How can I tell when my Blues Junior (2003 - cream board) needs to be racapped?

[Dolmetscher007]

I have a Fender Blues Junior, made in 2003, that has basically pooped the bed! I've never really pushed this amp too hard. I played 10-15 gigs with it 15+ years ago, but for the most part it has been a bedroom practice amp.

A few weeks ago, it started popping and crackling a little bit. I thought it was the input jack, because the plastic bushing of the garbage input jack is broken, so you have to put the guitar chord in "just right" or else it won't connect. But then the racket started to get worse. A week or so ago, the popping and cracking turned into total chaos after the amp has been on for 2-3 minutes. Every day, I'd have 2-3 minutes of time to play the amp, and then it would start crackling and buzzing very loudly. I tried removing all the tubes and putting them back in. Seemed to work for 1-2 days... but then... same story as before.

Then I noticed that the smaller tubes are not 12AX7s like they are supposed to be. They are 12AT7s. I had the tubes replaced at a Guitar Center 13 years ago and the guy must have just used the "wrong" tubes. I know that 12AT7s work in place of 12AX7s, but I read that the 12AT7s cause a much larger current draw. Maybe it means nothing, and that's a dead end. But I went ahead and ordered a full new set of JJ ECC83S (*12AX7s) and new EL84 power tubes.

Yesterday, I took off the back plate > took measurements of the filter caps. There was less than 1 Volt on the filter caps. I used my 25W 860 Ohm capacitor bleed resistor tool that I made to drain the filter caps, and I replaced the tubes. I then plugged the amp back in and turned it on. Now there is just a super loud 60 cycle hum. When I plug in a guitar cable and touch it with my thumb, it does buzz, but the 60 cycle racket is way louder than the buzz.

The stupid tube socket PCB doesn't look at all heat damaged. It looks brand new. But... granted... I haven't taken it out to see the other, more important, side.

I bought a full new set of F&T filter caps, but... before I go through trying to perform my first re-capping... I thought I'd ask all you guys if I'm missing something.

[Carlsoti]

The tube boards in those are notoriously crappy. Have you done much tube swapping in it? Just rolling tubes can pull the trace from the board. When that heats up, the solder gets all loosey goosey and can end up making all kinda of nasty noises and potentially damage the amp. Also, the original caps in those are notoriously crappy and could've been on the front line of the capacitor plague. It's generally bad advice to tell some to just recap it and trouble-shoot it from there, but I don't know what your capabilities are. If it were on my bench, I'd measure the electrolytic caps for shorts as a first step. After that might be to pull the tubes, bring it up to voltage safely and take a few measurements to know what the PT and rectifier is doing. Tubes back in, again brought up to voltage safely, and start checking voltages throughout the amp.

[tubeswell]

Fender ‘designed’ the factory stock BJs with the EL84 bias deliberately insanely too hot, so the tubes sound nice but they destroy themselves faster. Many guys mod their BJs by installing adjustable bias (trim pot). Crackling and hissing noise is a sign of bad output tubes. Hum is a sign of tired (or open/failed) filter caps (including the bias supply filter caps). (However, these are not only symptoms of these failure modes - need to do detailed inspection of the amp) But in saying all that, you should probably at least replace all the electrolytics. Beware the fragility of the stock PCBs - they’re designed to work for the duration of the factory warranty but not designed to be handled or repaired.


Many guys mod their BJs with improved replacement boards /kits to make their amps more reliable. See information on Doug Hoffman’s BJ mods on this site.

[Latole]

Power tubes are worn out . Fender original bias may be too hot.
Don't put new tubes if you cannot check and adjust bias . I adjust bias at 60% power dissipation, not 70%