Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: newguitarsmell on April 29, 2020, 12:45:42 am
-
Hi guys.. working on a Blues Jr III. It never worked so I said I’d take a look. There were two resistors noticeably fried - see pic
I measured B+ at 350 after I replaced these and R47 burned up again. I tried it with new tubes and 2 different speakers.
Any thoughts? I will attach the schematic in the next post.
-
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-III-Schematic-Rev-D.pdf
Sorry I can’t get the schematic to copy - will keep trying
-
R47 is the screen node supply resistor (between the reservoir cap and the smoothing(screen supply) cap. If its burned (and keeps on burning), there's been too much current through it. The cause is some kind of short to ground directly on the screen supply side of the resistor, such as: a shorted screen grid in an EL84, a burned EL84 tube socket on the tube socket board (look for signs of arcing there), a shorted screen node filter cap (C26), or a shorted trace to ground through charred material (e.g. between burned traces on a PCB) caused by arcing.
R35 is the screen grid resistor on V4. The schematic calls for a 100R flameproof (fusible) 0.5W here, so that they can 'burn up'/go open without catching fire if there's too much screen current.
So the most likely failure mode is a screen grid failure in V4. (so it probably needs new EL84s). In any event, check socket V4 on the tube PCB for signs of arcing around the socket. This will need to be cleaned up and/or replaced if there is arcing damage here. May even need a replacement tube PCB.
However, once you get all this stuff fixed, there is still one more thing I'd suggest. These amps are known for having too-hot-a-bias setup from the factory. So (if and when you get all the other stuff checked and fixed), change the bias supply circuit so you can get between -11 to -12VDC on the EL84 grid pins, before you pop new EL84 tubes in.
However, if the damage to the PCB (and/or tube board) is beyond repair (and let's face it, that board looks like its probably past it's use-by date), then you may want to suggest other options to the owner, such as a rebuild. There are a number of after-market BJ board/rebuild kits on the market. (Check out Doug's shop in the links at the bottom of this site)
-
I'm betting that C26 is shorted.
-
is beyond repair
:icon_biggrin:
looks like ~5+ hrs labor grinding, epoxying, re-wiring, re-installing. that assumes no self induced gremlins :laugh:
-
Thanks all. I basically told him it needs to be rebuilt. I don’t want to spend any more time on this amp. I see why people mod it. The quality is too funky
-
Right on the money Sluckey C26 had a reading in the pfs. Gonna swap that just to see..
-
Right on the money Sluckey C26 had a reading in the pfs. Gonna swap that just to see..
That tiny pf reading ain't burning resistors. Set your meter to read resistance and connect the meter leads across the cap leads. Probably gonna get a low resistance reading. If so, that's what is burning the resistor.