Super Champ XD: Swapping Op Amps

When I got my second Super Champ XD, I decided to explore the effect of the op amps on the tone. I installed sockets for the three op amps that are in the audio signal path and tried some different chips in there.

The incoming guitar signal first sees a TL072, and it goes through both stages. From there, it’s routed to the DSP. When it emerges from the DSP, the signal is balanced, with + and – outputs. Another TL072 integrates the signals and turns them back into a single-ended mono signal. Then the signal goes to the first stage of a 4560. From there it goes to half of the 12AX7, which buffers the signal and drives the phase inverter half of the 12AX7.

All three op amps apply bandpass filtering to keep the signal in the audible range and to prevent oscillation.

The TL072 has good specs and is a reasonable, low-noise op amp, but guys who hack their DVD players consider them “jellybean” parts– $.50 cheapies with barely acceptable tone. I generally don’t think about op amps having tone, but they do, just like tubes.

The question is how much improvement can you get, and is it worth the effort?

“Effort” is a definite factor. The SCXD circuit board is very difficult to remove and it’s hard to desolder the op amps from the board without damaging the board.

After successfully installing sockets, I tried two different kinds of op amps: the 5532 and 2132. The 5532 is well-regarded by most audiophiles, but the highest praise generally goes to the 2132. The 5532 is a bit more expensive than the TL072; the 2132 is a $4-$5 part.

I reloaded the amp with TL072s and a 4560 and fired it up–it sounded normal. I checked for oscillation on the op amps and there was none. I replaced the preamp TL072 with a 5532 and the sound was noticeably brighter and crisper. Not to an earth-shattering degree, but noticeable. Then I replaced the 5532 with a 2132. It was slightly better than the 5532–crisper bass, brighter highs, better note definition. I switched back and forth between the TL072 and the 2132 and the difference was fairly obvious.

Changing the second TL072, the differential integrator, to a 2132 added a slight bit of brightness, but the increase was not as dramatic as it was in the preamp.

Changing the 4560 to a 2132 again increased the brightness and crispness, but again, just a small amount.

I did all the bench testing through a well-broken-in Eminence Swamp Thang, which is a clean, loud, mostly colorless speaker. My son hears high frequencies better than I, and was there for all the testing to double-check my ears.

We then reassembled the amp and brought it upstairs to play against my other SCXD. I connected them with an A/B switch and adjusted them for identical volume levels. The first thing we noticed was that the SCXD speaker was not as crisp or articulate as the Swamp Thang–it added a certain blurry quality to the tone, which tended to make the amps sound more alike.

After switching back and forth a bit, though, we could easily tell which amp was playing. The difference is not as dramatic as upgrading the speaker to a Ragin Cajun, but it’s there. Pick attack is cleaner, note harmonics bloom more fully, and the amp is brighter, with more top end.

Would I recommend that you do this mod? No.

It’s a lot of work for a little bit of progress–half a speaker upgrade’s worth of tone. The chances of ruining your amp are considerable, especially if you don’t have desoldering equipment that heats and applies suction at the same time. I work on PCBs all the time, and I still managed to tear a trace when a pin got stuck.

I will say, however, that combining the upgraded op amps with the Ragin Cajun gives the amp a degree of transparency, the open, airy tone that the audiophiles talk about, that wasn’t there before.

Putting it in perspective, however, as soon as you turn on effects, you blur out most of the gain–reverb, chorus, delay all overlay the sound with more sound, and no more crisp edges, many fewer ringing, airy harmonics. And we are talking electric guitars here, not critical listening to a flute and harpsichord concerto.

There’s still more tone to be had from the SCXD; my exploration isn’t over yet.

72 Comments

  1. Tim says:

    I am just curious about all of the recommendations I keep seeing about use of the Ragin Cajun in the Super Champ XD.
    I have a band of advance musicians and we have two Champs, one with the stock speaker and one with the Ragin Cagun. We ran them side by side with an A/B switch and the entire band said the stock speaker beat the Ragin Cajun hands down. I was stunned since I owned the upgraded Champ. I clearly agreed after hearing it. Whats your take on this? Also do you know where I can get a deal on a use stock speaker?

    Thanks,

    Tim

  2. Chris says:

    Hey Bill,
    Have you considered a Presence control mod on the Super Champ XD? I have replaced the stock speaker with an Eminence Legend 1058, tubes with Tung Sols and the speaker cable with Van Den Hul. I’m very happy with it but was curious if the addition of a presence control might add some sparkle and liveliness to some of the modeled voices, in particular voices 1 and 3 which are my favorite. I use Voice 3 primarily with Gain at only 2 for a dirty clean Fender breakup tone with my Strat and it is great but a little dark even with the bass rolled down to 6. Any thoughts or other ingredients I could add to this setup I may have overlooked?

    Thanks!!

    • bill says:

      The SCXD uses a very simple negative feedback loop to improve output stage accuracy. It doesn’t have any of the tone shaping components found in the all-tube amps, with or without an adjustable presence control. Since the PCB is so crowded and hard to work on, I think it would be a bad idea for most owners to attempt to add a presence control.

      Voice 3 is a tweed amp and I agree that it is a little dark compared to the blackface voices. Voice 1 (Champ) seems plenty bright to me, but we all have different ideal tones in our heads.

      • Chris says:

        Thanks Bill. Have you tried any OD pedals into this amp? I wonder how an OD pedal would sound into Channel 1 or Voice 1 since those are Blackface cleans.

        I appreciate your feedback!

        • bill says:

          I don’t use OD pedals with mine. Since there are bunches of OD voices on the other channel, I can’t think of a reason why I’d want one.

        • DetroitBlues says:

          I use a Digitech Bad Monkey. A very inexpensive OD pedal. Sounds awesome on any channel. The higher gain channels have a bit of a volume boost. On the tweed and blackface settings, it has a noticable breakup making the amp sound so much sweeter.

      • donnyboy says:

        **do this** use a spst toggle switch to lift out R23….early marshalls(18watt/tmb/etc) don’t use the feedback. OR replace with 25K-50K pot…all can be done from the top of the pcb…by cutting up one end of the R23 after jumping it with the pot. if you no-like-y….solder the resistor back in place…AND since this is a pain PCB having eyelets….resoldering wont screw up the integrity of the resistor connection..sure the cap/pot presence is more traditional approach can work…have it on my BJr….the bottom seems just fine..IMO

        • bill says:

          That’s fine, as long as you realize that running without negative feedback on amps that are designed for it is an “effect,” an intentional distortion of the waveform. I find it too harsh for most of my music.

  3. Brent says:

    Bill,
    I have a SCXD and I also replaced my speaker with a Eminence Ragin Cajun. Just changing out the speaker is a big improvement. I have not tried changing any tubes. Are there any other tubes you recommend over the stock ones?

  4. Jason says:

    Bill,
    Is the scxd fixed bias like the blues jr?

    • bill says:

      The SCXD has adjustable bias and a 1 ohm resistor for measuring idle current. You can download the schematic from Fender.com–the specs are on the schematic.

  5. adrian says:

    Is changing the stock 12AX7A worth the mod, in that it’ll significantly improve the sound?

    If so, what’s a good model to buy?

  6. Danny says:

    Bill, I was wondering if there is a pedal or could a pedal be made that could be switch between the various voicings and FX of the SCXD. My SCXD does a great job creating the sounds our band needs, but we need to be able to access several different voicings on demand during our live performances. If you know of anything we could do please let me know. thanks

    • bill says:

      That would be a bit of a hack. The knob generates binary codes that are read by the DSP. Each code (1-16) has a voice associated with it. You’d need to preset the codes with dipswitches or additional knobs that generate the binary code. Then you’d need a circuit that switches from preset to preset. Then you’d have to make it accessible from a pedal.

      While you’re at it, it would be cool to store away the tone, gain, and volume settings.

      Sounds like a Cyber Deluxe. Or a good multi-FX pedal.

    • Sweidner says:

      Hi Danny.

      This is exactly the bit I’m missing too.
      Does anyone here know if the Mustang floorboard would be able to generate the right binaries to switch the voices. In this case one would simply need to replace the switch in the amp with an externally connected “controller”.
      Any thoughts??

      Thx. Stefan

      • bill says:

        The SCXD footswitch simply generates two voltages that throw two switches inside the amp: Effects on/off and channel i/2. To change voices, you would have to tap into the rotary switch and generate a 4-bit code. The Mustang floorboard is entirely incompatible with the SCXD. If you want a Mustang, get a Mustang! 🙂

        • Sweidner says:

          Thx Bill.

          All clear. I just thought that one could use the mustang foot switches as they allow to switch between voices. If this switching would work based on binaries as ncessary for switching voices on an SCXD this might be a solution. Btw don’t want a Mustang, love my SCX2 🙂

  7. Andy says:

    I’m interested in running my SCXD through an external 4 ohm cab. I see that the low profile TO20 transformer allows occasional 4 ohm use, but I’m concerned about the “occasional” qualifier. Could the TO20 be special ordered for 4 ohms or could a version of the TO22 be ordered for the SCXD?

    I’m running a Ragin Cajun right now but I would consider something with a lower profile if needed to fit a larger transformer. At that point, though, I would also consider swapping speakers or swapping cabs to run 8 ohms.

    • bill says:

      Your best bet would be to use the TO22 transformer, which has a 4 ohm tap. You’d have to drill an additional mounting hole for it and find a place for either a second output jack or the 4/8 ohm switch that I usually provide with the TO22. I have not done this on a SCXD, so I can’t offer much guidance in where to put the jack or switch. It’s a little crowded back there, and you might have to loosen the circuit board to drill the hole for the larger footprint (1/2 inch) of the TO22.

  8. Buddy says:

    I replaced the stock speaker with the Celestion Greenback G10 and had a great result. I also have an extra Fender Superchamp Cabinet that had the amp removed. I installed a Ragin Cajun in it. I bought a 1/4 to 1/4 speaker cable at Radio shack and stripped one end and wired it to the Rajun Cajun Speaker, so now I can just swap the speaker cables between the two speakers. I can get a variation in sound without a major hassle. The Ragin Cajun has a little more edge to it, and volume. The Celestion keeps a little of that Fender Mellow vibe and warmth.

    Now if someone can tell me how to run both speakers from the same amp or how to make it into a cab extension.

    • bill says:

      The aux jack kit would let you plug in a second speaker. You’d have to unplug the speaker from the amp in order to use the internal speaker as an extension speaker.

      • Dugbert says:

        I’m confused. If the aux jack is in parallel with the main speaker jack and is intended for use with a second 8 ohm speaker or cabinet won’t the total load be 4ohms? And isn’t that too low?

        Thanks

        • bill says:

          Yes, the load is 4 ohms, and no, it’s not too low. Any tube amp can handle a 100 percent mismatch–an 8 ohm output transformer will work fine with a 4 ohm or 16 ohm load.

  9. Jeff says:

    Bill, I’m all new to biasing. What is the Max i can set the bias at on my SCXD? Bias was factory set at 65 (resistor and pin 8 values). Pretty hot from what I’ve been reading. I biased it to 40 (20 per tube/GT’s stock) and I didn’t like that as much. I found a calculator online (http://www.ax84.com/contrib/biascalc.php) and if I am correct the 6v6’s are at 400v plate voltage so @ 70% I could set it to 49? Could I go 80 or 90%? – realising I’d have to replace tubes more often or would this put too much stress on other components that would toast the amp.

    Thanks

    • bill says:

      Many manufacturers consider the 6V6 a 12 watt tube, which is why Fender uses a more conservative 40 millivolts. In a way, it’s kind of a shame that you got used to the 65mV tone–it’s mushy and inaccurate. Maybe it adds something to the distortion tones, but I think it takes away from the clean tones. In any event, you shouldn’t consider the bias a tone control. It’s an operating parameter. If you bias at 80 or 90 percent (that’s the “idle speed,” remember), your tubes will easily go over 100 percent when you play. That seriously shortens the life of the tubes and doesn’t allow them to “hand off” to one another the way push-pull tubes are supposed to. The screen grid is far more likely to fail if you hot-bias, and that’s not pretty. A shorted screen grid usually takes a couple of power supply components with it when it blows.

      I bias JJ 6V6s at 50mV. They’re much heavier-duty than typical 6V6s; I can believe that they’re really 14 watt tubes. And they sound great at 50mV.

  10. Joe Sketchley says:

    I’ve both Super & Vibro XD’s. The Super has a Weber 10A150 speaker (a warm Weber with Alnico magnet). Stock bias should be 40ma, came at 36ma, which as is said above, a lower bias, is cleaner with later breakup. Tried higher and will confirm, an earlier breakup and more warmth/mush. I tend to wince at treble bite, so a brite cap C43@470pf on the Super circuit has been disabled, by unsoldering one leg from the pcb.
    The Vibro has a 10″ Celestion Greenback, and also the bright cap disabled. C47@470pf on the vibro circuit. Both speakers have warm ribbed cones, with the Celestions ceramic magnet being more precise. The Weber’s alnico magnet is warmer & I prefer its sound on a hollow bodied guitar. Over all though the Greenback is superior, particularily for a solid body guitar.
    Reinstalled the bright cap on the Super, but I didn’t like it, & took it out again. 10″ speakers have enough inherent brightness for me. Also the Blackface voice (the real gem in these amps) is plenty clean & bright anyway. Installing higher watt rated speaker in these cabinets produces a richer more substantial voice.

    • Greenman1976 says:

      I tried that as a mod on my SuperChamp XD with a small switch to control it and either my switch is malfunctioning or i cant hear a difference.

  11. Scott Fowler says:

    I have been reading some of the comments on here, as well as some of the forums, about speaker upgrades for the SCXD. I have shopped around, read and scratched my head about the right replacement speaker. I went on the web to a place called http://www.mojotone.com and purchased a speaker called the American Vintage AV10C. It is the perfect speaker for the amp. After I purchased the speaker I took the amp back to the store I purchased it from and several of the employees there tried my amp out compared to the stock amp and they all agreed it is a completely different amp with the speaker. One of the guys has a SCXD with a Rajun Cajun and he ordered this speaker to replace it. I know tone is a matter of taste but this speaker makes all of the settings sound much better. Just a option if you are looking for a great replacement speaker.

  12. Matt Thorpe says:

    Just wanted to share my experiences here.
    The Super Champ XD is an awesome little amp. I added an Eminence Ragin Cajun speaker and a JJ gold pin 12ax7 which I found to be brighter than the EH tube I also tried. The original ‘fender’ marked tube was very poor. The amp was louder with the EH tube but sounded more refined with the JJ. The JJ sound was more appealing in conjunction with Ragin Cajun.
    I don’t normally use the Gain channel of the amp except maybe mucking about at home. I use the XD with a Blackstar HT-Dual, a tube driven 2 channel, pedal into the clean channel. In use this setup is like having a three channel amp. The pedal off being the SCXD clean sound, then a nice bluesy rhythm sound and a solo sound from each of the pedal’s channels. In a band context or playing blues rock at a jam night the amp rarely goes above vol 4.5 as it is just so loud !. People can’t believe the sound that comes out of such a small box. The only real complaint would be that using a Les Paul the clean channel could do with being a little brighter – not by adding more treble as such, but somehow harmonically brighter. With the pedal kicked in the harmonic richness is there.

  13. Tony Bernitz says:

    I am very impressed with my SCXD except that with the stock speaker it is very trebely.Is there some speaker you can buy (short of running it thru a 12″)that would give more bottom and mid range.

    • bill says:

      Try an Eminence Copperhead.

      • telebender1956 says:

        Thumbs Up +1 for this recommendation. I played my SCXD with the stock speaker for about 2 hours and then knew I needed a different speaker for Fat,CLEAN, vintage Fender Tone.The Copperhead is just what the tone doctor ordered.Great for gigging and pedals work very good out front.

  14. Franksalot says:

    Has anyone tried a Jensen JN10-100TR Tornado in their SCXD…the sound bites on the Jensen tone chart sound pretty nice.

  15. bytewax says:

    Hey. I much appreciate your blog.
    I own a SCXD and it’s the very first time I use tube gear ‘in this case hybrid gear’.

    My question here is: I don’t have the time to learn that bias thing but how can I replace tubes by myself if they fail?
    I’m a tube noob. I know the pair should be matched. My concern is while i’m on stage and I need to replace the tubes in 10 minutes?
    I love tubes. 🙂

    Sound question: I noticed that channel 1 crancked up is damn good but to play some blues and cover a drummer wich setting/voice is for a good blues distortion while mantaining good definition .

    Thanks and keep up the great blog. Cheers from Portugual – yes fender is here too…

    • bill says:

      You can replace the output tubes with a matched pair and worry about the bias later. You can finish your set with no problems.

  16. captbobc22 says:

    Bill thanks so much for your work…
    My 2 cents worth- and have had GREAT results…changed the tubes to JJ 6V6s & 12 AX7. Changed the speaker to a Weber ceramic signature 10, and biased the amp @ 40 Mv. All recommended settings and parts- but they work very very well with no other dramatic changes required! This is truly an awesome little amp that will only get better with age…
    Be well all,
    Bob

  17. droto says:

    Bill,
    I’ve seen so many speaker recommendations for a SCXD…have you heard of anyone using a Eminence Lil Buddy…seems to add warmer rounder tones versus the kind of think sounding stock speaker. everyone seems to lean toward the RC and you mentioned the Copperhead. thoughts?

  18. djs419 says:

    Hi Bill,
    So I decided to go for the op amp mod, as you describe a remedy to a problem I have noticed with the amp. My question is this, are the op amps you are talking about labeled u1, u2, u3, respectively (on the circuit board)? I had some difficulty reading the blurry schematic I got from Fender. I’m guessing the first one, which you describe as having the greatest effect on the sound, is labeled u1. Thanks for your help!

      • Al says:

        Hi Bill,

        I just got a Super Champ XD and want to change the speaker and tubes. I’ll go with the JJ 6V6s and a 12ax7, however, can you tell me if I need to totally remove the chassis to get to the trim pot? Where exactly is the trim pot? I looked at the schematic and I cannot tell where exactly it is (is there a number on the schematic that you can provide to me as some kind of reference point to locate the trim pot)? Also, the stock tubes say they are 6V6GT’s, should I get the JJ 6V6GT’s or the 6V6s’? Regarding the speaker, I see you recommended the copperhead but if I was thinking of a Celestion, which would you recommend or is the copperhead the way to go with the Super Champ.

        I see you responded to an earlier post above to replace the tubes with a matched pair of new ones and forget about biasing them until later. would a new matched pair of JJ 6V6s’ really need to be biased?

        Thanks! Al

        • bill says:

          You have to remove the chassis to get at the trimpot. It’s no big deal, just the back and top screws. The trimpot is in the back of the chassis by the output tubes. It has a screwdriver slot; you can’t miss it. Use JJ 6V6-S tubes.

          It’s always worthwhile to check the bias. You never know how far off the factory setting might be or how different one pair of tubes might be from another.

          I understand that some users really like the Celestion G10T. I haven’t tried one myself.

          • Al says:

            Hi Bill. Just trying to understand the 40mv bias setting for the JJ 6V6s. I do see on the Fender amp schematic that it says “Adjust RB to achieve 40mvDC at TP22”. What does TP22 mean/stand for? Also, on an instruction sheet that came with a dual bias tester I purchased from AMP Head says 1 mV = 1 mA and that “6V6 tubes bias range is 22-28 mA”. There is also a Plate Voltage table that lists plate voltages from 250-600 and the associated 6V6 mA range for each plate voltage value. None of the plate voltage values from 250-600 have a mA or mV range that exceeds 39 mA (the 250 plate voltage shows a 39 max mA value. So the questions I have are: 1) Plate voltage is a measurement of the JJ 6V6s tubes and not a measurement of the SCXD amp, right? If so, what is the plate voltage for the JJ 6V6s tubes, or does that have to be measured/tested on each tube? 2) Where does the 40mv or 40ma recommended both on the amp schematic and referenced here come from/how was that value determined? 3) On the boxes of the JJ 6V6s matched pair I bought from Tube Depot there is a sicker with a 23 on both. Does that mean that they both tested out at 23 mA or mV?

          • bill says:

            First, the number on the box is just a number. It has nothing to do with milliamps. But a 23 from the same vendor should drop into your amp without rebiasing.

            The 40 millivolts measured at TP (test point) 22 tells you that 40 milliamps are flowing through the 1 ohm resistor. That same 40 milliamps are flowing through the plates as well. Multiply the amps (.040) times the plate voltage, 375V, and you get the watts that the tube is dissipating at idle. That’s 15 watts, divided by two tubes, so 7.5 watts per tube. Since the 6V6 is rated at 12 to 14 watts full power, that’s a good idle setting.

        • donnyboy says:

          re-bias ?? YES…did bills 6V6 convert on my BJr(awesome) and bought some chinese 6V6s…good tubes..only problem is that they were so far on the “hi” side that -26 was not enough …screamin warm…BUT the tubes(6v6) i pulled from a circa’59 lowery organ worked perfect! LOL…see demo on donnyboy911 (my youtube)

  19. kansascity7 says:

    I agree with Tim on the stock speaker being good. Especially if you play chords, very smooth. I tried the Ragin Cajun and was disappointed. It is definitely louder. Good for lead playing. The stock speaker has more of a compressed sound and the cajun has a in your face honk. I also changed the 6v6’s to JJ’s and they are good, more rounded sound with smooth harmonics. I can turn it up to 5 with no major breakup. I did not adjust the bias.

  20. Al says:

    Hi Bill. With the dual bias tester from AMP HEAD, I checked the the stock Fender Groove Tubes and got a reading (on a meter set to measure DCV @ the 200m setting) of 18.6 for one tube and then 17.7 for the other. Does that mean that these 2 numbers added together (18.6 + 17.7 = 36.3) is how many millivolts the amp is currently running with the stock tubes? If so, then is what I’m trying to shoot for an individual tube reading of 20 for each tube (on a meter set to measure DCV @ the 200m setting)?

    I then put the new JJ 6V6s tubes in and got a reading of 12.1 for one tube and 12.1 for the other. So what does that tell me? Is the amp only running @ 24.2 millivolts with the JJs? Should each new JJ be increased with the trimpot and if so, to what number for each tube, 20? I’m still learning about all of this and still really do not know how what I’m measuring in each tube relates to the 40 millivolts recommended on the amp schematic, and to the bias range for 6V6s which is 22-28 mA (where 1mV = 1mA).

    Thanks,
    Al

    • bill says:

      First things first, the 22-28mA number is meaningless without knowing the plate voltage and the operating class of the tube. The bias meter is pretty much unnecessary, since you have to open the amp to set the bias. The 1 ohm resistor is right there, and it’s shared by both tubes. So when you read 40mV at the resistor, that’s 20mA per tube. That sets the idle at 7.5 watts per tube, which is just right for a 6V6 12 watt tube.

      Your bias meter is telling you that your tubes are not perfectly matched (they never are) and that you should turn up the bias until the hotter one is at 20mA.

  21. wschafer says:

    I bought a SCXD a few months ago and when I brought it home a noticed a buzzing sound when playing certain notes and chords. Took it back to my dealer’s service shop and of course could not hear the buzzing. Brought it home and took the back off to expose the tubes and the noise was coming from the 12ax7. I would play the notes/chords and put a cloth and put a little pressure on the 12ax7 the buzzing would stop. Took it back to the service shop and he checked the circuit board for loose connections and replaced the original 12ax7 with a JJ. Got it home and still buzzes. I also replaced the 6v6s with JJs and had the service department rebias at 50mv. Any suggestions as to what I can do. I’ve thought about asking the dealer to return it to Fender, however, I would think they would be reluctant since they can’t produce the same noise I hear.

    • bill says:

      50mV is probably a bit too hot, even for JJs. Cool it down to the factory-recommended 40mV.

      Where to you hear the buzz from? Does it come through the speaker? Is it possible that you’re picking up hum or buzz from something in your home? Do you have an outlet tester? Is your outlet properly grounded? If it’s just a resonance from the power transformer, perhaps it’s just too quiet in your house (compared to the guitar store) and you’re being (no offense) hypercritical.

      • wschafer says:

        It does not come through the speakers. My home is only 25 years old and I’ve checked all the outlets and they are properly grounded. Again, it only buzzes on certain notes and chords and it is coming through the 12ax7. I quess it’s an unsolvable mystery. I also have a greenboard BJR with several of your mods and never have a problem. It sounds great and use it primarily at rehearsal. Thanks for your responses.

  22. Al says:

    Hi Bill. I hear some kind of “hiss” or “filament rattle” type noise when playing certain chords or notes similar to what “wschafer” describes above. The hiss was present when I got the SCXD stock and is still there after I changed all the tubes and speaker. You mention a possibility of a resonance coming from the power transformer. Is that common in all power transformers or would changing the stock power transformer eliminate the hiss (if so, which power transformer would you recommend). It’s really not a big deal unless you sit behind the amp while playing alone (if you have it cranked up and/or are playing with a band, you’ll never hear it).

    Also, here’s what I put in the amp which sounds pretty good: Celestion G10 Vintage, 2 JJ 6V6s power tubes, 1 Tung Sol 12ax7.

    • bill says:

      Changing the output transformer is a moot point because there are no aftermarket transformers with the same voltage. You can try to have it fixed under warranty; that might be your best bet. Fender doesn’t field service these amps, they just replace them.

  23. donnyboy says:

    my like for spkr is the Em 1518(yes15″…go with a bigger cab!) or the 1218 ..both will automatically solve the XDs EQ problem..can’t dial out the HIs!..plus the breakup is great AND these are 101db spl spkrs…NOW you ask” : why would i do that”? its simple: fender knew that if they marketed this amp w/12″ spkr(chassis is?17″ so no prob there!) it would blow the BJr sales ;so they pit it against its name sake “the champ” .NOW again you ask….SO…if you replace R5 /10K fixed resistor with a 500K pot….just like the 5E3 …this amp KICKS ASS….see my demo on youtube chan “donnyboy911 ” ya the vol gets compressed ;but you’ll get the point…this amp is the best bang for the buck…the mod is : 1 pot, 3 wires,1 hole..AND all done without disassy…i believe bill has pics i sent

  24. donnyboy says:

    ***here is the mod i’m looking into***
    “how to choose diff voices for chan 1″…cool huh…has just got to be a parallel bit switch that alternates position with the use of a rely or J111….anybody have thoughts on this?

  25. WarrenPax says:

    Bill,

    Picked up a new SCXD on the cheap the other day. Put in a pair of JJ’s, biased to 43mV (~60%). Replaced the stock ax7 with a 12ax7wc. Straightened out a few components that looked like a disgruntled employee had his/her way for a moment. Pin 4 on one of the 6v6’s had a noticable cold solder joint. Fixed that… For what it is, the amp sounds excellent. I’m happy. I do have one question: I’ve noticed that the PT gets a little more than warm. (It’s tolerable to the touch.) Is this normal?

    Regards,
    Warren.

    • bill says:

      Yes, the power transformer does run on the warm side. It’s just large enough for the job, nothing left over.

    • barfoden says:

      Hard to say with the PT heat up. You are not running the amp hot at 43mV so it should not be drawing to much idle current. But I remember that Bill tried various 6v6 tubes and a 5881 and meassured their filament draw, and the jj6v6 seems to draw a bit more than “regular” 6v6gt tubes, so it might put a bit more strain on your filament line.

      My laney cub 12r (after i removed a faulty polyswitch) runs very hot on the PT, but the Laney tech guy ensured me that they do that in the Laney cub 12r, Laney LC15, and Laney vc15. He said that they run the PT at 60-70 degrees celsius in this amp and the PT is spec. at 120 degrees.
      The Laneys I mentioned are amps with 3x12ax7 and 2xEL84 in fixed bias. I run the EL84 at 22mA each at 297V plate (6.5W) or 55% idle dissipation. I run my tubes fairly cold but the PT does get hot. I run jj EL84 and they do also draw a bit more (830mA)filament current than the Mullard spec (~750mA). I pulled one preamp tube (300mA), and the idle draw went up ~0.5mA on the EL84 so it does not seem that I am dragging the PT down to much.

      They (jj6v6) do sound like a bigger 6v6 tube with their larger plates and higher voltage/current handling. I have seen people run them in cathode bias at their rated max dissipation (14W) and they lasted for years. Another person ran them at 70% (9.8W) of idle in a fixed bias amps and they did not seem to show any signs of wear of tonal degradation over a long period of time.

  26. Dugbert says:

    Bill, I’d like to run a external cab with my SCXD. Here’s my plan. Using the original speaker in parallel with a second 8 ohm speaker and a 4 ohm dummy load in series. Would the dummy load have any negative effect on the amp? I would be unplugging the original speaker and plugging it into an external cab to complete the circuit. Any thoughts?

    Thanks,
    Doug

    • bill says:

      Don’t use a dummy load; it will just waste power.

      Just run the amp with a 4 ohm load. It will be fine.

      • schotter says:

        Hey Bill,
        new to this so ‘howdy’ and thanks beforehand; incredibly helpful advice here.

        You seemed to say it clearly but lemme ask again, since I am not keen on frying my X2:

        Can I just use a splitter cable and run an additional 8Ohms cab (additional to the internal 8Ohm speaker), thus reducing the load to 4Ohm?
        Would that be o.k. with the amp if I play at lower (room) volumes?

        I like the idea of making this amp a 2x 12/10 combo by simply adding the sc112 cab.

        cheers
        kai

        • bill says:

          The 4 ohm load won’t hurt your X2.

          • schotter says:

            Super. Thanks. Will probably go for the SC112 cab for looks and at some point replace te speaker with something more middy / brit. Could mak for a nice 1×10 / 1×12 combo.

            Anyhow, here’s another one: Got a set of TungSol 6V6 and EV 12AX7, biased to 40MV and played happily for two weeks but then the amp started making a popping sound after turned on and nothing else (no amplification).
            Swapped back the 12AX7 to the GT the amp came with and it works again but the popping /crackling sound is still there (when warming up, first 20sec).

            Anything to worry about? Transformer maybe?

          • bill says:

            The transformer is an unlikely suspect for crackling. Tubes, tube pins, sockets, solder joints, in that order, are the likeliest causes.

  27. chrills says:

    Hi Billm

    I have just bought an SCXD after reading lots of good things about it. I just have a few issues about it, witch I hope somebody can help me with.
    1. It’s very hard to adjust the volume in the low end. I use the amp to jam with a friend in his house, so I never go higher than 4. I read somewhere about changing the 12AX7 to 12AY7 or 12AU7 – would that help?
    2. As many other, I think the speaker lacks depth and smoothness. I want to change the speaker, but I keep reading that the Ragin Cajun will make the amp to play louder, and that is not what I’m looking for. Can you recommend another speaker in the same or lower price range?

    Thanks,
    Christian

    • bill says:

      It’s hard to find less-efficient speakers… the trend is towards greater efficiency. The Eminence Alpha 10A might work OK, but I’ve never tried it. A lower-gain tube like a 12AY7 or 12AU7 would reduce the drive to the output tubes, but might also darken the tone.

  28. Greenman1976 says:

    Bill,

    What are some mods i could do to my Super Champ XD to take away some of the briteness? Maybe give it a more Marshall-esque sound?

    GreenMan1969

    • bill says:

      Turn down the treble and use the British distortion voices. Fender does Fender voices best, but since the early Marshall amps were copies of the Bassman design, try that voice.

  29. GeeDub says:

    Bill,

    I’ve been looking to upgrade the stock output transformer in my scxd. Would the T020 or T020-low profile do the trick for me?

    G

    • bill says:

      A number of people are using the TO20 in the SCXD and like it. It’s a drop-in replacement, no drilling necessary.

  30. gas89 says:

    Bill,

    I have the SCX2. I discovered that board is laid out differently. It has three tl072s, two that are easy to get to and one under the dsp. That one did not have enough clearance to put a socket in. I changed out the easy ones for Ti tle2072s. I noticed better performance and more quiet operation. I am waiting to get some opa 2132s to see if there is any improvement. I am pleased with sound. The board is a pain to remove but if you are careful it is fairly simple. Thank you for your suggestion,

    Gas89