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Comments
The control plate can be touched for a while (it just feels hotter than the AC15 did).
I have read reports of burnt boards in older Blues Juniors. Maybe version IV of the Pro Junior won't have that problem.
Not sure if the Pro Junior is affected in the same way but there can also be problems with a lot of the newer Fender amps whereby they burn the boards due to some poor choices with component settings and installation.
I'd suggest having it looked at to see if it is running hot and if yes have it modded to bring it back closer to the specs for EL84s in sensible operating mode.
Export transformer has all the taps, as seen on the schematic
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Pro-Junior-Schematic.pdf
Getting the amps bias checked would be a good idea too.
@savbalac, if the heat is really bothering you you could get a low noise fan and install it to push the air out from inside the amp. I would get the mods suggested above done first though.
At the end of a 2 hour or so gig you can fry an egg on the back of my Hot Rod Deluxe.
Biasing cooler is a good idea. The easiest way to achieve it without too much work is to add a resistor in parallel with R30 - or a trimmer, if you don’t want to be bothered calculating it or doing it by trial and error. A better way is to increase the value of R29, but that’s more difficult without taking the board out - although if you’re careful it should be possible to snip one lead and add an extra resistor/trimmer in series. I can’t remember the right values right now, but probably about 100K for paralleling R30 or 10K in series with R29 will be in the right ballpark.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
230volts in the UK our voltage is 240volts so they run hot ! I have done the tap mod on my 68 Custom Princeton Reverb
& my Blues Junior and they run a bit cooler and I think they do sound better with less background noise. Hope this helps.
The Pro Junior doesn’t have a 240V winding, it can only be set for 100, 120 and 230V (and oddly appears to have an unused tap for 220V as well) unless the MkIV is different.
I’ve wondered if it would be possible to fit a Blues Junior power transformer, but I don’t know if the chassis cutout is the same size - and it’s a lot of cost compared to modifying the bias circuit, which gives most of the benefits.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein