Amplified Parts Lollar Pickups

Amplified Parts Lollar Pickups Guitar Pickups

Amplified Parts Lollar Pickups Guitar Pickups

Join Strat-Talk Today

How to Bias a Fender Super Champ XD (SCXD)

Discussion in 'Amp Input - Normal or Bright' started by NEStrataholic, Apr 2, 2013.

  1. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    I wanted to share my experience retubing and biasing the Fender Super Champ XD (SCXD). There appears to be limited pictorial information on the web on this and so hopefully, this will help someone.

    Here is a great link – thanks to Mickey for the procedure outlined below:

    Agile Guitar Forum Archive - - How to bias the SCXD

    There is a lot of discussion around the web on how Fender often ships the SCXD biased a tad cold. Many have indicated that a simple rebiasing makes all the difference in the world. I chose to go for new tubes all around and rebiasing for two reasons – 1) I was having a weird microphonic sound and 2) the Fender supplied tubes, particularly the Fender/Groove Tubes 6V6s are not highly regarded (although I acknowledge there are those that say it doesn’t matter at all).

    I went with a new manufacture Tung Sol 12ax7 and two used but “tested new” matched RCA 6V6GTA grey plates with dual bottom “D” getters for the power tubes (5,000 each on Hickok tester).

    [​IMG]

    Now, for those new to the subject, bias controls the current flowing through your output or power tubes. You set the bias of your amp for a certain amount of current at “idle”, that is, when not pushing the tubes with lovely guitar sounds. You set the SCXD to run at idle at 20mA current through each, but measuring at a point that sums current going to both tubes. Also, the SCXD conveniently uses a 1 ohm resistor in the current path so you measure voltage drop at that resistor in the equivalent number of mVs DC. Thus, you measure 40 mV DC at the test point, which means there is 40mA current going through there, or 20mA per tube. (This setting, slightly conservative, sets the “idle” at about 7.5 watts per tube, which is good for a 6V6 12-watt tube so that power spikes during playing won’t reach 12 watts and you won’t see fireworks, unless a tube or other part fails!)
     
    rmendozajr likes this.

  2. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    BEFORE PROCEEDING - THERE ARE DANGEROUS VOLTAGES IN THE AMP, EVEN WHEN POWERED DOWN AND NOT PLUGGED IN. YOU COULD DIE IF YOU ARE NOT CAREFUL.

    That said, this is an easy enough procedure and if you follow steps and do not take short cuts or get careless, you’ll be fine. Always keep one hand in your pocket. By doing so, you won’t create a current path from one hand, up your arm and across your chest, and down the other arm – thereby not creating a current path directly over your heart. That can give you a bit of a chance if you touch the wrong thing.

    First, power down the amp while sounding a chord. Then pull the power cord. Then go have dinner or better yet, go to work and come back to this job in the evening. The thing you have to worry about is high voltage (> 350 VDC) stored in the capacitors onboard the amp chassis. There are those who believe that over time, some voltage dissipates. I don’t know if that is true, and there is no fool proof way to bleed the caps, but it can’t hurt.

    Second step is to remove the back panel/tube cage. Four simple screws. I found I had to pull quite hard to separate the panel from the mounting braces – the Tolex on both pieces was really stuck together. Be careful not to whack the tubes with the cage on the way out.

    [​IMG]

    Third, I installed the tubes with the chassis bolted into the cabinet. I didn’t want to be pushing against nothing once I had the chassis out, and I didn’t want to be sticking my hands in places on the open chassis to get those suckers in. I figured it was better to be careful not to whack them as I removed the chassis than get electrocuted later. Just make sure you line up the pins properly before pushing hard. There is a rib on the center alignment pin to ensure you put the tube in correctly. Make sure they are completely seated all the way to the chassis. You will have to rock them back and forth a tad – and maybe push the spring clips apart a bit – to get the tubes all the way in.

    [​IMG]
     
    rmendozajr likes this.

  3. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    Removing the chassis was fairly easy. Four screws on top loosens it (don’t remove the handle screws). The chassis conveniently drops down and rests on some wood blocks.

    The trick in removing the chassis is to not rip the front grill cloth. The front face plate (metal piece with the lettering for all the controls) hangs down below the grill cloth just a tad. If you pull the chassis straight out, it will rip the front grill cloth. You have to tilt the chassis down in the back as you ease it out.

    [​IMG]

    Here is a shot of the edge once I had removed the chassis – you can see the grill cloth that goes over the wood frame and how the bottom edge of the face plate could rip it easily.

    [​IMG]
     
    rmendozajr likes this.

  4. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    With the amp chassis out, I placed it on some blocks to get it up so the tubes, transformer, etc. can hang down freely. You’ll want to do this in such a way that you can reconnect your speaker and have access to a wall outlet with the power cord. A real workbench would be ideal! Or you can use a dining room table as I did.

    [​IMG]

    The next step is to plug the speaker cable back in. You need the proper load on the amp. Don’t turn on the power switch without the speaker plugged in!

    [​IMG]

    Leave the power supply disconnected and the power switch in the off position for now.
     
    rmendozajr likes this.

  5. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    Looking at the circuit board with the face plate nearest to you, you’ll see three circular areas which are where the three tubes reside. Between the V1 and V2 tubes (V1 in the middle, V2 to the left of V1), you’ll find resistor R20 – the 1 ohm resistor that you use to test the bias.

    [​IMG]

    Turn on your multi-meter. You want to set the range to measure more than 40 mVolts DC. Each mutli-meter has a range that will work for this voltage level, so check your manual. If in doubt, set to a high range and work your way down to avoid damaging your multi-meter. (Also check your leads and make sure they are plugged into the right places for the range and for DC voltage.)

    I recommend a quick practice before you plug in and turn on your amp. That way you’ll be familiar with the process and ready to go without making a mistake (and potentially hurting yourself).

    Take the black lead and connect it to the chassis which is ground. Either set it into one of the screw holes so that it is secure (OK), or use an alligator clip to hold it to the chassis body (BEST). Don’t let anyone else hold the ground lead either! Then place one hand into your pocket and use the red lead to touch the R20 resistor connection. Don’t touch anything else on the way in or out and you’ll be fine!

    [​IMG]
     
    rmendozajr likes this.

  6. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    Time to do it! Plug the amp in and double check that you have the speaker cable plugged in. Make sure you DON’T have a guitar cable or patch cord plugged into any other jacks. I turned the amp to Channel 1 and turned the volume down, but it shouldn’t matter what the front panel settings are.

    Then turn the amp on.

    With the multi-meter on, make sure your black lead is secure to the chassis, put your hand in your pocket, and take your red lead and touch the R20 resistor connection. My measurement was a very cold 22.5 – almost half of the desired level.

    [​IMG]

    The next step is to adjust the bias trimpot. The trimpot is a blue disk with a screwdriver slot under a bunch of wires to the right of the V3 tube, just inside of the speaker connection.

    [​IMG]

    [​IMG]

    You want to adjust this with a fairly small INSULATED blade screwdriver (blade no bigger than about 3/16” wide). Put the multi-meter lead down, away from the amp chassis (you can leave the black lead connected to the chassis). Like with the test leads, put one hand into your pocket and use the other to slightly adjust the trimpot. Turn the trimpot clockwise to increase the idle current or turn the trimpot counterclockwise to decrease the idle current. Then, put the screwdriver down and take up the multi-meter red lead and test again (keeping one hand in your pocket). Repeat until you hit 40 mV reading.

    I found it very hard to get the trimpot “trimmed” to exactly 40 mVs. The slightest nudge would kick the current up to 41 or drop it down to 38. When I got really close, I quit.

    [​IMG]
     
    rmendozajr likes this.

  7. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    YOU’RE DONE! Put the test lead down and turn off the amp. Unplug it from the wall. Remove the black (ground) multi-meter test lead.

    (It’s actually not safe to test the amp in this set up – too many places you can inadvertently touch while things are open. But of course, I did.)

    At this point, you should walk away for a while (if capacitor draining does happen, you’ll want to give it enough time to happen). Sleep on it and put the amp back together in the morning.

    When you are ready to reinstall the chassis, unplug the speaker, carefully slide the chassis into the body. Again watch the grill cloth to make sure you don’t tear it with the face plate on the way back in.

    Lining things up is pretty easy if you have a strong light above the amp to see the screw holes in the chassis through the cabinet holes. Do all four screws concurrently and tighten the four screws in a diagonal pattern – don’t tighten one fully, then thread in the next one and tighten it, etc.

    Then, plug in the speaker and replace the back panel/tube cage being careful to make sure the speaker cable goes outside/left of the tube cage (but inside the back panel – it should lay there naturally). Again tighten all screws concurrently and finally tighten fully in a diagonal pattern.

    One note about starting screws: turn them counterclockwise until you feel it drop into the hole. Then screw it in clockwise. Doing it this way will ensure that you ride on the existing threads. If you don’t, you could end up damaging the threads or the wood.

    Now - Plug ‘er in and power ‘er up.

    I found the old RCA 6V6GTAs sound fantastic. I had replaced the V1 12ax7 earlier (doesn’t require a rebias) and it had a very subtle effect. Replacing the power tubes and rebiasing the amp made a big difference. I don’t find the amp to be much louder (although I can’t say what the ‘before’ bias setting was). I do find it to be much clearer and cleaner.
     

  8. Freeway Jam

    Freeway Jam New Member!

    3
    Sep 9, 2012
    Canada
    Bleeding Caps

    "I don’t know if that is true, and there is no fool proof way to bleed the caps, but it can’t hurt."

    Actually, you can drain your caps in a very simple way, but ONLY if the work you are doing does not require the amp to be powered on

    First, many thanks to NEStrat for these terrific, and detailed, instructions, with fantastic pic's of each process, always invaluable when doing these sorts of procedure's, especially anyone doing it the first time.

    Now, onto bleeding the caps. In the interest of accuracy, and more important, safety, rather than me trying to explain what to do to discharge your caps, i think it is preferable to point you to a link at Justin Holton's amp site., that gives you details on how to do it. All you need are some cement resistors, and clips, it is quite easy to do, and discharges them down to, if not zero, the point at which it would just give you a mild shock.

    The Unofficial Fender Hot Rod Deluxe Owner's Guide! | How to Drain the Filter Caps - when you go to the link, go to frequently asked questions, and you will see the sub heading 'Draining your filter Caps'.

    Also, of important note, this is only practical when working on an amp that does not need to be powered up while working on it. So if your amp needs to be powered up while doing the work, then as NEStrat said, indeed, there is no foolproof way to drain your caps. The above is only applicable ON AN AMP THAT IS SHUT DOWN.

    ALSO VERY IMPORTANT WHENEVER WORKING ON ANYTHING WITH LETHAL VOLTAGES !!!! - Even if you use the method outlined in the link, and go through and do as detailed and drain the caps. You should still heed the advice that NEStrat outlined already, and that is: ALWAYS KEEP ONE HAND IN YOUR POCKET when working inside the amp. Always better safe than sorry.

    I just wanted to mention this method, as it gives me a bit of a secondary safety net when inside the amp chassis, having done the cap draining. I still always treat it as though they haven't been, and you should also. But i am not a tech, yet i do like to do some of the more basic DIY things that are not to technical, and this is good piece of mind, for me anyway.

    Hope NEStrat is ok with me jumping into his superb thread with this, i was very happy to learn of it, and maybe someone else new to their amp will too.

    And thanks again to NEStrat for the superb thread.

    Cheers, happy playing all !
     

  9. pauln

    pauln Senior Stratmaster

    Sep 14, 2011
    Houston
    How do you know what dissipation you are subjecting the tubes if you are not measuring the high voltage from the transformer first?

    You don't. You have to do the measurement and then the calculation in order to identify the correct bias for the particular tubes in that particular amp with that particular transformer.
     

  10. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    Freeway: thanks for the kind words! And happy to have helped.

    Pauln: that's interesting. Can you say more about this? Everything I've researched, including what limited instructions I've received from Fender says that it is 40 mV at the test point for this amp regardless of tube manufacturer/specific tube.
     

  11. pauln

    pauln Senior Stratmaster

    Sep 14, 2011
    Houston
    Bias Calc

    Scroll down and look at the charts where the different voltages are shown.
    A particular bias level setting is meaningless without knowing the voltage.
    Because of the windings, transformers have variability in their ratings, and there may be variability in the utility service voltage from region to region. Individual circuit components typically have a spec design tolerance of +/- 20%. So, tubes and bias are not "plug and play". Measurements and calculations must be performed to set bias.

    What I do is use a bias tester with a meter. It has a pair of adapting sockets that fit between the tubes and the amp sockets and takes the measurements right at the base of the tube. I measure plate voltage, decide on what dissipation I want, calculate the correct bias for that dissipation based on the measured voltage, make the adjustment and watch the meter to confirm.

    It is a safe method that does not require pulling the chassis, and it allows you to play through the amp as you are adjusting it to fine tune the sound. I have a half dozen Fender tube amps and I can bias them easily in a few minutes - except the Super Reverb. For that one I have to remove the speaker baffle to get to the adjustment hole under the chassis because one of the speakers is in the way.
     

  12. ripgtr

    ripgtr Most Honored Senior Member

    Feb 16, 2012
    austin
    What Pauln said.

    The dissipation it the amount of heat the tube can safely get rid of. It is a function of power. You need to know the voltage AND amps to know - you know the amps, but the same reading you are getting could be cold or so hot it would burn up the tubes, depending on the voltage. Well, not really that extreme usually, but it could be.

    20 mA isn't all that high anyway. I run my 6v6s at least that hot, hotter, in a deluxe, which is notorious for way high voltage. Hasn't been an issue yet. But I use old RCA tubes, which can take a pounding, or Tung Sol reissues, which actually have a 14 W dissipation.

    But you can't correctly bias without knowing the voltage.
     

  13. Namelyguitar

    Namelyguitar Most Honored Senior Member Strat-Talk Supporter

    Jun 3, 2009
    Mobile Bay
    In this case, I believe you can adjust bias correctly without knowing both the voltage and amps. Fender did their homework.

    Refer to the Fender schematic for this amp which clearly states in the lower right hand portion, "Adjust RB to achieve 40mvDC at TP22." On the schematic, a short line is drawn from TP22 directly to the resisitor (R20) discussed (also, see photos) in the OP's posts. Reference...Title: Service Diagram, combined (schematic) Super Champ XD.
     

  14. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    You actually do know the plate voltage in this amp = 375V. So, a nice, safe idle dissipation of 7.5W results in an amperage requirement of 20mA for each power tube, or 40mA summed for both and a 40mV reading at the test point. (7.5W / 375V = 0.02A)

    Now, there are line voltage considerations as well as component tolerance issues within the amp. Most components are built with a tolerance of +/- 5% (some go as high as +/- 20%). Output transformers are usually spec’ed to +/-5% although some “ok” units reach into the +/- 10%-15% range (higher and you probably have a bad OT). Typically, U.S. line voltages range from 110VAC to 125VAC.

    Assuming there is a plate voltage difference of 375V +/- 7.5%, the dissipation at the tube for a bias setting of 20mA would range from 7.0W to 8.0W. That is a perfectly acceptable idle dissipation range for 6V6s and won’t impact your tone in any way with this amp.

    (This assumes a matched set of power tubes. If they are not a matched duet, you run the risk of one tube running hot and one running cold given that this amp delivers the same amperage to each tube controlled by the common bias trimpot.)
     

  15. Donald77

    Donald77 Senior Stratmaster

    Nicelly done post, super!
     

  16. Vindibona1

    Vindibona1 Most Honored Senior Member Strat-Talk Supporter

    Late again to the party.... But thanks for the detailed illustration and instructions.
     

  17. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    .

    Never too late for a good party! Glad it was of help!
     

  18. David2803

    David2803 New Member!

    1
    Nov 15, 2014
    Belgium
    great post. Followed it completely, but my VU meter indicated 1.98 mV which I tuned to 4.00 mV, but far away from 40mV. I know how to use a VU meter, it was put on V DC 200mV max selection. I have seen on other forums that other people also have a 4mV versus 40mV. A mystery to me.
     

  19. NEStrataholic

    NEStrataholic Senior Stratmaster

    Feb 8, 2012
    Massachusetts
    That's interesting. I'm wondering if either there is a multiplier on your circuit board that is different from others, or if there is a 10 X switch somewhere on your VOhm meter. I would only worry that I was not setting the bias to the correct level. But otherwise I think it sounds like you are good to go!
     

  20. wozzymoto

    wozzymoto New Member!

    6
    Dec 6, 2014
    WI
    bias scxd

    the highest reading i am getting at r20 is 22 mA,,,thats with the pot cranked all the way,,,is my meter set wrong,,i have tried otherways of setting meter but nothing that gives me close to 40,,,,,help please