Blues junior blog - noise! (buzz, not hum)

noisepolluter

In case it's of interest to anyone else with the same amp. 

I've noticed a harsh buzz, over the top of the usual mains hum, which creeps in a few moments after the amp has been switched on. It seems to get louder the longer the amp is on, and still happens when the amp gain and volume is at zero with nothing plugged in. This amp is absolutely brilliant with drive pedals, both for home practice and rehearsals/small gigs, but the buzz is getting really annoying.

The buzz persists using the amp in different locations. 

I wouldn't swear to it, but I don't think it used to do this - I got the amp around 2007 and don't remember noticing the buzz.  

Last new set of valves was around 2008. The amp's only been cranked a few times since, but has seen almost daily use most of the time.

Step one (thanks to jpfamps for his guidance) was to push the ribbon cables from the output valves away from the blue lead from the OT. Unfortunately the problem still persists. 

Next step - I've ordered a fresh set of valves (JJs with matched pair of EL84s) - at the same time as I fit them, I'll change the voltage to 240V, which as I understand it is a matter of swapping a couple of connections. I unplugged the amp yesterday so hopefully caps will be fully drained! 

Fingers crossed - will check in with results. 

Jalapeno

NP & high voltage capacitors, what could possibly go wrong ?

thermionic

If it happens with gain and volume at zero, could it be mechanical vibrations from the transformer?

Kylef

I have the exact same problem, exactly as you describe. I thought it was the old valves but I re-tubed the amo and it's still there. I asked BillM for his advice, asked me to try a few things but it still didn't work. He said it must be the capacitors, as they can eventually leak etc. I've been wanting to take it to my tech for a while, as I also want to get the amo biased.

jpfamps

I've seen several Blues Juniors with knackered filter caps, which really shouldn't happen in an amp that age.

It could be due to bad batches of caps (IC caps don't have a great reputation and I've seen cap failures in other recent production Fender amps), but could also be due to the BJs propensity for ultrasonic oscillation, typically around 50kHz.

Caps have rising equivalent series resistance (ESR) as frequency increase, so a 50kHz signal will reduce the life of the cap due to internal heating.

noisepolluter

Valves haven't arrived yet, but I'm getting a sinking feeling. 

Oh well, I ordered 4x 12ax7's which will do just fine in the DRRI. 

noisepolluter

It's definitely not the valves

Oh well, off to the tech then. I'd really like to get this sorted, as otherwise it's perfect for home with a pedal.