Checking Fender Blues Junior Bias

[dadroadie]

Hi to All,
Can some one advise me of the proper way to check the bias on the Blues Junior on an amp that does not have the bias adjust mod ?
I have a 2015 creme board tweed model that I got used. The Grooves Tubes EL84s are marked 4 and are not showing any signs of over heating or red plating.
The amp is working fine but the tubes are original and I would like to have a back up set.
I run the amp with the master at 6 and the regular volume at 2.

The amp repair people that are in the area want $150.00 ($50.00 tubes $100.00 labor) to put in a bias pot and adjust the new tubes. I cant afford that.

I have a good Fluke Multimeter and feel comfortable using it and soldering . I also own a socket saver for the EL84 tube with outside pin contact tabs so I believe I won't need to pull the board. .I don't want to install the bias mod myself because of all the horror stories I've heard about how delicate the board and ribbon cables are.

I would like to install a new set of #4 Groove Tubes (same as that are in there now) and check the bias to make sure the amp bias is within a working range.

Could some advise the proper procedure to do this. also what kind of bias I should be seeing to make sure the amp won't self destruct.
Thanks to all
dadroadie

[Leo_Gnardo]

A method I learned right here on MEF (thanx guys!) With the amp OFF, measure resistance between output transformer center tap (red) and each plate (brown and blue.) Then with the amp on and warmed up say 10 minutes or so, measure DC voltage in the same locations. Ohms Law gives you current I = V/R easy cheezy. Of course that will be in amps, so multiply that figure by 1000 for milliamps. I like to see EL84's in Blues Jr running 26 to 29 milliamps, just barely past the crossover distortion area. That will give you good tone and maximum lifetime.

I have managed to install a small bias pot without removing any boards, using a 10K Bourns mini blue cube half watt trimpot. I get 'em from Mouser, they show a price of $1.60 each. It takes a fine-tipped wire cutter, a good pencil-tip soldering iron and steady hands to do this. Mouser part # is 652-3386W-1-103LF . Don't be fooled by their illustration. The style I recommend has 3 small wires sticking out the side, much easier to deal with than out the back as the picture shows.

Those are some way gold-plated tech prices! Where in the world are they charging that? Even at list price a pair of good JJ EL84 should set you back $29 plus tax. I typically charge 1/4 hour for simple fast tube changes & bias, 1/2 hour if it's a bit more complex & starts eating up my time. 3/4 hour if I have to unass a circuit board.

Tell us where you are and maybe some helpful MEFster will offer help if you think it's beyond your capabilities.

[drewl]

Unless you're using the original bias resistor in series with a 10K pot, you're gonna want a larger value pot.

[Leo_Gnardo]

Unless you're using the original bias resistor in series with a 10K pot, you're gonna want a larger value pot.

Correct, and that's how I do it, leaving the resistor in series so the minimum bias voltage is the factory setting. Snip one resistor lead leaving enough to attach the trimpot to both ends. Not quite brain surgery but as close as I dare to get.

Bias voltage at output tube grids usually winds up in the -11.8 to -12.0V region. Depending on tubes used results may vary. As they come from the factory bias voltage in these amps about -11V, with bias current around 35 to 40 mA sometimes more.

[nevetslab]

A method I learned right here on MEF (thanx guys!) With the amp OFF, measure resistance between output transformer center tap (red) and each plate (brown and blue.) Then with the amp on and warmed up say 10 minutes or so, measure DC voltage in the same locations. Ohms Law gives you current I = V/R easy cheezy. Of course that will be in amps, so multiply that figure by 1000 for milliamps. I like to see EL84's in Blues Jr running 26 to 29 milliamps, just barely past the crossover distortion area. That will give you good tone and maximum lifetime.

I have managed to install a small bias pot without removing any boards, using a 10K Bourns mini blue cube half watt trimpot. I get 'em from Mouser, they show a price of $1.60 each. It takes a fine-tipped wire cutter, a good pencil-tip soldering iron and steady hands to do this. Mouser part # is 652-3386W-1-103LF . Don't be fooled by their illustration. The style I recommend has 3 small wires sticking out the side, much easier to deal with than out the back as the picture shows.

Those are some way gold-plated tech prices! Where in the world are they charging that? Even at list price a pair of good JJ EL84 should set you back $29 plus tax. I typically charge 1/4 hour for simple fast tube changes & bias, 1/2 hour if it's a bit more complex & starts eating up my time. 3/4 hour if I have to unass a circuit board.

Tell us where you are and maybe some helpful MEFster will offer help if you think it's beyond your capabilities.

And, of course, either null your DMM in the Ohms mode, or measure the probe lead resistance, so it can be subtracted from the DCR measurements of the primary leads from C/T to the plates.

[dadroadie]

Thanks to all that replied.
I believe I can handle testing the bias but just wanted to go through the procedures step by step before I check this.

1: zero the volt meter in the ohms scale and write down any residual reading for reference.

2: with the amp unplugged measure resistance between output transformer center tap (red) and each plate (brown and blue). Write down these measurements.

3: Turn amp on and warmed it up for 10 minutes or so, measure DC voltage in the same locations . Question, since I’m measuring DC voltage is the center tap of the output transformer ground or do I need to have the negative lead to ground and only check the brown and blue wires? (Schematic attached). Write these voltages down.

4: Use ohms law to calculate milliamps: Ohms Law gives you current (I = V/R) Convert amps to milliamps, multiply that figure by 1000 for milliamps. Write down these measurements. I should see the factory bias voltage of about -11V, with bias current around 35 to 40 mA sometimes more. The lower the bias the longer the tube life. As mentioned EL84's in Blues Jr running 26 to 29 milliamps, just barely past the crossover distortion area. That will give you good tone and maximum lifetime.

5: Install a new set of Groove Tubes with the same rating as the old tubes (#4) and repeat the procedure. If I get the same or a little lower MA readings they should work like the old units. The original Grooves Tubes EL84s are marked 4 and are not showing any signs of overheating or red plating. I do buy all my stuff from Antique Electronics (GREAT PEOPLE!!) and notice they have several types of match pair tubes and testing service available. (except for the Groove Tubes?.) . Is there an EL84 matched set of another brand that someone could recommend would be more robust (longer life ) in this circuit that would work better? I play the amp clean the master at 6 and the regular volume around 2. No outboard effects. I would order them with the APEX testing and 24 hour burn in if available.

It was mentioned to install a small bias pot without removing any boards, using a 10K Bourns mini blue cube half watt trim pot. If I work up the courage to install this mod could you advise me which resistor on the board would I modify? Also could you advise which leg (side of the resistor to modify? Or does it make a difference. (Possibly a picture of the arrangement?)

Sorry for this lengthy post and thanks to all that are offering assistance. I just want to be sure I understand the instructions correctly and don’t blow up the amp or myself in the process.
Regards
dadroadie.

Blues Junior Schematic (2001).pdf

[Dave H]

Question, since I’m measuring DC voltage is the center tap of the output transformer ground or do I need to have the negative lead to ground and only check the brown and blue wires?

The centre tap of the output transformer isn't ground, it's connected to B+ which is several hundred volts so take care!

With the amp off measure the resistance between OT red and blue (call it R1) then measure the resistance between OT red and brown (call it R2).
With the amp on and warmed up carefully! measure the DC voltage between OT red and blue (call it V1) then measure the DC voltage between OT red and brown (call it V2)
NB. no measurement is with respect to ground.

Calculate the bias current of V4 as IV4 = V1/R1
Calculate the bias current of V5 as IV5 = V2/R2

[Leo_Gnardo]

Question, since I’m measuring DC voltage is the center tap of the output transformer ground or do I need to have the negative lead to ground and only check the brown and blue wires?

One probe of your meter on the OT primary center tap, the other on brown or blue plate leads. If you want to be super safety conscious, you can apply alligator clip leads applied while the amp is powered off, switch on, make your measurement then power down before moving the gator clip to the other plate.

Doesn't really matter much which meter probe goes to the OT center tap - if you read it "backwards" and get a negative voltage, no matter, just ignore the - sign. Happens to me oft... well sometimes. No biggie.

Regrettably I haven't any photo of my bias trimmer installation.

In the schemo you posted, you want to install the 10K trimmer in series with R52, the 22K resistor.

(In some "pro" mod kits they have you replace R52 entirely with a trim pot. WHY? You could dial your bias voltage all the way down to zero... Much better to limit the adjustment range of bias voltage within a safe area. Thanx for consulting MEF where we do try to keep the safety of your amp AND you in mind. Most of the time.)

[Pedro Vecino]

If you connect a multimeter between transformer center tap in presence of high voltage it´s very likely that on one of the sides (red-brown or red-blue) you will read strange measurements due to an oscillation that occurs when extending those cables outside the amplifier. In this case it would be more convenient to identify the bad side and read on the good side twice by swapping the tubes between them.

[Dave H]

If you connect a multimeter between transformer center tap in presence of high voltage it´s very likely that on one of the sides (red-brown or red-blue) you will read strange measurements due to an oscillation

You could always pull the PI tube.
I use the shunt method myself. I've not had any oscillations with the OT primary shorted and there's no calculations to do do.

[Helmholtz]

I've not had any oscillations with the primary shorted

No wonder, shorting one side of the primary makes all transformer windings look shorted. Result: no gain - no oscillation.

[Pedro Vecino]

You could always pull the PI tube.

Good alternative!

[dadroadie]

Thanks again to all that are helping me with this project. I’m not sure if I understand the shunt method and would like to go through it to make sure I understand the correct procedure.

1: With the amp unplugged measure resistance between output transformer center tap (red) and each plate (brown and blue). Write down these measurements.

2: Turn amp on and warm it up for 10 minutes or so. This is where I get a little confused. Using the shunt method do I pull one of the power tubes and THEN measure from the red lead and brown and blue wires or just measure the voltage at the pulled tube and then put the tube back in and pull the other and measure that side? That would seem a little dangerous. Write these voltages down.

I think I may try to work up the courage to try the bias mod mentioned. I found this on Ebay. I believe its it’s a ½ watt like was mentioned. The part is listed as 3296Y-1-103 BOURNS TRIMMER 10K OHM 0.5W PC PIN and I have attached a photo.

This forum and its members are GREAT!! I don’t mean to appear stupid. I just want to do this safely and try to get the amp to run in the most reliable manner possible.
Thanks again to all!
dadroadie

[Leo_Gnardo]

Thanks again to all that are helping me with this project. I’m not sure if I understand the shunt method and would like to go through it to make sure I understand the correct procedure.

1: With the amp unplugged measure resistance between output transformer center tap (red) and each plate (brown and blue). Write down these measurements.

2: Turn amp on and warm it up for 10 minutes or so. This is where I get a little confused. Using the shunt method do I pull one of the power tubes and THEN measure from the red lead and brown and blue wires or just measure the voltage at the pulled tube and then put the tube back in and pull the other and measure that side? That would seem a little dangerous. Write these voltages down.

I think I may try to work up the courage to try the bias mod mentioned. I found this on Ebay. I believe its it’s a ½ watt like was mentioned. The part is listed as 3296Y-1-103 BOURNS TRIMMER 10K OHM 0.5W PC PIN and I have attached a photo.

That trimpot isn't the one I mentioned. But I'm sure it will do. Main difference is, the one I specified has a nice wide slot built into the body, less likely for your screwdriver to slip when making adjustments. OTOH the one on the photo will work the same electrically, you just have to have a steadier hand when turning the tiny adjustment slot.

Shunt method: nowhere near as complicated as your descriptions. In fact you have described the ohm's law method, not shunt.

For shunt, you set your meter to a current measuring scale, say 200 mA. Then clip one lead to the OT center tap, the other to a plate lead in this case either blue or brown. The assumption is, resistance thru your meter is way less than resistance thru the OT primary lead, therefore current is "shunted" thru your meter. IOW most of the current passes thru your meter, and only a small fraction through the OT primary. Kind of like a highway overpass to speed your way past a crowded town center. Yes there is some error induced because the meter isn't a perfect zero ohm shunt. But in most cases the error will be relatively small say 2 to 10 percent. Close enough for rock 'n roll. The dangers are, your meter leads are at the amp's B+ voltage (but they are anyway when you measure using the Ohm's law method) AND if you make an error placing your clip leads or slip whilst poking your probes, you will likely roast the internal fuse in your meter. Assuming it has one. That doesn't happen when you're measuring voltage.

Tell you what, I gave up using the shunt method after many a slip, and spending probably upwards of $100 over the years on meter fuses. For someone who isn't doing this every day in a pro shop, and has the presence of mind to keep a batch of meter fuses handy, it's a PITA to go out & find spare meter fuses especially now Radio Shack has gone the way of the dinosaurs.

Bottom line, if you're feeling adventurous, and you know your meter has a fuse in series with the ammeter function, and you have spare fuses, and you won't be fazed by a spark and a zap should you slip with your meter probes, then be my guest and shunt away.

P.S. We're still curious where your Cadillac-price amp tech is, to quote such high $$$ for a simple job. Palm Beach? Manhattan? Amelia Island? Kau'i?

[dadroadie]

Thanks for the reply! I think I'm going to go ahead and order the trim pot then use the "ohms law" method to check the voltage, convert to current to have a base line of what the amp is doing stock . Then install the trim pot and if I understand correctly adjust the current to around 26 to 29 milliamps to get the longest tube life and reduce the heat buildup. Also that way if I order another set of Groove tubes #4 I will be al set. Question if I do the bias adjust could I use another brand of tubes and just set the bias to the 26 to 29 milliamps? possible go to a #5 for a little cleaner headroom or Tung Sols ?

I live in the Chicago northwest suburbs and found that all 3 of the repair shops that had qualified people wanted this price. I did find a few that would do it for less but after talking to them I felt rather uncomfortable in their knowledge. One guy said he has played rock and roll for 20 years and and understood what tubes should sound like . When I ask him how he would do the mod he said he would look it up on You Tube! I really don't mind paying someone for their training and knowledge. I just don't have these kinds of funds available.Plus if I can do this it teaches me something about the amp and keeps this old mind young!

I got this amp in trade for a acoustic guitar to give bring back the memories of my old Fender Twin. I really do like the sound that I'm getting I just want to make it as reliable of possible. I still have my old solid state Peavey amp that has served me well for many years so if all of this starts getting to crazy I can always sell the Fender and go back to using the Peavey. After all playing music is suppose to be fun and relaxing not stressful.
Thanks again
dadroadie