TO20 Low-Profile Output Transformer

This brilliant new output transformer, co-developed with Allen Amps’ David Allen, is a breakthrough for the Blues Junior, Pro Junior, Princeton Reverb Reissue, and Super Champ XD. Designed from the ground up to provide big-transformer performance in the least possible space, the TO20 is built by highly respected US manufacturer Heyboer, fits the stock mounting holes in all of these amps and has the same height as the stock transformer. The “widebody” core is made of premium M6 steel, which has superior magnetic properties, allowing maximum efficiency and power transfer from primary to secondary.

Princeton Reverb, TO20, Blues Junior/Pro Junior output transformers

Princeton Reverb, TO20, Blues Junior/Pro Junior output transformers

At 1.5 lb., the TO20 is more than half a pound heavier than the stock transformer for any of the above amps. The core size is carefully balanced against the wire gauge and turns ratios. The design increases the “Q” of the output circuit and provides a higher damping factor. The amp thus controls the speaker more accurately, which is especially beneficial for low frequencies. Flaws like woofy, flabby or farty tone are a thing of the past. Overdrive tones are sweeter, harmonics are richer. Frequency response extends well past the range of hearing, so nothing is left behind.

Output transformers seldom make an amp louder; they’re mostly about tone quality. But they do improve headroom by reducing distortion.

Below is a power spectrum sweep, using a stock Blues Junior OT:

Power Spectrum, stock Blues Junior OT

Power Spectrum, stock Blues Junior OT

Next is the same sweep, same amplifier, nothing changed but the OT:

Power Spectrum TO20 M6 core

Power Spectrum TO20 M6 core

You can see that the power curve has shifted up several dB; the increase in loudness is just noticeable, not overwhelming. The real difference is in the distortion-free bass and improved tone throughout the range.

The only “caution” is for Princeton Reverb users, who may like the classic grind of the stock, undersized output transformer, what Fender’s Shane Nicholas calls the “aaaaaat!” tone. But it’s the same tone that forces you turn down the bass as you turn up the volume. With the new OT, the bass comes through in a way that you’ve never heard—unless you’re one of the old-timers who tore out the stock PR OT and installed a Deluxe Reverb OT. The TO20 gives you all of the performance of the DRRI transformer or other oversized aftermarket transformers without having to drill holes, find and attach the right quick connects, or worry about interference with speakers. (The “20” designation is just to differentiate it from the TO22; either OT can easily handle 25 watts.)

The Billm TO20 kits fit the Blues Junior, Princeton Reverb, Pro Junior, and Super Champ XD (SCXD available end of October), with the correct wire lengths and the proper push-on quick connects for each amp.

For reference, here is the power spectrum for the TO22 transformer, which has a 4 ohm tap as well as an 8 ohm, and is preferable for maximum efficiency running two speakers or a 4 ohm cabinet. You can see that the TO22 is very similar to the TO20, and both are superior to the stock OT:

Power Spectrum, TO22

Power Spectrum, TO22

You can order either output transformer from the Mod Kits and Services page.


  1. fp2000 says:

    Hi Bill,
    what is your opinion for an octal socket conversion? If I decide to use 5881’s and 6V6’s, would the TO26 be an upgrade worth pursuing since it has both 4/8 ohm taps? or would the TO22 or TO20 will suffice?
    Do you account for TO input impedance when you switch between 5881’s and 6V6’s.
    Please let me know

    • bill says:

      The Blues Junior power transformer can barely provide enough power to run 5881s, so the impedance doesn’t matter much (it gets more important at full power). The TO26 isn’t necessary because the 5881s can’t pull more than 25 watts from the power transformer.

      Remember, you run 5881s at your own risk. The BJr bias supply can barely provide enough voltage to run them properly and they draw very heavily on the heater supply.

  2. gjcamann says:

    Other than the proverbial doorstop, is there any other good use for the original OT after it’s been upgraded. Could it be used in my Valve Jr?

    • bill says:

      The Valve Junior is a a single-ended (one output tube) amp. The BJr transformer is for push-pull (two tubes). They’re different in design, not interchangeable.

  3. gjcamann says:

    I know enough about electricity to be dangerous….
    I’ve got a 16 Ohm speaker I’d would be interested in trying with a Jr. Could I run the two 8 Ohm speaker outputs in series to allow it to drive an 16 Ohm speaker?

    • bill says:

      A stock BJr has only 1 8 ohm output. The other jack is the footswitch jack for the Fat switch. You can unplug the 8 ohm speaker and plug in the 16 if you want to hear what it sounds like. You could also run both speakers in parallel, not in series, for a 6 ohm load. The amp will drive either a 16 ohm load or a 4 to 6 ohm load without problems.

      • jonpowl says:

        Please pardon the confusion, but can my stock greenboard BJr run an old Fender 2×12″ (50w 8 0hm Fender Speakers by Emminence )wired in parallel to 4 Ohm safely? Any idea whether this will sound better, louder or both? Also, is there a link for installation charges when you to do the mods I am interested in?

        • bill says:

          Yes, it’ll work. The Blues Junior will drive a 4 ohm load without problems. But don’t go lower and don’t try to run the internal speaker at the same time.

          If this is something you want to do all the time, consider doing the basic mods and using the TO22 output transformer.

  4. lancer.303 says:

    I am trying to decide between T020 and T022 for my early 2010 PRRI and have read your descriptions. My main goal (other than tighter low end) is to safely run an external cabinet in parallel with the combo speaker for a total load of 4ohm, but not all the time. And I really prefer the installation that does not need drilling for an impedance switch as required for the T022. Your description for the T020 says, “for occasional use of an external 4ohm load.” Just so I understand, the T020 is rated 8ohm, so I would still be running an impedance mismatch from time to time when I hook up my external cabinet using this model OT. Can you explain how this works? Does the more robust T020 protect the amp’s power section from stress even when running the occasional mismatch? Or does the T020 have a second tap for 4ohm as well?

    • bill says:

      The TO20 only has an 8 ohm tap. Any tube amp can typically handle a 100 percent mismatch without breaking a sweat. So a 4 or 16 ohm load on an 8 ohm OT is OK. The tubes will run hotter with the mismatch, but only at full power. You can sacrifice a little tube life for more speaker if that’s your choice. There’s not a lot of room for an impedance switch on the PRRI, and you’d also have to drill a hole to mount the TO22. An alternative would be a switching jack that switches to 4 ohms when you plug in the second speaker.

  5. fp2000 says:

    Bill, thanks for such great transformer upgrade. Basic mods and OT upgrade have made a huge difference. Just the basic mods with the twin stack mod I thought the amp sounded awesome, and after the new OT was installed and rebiasing the amp sounded fuller. I can say now that I don’t even want to touch the amp anymore. I am not even a fan of EL84’s, but the amp sounds pretty good compared to what it was originally. My next mod will be the conversion to 6V6’s, but like I said, I want to enjoy it as it is. I’d wish I had a 6V6 modified bj to compare to and see if the conversion is worth for me.
    Thanks again Bill. By the way, installation was pretty straight forward.

  6. madvek says:

    Hi Bill, I recently picked up a ’78 SFPR with the TO20 already installed. Also came loaded with an Eminence Ramrod. The combination makes for a great portable LOUD and CLEAN grab and go Fender. I’m not sure what, if any other mods were done.


    Where does the TO20 put the power rating of the PR?

    Can you recommend a good speaker to give me some breakup? Or what power rating should I be looking for to get some breakup without relying completely on pedals. Thanks!

    • bill says:

      The output transformer doesn’t change the output watts unless the previous transformer was severely undersized or mismatched to the tubes. The power is the same, bu the tone is better.

      Your ’78 PR should start to break up naturally past 5 on the volume. If it doesn’t, it may need attention in other areas, such as fresh electrolytic caps, coupling caps, out-of-spec resistors. Do you really want speaker breakup? Or harmonics and compression from the amp?

  7. aschcsa says:

    Hello Bill
    Can I use this transformer in Australia ?

  8. musicman_atl says:

    Hill Bill:

    I just bought and installed your T020 transformer (and standby switch and presence control) , it sounds great. In the instructions which came with it it says to measure, after the install, the voltage drop between the red (CP2) and brown (CPI) leads which should be set to 3.4 My jr. had already had an adjustable bias installed, but this was the first time I have touched it. I used a multi-meter set to DCV 200V and recorded 4.xx. I adjusted the trim pot until it read 3.4 and then checked the Blue lead (CP3) which read 3.0 It would seem they are unmatched output tubes.

    Did I measure that correctly? The description of bias adjustment for the B jr on your previous site and on your bias adjustment video for the pro junior involved multiple steps. Should I use the multi step method and measure at the top of R23 (or R25, R24, R25)?

    Thanks for the speedy turnaround of the order, the quality of the products and a very enjoyable site.


  9. Erik says:

    Hey Bill,

    I like to use my BJ pretty loud, but still clean (pretty much the EQ and master cranked, volume below preamp breakup). Sounds like a better OT would be useful for me…

    However, I would like to know if I can install a TO20 without drilling or heating up my soldering iron?

    • bill says:

      The TO20 will improve the tone, especially in the bass, but it won’t make the amp substantially louder, at least not without further mods. If you want more clean headroom, you’d need to do the basic mods and high-voltage preamp in addition to the output transformer. The TO22 is a little brighter, more clean-ish than the TO20.

  10. Skyblue737 says:


    I have a cream board BJr, and purchased the TO20 transformer. In the description, it says “…fits the stock mounting holes in all these amps.” Well, my original transformer has four mounting screws at the corners. The TO20 has TWO flanges for mounting, center-type. Did I misunderstand something? Does the TP24 transformer have two or four? Please advise.


  11. eddie_bowers says:

    Does the bias need to be adjusted after upgrading the transformer (I already have the adjustable bias mod)?

  12. goldtop87 says:

    Bill, I have emailed david allen and he mentioned that the to20 for the scxd has caused some oscillating with this ot . Will this happen to all scxd’s? I wanted to purchase, but are they even available for the fender scxd yet?

    • bill says:

      I have heard of some SCXDs have oscillated. I have a TO20 installed in one of my SCXDs and it’s OK, but I have not had an opportunity to test more amps. It’s probably not worth the risk. I’m currently not selling the TO20 for the SCXD.

  13. livingwater says:

    I installed the T020 today.
    It was a perfect fit

    Also installed the basic , tone stack and presence mod’s

    I now have more bass, complete control of my treble , bass and mid pot’s
    And… the amp is extremely noiseless except for my playing

    Thank you Bill

  14. bluessr1 says:

    Bill, I have a PRRI and it is very farty on the low end when playing hot pickups. Someone referred your transformer and another person said to just replace the speaker?

    Is there a huge difference?

    Can you get the 30 watts like the new BJ transformer listed?

    I was thinking of doing both. What do you think of the Rajun Cajun or do you have a better one in mind? I want little to no break in the speaker. Also the cabinet has a BUZZ where the bottom back panel meets the bottom of the cabinet. This amp is BRAND NEW, I cant believe I have to modify it.

    • bill says:

      I think the Eminence Copperhead sounds better than the Cajun in a PRRI.

      Hot pickups can be a problem for any amp and they can certainly cause the OT to saturate. You have to turn the bass down quite a bit as you turn up to avoid that. The TO20 will certainly help with that; so will doubling the value of the first filter cap to 47uF.

      If you want 30 watts, you’ll need a different power transformer and a different output transformer — and 6L6s. Check with David Allen at Allen Amplification.

  15. SamZ says:

    Hi Bill – I’m a little confused on how ohms and loads work. My scenario: my amp currently does have your 8 ohm aux out. I ran the aux out to a 2 x 12 8 ohm cab. My BJr has a C Rex speaker which drowned out the 2 x 12 cab. Since the 2 outs are in parallel with each other, does that mean power is evenly distributed across the 2 outs, where the C Rex got 50%, while the other 2 speakers in the cab each got 25%? I am wondering if its possible to get a balance of volume between 3 speakers (assuming they each have the same sensitivity) using a 2 x 12 cab, via the aux jack….

    • bill says:

      If the cab was truly 8 ohms, the current was divided evenly between them. But what the speakers do with it depends on their efficiency. The C. Rex is probably more efficient, since it’s one of the loudest-per-watt speakers you can find. Two speakers with lower dB-per-watt ratings will hardly be heard over single, more efficient speaker. Also, if the cab was closed-back, it’s probably quieter because it trades off efficiency for tight, controlled bass.

      • SamZ says:

        Thanks Bill – this helps! I know the speakers in the cab were 75 watts @ 97 dB. Using th dB/watt ratio, that makes the CRex 2.04 db/watt, while the cab speakers 1.29 dB/watt. For argument sake, if the cab had 2 additional CRex speakers, an open back, and truly 8 ohms, could there ever be balance of output between the 3 speakers – or would the evenly distributed current between the 2 parallel outputs still hinder that? I wondering if the lesson of this experiment is that I need to use same # of speakers, with matched efficiencies (dB/watt) from each output jack if I want even volume across all speakers…

        • bill says:

          The maximum watts rating doesn’t matter. That’s simply the amount of power that you can put in before the voice coil melts. It sounds like your speakers are 97dB at 1 watt and I think the C.Rex is rated at 101 or 102dB at 1 watt. So it’s going to be louder no matter what.

          Adding a second, identical speaker increased the overall loudness by just under 3dB. A third identical speaker adds another 1.5 or 2dB. If you want it to be significantly louder, you need a lot more watts going into the speakers. And yes, it’s best to have speakers that have similar dB at 1 watt ratings. You can mix and match tones, but you’re never going to hear a 95 or 97dB speaker over a modern 100+dB speaker.

  16. isljms says:

    After playing my Blues Jr. for a good 3-4 hrs. The clean sound starts distorting a bit especially when I volume up. Do I need to mod my transformers? I also switched out the speakers to a 12″100w 8ohm. and it still does that.

  17. srwilk says:

    Hi Bill-

    I would like to upgrade my Superchamp XD with the T020 Low-Profile output transformer. Is this simply a “remove and replace”, (drop in) transformer switch? Any other mods necessary once I move forward with this upgrade? Thanks for your time! Steve

    • bill says:

      It’s a drop-in. Same screw holes, same push-on connectors. A few folks have reported oscillation with the TO20, most others love it. I haven’t had the time to do extensive testing to see if there are issues.

  18. sliberty says:

    I am going to build a Brown Princeton (6G2). trying to decide what OT to use. I have seen some listed at 8.5K, and this others at 6.6K like the TO20 and TO22. Any thoughts on this would be appreciated. Would either of them be a nice choice for a 6G2?

    • bill says:

      Anything in the 6K-8K range will work fine with 6V6s. We did some 8K prototypes of the TO20 when we were developing it, and there’s no audible or measurable difference in performance.

  19. willembad says:

    I have a TO20 in a Pro Junior. It is my frankenstein (learn about amps) amp and the latest mod is a SkipZ VariwattFX Dual to scale the whole amp. The Variwatt dual lets me adjust the bias individually for both EL84’s and I’d like to know if the TO20 is 100 Ohms as the stock OT. I’m planning on using your method of adjusting the bias but I’m guessing the voltage drop I’m looking for will be tied to the OT. Please advise what I should be looking for. Thanks.

  20. Danny says:

    Hey Bill,

    I’ve got a question for you – which output transformer would you recommend for me? I’ve got a cream board Blues Junior with a Cannabis Rex, all the basic mods, sparkle and presence control, and am planning on doing the cathode follower mod (I’ve got a 12DW7 in V2 in preparation for that) and installing the TP24. I’d like to have the ability of running another speaker, but it’s not terribly important to me. I know that you say that the TO20 can run an external speaker, but at what cost? Also, I’m wondering what the difference in tone is between the TO20 and TO22, and which you would recommend for my setup.


    • bill says:

      The TO20 and TO22 were designed to sound alike. If you can hear a difference at all, the TO20 has a slightly bigger bottom end and the TO22 has a slightly clearer top end.

      With the emphasis on slight.

  21. Paul says:

    I have a Champ II and would like to know if the TO20 would be the right upgrade and if you include some directions for wiring. Thank you.

    • bill says:

      The TO20 would be a great replacement/upgrade for the Champ II output transformer. The color codes on the wires are exactly the same, so it’s a straight replacement. The wires are soldered in, so there are no issues with connectors.

  22. modwiz says:

    Can I use the t020 for my SCX2?

    • bill says:

      It should work, but I’ve never been inside a SCX2. I can provide a TO20 with the proper connectors and wire lengths for an SCXD; if it doesn’t fit, you can return it for full credit.

  23. michele3b says:

    Bill–as per your recs. I replaced the speaker and transformer in my 2007 BJ with Cannabis Rex and T020 respectively and Mod reverbe and it sounds exactly like one half of a freaking Twin–unbelievable improvement–my tech uses your site for all the mods–many thanks!

    Question–I need a second amp and will likely get a BJIII and swap and hopefully sell the Lightning Bolt. Has there been ANY improvement on the transformer in the new ones, or is it the same old same old?

    Thanks again–Mic

  24. michele3b says:

    Thanks–will do the double swap again–Rex and T020–sounds VERY close to the old twins we used to retrofit with Altecs or JBLs, just naturally much lower power. But hauling two of these for us geezers is a breeze for the sound I can get–somewhat easier to haul than a twin. Thanks for all of your work in recognizing the potential for this mini amp with a big 12.

  25. Frets Repair says:

    Hi Bill, I put the TO20 on my PRRI, and am wondering what the included capacitor is for? Also, do you directions for me to install the Standby switch I ordered from you for my PRRI? And, also curious how I install the Switchcraft input jacks that I ordered, aka. Bypassing or tying into the ribbon connect on the board. Thanks so. Thank you, and you can email me at

    • bill says:

      The 100pF capacitor is for Blues Juniors. It’s not necessary on PRRIs. I think we emailed about the jacks; you would have to duplicate the setup on a non-reissue (original) Princeton, with two 68K resistors and a 1 meg resistor. I don’t see any advantage to doing this; the PRRI does not have any particular noise problems with the jacks and they have been reliable so far.

  26. radajimi says:

    Hi Bill, Is there any solder needed or anything else I have to do after installing the transformer?

    • bill says:

      No soldering is required to install the TO20. You have to partially remove the circuit board, as shown in the video on my site, so you can run the wires. but it just plugs in.

      But in terms of tone improvement, the basic mods kit makes a much bigger difference, and it does require soldering.

  27. Artgtr says:

    Hi Bill, Installed the TO20 in the amp and followed your directions,again fast, Medium easy and went perfect. Sounds great and less noise than before. Again thanks for putting out great products with great instructions.
    Next basic mod kit.

  28. andregodoycoelho says:

    Hi Bill!

    Can this be used on the Excelsior? I think it would be a worthy improvement over the stock OT.


  29. muzza says:

    hi bill, i put in the basic kit, recap kit, aux & lineout jack kit, t20 tranny, in my 230v bj, How does it sound…. Awesome!!, ive still got the stock speaker in although ive read how a vintage 30 would unleash the beast, Also theres no flabby farty sounds, just heaps of growl bark and and volume where you would expect, tried different output tubes, some old stock awa & phillips which are creamy, also a pair of 2ndhand jjs which were ok, some groove tubes which were great, The amp sounds best with my fender usa strat and pretty good wth the prs. Now ive just ordered the worksburger ie, octal sockets plus all the other mods i didnt have so here we go!! thanks bill for all your work in making these amp GREAT

  30. Clement says:

    Hi Bill,

    Just for information, Where can I find a TO20 and a TP24 in UK? Have you unit?

    Thank, Clement

    • bill says:

      There is no UK distributor for the transformers; you can order them from me. Priority Mail is expensive, but I use it because of the tracking and insurance. The TP24 is only available with a 120V primary; sorry.

  31. teza says:

    hi Bill,
    For you, what will be better the T0 22 from you or Mercury Magnetics Fender Blues Jr Output, I would a fair answer from you, as Mercury Magnetics got a very well known reputation.

    • bill says:

      The Mercury FLBJ-O is a good-sounding transformer, but it’s a huge beast. I should post a picture of the FLBJ-O. It doesn’t fit particularly well in the BJr, it needs you to drill mounting holes, and it doesn’t have bell ends. Oh, and it’s pricey. 🙂 We designed the TO20 to do everything that it does and be a drop-in.

  32. cam0070 says:

    What effect will the T020 have on the speaker? I love the tone I’ve heard from the C12R in my blues junior, and the Jensen website ( says its a 25 watt, and this guy who installed the T020 ( said his 25 watt chinese greenback blew (at 2:45). Also, sorry to get off topic, but do you have an update status on the availability of the tone shaper mod? 🙂


    • bill says:

      He blew the Chinese greenback because a) it was a Chinese greenback, not the real deal, and b) He was getting 20+ watts out of the amp and playing distorted, which greatly increases the load on the speaker.

  33. Forty4 says:

    Hey Bill does the TO20 for cream board B jnr work like for like with the Pro jnr?

  34. Jagmaster says:

    Am I correct in assuming that , if I install a TO20 in my Blues Junior (all basics mods done) , that should I ever do the octal tube conversion using thee TP24 power transformer, that I can continue to use this TO20 output transformer ? I love the basic mods, presence and sparkle controls ! Thanks

    • bill says:

      The TO20 and TO20B work with EL84s and 6V6s, with the stock power transformer or the TP24. If you want more power, from 6L6s or 5881s, you’ll need a larger output transformer, too–the TO26.

      • Jagmaster says:

        Ok, understood.
        Then is the TO26 compatible a Blues Junior that has only basic upgrades and still using the stock tubes ? (with an eye towards possible future 6L6 conversion) .

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