Pro Junior Adjustable Bias

I made the bias adjustable because the factory bias is just too hot on Pro Juniors and Blues Juniors. This particular amp was idling at just under 12 watts, which is supposed to be the maximum dissipation for the output tubes. As a result, it went much higher when you actually played. This “High AB” biasing seems to be fashionable among some EL84 designers, supposedly giving the same sonic attributes as a Class A amp. It doesn’t. It’s just more hiss, more heat, and shorter tube life.

I replaced R29 with a 50K 25-turn trimpot, configured as an adjustable resistor. This gives me a huge adjustment range for the bias. I set the idle at 9 watts. It sounds just as ballsy as it ever did, but the temperature on the glass of the EL84s is 70 degrees lower.

The lower leg of the pot is in the lower R29 hole and I drilled holes for the other two legs. The wiper leg is bent and soldered to the lower leg and a jumper wire connects the upper leg to the upper hole for R29. Simple.  Note that I bent CR5 just a bit to make room.

 

 

You can see here how easy it is to do the bias mod on the back of the circuit board. Note that the bottom edge of the circuit board in the picture above is now the top edge in this picture.

One leg of the trimpot goes into the existing R29 hole.

 

Drill an offset hole that matches the center leg of the trimpot. The offset hole comes out pretty close to the existing R29 hole, so all you need to to is bend the lead over so it touches the solder pad. I haven’t soldered them yet in this photo, so you can see them clearly.

For the other lead, a simple jumper does the job. Make a loop in one end and place it over the trimpot lead. Bend the other end 90 degrees and place in the top hole of R29. I crimped the loop a little tighter before I soldered this.

After you’ve put everything back together, it’s best to set the bias voltage to around -12V before you insert the output tubes. That way, you know they won’t immediately red-plate.

 

When you bias the amp, all you have to do is measure the voltage drop from B+ (the + side of the large filter capacitor) or from the red output transformer wire (they’re connected together).

To adjust the bias: Put a jumper clip on the red transformer lead (P2) and another on the brown (P3). Plug in speaker, insert tubes, warm up amp, turn master volume off. Adjust the trimpot until you get a reading of 2.4 to 2.7 volts across these two leads. Remember that the actual voltages may be over 330V and you’re only measuring the difference between them!

Because the output transformer resistance is 100 ohms, a 2.4 volt drop indicates 24 mA of plate current. Multiply that times roughly 330V on the plate, and you’ve got 8 watts of idle dissipation per tube, a good number. The actual plate voltage will change with your AC line voltage, but the difference in plate dissipation is small.

Here is a video of how to adjust the bias:

21 Comments

  1. barfoden says:

    Hi Bill.
    I have a laney cub 12r.
    It is fixed adjustable bias. There are no resistors on the screens. Just two 1k resistors in parellel dropping the 300V +B through ~ 500 ohm to a little lower screen voltage.
    I have changed the output transformer to a larger one. I do not know the primary impedance of the small stock one but the new is 8K ohm with a 8 ohm speaker coupled to the 8 ohm secondary.
    How do I determine current draw through tube.
    The stock tubes are ruby el84c (china). With -12V bias voltage i get around 300V plate and measure ~ 2,5V from pin 7 to pin 9. Decreasing bias voltage to -9.5V i get slightly higher voltage reading but no major increase. The PT is small and the voltage on the plate decreases also.

    I have some harma E84L tubes. These give plate voltage around 319V with -12V bias voltage. lowering bias voltage do not seem to increase the voltage difference over pin 7 to 9. But going down to -7V i can feel the tubes radiating more heat..

    • bill says:

      Measure the DC resistance of each side of the primary on your new output transformer. The impedance is unimportant. Then you can measure the voltage drop across one side of the primary, as shown in my bias video. Then use Ohm’s Law to calculate the current from the resistance and the voltage drop. That current is the same as the plate current.

  2. barfoden says:

    Hi Bill.

    I bought a kit for making a bias probe with a 1 ohm resistor. Seems more safe that way.
    Plugged into a multimeter I read mV which should correlate to mA. The stock tubes draw around 20 mA at 300V plate with a negative bias voltage of -11.5V.
    THe other tubes i have are low current draw tubes and the bias voltage needs to be dropped to -7V to get above 20 mA dissipation values at idle.

    THe chance of running my amp the laney cub 12r underbiased is very small,as it operates the EL84 around 50% dissipation at 300V. TO get a hotter bias towards 70% disspation the voltage drops to 287-289V plate which is a very safe operation for the tubes and will last for years.

    The Blues junior runs tubes at ~330V at a higher dissipation, which would require that when changing tubes one had to request for low current draw tubes (Cold rating)as EL84 with a hot rating would run closer to 90% dissipation which is to hot for class AB fixed bias.

    • bill says:

      If you’re talking about EL844s (the “low current draw” tubes), you are correct. They bias properly at around -7V in a Blues Junior. That said, I think they’re crap. The small plates noticeably constrain bass performance, and overall I think they sound wimpy and lame.

      If the plate voltage drops when you allow more current to flow/higher dissipation in the Laney amp, it would indicate that the power transformer is undersized. It should be able to maintain the plate voltage at most any current. An undersize power transformer causes voltage droop on the transients as well as when you bias hotter. It may be intentional on the part of the designer, to get some of that “old amp” distortion from voltage sag.

  3. barfoden says:

    Thanks for the reply Bill.

    I was not talking about the EL844 but just that regular EL84 can be rated differently and some need more or less bias voltage to draw the same amount of current. Like groove tubes three grades.

    Anyways I changed back to the tiny stock OT and the dissipation and voltage has gone up a bit. Now i get 301V as specified in the schematic with 24-26 mA combined plate and cathode current with fresh jj EL84 (~60-65 % dissiaption). Perfect..

    THe larger OT i used might have a slightly larger primary impedance (8K) and very different wire type and larger core and rated at 18-20W. So some power maybe lost ,,and the power transformer in the laney amp is not very large… It gets hot despite lowering the current draw at idle (maximum bias voltage -15V and 6 mA draw on tubes). I think it has a hard time supplying the filament current to the 5 tubes (~2.5A).

    The large and heavy 18 watt OT I tried is designed for a pair of EL84 in a hot cathode biased Watkins dominator clone that runs the EL84 at 90-100% dissipation at idle at 330-350V plate with a beefy power transformer.

  4. unhinged17 says:

    Hi Bill,

    I’ve got a tweed Pro Jr. that has an awful hum which is non-dependent on the volume level – I switch the amp on, it warms up, and the hum is there at the same level whether volume is 0 or 10. I’ve tried new 12AX7s and EL84s in it, in various combinations, but to no avail. If anything, it’s gotten worse over time, and I doubt all the tubes I’ve tried are bad. I would rather have another amp – never really been pleased with this one, I’d actually like something lower wattage as this one is loud for my purposes – but I just can’t afford it, and I need to make this one work for me now. Can you suggest anything? Grounding? Transformers? Rectifier? Can you suggest points to check in the circuit of the amp? From what I can tell, it sounds like a 120 Hz hum – I’ve seen others testing amps with hum on Youtube, and what they’ve identified as 120 Hz hum sounds much like mine. I have an oscilloscope and DMM, and I know the basics of using them.

    I know you’re not a big fan of these amps – like I said, I’m not either – but it’s all I’ve got to work with right now, and I won’t be in a position to replace it for another year or so. Not having an amp – my only one – is setting me way back in my playing!

    Thanks –

    Jason

    • bill says:

      Follow the procedure on the Tracking Down Hum page to see if you can isolate the hum somewhat. And then signal-trace it backwards to see where the hum comes in. Look for ripple on the two smaller filter caps. A sawtooth is normal on the first filter cap, and the push-pull tube configuration should cancel it out. Check all the AC and DC voltages against the schematic and make sure that all four diodes in the power supply are good.

  5. markt says:

    Hi Bill, great stuff here. I have an older PJ, the PCB is marked 1-94. It seems different then the one featured here. There is another resistor (R28) inbetween the diode and R29. Not a lot of space to fit a trimpot. What about changing the value? I think I got a good one, because most of the hiss is from the EL84’s, they cook! In the meantime, I’ll try another tube mfg. Thanks, Mark

    • bill says:

      Making the resistor larger will increase the bias voltage by shunting less of it to ground. If there’s no room on your board layout, you can locate a trimpot somewhere else and run both wires to it from the R29 holes.

  6. Chrismcgon says:

    Here’s a rookie question. Is there a particular value resistor that can be installed in the R29 slot that will set bias in the right ballpark without going the trimpot route? Again, if that’s a stupid question, then, I apologize.

    • bill says:

      Every pair of EL84s is different so there’s no magic resistor that works for all. If you change resistors a second or third time, you risk lifting traces on the circuit board. It’s easy to install the trimpot; you do it once and you can accommodate any tubes, any preference.

  7. rduval says:

    I know this has nothing to do with biasing so I hope you’ll forgive me… I just bought a used Mexican Pro Junior and it’s a great little amp, I can live with the hiss but the 60hz is crazy! In checking it out the ribbon cable to V1 (Far right 12AX7 looking at the back) is highly microphonic. I’ve changed tube so that’s not it. If I tap or even slide my finger from side to side it’s like I’m moving it an the head of an SM57. It’s crazy! Any ideas how to resolve/lessen this?

    • bill says:

      The hum is usually caused by tubes, especially V1, which has an unbypassed second stage. Adding a small bypass cap can help a lot (maybe 1uF across the leads of R11), or you can keep swapping 12AX7s into V1 until you find a quiet one.

      You can also offset the heater voltage, as I show on my site. It prevents heater hum from “printing” onto the cathodes.

      The microphonics on the ribbon cable are normal. Remember that every component in the amp is microphonic to some extent, but here it’s in a stage with very high gain. Since you don’t touch the ribbon cable when playing the amp, it’s not an issue. Running individual wires to the tube and separating them would cure the microphonics, but that’s a lot of work for no real improvement in the way the amp sounds.

  8. Tark says:

    Thanks so much for building a great resource web site for these amps.
    I have been repairing a Pro Junior for a friend. It had been crackling badly for a while. I already came up with the ‘bad valve base’ diagnosis myself and tried the re-tensioning and a squirt of contact cleaner cure before I saw your site. This improved things for a while, but eventually the amp started making really horrible noises. So I have just replaced the old phenolic bases with new ceramic bases – problem solved !
    However I then found your site and started worrying about biasing. The amp did seem to run hot. I checked the voltages – 331V on the plates and 4V across the transformer to CT with a 94 ohm winding resistance. Which gives me 14 watts on the tubes !!!
    So I’m thinking about fitting the bias adjustment mod. However according to my circuit diagram (maybe I have the wrong revision) R29 (18K for UK 50Hz mains it says on the diagram) runs from the cathode of the bias rectifier diode to ground. So its placing a load on the output from the diode. Your mod replaces that resistor with a variable resistor that as far as I can see is just varying the load on that diode. Apart from anything else if you wind the trimmer down to zero that shorts the diode to ground, which does not seem like a good idea. It seems to me that the variable resistor would be better placed across C14 so it forms the bottom leg of a divider to ground from R30 the 33K. But maybe I have misunderstood something somewhere?

    • bill says:

      The diode will not burn out because it is fed from a capacitor, not from a direct connection to the high-voltage supply. The bias voltage is more of a charge on the grid than an actual current that gets consumed to any degree. R29 divides some of the voltage to ground. The load on the diode does not change substantially.

      And if you took the trimpot all the way to ground, the redplating output tubes would tell you pretty quickly that you had done something wrong. 🙂

  9. Tark says:

    Doh! (slaps forehead) now I get it. The negative bias rectifier CR5 is as you say fed through a fairly small value capacitor (C12 0.047uF) so the resistor R29 in combination with the high reactance of that cap at mains frequency (67.7K) acts as a voltage divider. And since I need to increase the negative bias to reduce the plate current of the output tubes I need to make that resistor larger. A resistor across C14 could only make the bias smaller. Thanks once again.

  10. sosaman894 says:

    Hi Bill, i bought a MEX Pro Junior (i’m from mexico) i wasn’t very happy with that full of gain sound so after reading several pages and translate network decided to make a few mods to my Pro Junior that I found on the internet, the first and the one about I was not so sure was to reduce the value of resistor R6 which was supposed to have value 56k ( factory ) but when I opened it I took the surprise that had already repaired the amp and It had bad solder resistors and some burns (note that I bought it at a great price ) , plus the cables had tags and R6 was not 56k it was 22k but sounded very dirty still, I guess the other resistances that had changed was increasing the gain (perhaps the previous owner like gain , I do not) then I decided to go to bottom & replacement with a 10k resistor (R6) which sound clean 15% then I was not very happy . A friend bought an American pro junior but with the vol at 3 is already heard popping but clean, then continued researching about this amp resistors and a guy said that i had to lower the value to the R10 and R3 ( 100K) to 56K cleaned much sound, so i decided to change the resistance for a 56k and then the only thing I did was leave my amp dumb and decided to return it to normal ( already peeling off the copper circuit so unsoldered )i really hope you see this, and if you do sorry for the bad english. I want a cleaner sound but i just don’t know what else to do (i know about 12at7 valves but daaamn they are expensive in mexico)I’ll wait for an answer! Saludos!

    • bill says:

      The Pro Junior has been through six revisions, with many small changes. Some of them have the 56K resistor, some have 22K. But R6 is for voicing, not for gain. The smaller resistor makes the amp a little darker, less harsh.

      The newer Pro Juniors have .022uF in C1 for more low end in the signal.

      If you want less gain, you can change the plate resistors, R3 and R10 to 82K or 68K.

      • sosaman894 says:

        Oh i see, well i’ll give it a try, thanks for reading! and nice posts!

      • sosaman894 says:

        where can i send you a pic of my amp? i think there’s something really weird on it

        has a lot of changes in most of resistors and i really want to know if there’s something that makes the gain going up

        • bill says:

          You can send pics to billmaudio@gmail.com. Again, though, I’m not doing anything with Pro Juniors these days, and I doubt that I can take the time to go through the circuit and analyze someone else’s mods.