Blues Jr. distorts than dies

[DougF]

Hello,
Me and a friend where jamming last night and my Blues Jr. never sounded better. Slowly the high notes started clipping followed by the amp losing power and distorting every sound untill after about 30 seconds it almost completely died. I turned it off and let it sit for a minute and turned it back on. It plays fine for 5 seconds, distorts for another 5, than slowly dies to almost totally inaudible. The tubes all appear to be glowing normally and nothing smells like its burning. I did the BillM presence mod a few years back and that worked out great, I'm sure thats not the issue....any ideas?

[DougF]

After further investigation the amp only "seems" to die...it can't function at low volume. It operates the opposite as it should . As the volume is turned down it starts to distort more and more unti the sound totally fades away at about 3 on the volume knob. The louder I crank the amp the cleaner and punchier it gets. With the volume knob at seven or 8 the amp sounds fantastic, real clean and punchy with hardly any distortion at all. I'm baffled.

[Tele-Dave]

Sounds like a preamp tube. Check V1 by replacing with a known good tube.

[DougF]

Thanks for response Tele-Dave. I swapped every tube in the chassis but it still has the same problem. It is definetly heat related, as if the amp is cold it takes a good 15 minutes of hard playing for the problem to manifrest itself. I'm no electronics expert, but could it be possibly a bad capacitor in the gain stage somewhere?

[pchilson]

Could it be a cracked solder joint that when heated up expands and causes a bad connect? Or, maybe not. Just speculating...

[Tele-Dave]

Maybe Billm will chime in here. He is the BJr expert.


Might be tube sockets on those EL84's (solder joints). The heat does some bad mojo to them.

[bangntwang]

Mine did the same thing new tubes didn't help. Billm answered my guestion (thanks Billm). I have a greenboard BJr. it need a cap job. Took it to a friend of mine that does repair, It can walk and talk now. It seem that Fender uses cheap caps and last only 6 yrs or so if I remember what the tech told me. So he upgraded all of mine. With the amp unplugged take the back off but be careful it can still bite even when unplugged. See if the caps are leaking.
go to Billm web site. I think he even has some pix of leaking caps.
So you will know what to look for. Don't give up this amp is worth it.

[rob999]

Funny, I just got in a bunch of new caps for my Blues Jr.... I took a look inside a while ago (debating mods) and noticed that I had 2 of the 3 filter caps leaking.... still sounds fine and I have not replaced them yet, figured I'd adjust the bias and the tone stack while I was in there...the board looks like a PIA to remove.

[Billm]

If the tubes are good, I'd suspect filter caps next. They can steal away a lot of power. But the inverse volume thing is puzzling. Make sure the back of the volume control isn't cracked. There's a pic of a cracked control on the "aftermarket knobs are a bad idea" page. I'd post a link, but it appears that my hosting company is having problems tonight and I can't get to the Billmaudio site!

Getting to the back of the board isn't that bad. I have instructions on my site for that, too... if my hosting company is serving pages.