how does stepping on a tuner pedal stop the buzzing?

 

[Vizcaster]

I am confused, if the problem is the amp, then how does stepping on a tuner pedal stop the buzzing?

It would be like unplugging the amp and letting the input jack short out the input, muting everything. Good point though, try a different cable and guitar, without the effects.