How to measure bias in a Blues Jr?

[JustMike]

I know it's fixed fixed bias and I've read that they come from the factory HOT. I have one that has a blown fuse and a dead output tube. I replaced the fuse and threw in some test el84's and it works and sounds fine, but I'd like to check it for the too hot bias problem. I don't want to put in a bias pot. I just want to adjust R51. So with only a DMM and no 1 ohm resistors, how do I measure the bias?


Thanks,

[AlNewman]

You need to measure across the transformer.  So you measure resistance between b+ (red) and both brown and blue when the amp is off, it should be around 100 ohms.  Then measure the voltage drop when the amp is idling at both connections, between red and blue and red and brown.  Then measure b+ to ground, and do the math.

[Latole]

You work to repair an amp, you buy new tubes, why not operate in the rule of art by ordering resistors of 1 ohm that you can leave in the circuit permanently and that will allow you to measure the bias easily in the future ?

I have never heard of the method proposed by AlNewman. Why ?

[pdf64]

…
I have never heard of the method proposed by AlNewman. Why ?

Perhaps that’s a question best asked of yourself
Plug the readings obtained from the tests suggested in post#2 into a popular equation developed by Herr Ohm, and anode currents should be revealed.

[sluckey]

You work to repair an amp, you buy new tubes, why not operate in the rule of art by ordering resistors of 1 ohm that you can leave in the circuit permanently and that will allow you to measure the bias easily in the future ?

Easier said than done.

[JustMike]

My blue winding R is 93.9Ω and the brown is 93.4Ω.
 Voltages are -2.47 and -2.45 respectively. Math gives me 26.3ma and 26.2ma. Is that it?
 B+ is 334v measured at OT CT (red).

[mresistor]

Measure Bias With the Output Transformer Resistance Method    latole has never heard of it ?  seroiously?? 

[mresistor]

My blue winding R is 93.9Ω and the brown is 93.4Ω.
 Voltages are -2.47 and -2.45 respectively. Math gives me 26.3ma and 26.2ma. Is that it?
 B+ is 334v measured at OT CT (red).

well yes it might be     I haven't ever had a negative number   look over the procedure and make sure you are doing it correctly

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

[acheld]

Just be aware that BJ IVs are cathode biased.   

BJ III and older are fixed bias; as delivered, usually biased fairly hot.   Sluckey's cream board (2003) is fixed bias.   

I've had several BJ's over the years -- they are light weight, easy on my back and sound OK (no, they do not compete with heavier iron).  All of them were biased very hot; usually had to replace the EL-84's once or twice per year.   And all of them have been modded with adjustable fixed bias (a la Bill Machrone) with great success, meaning that I have not had to replace tubes since then. 

The BJ IV has fixed that particular problem as best I can tell, and they sound about the same to my ears.

[mresistor]

If you want to cool the bias by adjusting the value of R51 this video will show and easy way to do that ..   

[tubeswell]

You could easily add a small tag terminal to Doug’s BJ tube socket replacement board for mounting the other lead of 1R bias sensing resistors from the cathode pins.


https://el34world.com/Hoffman/Blues_Junior_Tube_Board.htm

[shooter]

I added some to a board like Sluckey posted, just wicked out the tube pin, raised it, soldered in the 1ohm'rs.  then I replaced all the ribbon cables then i bridged all the burnt traces then, then, I QUIT working on them 

[AlNewman]

My blue winding R is 93.9Ω and the brown is 93.4Ω.
 Voltages are -2.47 and -2.45 respectively. Math gives me 26.3ma and 26.2ma. Is that it?
 B+ is 334v measured at OT CT (red).

Well, your el84 plates are pushing 8.8 watts at idle.  Looks like they're rated for 12 watts across the plate.  So, you're running at 74 percent at idle....  Actually, that ain't bad, considering some of the horror stories I've heard.  Rock out.

There's a 33k resistor you could reduce, it's either r51 or r37, depending on what board you have.  You can reduce that to dial in your bias, anywhere down to 50 percent sounds great.