Champ Reverb Build 6L6/EL34

Discussion in 'Shock Brother's DIY Amps' started by bryankloos, Apr 13, 2021.

  1. bryankloos

    bryankloos Tele-Meister

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    The sound level out of the speaker is loud. It just gets ratty really real fast.
    I'll finish wiring up the NFB as designed and then take another round of voltage measurements, as I've tweaked the bias settings a bit.
    I will also check all my connections and repost a current and accurate schematic as it currently stands.
    From there, with some help and deeper study, I think we should be able to get this thing dialed in.
    jsmwhite691's thread was very informative and gives me fruit for thought.
    I'd like to marry his approach with some scope measurements to see what unfolds.
    Did I mention I'm a scientist by trade...?

    First things first, I'll button up the prototype and post voltages and schematics.

    Thanks all.

    BK
     
  2. bryankloos

    bryankloos Tele-Meister

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    As promised, I've updated the schematic as it currently sits.
    Most things were as previously indicated, other than the mixing resistor, which in fact is 3.0M as thats what I had in the box.

    Here are voltages as recorded tonight:

    6CA7

    B+A - 343.5
    B+B - 234.5
    B+C - 220.3
    B+D - 213.1

    V1/1 - 133.8
    V1/3 - 1.079
    V1/6 - 137.3
    V1/8 - 1.107

    V2/1&6 - 232.7
    V2/3&8 - 2.02

    V3/1 - 140.2
    V3/3 - 1.14
    V3/6 - 141.0
    V3/8 - 1.19

    V4/1&8 - 13.5
    V4/3 - 341.1
    V4/4 - 224.5


    6L6

    B+A - 348.3
    B+B - 271.1
    B+C - 254.3
    B+D - 245.7

    V1/1 - 160.5
    V1/3 - 1.24
    V1/6 - 157.7
    V1/8 - 1.26

    V2/1&6 - 266.9
    V2/3&8 - 2.33

    V3/1 - 161.2
    V3/3 - 1.31
    V3/6 - 162.4
    V3/8 - 1.36

    V4/1&8 - 16.44
    V4/3 - 338.6
    V4/4 - 264.0

    Hmmmm, nothing seems too crazy here.
    I didn't wire up the NFB, will do so tomorrow. I'm curious to see what a difference the NFB adds.
    Short of a miracle, I'm thinking its time to pull out the scope and signal gen. I need to get a dummy load...
     

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  3. bryankloos

    bryankloos Tele-Meister

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    Hey All,

    I wired in the NFB, playing with a couple values (4.7K, 2.7K) and noticed a marked improvement in the early breakup. I have much greater clean headroom in the amp now, and it is sounding pretty good with the 12" Cannabis Rex that arrived compliments of sweetwater yesterday.

    One thing I have noticed is that the bottom end of this amp is weak/flubby. Mids and highs are fine, if not a bit clangy but certainly manageable with tone adjustments. The bottom is just lacking. Low E just has no authority and is fuzzy.

    How might I diagnose the cause of the weak low end and stiffen it up?

    I'll retrace my tone stack values just to ensure they are correct.

    Thanks!

    Bryan
     
  4. 2L man

    2L man Tele-Holic

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    Single ended bass response can be inadequate because of OT saturation and because of too small coupling capacitor in power tube cathode. Third obvious reason is low or lowish power output.
     
  5. bryankloos

    bryankloos Tele-Meister

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    I’m guessing (and really guessing here) that the low end response is probably a design issue versus the iron. David tells me the OT handles el34 and 6L6 SE very well.

    Looking at my voltages, my suppressor voltage is a fair bit below my plate on the output tube. What could the effect of this be WRT sound? Could this be influencing the lower frequency response?

    I will confirm the coupling cap to the output tube tonight. I’m pretty sure it is as indicated in the recent schematic.

    I do need to change the cathode values in that recent schematic, 250 and 200 currently, as I forgot to update them a couple days ago.

    thoughts?
     
  6. 2L man

    2L man Tele-Holic

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    If your power tube bias circuit uses two 47uF like in last schematic you could connect them both between cathode and ground and use switch only for resistors and then higher capacitance increase bass slightly.
     
  7. bryankloos

    bryankloos Tele-Meister

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    Thanks for the suggestion.

    Interestingly enough, last night I probed around with my scope. Admittedly I'm new with the scope, but it seems with a 5K sine wave, I don't see much wave change when I adjust the bass pot. I'm going to have a good look through the tone stack to make sure all my values are correct. I may have mis-wired the TS???


    I also see what I think is assymetric cold clipping in V1b with moderate volume.

    Assuming the TS is correct, I may try dropping the plate resistor to 81K and to increase voltage and then re-bias to get a little more headroom.

    I guess this is going to turn into a real learning exercise for me. Time to read up on TMB Tonestacks from RR's site.

    At least I'm exercising my brain. This is still fun!
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I’m impressed by your journey on this amp. I probably missed something, but IIRC you were biasing your output at 60-65%? Here's the last mention I saw.

    I hadn’t heard that; seems to describe every Champ ever built. I’d be interested in what our pros here think. Again, I may be missing something. :)
     
    Last edited: Jun 15, 2021
  9. 2L man

    2L man Tele-Holic

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    Obviously because Bass adjust operates on low frequendy ;)

    When wave clips from the other end first it produce "warm sounding" second harmonic. Single ended power stage tends to produce 2nd too so amp can begin to sound too warm. I can't give detailed help but before you begin to tune V2 bias look also what power stage does to a sine wave.

    I like when power stage begins to distort signal before the pre amp. Power stage flatten signal when it produce about max power and then pre amp distortion which come later increase distortion still without increasing volume much anymore although it sounds it does. Obviously there comes bit more power although output peak to peak does not increase.

    If your signal generator can produce higher amplitude you can input it to the later stages but remember to use a capacitor on its hot output lead to cut HV which would "burn" SG. I have 0,1uF/600V on mine.
     
  10. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Yes, a 5k sine wave won't be affected by the bass pot.

    The 100K/1.5K combo usually turns out to sit right in the middle when it comes to bias of a 12AX7.

    Clipping in V1B on "moderate" volumes seems a bit strange, but I've never scoped that circuit so I can't say for sure. Do you have overdrive in downstream stages at that point?
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ime, you have not heard how this amp may sound because it is biased so cold.

    I am curious, do you notice a difference when switched between SS and tube rectification?
     
  12. bryankloos

    bryankloos Tele-Meister

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    Thanks for the replies fellas.


    I'm going to spend an hour tonight with the scope and make some measurements and report back.
    I'm going to make a list of priorities and confirm the TS is working as expected. Then I'll report back with some questions based on my findings of this evening.

    Briefly, I want to:

    1. Confirm operating voltages
    2. Conform the TS is working as designed. I noticed an error on my schematic, where mid pot is 25KA, should be 250KA. I'm pretty sure I built with a 250, but will confirm.
    2B. Run the amp through another cabinet, as the test speaker is sitting on my bench facing up, so bas response could be affected based on no cabinet and speaker loading.
    3. Determine what input voltage is clipping V1A
    4. Determine at what vol setting I get clipping in V1B.

    That's about it for today.

    I'm also wondering about the difference of plate vs suppressor voltage on V4. I may drop the 10K resistor in the PS to 5K to bring up the suppressor voltage... This would also bring up the voltage on V1 and V3 for some more clean headroom???

    I do notice a difference with the SS vs Valve Rectification. I haven't played/listened enough to really comment of the qualities, but there is a dif. I think the operating voltages are pretty close, so maybe it IS the sag. Dunno. Once I get things were I want them, I'll poke around more with the SS/Valve recto and see what I can see.

    I'm currently biased at I think 74% MPD. I want to up that a bit, closer to 85%. I'm not sure if the suppressor voltage will help that?
     
  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    With respect, why don't you want to bias at 100%? All sources I know suggest that as the norm, or at least the starting point / midpoint, in cathode bias? Even if you buy the "upside down tube near the PT" thing (which to me is like saying "it's an amp, in fact a Champ") you can certainly bias at 100% (or more, very safely) to see if the amp works/sounds better working at spec, and then go cooler *if* that doesn't sound too bad and you're still buying the 'longevity' argument.
     
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  14. bryankloos

    bryankloos Tele-Meister

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    Good question... Firstly, I don't have a low enough resistor on hand to get me near the 100% bias. The more I think about it and the more I read, I agree that more current wont hurt my situation. I want to get the suppressor up first...

    I'm going to work on the amp tonight and take a stab at my previous post concerns. All those issues will help me regardless of bias.

    I'll order up some more 20W resistors in the 120+ range for experimentation in the next couple weeks.

    I'm heading out on a weeks vacation this weekend with the family, first real vacation since my son was diagnosed with cancer 2 years ago... I'm looking forward to some quality time with him and the family. The resistors will be here when I get back!
     
  15. andrewRneumann

    andrewRneumann Tele-Holic

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    Yeah... do not worry about any serious tone tweaking until you've got this thing connected to a speaker and cabinet that you like. Same with guitars. If you have multiple guitars that are known quantities, that is better. This is just my opinion--but use the scope for trouble shooting and use your guitar and ears for the rest.

    I think you mean screen voltage, and yes if you increase the screen voltage you will increase current and dissipation given the same grid bias. (But increasing current will increase bias with the same cathode resistor, so it's a balancing act.)

    Have an awesome vacation man. Try not to read any amp books on the beach.
     
  16. bryankloos

    bryankloos Tele-Meister

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    Champ Verb 1.9.JPG Wow, that was enjoyable. Huge difference in sound.

    I dropped the first resistor in the PS to 1.5K from 10K.
    Voltages are V4 - 6L6, V1,3 - 12ax7, V2 12at7

    B+ is now:

    A - 339
    B - 328
    C - 304
    D - 294

    V4 with 200 ohm on the cathode
    Plate - 319
    Screen - 307
    Cathode - 18.09

    This should get me to about 87-88% MPD. Best I could do with the resistors on hand.

    This also beefed up the voltages on V1 V2 and V3

    V1
    189
    1.47
    187
    1.43

    V2
    313
    2.75

    V3
    191
    1.51
    194
    1.55

    I now have MUCH more clean headroom, with distortion staring at about 6 on the volume pot, as measured by my ears. The TS is much more responsive, and the Raw seems much more responsive in a pleasing way. I'm still missing a little bottom end but I'm guessing that will tighten up when I get the speaker in a proper cab.

    150 ohm 20W cathode resistors were ordered tonight.

    I still need to confirm the TS is correct and take some measurements with the scope, but I think I made major progress tonight. I played for about an hour with a smile!
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If you have another 1K5 resistor, parallel it with the 330R bias resistor. You can just clip it in with alligator clips for a test. That should equal about 270R.
     
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  18. Lynxtrap

    Lynxtrap Tele-Afflicted

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    250K plus the series Raw pot, that's a whopper! I would expect the Treble and Bass pots to be quite ineffective, if not useless, once you have that Mid knob turned up about halfways? At that point the Mid control starts acting more like a volume pot for all frequencies.
     
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  19. bryankloos

    bryankloos Tele-Meister

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    The TS was lifted from another amp I built in kit form. It works quite well in that amp, hence why I copied it. It was not my design.
    I'm going to download the duncan TS sim and play around with it in the future. I'm just now starting to study TS topologies/values to understand how they work and why.
    This has been quite the learning experience thus far. I'm still having fun!

    As always, advice and critique are welcomed.

    BK
     
  20. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Just out of curiousity I thought I'd try it in the TS calc with the Fender TS.

    Green is 15K log mid pot on 5, bass and treble on 10.
    Pink is 250K log mid pot on 5, bass and treble on 10.

    I used the extreme B and T settings to show how the mid resistance affects the B and T controls.

    I also tried to sweep the controls.

    With the 250K mid pot on 5, the bass and treble pots each have a range of about 5dB at the lowest and highest frequencies respectively.

    With the 15K mid pot on 5, the range is close to 25db.

    In my own experiments with this tone stack in different amps, I tend to find that the EQ gets too unresponsive for my taste already above 25K mid resistance. That's one reason I'm no fan of this tone stack.

    But I guess it's a matter of taste, and of course what the rest of the circuit is. If it works and sounds good to you, it is good.

    TS.jpg
     
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