Hello and old Blues Jr.

[ArturoQ]

Hello to All,
I don't remember when I purchased my Fender Blues Jr, but it was in the 90s with the green board. Pretty sure it is a Revision B. Changed speaker to Cannabis Rex way back, not sure why. I have changed the tubes several times. Have had to resolder the tube socket board a while ago. Decided to check it out the other day. Had all sovtek tubes. One of the el84s was redplating and there was alot of scratchiness going on as well as hum. V1 was microphonic so I placed it in V3, it was bad. Had old GT (sovtek) and put it in V1. The microphonics seems to have stopped. Have a JJ12AX7 on the way. Swapped el84s and that tube still got hot quick. Placed old GT in there until I get replacements. Read numerous articles and YT videos on reported high biasing of the amp and causing the early demise of the power tubes so I decided to check it out.
Remembering the mismatched el84s
(thanks BTW for a wonderful thread in this forum on how to test the biasing !!!!!!!)
Resistance was 97 ohms on both coils. Red to Blue and Red to Brown.
Voltage difference after 10 minutes warmup was
Red to Brown = 3.39 Vdc
Red to Blue = 4.24 Vdc
If I did it correctly that is 34.9 mA and 43.7 mA

As a test I clipped a 100K Ohm resistor across R37 with the following results
Red to Brown = 2.24 Vdc @ 23.1 mA
Red to Blue = 3.03 Vdc @ 31,2 mA.
Please check my methodology and reasoning. That seems like a 33% reduction in amperage. I am hoping that the balanced pair of el84s will even the differences out ???!!!
Don't see any caps leaking. Cleaned out the board mounted cable plug with alcohol and it seemed to have stopped that scratchiness.
Wil worry about more after the tubes are replaced.
What is the current trend of thought on the biasing of the amplifier. Should I put the 100K across R37 and solder it there? Trying not to pull the boards again if I dont have to.
Thanks for any help!

[g1]

If you are asking about mounting the added resistor on the component side of the board, that is acceptable. It also makes it more obvious that the bias circuit has been modified, to anyone working on the unit.
Matched tubes should even the difference out.
Schematic (attached) shows B+ around 330V for Blues Jr., so your later numbers look more reasonable, around 10W for the hotter tube.

[Jon Snell]

I always replace R31 with a 33k, whilst I have the chassis out and replace the screen grid resistors along with the under rated smoothing capacitors, after replacing the EL84s with Sovtek or Reflektor types.
In the UK these arrive set to 230volts, we are 240volts so I have to move the primary wiring to match 240volts.The EL84s last a very long time afterwards because they are no longer being over cooked.
Lots of mods to improve the sound are available, subject to my customers choice of course.

[Leo_Gnardo]

As a test I clipped a 100K Ohm resistor across R37 with the following results
Red to Brown = 2.24 Vdc @ 23.1 mA
Red to Blue = 3.03 Vdc @ 31,2 mA.
Please check my methodology and reasoning. That seems like a 33% reduction in amperage. I am hoping that the balanced pair of el84s will even the differences out ???!!!
Don't see any caps leaking. Cleaned out the board mounted cable plug with alcohol and it seemed to have stopped that scratchiness.
Wil worry about more after the tubes are replaced.
What is the current trend of thought on the biasing of the amplifier. Should I put the 100K across R37 and solder it there? Trying not to pull the boards again if I dont have to.
Thanks for any help!

Yes, you made a step in the right direction taming the bias current.

My method is a little different but results are similar. I place a 10K trim pot in series with the ground leg of the bias voltage divider (? R31). That way bias voltage can be fine tuned and set to a point where there's no excess current in output tubes but not so low that crossover distortion is evident. Typically 26 to 29 milliamps. You came close with your method. It would be a little nicer if output tubes were matched for emissivity, but the ones in there are "close enough for rock and roll." One problem with mismatched tubes is hum/buzz in the speaker. The more mismatched, the worse the hum. It's no big deal unless you need a super quiet amp for recording or playing quiet sensitive style. That hum is quickly swamped playing at medium or loud volumes.

One of the other troublesome behaviors of Blues Juniors is their tendency to develop ultra high frequency oscillation. It's not audible on its own because the frequency is too high for humans to hear, also would not pass through the speaker, but it can make output sound weak and distorted. There are two ways to prevent this. One is by placing a 100 pF capacitor in parallel with one of the resistors on plate of the output drive tube. Which one? I'm sure someone here will offer the answer. The method I used for years was instead to place a 470 pF cap across the plates of the drive tube, bridging pins 1 and 6. May seem a little severe but I never noticed anything to complain about in the way the amp responded. Perhaps a smaller cap would work just as well. But my method hammered the UHF oscillation into submission first time every time. Scores of amps can't be wrong. When I tried the other method it worked well too. Pick one and go with it. I use 1000V rated disc caps for this function.

Eventually that awful plastic input jack gives way. Replace with an authentic Switchraft 12A jack. Those last for decades in millions of Fender and other amps, can't go wrong.

Main filter cap can be replaced eventually with 100 uF unit. That will reduce background hum a bit. It's popular to replace ALL the high voltage filter caps with a better brand, F&T for instance. Long term reliability. But your caps seem to be holding out. You can wait for a failure or swap in new ones as a preventive maintenance procedure.

The 2200 ohm resistor in the power supply sometimes fails. That would indicate a weak or failing second stage high voltage filter cap. No need to replace the resistor if it's still functioning and showing no signs of distress like turning brown or black due to charring.

Good choice on the Cannabis Rex. Some of my customers have chosen that one for their Blues Juniors, also Deluxe Reverb. Different tone from the stock speaker. And the cone is made of hemp "for that mellow tone."

[ArturoQ]

Hello and thanks g1. For the info and for the pdf of the manual. I measured 320 Vdc at the main filter cap,, but the tubes are hot. I've read that they should be at factory settings and changing the bias kills the sound, thus the question on adding or not the parallel resistor to drop the bias. Thanks for confirming my hope that matched el84s will solve the difference in balance!
What measurements do I need to determine the wattage of the amp????

[ArturoQ]

Hello John Snell,
So far there seems to be a conscensus on dropping the factory bias. I was also thinking of using a potentiometer but at R31 this would affect both tubes, yes. Is that much of an issue with different brand tubes. I purchased some inexpensive JJ el84s as I was guessing at what I was doing. Just 2 replies here, have helped my point of view invariably!!
Tanx!

[ArturoQ]

Hello Leo Gnardo,
If I recall correctly your name was in the thread I found on the biasing, so thanks again! Fixing the tube board was a fairly bold experiment on my part an years ago. Of course, found the BillM threads. Tastes and ambitions have changed since then. I use it as the bedroom amp, well my only amp actually. I keep it's master volume fairly well down and not much volume or gain. If I were to choose I would like to replicate the sound of a 57 Tweed Bassman at the volumes I can live with :-) !
I feel like the potentiometer is a neat idea, can't see how to install without pulling the main board. Eventually I want to pull the board but am not equipped yet.
It has been indicated that the caps in the blues jr. are superior to the ones used after 2001 and are very good. I will eventually have to change them thanks for the brand recommendations as also for the other issues you seen and resolved. I will have to keep this thread for future investigation when I am ready to go all the way. Replace the tube sockets with hardwired one and such!!
thank you!

AS AN Aside, do you think a HewlettPackard 100 Mhz dual channel analog oscilliscope for around $130 tested with 30 day warranty is a acceptable value?

[ArturoQ]

Hello g1, bummer but that link is only giving me the first page of the manual. Can you help or point me to where I might get the full copy????
Thanks again

[Leo_Gnardo]

Hello Leo Gnardo,
If I recall correctly your name was in the thread I found on the biasing, so thanks again! Fixing the tube board was a fairly bold experiment on my part an years ago. Of course, found the BillM threads. Tastes and ambitions have changed since then. I use it as the bedroom amp, well my only amp actually. I keep it's master volume fairly well down and not much volume or gain. If I were to choose I would like to replicate the sound of a 57 Tweed Bassman at the volumes I can live with :-) !
I feel like the potentiometer is a neat idea, can't see how to install without pulling the main board. Eventually I want to pull the board but am not equipped yet.

I'm not surprised - I have sounded off on Blues Junior & Pro Junior improvements so a search was bound to turn up some of those. Not that I'm the total expert, just hope to add to the body of knowledge and promote what is practical. Some of the BIll M mods I agree with, some not so much. One problem, his mod for installing a bias adjust 1) uses a 50K 10-turn pot - way overkill for the purpose. and 2) that pot can set the bias voltage all the way to zero. WHY would anyone need to go there, or anywhere near? So much smarter to delimit the resistance so that bias voltage is always in a safe zone. Maybe Bill M got a huge discount on a quantity of 50K 10 turn pots, who knows? Now his son is saddled with the task of selling them off.

I have been able to insert a trim pot without removing the circuit board, but it's not for everybody. Takes some dexterity and you need to cut the 22K resistor carefully so that there's enough lead to solder to. Some may object to doing it this way. Admittedly it is a bit dicey but for me, so far, it has worked every time. Certainly a step more reliable to remove the board so that a lead from the pot can be inserted through the board and soldered to the land on the back. I use single turn "blue cube" Bourns trim pots. 652-3386W-1-103LF Mouser part number. Price now is $1.60 in single quantity and DRAT they are out of stock. Due to have them December 2. So... don't rush, get yourself one for Christmas. You may be able to find the same part at another vendor. Leave off the 652- prefix and the rest may lead you to the right trim pot. OR a similar pot, 5K instead of 10K will probably suffice. 3386W-1-502LF Those are showing IN STOCK, same price. Good luck!

[g1]

Hello g1, bummer but that link is only giving me the first page of the manual. Can you help or point me to where I might get the full copy????
Thanks again

It says the file has been viewed 0 times, so I think maybe your browser is trying to open it, instead of downloading the file? See if you can download the pdf. 1.67 MB is the whole 21 page service manual (1995).

[ArturoQ]

Howdy Leo Gnardo,
I don't know that I have the senses to appreciate all the changes to the tone that were suggested in his pages. Thanls for the links to the pots! Save that for when I feel adventurous again.

[ArturoQ]

Howdy g1,
Finally figured out what I was doing wrong and how to do it right, so thanks again for the manual. If I figured your post correctly, to get the wattage of the tube I would use my voltage 320 Vdc and multiply it with the mAs of each to to get the correct value.
So with the stock bias and current tubes as described, I am getting 11 Watts and 14 Watts, is this correct?
What is the mA values actually represent, the plate amperage?

[g1]

Howdy g1,
So with the stock bias and current tubes as described, I am getting 11 Watts and 14 Watts, is this correct?
What is the mA values actually represent, the plate amperage?

Yes and yes.
The wattage figures are 'plate dissipation' at idle. If you know the max. plate dissipation (from datasheet) for tube type, you can calculate % dissipation. For EL84, it is 12W, so 14W over 12W means one of your tubes is idling at about 117% of max.

[Helmholtz]

What is the voltage at TP30?

EL84 datasheet recommends -14.7V.

Do the different plate currents stay with the tubes when you exchange positions?

[ArturoQ]

Okay.. plate dissipation, cool! Thanks for the knowledge you shared!!! That plate dissipation doesn't sound to good. I have the 100K ohm clipped across it for now. So that is 7.4 and 9.99 Watts. Looks like I should make the change permanent. I have heard that idle should be about 70% of maximum. If my calculation are correct is is currently idling at 83% on the hot one and 61,6 % on the other. Sounds more and more like a potentiometer is the way to go if there is that much difference from 70%,
It is still loud with the lowered bias.