" The Blues Jr is a fixed, non-adjustable bias amp (like Mesa). This means that the bias supply was set at the factory, and changing it requires removing and replacing a resistor on the PCB to adjust it (or modifying the amp to make it adjustable).'
Phillip Holmes yep, factory circuit in the Blues Jr is fixed negative bias. And my preference is to mod the bias circuit and add a 10k bias pot to the chassis so it is easily adjustable in the future. But adding a resistor parallel to R51 or R52, depending if more or less negative bias voltage is needed, will get it done without having to pull the main pcb.
How exactly are you measuring the current? I saw another video where he calculated the current using ohm's law, measuring the transformer's resistance and voltage difference on red and blue/brown wires, which are connected to your probes... Thanks.
My roommates Blues Junior is bias very hot stock , it can eat tubes so I put the toughest sovtek tubes that can survive the bias ! I think the different versions vary , there are 3 different ones now !
@@racmusiccom I have a hot rod blues jr iv with a black pc board. It seems to be built much better than the previous versions. I have had it a little over a year now & I play it average 2-3 hours a day, everyday since I ordered it from fender. Never changed anything on it yet & it plays & sounds better each & everyday. When I decide to replace the power tubes, I will order the same tubes from fender & I shouldn't have to adjust the bias. So many people trying to reinvent the wheel.
@@MAP448 the output section of most recent revision Blues Jr (IE black pcb) is drastically different than earlier v4 green pcb revision. The most recent black pcb revision has a cathode biased output section. Generally this type of circuit is very forgiving as far as the bias point varying little regardless of the operating rating of the tube. Where as a fixed negative voltage biased output section (all Blues Jr revisions leading up to the current black pcb) is not as forgiving. So in fact the engineers have “reinvented the wheel” with your Blues Jr ;)
@@MAP448 the handful of early tweeds, a few brown tolex models, the black champ and bronco…and I’m sure a few more after that escape me at the moment. Let’s say “several” other Fender amp designs have utilized a cathode bias output section as to not be mis-lead and mis-inform others. My point was that you said “reinvent the wheel” as if to imply that adjusting fixed negative bias voltage for an idle plate dissipation around 60% would negatively impact the performance of the amp. More times than not, adjusting the bias will actually improve performance of most units with the non-adjustable fixed negative voltage bias circuit as with the one shown in this video. In fact most units typically operate in excess of 100% idle plate dissipation and eventually fail catastrophically causing excessive damage to the amp if such adjustment to the bias voltage is not done.
Hallo RACMusic Thanks for the the video. Could you give one mor information how you exactly conect the Multimeter for current and the ohter one for voltage.
One more reason I am glad that I moved my tweed blues junior down the line. The first reason was pots soldered to a pc board without nuts on the face of the control panel.
I bought a brand new pair of gt el84s grade 5 for my blues jr but when I put them on and turned on the amp I noticed quiet hum that wasn't there with the older tubes so I checked the grade o f the old tubes and they were number 1. Is this bad? Did i damage the amp or tubes?
Sorry for the slow reply. I obviously don’t check RU-vid account often. There is a chance you have a damaged resistor in the filament supply that is causing the hum. You should really have a qualified tech check it out. And these amps routinely average 90-110% max plate dissipation at idle. IE wayyyyy to hot. The short of it is that every Blues Jr really needs to have the negative bias voltage increased so the tubes will run cooler and not flame out like they often do.
Agreed. Which is why I adjusted the bias on this unit by adding a 33k parallel to the stock 22k resistor to make the negative fixed bias voltage more positive. Which increased it to 60% idle plate dissipation. While most tubes produced 60 years ago could handle biasing for increased idle plate dissipation in excess of 70%, I’ve found current production tubes become less reliable at these increased operation. In addition I personally have found most players don’t notice a significant tone / response difference to justify the increased idle plate current. In fact more have found they prefer the more moderate 60% IPD which allows for greater swing positive and negative without reaching saturation or clipping.
