Setting the Bias

After you’ve installed the basic mods, your final task is to set the bias. Much mystery seems to surround setting the bias, but it’s really quite simple. Bias controls the current flowing through your output tubes, and it’s well known that the Blues Junior’s original designer chose a large current, or hot bias. This causes excessive tube wear, limits headroom, and constrains the tonal range of the amp.

Each model of output transformer has a characteristic resistance and when current flows through it to power the tubes, there is a voltage drop across the resistance. All you have to do is measure across two leads of the transformer and adjust the bias trimmer potentiometer until you get the correct voltage.

This video, showing a Blues Junior with EL84s and a TO20 output transformer, demonstrates how easy it is to set the bias.

SAFETY REMINDER: You’re measuring the difference between two high voltages! Work carefully, with insulated leads, as shown.

Here are the target voltages for different tube and transformer configurations:

Tube Stock OT TO20 TO22 TO26
 EL84  2.6V 3.4V  4.1V
 6V6  4.2V  5.1V
 5881*  6.6V
 6L6GC*  7.2V

 

 

 

 

* With TP24 power transformer.

98 Comments

  1. Nuno says:

    Hello Bill,

    Just bought the basic Mod kit (cream board), and have one doubt, should I remove the amp stage valves to set the bias?

    Thanks
    Kindest Regards

    Nuno

  2. NEDiver2 says:

    HI Bill:

    I have a cream board Blues Jr. I performed your twin stack, tone stack, filter and bypass cap mods and set my bias by changing R52 to 27K. Problem is once I set the volume to 10 or greater and the master to 5 or greater I get a harsh bass/ lower mids distortion that is not musical at all. I’ve also previously done the PI Osc lead dress and added the 100pf cap across R30. Any ideas on what might cause this. I also watched your video here and checked the bias voltage. It measures 0Vdc with the EL84s pulled or a red lead disconnected. However if I leave the tubes in and the lead connected I measure aprox -3.0 to – 3.1 Vdc. I measure resistance of the transformer 100ohms from red to blue or brown, 200 from blue to brown, Any advise would be appreciated.

    Thanks Bill

    • bill says:

      It sounds like you’ve done everything right, but the amp is still running a bit on the warm side for the stock output transformer. I usually set the bias for a voltage drop of around 2.4 or 2.5V. Since you didn’t install the trimpot, you’re stuck with replacing resistors until you get it right.

      If your output tubes are old, you may want to try fresh ones first. They may bias properly with the 27K resistors– EL84s are all over the lot when it comes to the bias level they need.

  3. natethegreat says:

    Hi Bill,

    I am finishing up with the basic mod kit for my creamboard BJ and I’m ready to set the bias, however when I measure R31 or R32 to ground, I’m getting a reading of 0Vdc. In the above video you mention that it is important to do this test before turning the amp on. As I perform this test the tubes are in and the amp is off and unplugged. Any help would be much appreciated, thanks!

    • bill says:

      Yes, you should be getting a reading of around -13V if you haven’t adjusted the trimpot.

      • Tom says:

        Bill, I think there is a bit of confusion with the instructions in the video. In the video above you say before adjusting the bias to either put the amp in standby (if you have it) or disconnect one of the red leads or remove the output tubes, then measure the voltage. However, if I disconnect one of the red leads I don’t get any voltage to measure. I had to adjust mine with the amp on, the tubes in and the red leads connected. Can you please clarify? Thanks Tom.

        • bill says:

          Check the voltage at the tops of R31/R32 in standby to make sure you have a negative bias voltage. It’s just a double check before you switch on the high voltage measure the drop from red to brown.

  4. JRC4558D says:

    Hi Bill,

    Yesterday I turned my Fender Blues Jr on and it just did a noise and turn off, the fuse just blown, but after replacing it I noticed that one tube was doing funny electric arcs inside, I switched off the amplifier and I bought a new matched pair of EL84.
    Before intalling them I checked your web to be shure about the bias because the EL84 are getting bad too fast, but none of my readings were even close to yours so something is going wrong there.
    I did every measurement without the EL84 installed and with the multimeter’s prove to ground, this is what I get:

    B+: +375 VDC, Z:+364 VDC, Y: +317 VDC, X: +289 VDC, TP25 (D1-D3): 137 VAC.

    Then I measured B+ (positive leg of C25) with negative prove and Brown Cable (P3) and I got 0VDC or 374 VAC (there I should have -3.34 VDC, am I right?).

    What do you think Bill? is that correct? Thank you very much for you help.
    Cheers from London, UK.

    • JRC4558D says:

      Oooopssss, I didn’t read this post properly… I’ve omitted that and yeah mine haven’t got any TO-20 Output transformer, just the stock one. By the way mine is a BJr Cream Board, Rev C. Sorry about that.

      Now, I was digging a bit and doing some measurement and I have more doubts…
      Why trying to match tubes, adjust the BIAS but leaving the stock resistors: R31 and R32 (220K)? that actually are not matched, and in my case one is 216K and the other one is 219K, besides there is another couple: R33 and R34 that are there in the way and those should be matched as well, am I wrong?

      I’ll try to replace all of them with 1% MetalFilm matched resistors and accurate ones and I’ll post the results.

      Bye for now and thanks so much.

      • bill says:

        There is no need to balance the 220K resistors. They have a very minimal effect on the actual bias voltage delivered to the tube; their function is to isolate the grid signals from one another, since they’re inverted, while delivering the DC bias voltage.

        It’s not necessary to have the TO20 output transformer. All of the same measurements apply to the stock transformer, except that a good, tube-saving voltage drop (brown to red) should be no higher than 2.7 volts.

    • bill says:

      Are those B+ measurements with the tubes in or out? If the tubes are in, your voltages are quite high. Per the schematic, all voltages should be measured with the tubes in. If they are in, it may be that your power transformer is wired for 220V, while your mains supply is closer to 240V. That’s not necessarily a problem; higher voltage gives you more clean headroom. But it does require more negative bias to keep things under control.

      There is no DC voltage drop from brown to red if there are no tubes in place.

      • JRC4558D says:

        OK, there was a dodgy GROUND connection, I guess that was the problem that caused bad readings. Once I’ve that sorted out all was fine, still high voltages on X,Y and Z (Factory Set seems to be 230V) but I could adjust BIAS (brown to red) in -2.44 VDC and now the BJr sounds amazing, very quiet, clean and the tubes are not that hot than before and the tone is exactly the same but much much clean.

        Thanks a lot Bill for all your answers and also this useful web page.

        Cheers!

        Guillermo.

  5. JayDee says:

    Hi Bill,
    Just got the basic mods for a Blues jnr cream board.
    Included I have a small blue capacitor marked ‘101K 3KV’.

    The Easy Bias control instuctions say to solder the 100pF capacitor across the leads of R30.

    Is this small blue capacitor I have the right one to solder across R30?

    Many thanks,
    JD

  6. SamZ says:

    Hi Bill – the bias on my amp was adjusted on 2 different occasions: (1) when the basic cream board was installed, and (2) when the TO20 Output Transformer was installed. Do I need to adjust the bias when I replace the tubes (I still have the stock tubes)? And what other conditions would require adjusting the bias?

    Thanks!!

    • bill says:

      You only need to set the bias when changing output tubes. Sometimes new tubes will drift after half an hour or an hour of operation, so a quick check to make sure they’re not running real hot, then let it burn in, then do a final check.

  7. SamZ says:

    Thanks Bill! Your help is always appreciated!!

  8. dpm309 says:

    I just did the basic mods for a cream board, Rev. A, (EL84s) and am now getting string buzz on the low E and A strings. It is hard to tell if it is something rattling in the cabinet or the circuit. Plugged the amp into a separated speaker cabinet and it seems to almost disappear, but is still there. I read above that this could be a biasing issue. The amp has a small, no-name OT that the owner had installed before I did the mods. When I lower the bias to 1.8V, it improves somewhat but is still there. At 1.8V, I am getting -17.8V at R31 and 32 (with power tubes removed). At 3.4V, I get -16.1V and at 2.6V (Stock OT suggested voltage) I get -16.6V. If I try to adjust the trimpot to get ~ -12V, the string buzz is worse and the bias reading is greater than 4V. Could this problem be related to the OT? It does not seem to be acting like any of the stock or upgrade OTs.

    • bill says:

      I’d say that the sound on the E and A strings is oscillation. If it’s a no-name OT, how can you be sure it’s the right impedance? What’s the DC resistance of each side of the primary? You’d need to know that in order to set the bias properly.

      Also, a 100pF cap across R30 may help with the oscillation. See here: http://billmaudio.com/wp/?page_id=115

  9. dpm309 says:

    Do I measure the DC resistance with the power off or on? With the amp off, I get 400 ohms when I measure the resistance between both sides of the primary (blue and brown). I get 197 Ohms between the red and blue primary leads and 201 between the red and brown leads. Am I measuring this properly? Also, I did install the 100pf cap across R30 when I did the basic mods.

    Thanks,

    • bill says:

      Always measure resistance with the power off. It sounds like you made the measurement properly. I’d guess that this OT has too much impedance (something you can’t measure easily at home) and is causing more oscillation than the 100pF cap can cure. Or there is another problem. Have you tried a different pair of output tubes?

      If you want a stock Blues Junior OT, you can have one for the cost of postage.

  10. dpm309 says:

    Bill, thanks for the quick reply. I just finished going over your page about cream board phase inverter oscillation and made a couple of adjustments to the ribbon cables (after I hot glued them to the board as you suggested) and made sure the blue OT wire was not resting against the ribbon cable like before. Fired the amp up and tested it quickly and I am not hearing the buzz. I will test it out a little longer to see if this might help with the problem. I might also take you up on the stock OT if I am still having trouble.

    Thanks,

  11. dpm309 says:

    Well, I am still getting the distortion/buzzing on the low E and A strings even with the amp plugged into a separate speaker cabinet. Tried it at different bias values from 1.2 up to 4.1 with no noticeable difference. The tubes are new JJs that replaced the original Groove tubes. I may have to take it to another amp tech to see if he can figure out what the proper bias is with this OT.

    Dan

    • bill says:

      It’s not the bias that’s causing the distortion. You need to check for ultrasonic oscillation and address that problem first. That OT may have too much inductance for the BJr circuit.

  12. Alan007 says:

    Hey Bill,

    Thanks a lot for your input to the community and BJr users! I already did your TwinStack mod and it’s super easy and works!

    Now I wanted to see if my BJr III is also running too hot like most of the ones in stock form so I checked the bias (taking appropriate precautions) both with and without power tubes and the reading is – 11.8 Volts at those bias resistors. Was expecting something in the -10 range. Is it possible this one runs cooler somehow ?

    Thanks!

    • bill says:

      The voltage may be higher or lower than the factory spec of -10.5 volts depending on components and the line voltage in your area. That voltage doesn’t tell you anything about how hot the tubes are running. For that you need a bias meter or to measure the voltage drop across the red and brown transformer wires. a 3 volt drop, for example, would be 30 milliamps because the resistance of the output transformer is 100 ohms. Multiply the milliamps times you plate voltage (brown to ground) to get the idle power in watts. You generally want this to be 70 percent or less than the tube’s full rated power (12 watts), so around 8.5 watts is a good target.

      • CleanMAC says:

        Hi Bill, I hope you fell good today. About my OT, the resistance of the red / blue and red brown coil / is not the same. 178 ohm and 188 ohm. Consequently a tube is 8 watt and the other 9 (6V6). I read a page on your old site a way to adjust the bias separately but is it possible to do it with your board bias? Can I add two small potentiometers in series with R31 and R32 to adjust the voltage separately?

        Thank’s!

        • bill says:

          This is a non-problem. Layer-wound output transformers naturally have more windings and greater DC resistance on the outermost winding. The outermost winding is farther from the core, so more turns offset the weaker magnetic field. The inner and outer cores have different DC resistance, but the same AC resistance.

          • CleanMAC says:

            In conclusion, Vdc red-brown / Rdc red-brown x Vbrown-ground = Powers tubes. (Should be 70% of power Max)
            And it is not necessary to do for blue wire?
            correct?

          • bill says:

            It doesn’t hurt to check the blue wire, and remember that you can always swap the tubes for a closer match.

  13. Wolfe_BTV says:

    Hi Bill, I’ve installed the trimpot (and other mods) and am having a hard time getting the voltage drop reading on my meter. Everything is working fine, and the top of R31 is reading at about 12.8V.

    I’ve tried measuring VDC between the red and brown transform leads as well as from the positive side of B+ and the brown lead. I have a Fluke 83, which has always served me well, but I’m curious if it might be my meter. Any advice, or alternate approaches to taking the readings so I can dial in the voltage?

    Thanks much!

    • Wolfe_BTV says:

      After a little more poking around I found that I was able to get a reading off the blue OT lead rather than the brown. Everything seems to be working fine, but it leaves me wondering why the brown lead is consistently reading at 0.0 (it will register a very slight and temporary reading of about 0.001 when I turn the trimpot, but falls right back to 0).

      Please let me know if you think this is cause for concern, otherwise the amp sounds good and isn’t smoking/sparking/burning–so I’ll keep playing it 🙂

      • bill says:

        You may have a blown screen grid in that tube or the screen resistor may have failed.

        • aklamson says:

          I am having the same problem as Wolfe.
          No reading from the brown but was able to adjust from the blue.

          The amp seems to work just fine, I just wanted to make sure I’m not missing something or if there is something I should do about no reading from the brown wire.

          Thanks.

          • aklamson says:

            After looking around a little more, I found out that one of the wires in the ribbon for pw5a was broken right where it connected to the board. I repaired the connection by taking the board out, desoldreing the broken wire and pushing pushing the wire out a little so I could have something to solder the wire in the ribbon back to on the front side. Now I am getting a good reading from the brown (p3) wire. I also noticed that you can get a reading from blue wire (P1) but its a lot lower reading then from the brown wire (p3) so I would not suggest setting the bias off of the blue wire (p1).

            So I guess my question now is, do you think my fix on pw5a will hold or should I replace the ribbon wire?

          • bill says:

            Go with your repair for as long as it lasts, which may be for the life of the amp.

            If you remove the ribbon and replace it, that’s eight solder joints, eight possible traces to lift. No sense in making more work for yourself.

          • bill says:

            If you have no reading from red to brown, what’s the reading from brown to ground? Is the filament lighting up? Is there a broken wire in the ribbon cable? Is the bias voltage getting to the tube? Is the tube dead?

          • cmizell says:

            I too am having this problem. I get the reading off R31, but no reading between red and brown, or the B+ and brown! or either and blue. But the amp works. Sound comes out, sounds fine (haven’t played it much because I don’t want to hurt anything before figuring this out), all tubes light up, I can’t find any sign of broken leads… Any ideas why this might be? Anything else I should check? I’d love to get the bias set so I don’t burn through tubes so often.

          • bill says:

            I’ve found that this can happen if the screen grids in the output tubes are blown (common on EL84s) or the screen resistors, R35 and R36 are open/blown. With the amp discharged and off, check the resistance across R35 and R36. It should be 100 ohms. If R34/R35 are OK, try a fresh pair of EL84s.

  14. JerryK says:

    I have a green board, is the procedure for checking “Safe” voltage the same as in the video..I do not have a R31 and R32 to gnd voltage was 300ish.

  15. zifir says:

    Hello Bill,

    I am quite new in BIAS issue. I have replaced the stock 22K resistor R31 with a 47K trimpot (on greenboad) then check the voltage from the top of R21. I adjusted down to -10.0 and up to -13V and tried to hear changes in tone. However couldn’t hear any audible difference that you mentioned. Do you think that something wrong? Thanks

    • bill says:

      Bias isn’t a tone control. It just keeps the tubes from killing themselves with too much current and internal heat. There is an audible difference between a tube that’s running too hot and one that’s running right, but it’s subtle. It’s not tone, it’s articulation or crispness.

  16. AcousticBruce says:

    Greetings Bill,

    I put a few of your mods in (T020, sparkle, twin, basic mod.) I am now trying to figure out an issue with tubes.

    I used to get this loud screem for brief moments randomly, then no sound at all and the tubes were glowing. So last night I felt inclined to fix my amp. I took the back off and turned the amp on and watched the tubes…. I noticed that V4 power tube was not glowing like the V5. It took me a while to find the glow 30 seconds or so. I noticed a glow near the base of the tube first then towards the tip on V5. V4 the tip started glowing. I put my fingers on both tubes intermittently and noticed V4 was getting hot fast. In fact it was almost too hot to touch while V5 was lukewarm. So I waited a bit and swapped V5 and V4. I turned on the amp and noticed V4 (original “lukewarm” V5) making weird noice and when the noise was heard I saw a plasma-like effect very slightly in V4. I swapped back and V4 disnt do it anymore.

    I think I need to make sure nothing is up with V4. What is the best way to go about this?

    I have 2 extra 12ax7WAs and two matched jj el84s.

    I am afraid to put these in the amp because I feel more trouble shooting needs to be done. Any suggestions?

    Oh by the way here is a picture of how hot V4 apparently had gottten on day.

    http://db.tt/oipmXwxd

    • bill says:

      V4 runs hotter because it is oscillating at ultrasonic frequencies, and sometimes audible frequencies. See here: http://billmaudio.com/wp/?page_id=115.

      The tube is wasting large amounts of power and the heat sometimes damages the tube and can cause the tube to lose bias and overheat further.

      If you installed the 100pF cap on R30 and re-bent the ribbon cable, it should not be oscillating. But you would need an oscilloscope to know for sure. Check to see if the heat has caused the solder joints to crack on the V4 socket. That can cause loss of bias and overheating, too.

      • AcousticBruce says:

        If I am correct that 100 pf Capacitor would create a low pass filter with a cutoff freq at about 16kHz. Is that correct ? I did the whole 1/(2pieRC) Equation. Truth is I am insanely into audio electronics and I am looking for a good analog Scope. And I really like to understand the details. Is the ribbon cable mod necessary with the Filter? Also… Where can I learn how to test the oscillation With my oscilloscope? Would I just test R3O?

        Thanks again Bill!

        • bill says:

          Bend the ribbon cables as shown. The closer they are to ground/the back of the amp, the better. Oscillation shows up most strongly on the output tube plates/transformer connections, especially V4/blue.

          Yes, it’s a low-pass filter that bleeds ultrasonics to ground, before the output tubes have a chance to amplify them and waste power.

  17. roseblood11 says:

    In the video, it looks like R51 and R52 have been replaced by the trimpot, wired as voltage divider. So the bias voltage is taken from the wiper? What happens if the trimpat fails and the wiper looses contact?

    • bill says:

      It hasn’t happened yet, but if it did, and you didn’t shut off the amp when it started making garbled noises, you would lose a pair of output tubes.

  18. 1fastbullet says:

    I just bought this cream(2003)board BJr that I knew needed a replacement treble pot. (The amp was working, albeit with a noticable deficiency in the highs). I got the new pot (0041913000) and disassembled the amp to replace the broken one when I discovered some things that look all wrong.

    Someone has been inside this amp before me and cut out resistors R51 & R52. They then bridged the top leads of R31 & R32 and have run a jumper from the negative side of C27 to these bridged resistors. I have no idea what purpose this “modification” was intended to serve, but from studying your pages here, I have a horrible feeling that this is not a good thing.

    Is this as bad as I suspect? Shouldn’t I “un-modify” this before ordering the kits I want to incorporate into the amp?

    • bill says:

      The negative side of C27 is ground. It wouldn’t make sense to ground the C- point (bias), even if the amp has been converted to cathode bias. The previous owner may have added a resistor between C27 and ground to generate a bias voltage.

      I would not attempt to mod this amp further without knowing what else has been done to it. For such a radical change, I have trouble believing that the owner stopped there.

      If you like the way it sounds, play it and enjoy it. If not, sell it.

  19. poerum says:

    Hi.
    I have changed EL84′ s in a friends Blues Junior.
    The -C shows – 10,50 v
    If I measure the voltage drop at the brown wire it is – 4,0 V

    Any comments to that?

    Best
    Per

    • bill says:

      Yes, that Blues Junior is running very hot! 4 volts across 100 ohms (the DC resistance of the output transformer) is 40 milliamps. 40 milliamps at 330 volts (approximate plate voltage) is 13.2 watts. The tube is rated for 12 watts.

      You should install adjustable bias or increase the resistance of R52 to get the dissipation down to 8 to 10 watts.

  20. poerum says:

    Thank you Bill.
    The R52 has driftet/is not accurate – measures only 19 K?
    Will install a new resistor nd make the bias adjustable.

    Per

  21. poerum says:

    Hi.
    Now the -C is 12,8 V
    Voltage drop is – 26,5 V
    Thank you for your advice.

    Per

  22. NGW says:

    Hi Bill,
    I have just set the bias on a 2010 Blues Jr., after fitting new JJs.
    The GTs were past it and needed replacing.
    I will be ordering some of your kits, for future use, but for now I needed to put this amp back in service for it’s owner, with a sensible bias.
    I think your bias pot mod is the best option, but for now, something needs to be done.

    Instead of pulling the board and replacing R52, I set up test leads to which I could attach resistors.
    Using the lead clips I was then able to piggy-back resistors across R51, to alter the voltage divider offset and achieve the required bias.
    Simple calculations give the approximate resistances needed. The resistor of choice can then be soldered in place.

    This worked well and with the actual measured bias supply of -26V, an 82K resistor across the 33K of R51 (now 23.5K), gave a bias of ~25mA.
    I found this to be much easier than replacing R52.
    I wondered why this method isn’t adopted, instead of replacing R52, for those who need to reset the bias now and don’t have the bias pot mod available.
    Better, at least in the interim, than running EL84s at the ridiculous ~40mA bias.

    The only worry I can see with this method is; ensuring that not too much heat is used, when attaching the resistor to R51, so that the board joint isn’t compromised.

    What do you think?

    Cheers, Noel.

    • bill says:

      It’s a few minutes’ work to install a bias trimpot. No more soldering; set the bias wherever you like, hot or cold.

      • NGW says:

        I totally agree with you about fitting the the bias trimpot.

        I didn’t have time to pull the board and fit a trimpot, or replace R52 and thought this would work.
        A quick interim fix, for an amp that didn’t sound very good; it had no clean headroom, distorted way too early and sounded crap, due to the high bias current.

        I will be ordering your kits and transformers, to rework this amp, but for now it sounds quite good and the owner can actually use it.

  23. Perfusionist says:

    Hi Bill,

    Help! I have just installed a few of the mods. With the amp in standby I can only get a multi-meter reading with my little metal screwdriver touching the bias adjustment screw…? While touching it I have a good reading (-12.8v).

    When I tried the red and brown lead measurements out of standby the reading would keep going up unless I had the screwdriver touching the bias adjuster. I think I’m going to blow something up…

    Your help is greatly appreciated.

    JJ

    • bill says:

      It sounds like you have a bad solder connection on the trimpot. Pressing down on it is making the connection, releasing it loses the connection. It’s also possible that you overheated the trimpot when installing it, and that it’s now damaged internally. Check your connections first; let me know what you find.

  24. Coen Bijpost says:

    Hi Bill,

    I’ve received your basic mods, master volume and input last week. Put everything in my new BJ III green eagle straight away. Blown away by the improvements in tone! I just could get myself to put a drill to the pristinely new amp… Is there any way to fit the bias trim pot without drilling? I was thinking of using a small strip of vero board and soldering that in place and then soldering the trim pot onto that. Does that sound like a bad idea?

    Thanks for all the useful stuff you provide 😉

    Cheers,

    Coen

    P.s. if you haven’t tried the Eminence Screaming Eagle in a BJr yet, you should. I’m really digging the sound, especially with your mods.

    • bill says:

      You could certainly build a little interface board, but it’s not necessary.The two holes are trivial, kind of a non-event after you’ve done it. Many folks do it with just a pin vise, twisting it in ther fingers.

  25. Chrismcgon says:

    Hi Bill,

    As a temporary/quick fix, I installed a 27k resistor in R52, now my voltage drop reading is -1.96v. The amp is completely stock. Is that too cold? I have to figure out how to calculate this stuff. Thanks in advance for your response and expertise.

    Chris

    • bill says:

      Yes, that’s pretty cold! You’re fortunate to have cool-running tubes, and if you had adjustable bias, you probably would wind up setting that pair pretty close to the stock bias. It’s hard to dial in just the bias you want with resistor swaps; the jumps can be a little too large. Or you wind up changing both resistors.

  26. Chrismcgon says:

    Wow! Thanks for the lightning quick reply. I am going to be ordering the basic mods, mainly for the bias trimpot but as I said, this was a quick fix as my V4 tube red plated on me.

  27. Chrismcgon says:

    Thank you Bill. I’ll investigate your suggestions.

    Chris

  28. Frankli says:

    I bought a cream board kit and the TO20 transformer.
    I live in Brazil and here the AC voltage is 127v, above standard U.S. 120v.
    What would be the ideal tension for me in adjusting the bias?
    I have a Blues Jr III NOS
    Thanks 🙂

  29. mdesign says:

    Setting/checking the bias on a green board – would the same leads apply to check voltage drop (main filter cap and brown wire?) I know the resisters to check current bias are labeled R21/R22 (green board) instead of R31/R32 (cream board).

  30. blackstone says:

    Just want to share my experience with biasing my blues jnr.

    It’s first worth noting that I’ve measured variation in B+ voltage on different tests from 331V-342V, this is due (I think) to fluctuation in mains voltage.

    I decided to put in variable bias which is a much better solution to set accurate bias. I think that caution should be taken when just replacing R52 with a fixed 27K resistor as variations in different amps, different tubes and fluctuating mains voltages might mean you’re still running the output tubes too hot.

    With adjustable bias you are also able to hear the changes in tone at various settings.
    I opted to set static dissipation at 62% of maximum dissipation for the EL84 – this sounded the best to me.

    Out of curiosity I measured the bias resistance, turned out to be 38K! (well above any fixed resistor recommendations I’ve seen).

    Thanks 🙂

  31. Nithlian says:

    Hello Bill !

    I’m contacting you because I have some sort of a mystery to solve. I have ordered several kits to mod my BJ3 (Basic, presence, sparkle, jack, audio taper, standby, octal with 6V6, TO20, cathode follower, and recap)a few months ago and I only have found the time to install it last Friday. Everything went pretty well up to the very last step: Setting the bias. I’m not able to obtain a reading on my multimeter between the + of the cap and the brown, as you show it very well on the website video. I’ve double-triple-quadruple checked the connections, the continuity, if the tubes light up and if I have forgotten something when installing the mods and I absolutely don’t see where something goes wrong. I even don’t get a reading between the red (or its equivalent at the cap) and the blue. I’ve also checked the video a considerable number of times to see if I’ve missed something and here also, I don’t see where I made a mistake, if I made one. I’ve read the thread and I’ve seen that there is people getting the same issue but I also checked their solutions and it didn’t work. By curiosity, I played the amp: it sounds great and plays perfectly, even at high volume!!!! I’ve checked if I can get a reading when not on standby, and obviously I get one !!! So I set the trimpot until I got -4.2V under these conditions but then, when I return to standby, I still don’t get any reading between red and brown and the reading between C- and ground is -22.45V. But I want to use this amp in safe conditions so I really want to sort that out.

    Since I’m pretty new to analysing circuitry, I’m completely lost on that one! Could you help me? I’ve tried to see on the schematics (I got it on Fender’s website…) but I don’t know where to look for the problem after checking everything in the above. By the way, on the traced side of the board, this is labeled “Rev. E”.

    My second question is about something else: In the Clean Boost thread, I’ve read that this is possible to alter the sound of the FAT mode by changing the value of the 22uF bypass capacitor in the FAT circuit. Which one is this on the board? According to the schematics, is it C4?

    I thank you in advance for your time and I wait to hear from you.

    Best regards.

    • bill says:

      You only get a voltage drop when the amp is in play, not in standby. It sounds like you’ve done everything correctly.

      And yes, the capacitor in the Fat circuit is C4. If you find that it gives too much boost, try 4.7uF or even 1uF.

  32. Studrak says:

    Hi Bill,

    I’ve just completed all the mods on my 1997 green board Bjnr, it’s absolutely amazing, all my guitar playing buddies can’t believe the transformation. I know one of them who has had a Bjnr in the past is sourcing another right now and he is going to do the Billm mods, only last night I told another guy about it, he is a a pro guitarist/tutor and plays a lot of jazz he’s going to listen to my mods, and I know he’ll be modding his amp.
    I have just one question Bill, before I set the bias I checked the voltage at R21/R22, it read 13.7 V then I set the bias as your instructions to -3.4 V (it was originally way high at about 7 V, nowhere near 2.2 V that you show on video) I then re-checked the reading at R21/R22 but it remains unchanged at 13.7 V, what do you think?
    I had a delayed sharp pop or crack when turning the amp on from stand-by, I re-arranged th 12ax7’s, now it’s quiet.

    Thanks for everything Bill, your reputation is growing in the UK.

  33. birdseye says:

    Hello Bill,

    Finally got a chance to install a number of your mods including the TO20 OT, re-cap and basic mods including the bias trim pot on my cream board Blues Jr. When trying to set the bias I can see the voltage at R31 and 32 change as I adjust the pot but when I try to measure the voltage drop as described at either the brown or blue transformer wires I get 0.0. The amp seems to be working though I did not want to run it long due to the bias issue. Tried the measurements in standby with the power tubes mounted and again in the on position with them out, same results. Something I am missing?

    Thanks,
    Jeff

    • bill says:

      Hi Jeff,

      Check the voltage on both sides of the screen resistors, R35 and R36, from the lead to ground. The voltage should be nearly the same on each side, since there is very little current draw. If this resistor is open/broken, you won’t get a voltage reading between red and brown. Also, if the screen grid in your EL84 is burned out (it’s usually the first thing to go), you’ll get no reading or wacko readings on the voltage drop. If you have a fresh set of EL84s, try them after you’ve checked the screen resistors.

    • bill says:

      Hi Jeff,

      Check the voltage on both sides of the screen resistors, R35 and R36, from the lead to ground. The voltage should be nearly the same on each side, since there is very little current draw. If this resistor is open/broken, you won’t get a voltage reading between red and brown. Also, if the screen grid in your EL84 is burned out (it’s usually the first thing to go), you’ll get no reading or wacko readings on the voltage drop. If you have a fresh set of EL84s, try them after you’ve checked the screen resistors.

      • birdseye says:

        Thanks for the prompt response Bill. I think I made the mistake of thinking you did this test in standby or with the power tubes out of the amp. Since you mentioned that the screen grid in the EL84 could be the problem I figured that I misunderstood the instructions and should be doing this test with the tubes in and the amp fully powered up, all seems good now, sorry to have bothered you with such a foolish mistake.

        Thanks again for the great upgrades, website and service!

        Cheers,
        Jeff

  34. poerum says:

    Hi.
    I have chances some caps and have had the printboard off. When putting everything back i forgot to connect the two Brown wires. Internet the amp on and it was on for some minuts.
    Wien i realized that I have forgodtbefindende the wires I turnes it off. Connected the wires and switched it on again.
    Then there was only a Well Sound Wien both volume and master is full up.
    I suspekt there is a problem with the input jack but haven’ t had time to check it.

    Could the mistanke have done any harm to the amp?

    • bill says:

      The brown wires provide the bias, so the EL84s may have gotten hot. But they will probably be OK.

      The output jack must be installed in the chassis. Otherwise, it will make a loud hum or feedback.

  35. Studrak says:

    Hi Bill,
    In the near future I am abuout to modify another Bjnr, its for one of my buddies who has just bought a cream board, I think its a 2003/5. The amp has hardly been used, its like new, Iv’e been steering him through all your mods and soon he will place an order.
    I have a question regarding the Lexan drill guide that you mention in “Billm’s Blues Junior Head Unit”, the thing for getting the holes drilled accurately when fitting the Bias Trimpot.
    I can see that this would be a great advantage and I wonder either if you can supply one with an order or, could you let me have the precise hole positions and overall dimensions so I can make one.
    Thanks Bill for a brilliant service

    Regards
    Stuart

  36. Studrak says:

    Thanks Bill for fast response and the good advice

    Stuart

  37. CleanMAC says:

    Hello Bill, I shall soon have the TO26 and the TP24, with Tung-Sol 5881 23Watts.
    I wanted to know, can I put back my 6v6’s jj that I use at present with the TO22? If yes, what will be the value of the BIAS volatage?

    • CleanMAC says:

      update!, I found out how more need to ask you Bias voltages.
      I found out how to know the cathode current with a resistance of 1 ohm. ( Ic)

      Does the voltage measure on R35 or R36 represents the current screen grid? (XmV/470ohms = Ig2)?

      Because I’m still not sure how to know the current screen grid.

      Ia = Ic – Ig2,
      P = Ia .Va, …P=70% max Power

      • bill says:

        I don’t consider the screen current very important because the output tubes are operating in Class AB1, not at full power (Class A). The screens do not consume significant power at idle, which is where you are setting the bias. If you wish, you can measure the voltage drop across the screen resistors, but you will see that the consumption is rather low.

        The screens only consume significant power with a strong AC output signal.

    • bill says:

      The 6V6s are the wrong impedance for the TO26.

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