Installing a bias pot in a Blues Jr

[dude]

I’m installing a bias pot in a Blues Jr. Most of the info out there does not show a resistor to ground to prevent the bias knob being turned to 0, not good. I found Sluckley’s schematic using a 50k pot and a 12k to ground to prevent a zero setting, l’ll go with this. My question is l’d like to install 1 ohm resistors between pin 3 and ground on the two EL84’s. Since this board is a PC with ribbon cables close to the pins, looks like a well laid out plan is needed but l’m sure others have done this, any suggestion on the best way appreciated. See pic.

[dude]

Rather than installing 1 ohm resisters on the EL84’s, how accurate is taking the voltage difference on OT from primary and one plate, red primary and blue plate with meter set on DCV, say one gets 3.8v difference, would that transform to 38mv bias on that tube..? Is this method accurate.

[dude]

I've read here about cutting traces and adding the 1 ohm resistor, hopefully looking for a picture of this surgery, as what I'm looking at is the ribbon cable would be in the way when trying to measure across that 1 ohm R..?  I don't think the OT method is reliable and don't like messing with HV.

[tubeswell]

Best way on a PCB is to cut the traces and solder resistor between. But if you really want to do it properly - convert the socket board to separate tube sockets - and do it properly. YMMV

[dude]

Best way on a PCB is to cut the traces and solder resistor between. But if you really want to do it properly - convert the socket board to separate tube sockets - and do it properly. YMMV

I hear ya, but not my amp. I can’t see the traces on the tube pins on board without flipping the board. But looks like the cathodes are shared with the same trace from one tube to the next then ground. Does it make a difference if each resistors doesn’t go directly to ground?

[shooter]

pin 3 gets to ground,
I did a couple, no notes, but I had the board out on the bench.
I believe I cut the trace, leaving the pad only, maybe 1/4" of removed trace, raised the resistor 1/4" for each clipping
at least I think it went like that 
 

[sluckey]

looks like the cathodes are shared with the same trace from one tube to the next then ground. Does it make a difference if each resistors doesn’t go directly to ground?

Yes it does. Best to just install one 1Ω resistor between ground and the first tube. The second tube will still be connected to the first tube, so the current through the single 1Ω resistor will represent current flowing through both tubes so divide by two to get the current for one tube.

Better yet, forget all this nonsense and learn to measure the current by measuring the voltage drop from each plate lead to the center tap on the OT. You also need to measure the resistance of each plate lead to the center tap. Then divide voltage by resistance to get the current. In the case of the BJ this is the fastest and easiest way to measure plate current.

[scstill]

Here is a thread on my Pro Junior bias mod. Lots of good forum guidance here.
https://el34world.com/Forum/index.php?topic=29413.msg323749#msg323749

[JPK]

looks like the cathodes are shared with the same trace from one tube to the next then ground. Does it make a difference if each resistors doesn’t go directly to ground?

Yes it does. Best to just install one 1Ω resistor between ground and the first tube. The second tube will still be connected to the first tube, so the current through the single 1Ω resistor will represent current flowing through both tubes so divide by two to get the current for one tube.

Better yet, forget all this nonsense and learn to measure the current by measuring the voltage drop from each plate lead to the center tap on the OT. You also need to measure the resistance of each plate lead to the center tap. Then divide voltage by resistance to get the current. In the case of the BJ this is the fastest and easiest way to measure plate current.

Agree, I did all that fancy stuff on my AX84 October but not on my next build. I want the circuit simple as possible. It's been a while but I now have a 5E3 build coming up so your comment made me go review it. Pretty simple. 


https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm#:~:text=To%20Calculate%20the%20Bias%20Manually&text=The%206V6%20is%20rated%20for,watts%2C%206L6GC%20is%2030%20watts.&text=The%20Plate%20Current%20must%20be%20in%20amps%2C%20not%20milliamps%20so,cathode%20voltage%2C%20not%20plate%20voltage.
.

[dude]

looks like the cathodes are shared with the same trace from one tube to the next then ground. Does it make a difference if each resistors doesn’t go directly to ground?

Yes it does. Best to just install one 1Ω resistor between ground and the first tube. The second tube will still be connected to the first tube, so the current through the single 1Ω resistor will represent current flowing through both tubes so divide by two to get the current for one tube.

Better yet, forget all this nonsense and learn to measure the current by measuring the voltage drop from each plate lead to the center tap on the OT. You also need to measure the resistance of each plate lead to the center tap. Then divide voltage by resistance to get the current. In the case of the BJ this is the fastest and easiest way to measure plate current.

I'm having issues doing the resistance method. I get 95.8 ohms on blue plate to CT of OT and 94.4 ohms on the brown plate. But having issues getting the difference in voltage from the CT of OT, 335.3v, (B+) and each plate, using DC volts with black probe on transformer bolt.. I get 335.6v on blue and 332.0v on brown, blue more voltage than B+, results are -0.3v blue, other plate 3.6v.  (bad tube on the blue plate?)
I tried the shunt method, set my meter as below picture, The blue plate mA current goes up to 3.36 then the meter changes to ac.. and reads 0, I then turned off the amp. The other brown plate on current mA stays a .001.
What am I doing wrong.

[sluckey]

Connect the red lead to the OT center tap. Connect the black lead to the tube socket plate pin.

[dude]

Connect the red lead to the OT center tap. Connect the black lead to the tube socket plate pin.

Current reading mA's, shunt method, red on CT of OT,  black on pin 7, plate of el84. Auto read meter starts to read current for a few seconds then switches to a"AC" mA's..?
Meter inputs as in above picture for mA current readings

Tried another non auto meter, reads 0 mAs, the tube on that plate isn't red plating but bottom is way more red then the other, tried a different tube the same it's that socket.
EDIT: Time for a time out, one tube's plate seems much brighter than the other which seems the plate isn't even lighting up, middle of tube is dark. Let me retention the pins and try again.
The voltage at the filters down the rail look good., -11.8 volts bias.

[sluckey]

Connect the red lead to the OT center tap. Connect the black lead to the tube socket plate pin.

SET THE METER TO READ DC VOLTS.

[dude]

Connect the red lead to the OT center tap. Connect the black lead to the tube socket plate pin.

SET THE METER TO READ DC VOLTS.

I f**k'd up, sorry about that, just never measured bias by the shunt method, thought I had to set meter in current mA setting.  , I got 2.77v and 2.982v, difference, comes out to 28.9mV on one tube and 31.2mV, way high. I might as well flip the bd. as one of the filters has some leakage on the + end, add the 50K bias pot while I'm at it, then bias to around 65/70% dissipation. probably around 24 mVs.I noticed that most of the bias mods on these amps don't have a R to ground off one leg to prevent 0 bias, your schematic has a 12K to ground to prevent this. I guess that 25 turn tiny pot they figured you won't turn it to 0, I'll just use the 12K.
I don't know why I didn't get the proper voltage difference just measuring the OT CT to ground and plates to ground then subtracting. I did retention and clean the pins before the shunt method though, maybe loose pin connections caused bad readings.

They say it's dangerous to put high voltage on both leads, I had to leave the leads on for a few minutes to settle the reading, kept jumping around but settle within a few thousands of a mV, such a small reading is probably impossible on even a good meter. 
If this amp was mine, I'd just tear out that Bd. and get Doug's bd and have an easy amp to work on, I noticed the half of a 12ax7 is not used, maybe a cathode follower..? 

Thanks for your patience and help, all this because I didn't have my meter set right.