TP24 Power Transformer

Billm Audio’s ongoing collaboration with David Allen of Allen Amplification pays off again–a real upgrade power transformer! The new TP24 power transformer fits exactly and addresses a number of Blues Junior modification issues:

  1. More heater power for octal conversions. While it’s not necessary for 6V6 output tubes, it’s essential for getting full power from the 5881 or 6L6GC.
  2. More plate voltage. An additional 25 volts of B+ provides more headroom without exceeding the 400 volt rating of the coupling capacitors.
  3. More reserve power. There’s lots of current on tap for a powerful, effortless sound.
  4. Designed for the Blues Junior’s bridge rectifier power supply and includes the bias/solid state winding.
  5. Cooler running under load, no overload or sag issues, as you would get with the stock PT and 5881s or 6L6s.

Click for larger image.

As you can see, the TP24 has nearly twice as much core as the stock power transformer and has an internal bell end for maximum hum protection. The TP24 benefits any Blues Junior, but it delivers the most with 5881s or 6L6s. There’s no particular reason to choose a 5881 over a 6L6GC, unless you like the sweet tone of a particular tube, such as the Tung-Sol reissue 5881. The 6L6 delivers more bass and more clean headroom:

Tubes Power
EL84 Stock 15W
EL84 TP24 18W
JJ 6V6 Stock 18W
JJ 6V6 TP24 20W
5881 TP24 23W
6L6GC TP24 30W








When paired with the TO26 output transformer, the TP24 can drive a pair of 6L6s to a clean 30 watts, with all of the effortless, big-plate sound you expect from a 6L6 amp, but still in the Blues Junior’s convenient form. We provide the TP24 with leads trimmed for the Blues Junior and high-quality, double-crimped push-on connectors for easy installation.

With EL84s or 6V6s and either the TO20 or TO22 output transformer, you’ll hear additional clean headroom and the additional punch you get from having all the current on tap that the tubes can use.  There is no sag.

New video: Here’s a link to a demonstration of the TP24 with EL84s. The higher voltage brings out the glassy nature of the EL84s, which the Blues Junior otherwise masks. The presence and sparkle controls help you dial back if you find it a bit too aggressive:

New! The bias board gives you proper bias regulation for all octal tubes with the higher voltages produced by the TP24. It, and the basic mods kit, are essential for proper operation of the TP24.


  1. catscratch says:

    Hi Bill

    What affect does the high voltage preamp mod have when used in conjunction with the new power transformer and is it necessary (cream board version III)?

    Thanks and regards

    • bill says:

      I have the TP24 and the TO26 installed with 6L6s in a Series II, along with the high-voltage preamp. The brightness is much like a Deluxe Reverb, but when you turn up the volume, there’s still plenty of grind. If you like headroom, go with the high voltage preamp mod.

      • Eric Bernhardt says:

        Will the new power transformer increase all the voltages? I’m looking for maximized non-detrimental voltage levels. Even though I installed the high voltage preamp kit, I wouldn’t mind some more volts so I can get even closer to Blackface territory.

        • bill says:

          Yes, the new PT increases all plate voltages — power and preamp. There’s noticeably more clean headroom.

          • Eric Bernhardt says:

            Grand! I measure the B+ currently at 324 volts. How much can I expect with this new transformer? I’m sure 415 is asking too much but even higher 300’s will please me. Thanks Bill. My amp is getting more fierce with every mod.

          • bill says:

            415V is not only asking too much, it is too much! The coupling caps in the Blues Junior are rated at 400V, so you want to keep it under that. Depending on the line voltage, you’ll see 355-365V on the B+.

  2. catscratch says:

    I just noticed that this transformer is only available in 120v format.. Any plans for a 220-240v model?

  3. Richard Wetzel says:

    Hi Bill,

    I have many of the mods done including the T020 output transformer. I have not done
    an octal conversion and not sure if I will. My Blues Jr is a green board and I’m curious
    what changes/ improvements I would expect with the PT upgrade, including estimated wattage increase.
    Thanks! Richard

    • bill says:

      As the chart shows, you only get a few more watts with the TP24. But the headroom increases and you get a bit more depth and slightly brighter tone. The EL84s are the limiting factor.

  4. Phil Connolly says:

    Do you have any plans to post some sound clips? I.e., perhaps various before and after combinations: TP24 with EL84s, with 6V6s, with 5881s. And a clip of the TP24 + TO26 + 6L6s. How does that combination line up to the HRD?

  5. Johnny Blues says:

    Would KT66 tubes work with this setup?

    • bill says:

      No. The tubes are too close together, too close to the back of the amp, not enough ventilation for the excess heat, and not in the KT66 sweet spot for plate voltage. And finally, are you freakin’ serious? 🙂

  6. wbyoung2003 says:

    In order to make the jump to 30W, I would need the basic mod (which I have & in the process of installing), the TO26 mod & the TP24 mod — Correct?

    Would I also need to perform the cathode follower mod?

    Does the TO26 kit include the material to upgrade to the 6L6 tubes?

    And finally, Will you make some recommendations for speakers that will handle the 30W ?

    • bill says:

      Yes, to get 30 watts, you need the TP24, the TO26, the octal conversion kit, and a pair of 6L6s. The octal conversion kit is separate. It’s listed on the parts page.

      I recommend the cathode follower mod because it improves the tone–adds some harmonic richness. It is not essential.

      Pretty much any of the speakers on my speakers page can handle 30 watts. The stock speaker sound pretty darn good when you get some power behind it.

  7. roger says:

    Bill, since I don’t think I’ve seen this spelled out on the site, what is your advice on when to keep modifying the BJr vs upgrading to a “bigger” amp or adding amps or other? Here are the options in my mind:

    -Base: do main mods and replace speaker
    -Option 1: add Clean Boost for more DB and to minimize unwanted distortion at max volume
    -Option 2: mike the amp to add more DB (sounds like would still need clean boost)
    -Option 3: Add an extension speaker (mod) to add more DB (sounds like would still need clean boost)
    -Option 4: do this 30W mod (TP24)
    -Option 5: add a second Bjr or other amp (split the signal giving a lot more flexibility in tone)
    -Option 6: use a “bigger amp”

    What is your recommended approach to navigate these options? Thanks! Roger

    • bill says:

      They’re all options. The Clean Boost works at moderate volume, but when you’re playing loud, it increases drive to the output tubes. Hey, it’s still just a 15 watt amplifier! Mic’ing the amp is always the least expensive way to get loud and get over the drummer and the bass.

      An extension speaker is worth 3dB.
      The 30 watt version is very loud. But it doesn’t sound like a normal Blues Junior.
      Two Blues Juniors sound like a Blues Junior, but the sound stage is wider. Surprisingly, it’s not as loud as the 30 watt version. There’s less clean headroom with two amps than with a full-on modded Blues Junior.
      Using a Blues Deluxe will give you lots of volume and Blues Junior-like tone. I haven’t had a chance to compare them head to head. But if you like the form factor of the Blues Junior, the 30 watt version is the way to go.

      That’s no help to your decision-making process. Sorry! 🙂

  8. fp2000 says:

    Bill, I did the TO-20 installation earlier this year, and then the octal tube socket installation as well. Now, I never tried my 5881’s in it because of the PT not being able to handle it, if I do the upgrade to this new PT24, will the TO-20 be able to keep up? Please let me know. I have the stock 8ohm jensen speaker.
    Thanks Frank

    • bill says:

      The 5881s are an impedance mismatch for the TO20, so you lose some efficiency. The 5881s also put out a bit more power than the TO20 is comfortable with, so if you play loud and proud, the TO20 will get warm. For around the house and occasional jamming, no problem. but you’ll get some saturation when you turn up.

      Also, at full power with 5881s, there is measurable voltage sag, which means that the power transformer is maxed out. That’s good for some funky distortion tone, but if you’re playing out, you’re pushing that PT to its limits… and beyond.

  9. Ralph says:

    Hi Bill,
    I’d like to do the 30 watt mod with 6L6s, (among other mods), to my “Blue-J” but from what I’ve read on your parts page it won’t work with a green board. Is this correct? (If that’s the case, I have access to the cream PCB assembly).

    • bill says:

      That’s correct. The green board, unlike the cream board, has components on the tube board that need to be moved to the tube sockets and the main circuit board, with new connections. There are multiple ways for owners to mess up and the instructions become too complex, so I don’t offer the octal conversion for the green board.

  10. leslie says:

    Hey Bill,I just thought I would come over here on your web page and see if you had come up with a power transformer for the 6l6’s, and low and behold there it was you done it ….Great news!! I can’t wait, You will be hearing from me real soon!!!This coming weekend……I assume the to 22 isn’t an option for use with the new pt?…..Thank’s for all your help!

    • bill says:

      You can use the TO22 with 6V6s and the TP24. If you use 6L6s, the impedance mismatch will cost you several watts and the TO22 will get a bit warm at higher volume. It’s being pushed a bit beyond its design limits.

  11. Jeff says:

    Any physical interference issues with speakers or the chassis to plan for with the TP24?

  12. Billatl says:

    Hey Bill, I have ordered the parts to upgrade to 30 watts. Will it be possible to switch from the 6l6 to the 6v6 tubes with a simple re-bias? Do any of the modifications to accomodate the 6l6 tubes make the amp un, or less suitable for the 6v6’s? Thanks, Bill

    • bill says:

      The 6V6s will lose a couple of watts because of the impedance mismatch, but you can run them. You’d need to reduce the bias by around 3 volts.

      • Phil Connolly says:

        What is it about the T026 that is different from TO20 / TO22 that you decided to recommend the TO26 only for 6L6’s? I saw on the Allen website that all three very close primary impedance (7000 ohm for TO26 and 6600 for the other two), all basically similar in price too (within a few bucks of each other).

        • bill says:

          There’s more to transformers than impedance. The TO26 is heavier-duty. More steel, more windings. 6L6s will make the TO20 or the TO22 get warm, and both will saturate under load. They don’t allow full output power.

          • Phil Connolly says:

            I guess my question was confusing. I wasn’t asking if the smaller OT’s could handle 6L6s. If a modder wanted to upgrade the OT, would the T026 work regardless of which power tubes are chosen? Each of the Allen OTs is in the same price range, similar impedance. Is it possible for an OT to be too big (more steel, more windings) for EL84/6V6s ?

          • bill says:

            The TO26 will work with lower-power tubes. The impedance match is somewhat off, but it would work.

  13. wbyoung2003 says:


    I just finished the basic cream board mod, presences knob & master volume taper mods.
    Very Pleased!!! thank you!!

    I was looking at bumping up the wattage to 30W. However, I really don’t need more volume(I play upstairs in the bonus room), but would like to get the most out of my BJ. In order to maximize tone…

    1. Is the 30W upgrade the way to go or can I just replace the output transformer?

    2. Do I need to make the octal socket mod to change the tubes?

    3. And for the final mod, replace the speaker?

    Thank you in advance for “spelling it out for me”

    • bill says:

      If you play at home, there’s no need for 30 watts, unless you’re in love with the tone of 6L6s. There’s plenty of good, quiet tone on tap in your Blues Junior, and you can bring it out with just the basic mods and a TO20 output transformer. The audio-taper master volume control might be a good idea so you can dial in nice tone at low volume.

  14. B.Lindsay says:

    I see in the comments that the 30watt 6L6 version is much louder (more so than an extension speaker) and that the tone is quite different. I’ve done almost all your other mods, and I love them, but I haven’t felt the need to do the octal sockets. How much louder does the EL84 18watt version with TP24 get? Is there a big tone difference in it? As a matter of your personal preference, would you endorse the tone change in the 30watt version (I mean, am I really missing out on something)?

    • bill says:

      The TP24 in an EL84 amp with the basic mods and one of the upgrade transformers is not noticeably louder than the stock transformer. You can’t hear the difference that 3 or so watts make (and that only comes at full power). There is a difference in tone, mostly in clean headroom. The amp feels a little “faster,” more responsive to pick attack.

      The 30 watt version sounds much different than a stock Blues Junior. It has robust bass and the glassy transparency that you only get with 6L6 tubes. And it can get quite loud.

  15. leslie says:

    Hi Bill, Just got the T24 – The T26 and the 6l6 conversion kit today. I was wondering about the extra wire green with a yellow stripe? Where does it connect?….There was no information about it on the mod pages. Thank’s….

  16. mattydubs says:

    Hi Bill, I’ve been near convinced to pick up a new Blues Junior and go haywire with mods. Historically, I’ve only owned heads/cabs (a Twin Rectifier back in the post-hardcore hay day of the early aughts, a MIG-50, etc) and am bent on a more retro garagey sound (MC5 like), which has led me to Fenders with pedals (if needed). The Blues Jr seems like the best starting point (save the heavyness of a bigger 4×10 for the practice space). What mods would you recommend? 30W might make it strong enough for shows. I’ve not worked on amps before but I do have a little soldering experience in my past (surface mount stuff mostly). Would this output transformer be too big of a task for a relative newbie? I want to run with the amps natural distortion most of the time and kick on a pedal (my Ram Head kit pedal or the Keeley DS-1 I use with my bass) occasionally.

    Thanks, I’m super stoked on this project (and a little afraid of zapping myself…).

    • bill says:

      I think you should make sure that you like the basic sound of the amp before you dive in with mods. They’ll make it a better-sounding Blues Junior, but I can’t be sure that that sound will be the sound you want. Also, do you need more volume, or can you mic the amp through the PA?

  17. Billatl says:

    I finished the 30Watt conversion, and I have to say that it rocks! Most importantly, my wife likes it, also. It’s her amp, and she only plays it for her church gig and various get-togethers. She thought that the sound was far better than my modded vibrolux. And she can still can carry the damn thing.

    Question 1: Is 6.5 volts (brown/red wires) about right for the JJ 6L6 tubes? (My wife has claimed my vibrolux JJ’s as her own. I’ll have to buy more.)

    Question 2: I tried a Burr Brown 2134 (on a socket) as the reverb op amps, but with all the changes, I can’t tell what difference it made. Have you played with different op amps?

    This is a Blues Senior, now. It’s got the big Fender sound. No other way to describe it!!!



    • bill says:

      Hi Bill,

      Yes, the 6.5V drop is just right. The tubes are stressed enough to sound funky, but well within their capabilities.

      The op amp for reverb send/recovery doesn’t make any audible difference. It’s driving three floppy coil springs through a magnetic transducer–there’s so much loss and slop in that system that there’s no hope in hearing a “fidelity” difference.

  18. Dustinfee says:

    Bill, Which 6L6 tubes can be used with the new power supply? Like Tung sol 6L6GC-STR?, GT 6L6-S, GT 6L6-R(B), JJ 6L6 GC? Can all of these be used and if not why not and which ones can be used and what do you recommend? Sorry thats a bunch of questions.


  19. Billatl says:

    On my amp the bais was ok, [ just barely, I think ) for the JJ 6l6gc tubes. I tried some old RCA 6l6gc tubes and it was way to hot! I am not sure , but I think they went into a run-away condition after only a few minutes on. The brown/red wire voltage was over 10 volts and rising fast. I could not set the bias down any more without changing ( let Bill say which ) a resistor.?? Put the JJ tubes back in and the amp is fantastic. The sound is NOT the same, there is the 6l6″ bloom “thing going on!

    • bill says:

      Yes, the 6L6s change the sound of the amp in several ways — the bloom, as you mentioned. Also, it has far more dynamic range from soft to very loud, and it’s noticeably brighter than the EL84 or 6V6 setups. If you want to use NOS tubes, which require a higher bias voltage, we’d have to build a voltage doubler into the bias circuit or cathode-bias them. David Allen has done this with the 6L6s and says it sounds great. He likes the compression you get with cathode bias, even though it costs a couple of watts of power.

      • Billatl says:

        Wow, you really got me thinking. At 30 watts, I can lose a few to cathode biasing. With the spare parts off the Blues Junior, I repaired/modded a old silvertone 1482 amp chassis I had been given 25 years ago.It is cathode biased and has
        the compression and smooth breakup that good distortion pedals are trying to emulate!
        Looking at the schematics for the BJ and some cathode biased amps,it doesn’t seem so difficult. Maybe it could even be switchable from fixed to cathode biased. Have you looked at how to try this?
        To me, the steps to convert would be; unhook existing bias supply,add a ground between r31 and r32, add the bias resistor and capacitor from the cathodes,( pin 8), of both the power tubes and connect to ground.??? If this is correct, how do we determine the value and wattage of the bias resistor? Am I in the ballpark???? Thanks for any help! Bill

  20. Billatl says:

    Hey Bill, I got the cathode bias done and it worked out pretty good. I ended up with a 300 ohm resistor and a 75 uf cap. 330 ohm lost a little harmonic richness, 220 ohm was unstable, 260 ohm still had ghost notes. The sound was fantastic, quite bright, the larger cap seemed to fill in the bottom end better. The tone controls had less influence than before, but the sparkle control maybe had a little more. The overall perceived volume seemed to be much lower, maybe 50 or 60%,possibly deceiving because of the compressed tone, smooth attack and sustain like a good pedal. Notes sparkled, making reverb optional. Great boutique blues sound, the notes floated around the room.
    The bad news, my wife missed the bold, clean sound and volume of the fixed bias, so I changed it back!( It’s her amp!). She did tell me, though, that she appreciated the sound.
    I think I will still buy a 10 watt resistor ,a 100uf 100 volt cap and make it switchable back and forth!
    Thanks for your help and encouragement, Bill

    • bill says:

      Yes, that’s why I don’t like cathode bias all that much. The compression is nice, but you give up a lot of power, too. At some point, the size of the bypass cap stops mattering. It’s very unusual to see more than 22-25uF in most cathode circuits.

  21. duncan says:

    Do I need to have any other mod installed before I install the TP24 upgrade?

    • bill says:

      What are you trying to achieve? Just putting in the TP24 won’t change much — a little more clean headroom, that’s about it. The amp would still be constrained by the stock coupling caps and tone stack, as well as the output transformer. You get much more improvement from the basic mods and a TO20 than from a TP24. I would recommend the TP24 primarily for the higher-power octal conversions because the amp with EL84s is going to be limited by the tubes. Final point: If you change the plate voltage, you have to change the bias voltage. So you would need the adjustable bias mod that’s part of the basic mods.

  22. duncan says:

    Thanks. I just ordered the cream basic kit as well as the recap kit since i found that mine were leaking. I was mostly drawn to the tp24 when I read that it enabled 30watts. I’ll order the tp24 and to20 after I get the basic mods in place! Thanks.

  23. DOUBLE J says:

    Love the site. I’ve pretty much read everything now and had thought to buy a Green board BJ like I had in 95′.

    One of the things I liked about it compared to other Re-issue type amps was it wasn’t too brittle or painful in the high end.
    Kind of medium.(RI deluxe reverbs have way too much treble for me).
    I had played out with my old BJ and it worked well enough for small clubs.
    Right now I am planing on doing the basic mod/pre-amp voltage/taper master v & reverb controls/presence/ and adding a TO20.
    Later I could consider adding more watts(tube and PT up grade) but I read were that is not possible with the green board units, correct?
    The main reason I am concern is I see the possibility of wanting to try the 5881’s. As I am basically only playing Blues in tone(attitude). How different does the 5881’s sound compared to the EL84’s?
    I realize it isnt going to sound like a super reverb. However I like fat and glassy with natural distortion. 23 watts is a nice idea too if I play out again.
    If the series II juniors are as treblely at the RI deluxe reverbs, I may have to stay with the green board.
    Am I wrong to be concerned out the cream boards high end?

    • bill says:

      I don’t sell the octal upgrade for the green board as a kit, but I can do it here. There are too many ways for it to go wrong in owners’ hands, outside of my control. Also, the TO20 is fine with 6V6s and OK with 5881s and the stock PT, but if you go to the TP24 and 5881/6L6, you will need the TO26 output transformer.

      The cream board can be just as mellow as the green board; just add the presence control. And if that’s not enough, add the sparkle control.

      The tone of the 6V6, 5881, or 6L6 is much different from the EL84. There’s more bass, a smoother, glassier top end, more authority.

  24. Matthew says:

    Thinking about grabbing a BJr for some mod’n fun (my Limited Edition one still has few years on the warranty). If I do I will definitely want to do this mod for 6V6 tubes. I’m wondering if upgrading to the new OT that you offer will give me the option to use a 16ohm speaker instead of 8ohm. I have a great English made V-30 that is 16ohm that I would like to use for the project.

  25. stevewdewitt says:

    Can this power transformer be run with the T020 output transformer and the octal conversion? I have purchased the T020 and I am now thinking about the octal conversion and what that would take. I have not installed the output transformer yet. Thanks for all the great work!

  26. scott says:

    I just added the TP24 to my blues jr. This was an excellent upgrade i’m glad I got it. I’m still using EL84’s so I wasn’t really sure if it would be worth it or not.
    I already had done the basic mod, TO20, upgraded tubes & a Texas Heat a while ago, so here is the difference I noticed after recently adding the TP24:

    1. It sounds much cleaner/ clearer, even when everything is turned up the notes are still clear through the distortion. (crunch sounds more musical).
    2. bass is not mushy at all. more punchy and powerful.
    3. the low and upper mids sound a little brighter, but not too bright (the amp was a little dark before with a les paul)
    4. response is better, by this I mean I have better control of whether it plays clean or crunchy by how hard I pick and easier use of the guitars volume control.
    5. fat switch “on” sounds tighter. (I’ll just always leave it on now).

    TP24 didn’t really make the amp any louder, but the quality of the sound at loud volume is nice. (I always play it as loud as it will go and still maintain a cleanish sound).

  27. scott says:

    Oh yeah, I had a question! All this talk of 30 watts in a blues jr makes me wonder why not just add 2 more EL84s? Would it be possible? And wouldn’t (4) EL84s still sound like a blues jr but be louder.
    If this sounds ridiculous, Please forgive me, it’s because I’m a musician NOT an electrician. Have you ever considered this?

    • bill says:

      Four EL84s take up more room than a pair of 6L6s, suck down more heater current than a pair of 6L6s, and generate more waste heat than a pair of 6L6s. You would also need a different output transformer to match the impedance of paralleled tubes. It’s far less work to replace two sockets than to add two sockets. And in my opinion, a pair of 6L6s sounds better than a quartet of EL84s.

  28. leo says:

    Can I install only the TP24 and the TO20 or TO24 in my blues junior and get decent results without having to do all the other mods. I can install the transformers but I dont have the skill to do the mods. I already opened up the amp and changed the speaker and it is a creme board 2008. I dont have a year to wait for the amp either.So is it possible to upgrade my tone with only the transformers. Thanks bill

    • bill says:

      Doing the transformers alone would be a huge waste of money. The tone is still bottled up in the amp, and you need adjustable bias to compensate for the higher voltages from the power transformer.

      The turnaround for modding the amp here is only a couple of weeks these days.

  29. blues4U says:

    Octal conversion is only for V4 and V5, right? The first 3 tubes remain 12AX7?

  30. blues4U says:

    You recommend the cathode follower mod with the octal conversion no matter what the OT/PT configuration, right? So if I did the octal conversion with a TO22 and stock PT with 6V6’s, you’d recommend the cathode follower mod?

    • bill says:

      The higher plate voltage with the TP24 stresses the cathode follower pretty hard. You need to use a 12DW7 or ECC832 tube, which can handle the voltage better. Even with that, some circuit alteration may be necessary.

      • blues4U says:

        I’m sorry, I didn’t follow that. If I stay with the stock PT, but upgrade to the TO22 and octal 6v6… should I do the cathode follower mod?

        • bill says:

          Yes. A 12DW7/ECC832 will still perform better. If you go with the TP24, there’s an additional mod to reduce the load on the follower.

          • blues4U says:

            I think I get it now… you want to put a 12DW7/ECC832 in V2 since that’s the one that is only being half used. 12AX7’s stay in V1 and V3.

          • Billatl says:

            Bill, I did the tp24 transformer when I did the 30 watt conversion 6 or 8 months ago. I had already done the cathode follower mod with a batch of previous mods. The amps has been performing flawlessly and sound better than I could have hoped!
            Should I be concerned about V2 tube life? Is there a way to tell if I am stressing it too hard? Is the hybrid tube a better solution? Otherwise I am inclined not to worry because it has been working so well… Thanks Bill

          • bill says:

            The tube will let you know if it’s being overstressed by failing. 🙂 In my experience, the amp will go quiet, then will resume working once the tube cools down.

            An ECC832 or 12DW7 (same tube) will stand up to the strain placed on the cathode follower better. I’ve started using the JJ ECC832 with the cathode follower.

            There’s also a voltage offset mod that you can do, but it requires pulling the board. It reduces the stress on the cathode follower by putting a DC bias on the heaters. It can also reduce hum with noisy tubes, but Blues Juniors are pretty quiet in the hum department already.

  31. JazzyRay says:

    Bill, I am absolutely impressed with each new mod I install. The sound just gets better and better…just as you describe.

    I recently purchased the TP24 power transformer along with the high voltage mod kit. I want to initially experiment with the sound using EL84’s and the new TP.

    My question is do I need the bias board for the higher voltage setup using EL84’s, or can I properly set the bias with the installed basic mod potentiometer?

    If I can use the basic mod trim pot with the TP24, at what level should the bias voltage be set with the TO20 output transformer?


  32. blues4U says:

    Can you run the TO26 (with basic mods) with the stock PT and EL84’s?

    • bill says:

      The TO26 is too low in impedance for EL84s. It’ll work, but you’ll lose some efficiency.

      • barfoden says:

        The TO26 is listed as 7k when using the two secondary outs as 8 and 16 ohm. Perfect for 6v6 and EL84.
        If you use the same outs as 4 and 8 ohm out the reflected primary impedance to the output tubes is halved to 3.5K which is good for larger tubes such as 6L6 and EL34 especially when run a those ~360V plate (TP24). Infact the EL34 with the TP24 which have enough heater current, a fixed bias amp with 3.5K primary and 365V plate would be damn loud..

        This info is from Allens webpage..

  33. blues4U says:

    I currently am running 6v6’s with the TO20. If I got the TP24 and bias board, could I run 5881 or 6L6’s at say 50-60% ? I don’t need any more power, it’s plenty loud now. I just want to compare the tone between the three.

    • bill says:

      The TO20 is a little too high in impedance for 5881s or 6L6s. You can get away with it, but you’ll find that the transformer saturates when you turn up.

      • blues4U says:

        Thanks. So then I’d need the PT24, bias board and TO26. If I get all that, can I run 6v6’s with it? Would you’d say it’s better to run 6v6’s with the TO26 than 5881’s with the TO20? I’d like to be able to run all 3 tubes at different times.

        • bill says:

          “I want to run anything with anything” is fine, but you’re going to settle on something. Tube swapping gets old. If you set up the amp for maximum power, you’ll never wonder what it was like to have 30 watts. And there’s only one way to get there.

  34. blues4U says:

    Question about the power ratings for the TP24. 6v6’s have a max dissipation of 14W. General rule of thumb is 70%… which is 10W each, which matches up with your rating of 20W for 6v6’s. Max dissipation for 5881’s are 23 W, your rating of 23W total suggests running them at 50%. Max dissipation for 6L6GC’s is 30W, so again your rating suggests running them at 50%. Couldn’t you run the 5881’s and 6L6GC’s hotter? Or is that a problem with the TP24/TO26?

    • bill says:

      With the TP24 and TO26, I measured the clean (10 percent THD) power available. A larger OT would probably get a bit more power out of the 6L6s, but they and the 5881s prefer a much higher plate voltage for maximum power. That would require a full re-cap for the Blues Junior; not feasible.

      All of the tubes were biased at 70 percent. It doesn’t have much bearing on the total output available.

      • blues4U says:

        So I’ve been running 5881’s with the TO20 and stock PT for a week or so now (I know, at my own risk!). It sounds great most of the time, has a fuller more rounded bass than the 6v6’s… but the 6v6’s are a tad cleaner I suppose, mainly in the bass I think. But some times it seems like someone turned the volume down a notch or two. Almost like flicking a switch… one minute it sounds loud, the next it’s down a few notches. I think I’ve observed this while making voltage/current measurements too… one minute it reads one thing, the next it takes a big jump and the speaker noise floor changes. Maybe this is what you are referring to about voltage sag, and fully taxing the PT. With the bias maxed out (-27V), I measure 6V across the OT and 45 mA/tube for 5881’s. That’s almost 15W each at 330 V of plate voltage. The tubes can handle that, but I can see why you say the PT can’t.

        Hopefully I can save up some coin and get the TP24 and bias board and see how that does with the 5881’s.

  35. californiajeff says:

    Hi Bill,

    Will all the parts for this mod work in A Blues Jr. III Red Nova with the Red Coat Wizard speaker?

  36. TheRealGuybrush says:

    Hi Bill,

    I’ve got a Ltd Ed Relic Blues Jr. but I live in the UK, so I’m running an external step down transformer to get the right voltage. If I ordered this power transformer would I be able to wire it for 230-240v rather than the 110v I believe it’s wired for now? That way I can do away with the extra transformer.

  37. Danny says:

    Hey Bill,

    I’m planning on getting a couple things from you for my Blues Junior – just the octal conversion kit, TP24, and TO26. When do you expect to have the backlog cleaned up?

  38. barfoden says:

    Hi Bill..

    I got 229-233V (rather stable) AC wall outlet here in sweden.. The power transformer for my project amp (UK made), has either 220V or 240V primary. The PT is a torrodial one with 0-190-275V secondaries (200mA AC ~ 127 mA DC), and heater current is 3A at 6.3V. I am not concerned about the high voltage secondary, it is the heater voltage I am concerned about. I would like to use the 220V primary to boost the B+ as much as possible for headroom (~405V B+ unloaded), but that would give me in the worst case a 6-7% increase in filament voltage to around 6.75V. Will this damage the tubes ???

    • bill says:

      I think the tubes would be OK with 6.75V on the heaters. If the voltage turns out to be higher than that, you could put power diodes in the line. The junction voltage loss is typically .6 or .7V.

  39. mojeguitars says:

    Hi Bill..
    Sorry for my bad Italian-english..
    I have modded my blues junior cream board. with your ideas and kit and it’s a wonderful amp!!
    And I have replaced all caps with Orange Drop 716p and Mallory and.. WOW, It’s Incredible!! the sound is more more open and the armonics three-dimensional!!
    The bass freq is very definited without any distorsion and dirty sound… WOW!!
    The High is definited and very musical!!! WOW WOW!!
    Try this.. With you mods and new quality caps, if compared to an original BLJR, the difference is ridicolus…without high cost!!!
    My question is about Power Tranformer 220v for a 30 Watt BLJR with 6L6.
    Allen Amplification not provide an PT for 30 Watt 220 volt… Anyone have a solution..
    MercuryMagnetics FBLJR-P/E – Universal Voltage Primary- it’s the solution?

    • bill says:

      I prefer the 225 Orange Drops (as used in my kits) to the 716 Orange Drops, but the real improvement comes from the size, not the brand of the caps. No one makes a higher-voltage power transformer like the TP24 for 240V operation. The Mercury Magnetics transformer is just a replacement for stock. It won’t help.

  40. Danny says:

    Just curious… Was reading barfoden’s post above, and was wondering whether with minor modification you could run el34s with the TP24 and TO26. To my uneducated eyes, looking at diagrams, wouldn’t you just connect pins 1 and 8?

  41. Danny says:

    Did all the mods, and it turned out amazing. It’s kind of like with EL84s, the amp was like a club, the tone going up and bashing you. Can be a good thing, but wasn’t what I was looking for. 6L6s are more like a sword – you can still broadside and get some of that in-your-face tone, but you can also do so much more with the precise, stabbing, airy tone you get. It clears up the amp like no other – there’s more presence. It sounds big and effortless now – none of that boxiness any more – very Fender-y. Overall, very pleased. Totally worth it.

  42. muzza says:

    hi bill, would a mojo761ex 230v (replacement for a Blackface Deluxe Reverb Power Transformer (Export Taps) PT fit in to run 6l6s and 5881 with a t26 o/t?

    • bill says:

      No, the voltages are too high. It’s set up for a tube rectifier, not the Blues Junior’s solid state bridge rectifier. Also, there’s no 20V winding for the reverb, Fat, and bias.

  43. ingobrun says:

    Hi Bill. I recently ordered the Cream Board basic kit, the TP24, the TO26, the bias board for the TP24 and the octal conversion kit. My 6L6’s are on the way so I can do my entire upgrade at once. I have a few questions before I get started.

    First, since I’ve got the TP24 and Bias board, this obivates the need for me to install the adjustable bias trimpot that comes in the basic kit, correct?

    Second, since I’ve got two new transformers, is the power stiffening capacitor that comes in the basic kit also now unnecessary to install?

    Third, I assume the orange tone and coupling capacitors in the basic kit are still key to better tone, correct?

    Lastly, did I read correctly above that the proper bias with the TP24, the TO26 and a matched pair JJ 6L6GC’s would be 6.5V?

    Many thanks for such a great site and for your dedication and expertise on such a dear topic as hot Fender tone!


    • bill says:

      No, you must install the 100uF stiffening cap, too. And everything else in the cream board kit except the bias trimpot.

      For the TO26 and TP24 you need the bias board instead of the cream board bias mod.

  44. bobber222 says:

    Hi Bill,
    simple question from a beginner, I have successfully completed a few of your mods and everything sounds great! however before I continue with installing the tp24 I wanted to know which leads belong where? in regards to their being two of each color… I am sure this is pretty basic but just want to be sure since there was no instruction included. Thanks

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