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Champ Reverb Build 6L6/EL34

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

  1. bryankloos

    bryankloos TDPRI Member

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    Hey All,

    I'm working on a build... First real build from scratch using various bits of circuit. I'd love to get some feedback on the design before I start drilling holes in G10.

    Many Thanks,

    Bryan
     

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  2. printer2

    printer2 Poster Extraordinaire

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    No point to have two pots for mid and raw, one 50k would do. How many volts does the 5AR4 drop (less than 20V I think)? Is there much added voltage going to SS diodes? Maybe put them in series with the tube? How much current will be drawn by the screen and the reverb? What will the voltage drop across the 10k be? What will be the effect of the 220k after V3b?
     
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  3. bryankloos

    bryankloos TDPRI Member

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    Thanks for the quick reply... and all very good questions. Again, this is my first piecemeal attempt at building up an amp so I'm new to this... I picked preamp, reverb, and output stages from various donors.

    My goal with the SS and Tube rectification was to be able to experiment with both options, in a switchable manner. I have 5AR4 in the schematic, but may play with other rectifiers to adjust drop to my liking/performance. I wanted to put in a dropping resistor in the SS rectification path to get me "close" to the final voltage of the tube rectifier, so I'm comparing apples to apples. Though this may ultimately defeat the purpose of the SS and its slightly higher working voltages?

    I'm hoping to draw 5ish mA from the screen and a few more through the reverb. I wanted to get the screen voltage a fair bit below the plate on V4 for longevity. Is this wise?

    The 220K of V3b was remnant from maybe the bassman or bandmaster amp? I cant recall where I pulled this from... I was wondering if it was not necessary in this application as it looked redundant to me. What is its purpose, and what would having/eliminating it do?

    I'm trying to wrap my head around this, reading and learning. I appreciate the criticism and guidance.

    Thanks!

    Bryan
     
  4. bebopbrain

    bebopbrain TDPRI Member

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    Ditch the tube rectifier; class A don't sag.
     
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  5. bryankloos

    bryankloos TDPRI Member

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    interesting point... I didn't think about class a being full throttle and not having any sag...
     
  6. printer2

    printer2 Poster Extraordinaire

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    I just did a quick calculation for the 10k, it should be fine. Does Class A sag? Maybe not if you play clean. Not as much as a Class AB but depending on the PS it could drop some. The 220k has to go.
     
  7. bebopbrain

    bebopbrain TDPRI Member

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    A few thoughts about the power supply. I like how you have plate current going through the inductor. Since this is not push pull, you need smooth voltages to avoid hum. Should be good.

    I also like how you have the TBD resistor after the diodes. This does two things: filters the voltage and reduces the peak capacitor current. Big current pulses can make grounding noisy.

    Going farther in the same direction, I might be inclined to use the smaller cap (22uF) before the inductor and the bigger one (47uF or as big as possible) after. The smaller cap conducts for more of the cycle, reducing peak current.
     
  8. Ten Over

    Ten Over Tele-Holic

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    The NFB needs to come off of one of the speaker taps instead of coming off of the speaker, i.e. before the impedance selector. Presuming you are going to eliminate the 220K resistor after V3B, 4.7K would be a good NFB resistor to try between the 8 ohm tap and the 47 ohm tail resistor on V3B. The 22K resistor shown is way too big.

    The 4.7K cathode resistor on V2 is too large and it will cause one side of the reverb send signal to clip prematurely. I suggest a 2.2K.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ime the ground nodes should be separated. Each color on the schematic below should be routed together. They should not be mixed.

    InkedChamp Verb 1.5_LI.jpg
     
  10. AlfaNovember

    AlfaNovember TDPRI Member

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    I like the choke in the powersupply. For experiment's sake, you can get different values of B+ by adjusting the value of the first filter capacitor. Smaller values will bring you closer to a choke-input filter, which was a popular design way back when capacitors were small in value and big in size. The choke, as you know, regulates the AC current drawn into the filter caps, which can make life easier for the PT.

    Second, and utterly trivially - the 100v spec on the cathode bypass caps is needlessly high. Use 'em if you've got 'em, but the cath on the 9-pin minis are only at about 1.8V, so a 25V rated cap is easier to tuck into place.

    I didn't look - Are you elevating the heaters? That's a good thing to do. I usually tie in via humdinger pot from the top of the powertube cathode resistor.
     
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  11. bryankloos

    bryankloos TDPRI Member

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    Excellent suggestions, and thanks!

    I did make note to change the NFB resistor but forgot to update in the schematic. I had 2.2K as a value in my notes, but can easily play with these to get me in a good zone. Thanks!

    Excellent point on the grounding scheme. Out of curiosity, where would you tie the ground of the first 22uF cap in the PS? Would you tie the main power cord earth to the purple dots of the PS or give it its own location on a PT lug?

    Noted on the 100v cathode caps. I have a bunch of 25V that just came in, which I will use.

    I wasn't planning on elevating heater... This would certainly be easy enough to do. I'll play with both options and see what change it makes for hum.

    I will rework the schematic tonight and report back. I appreciate all the help. this has been a fun project so far... Building the cabinet is going to be a different story. Time to learn how the dove tail jig works!
     
  12. bryankloos

    bryankloos TDPRI Member

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    I've updated the schematic with the above suggestions. I also added a voltage divider on the plate of V3b so I can fine tune output into V4. Probably overkill, but will be a good exercise for me to manage gain and output of each successive stage. Is this lunacy?

    Lowerleftcoast.... with your grounding scheme, I'm assuming the second cap in the PS is tied with the first and the PT center tap?

    Any other thoughts or concerns?

    Thanks!
     

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  13. Ten Over

    Ten Over Tele-Holic

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    That voltage divider changes the open loop gain of the negative feedback loop which has a dramatic effect on the amount of negative feedback when the NFB resistors are held constant.

    The 6L6 and the EL34 require different levels of grid 1 signal in order to obtain full output. What level you fine tune for one tube may not be what you want for the other tube.
     
  14. bryankloos

    bryankloos TDPRI Member

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    So then, should I ditch the divider and keep the 100K load and NFB value consistent or stick to a single output tube? I'd love to find values that work for both tubes, and allow them to be swapped...
     
  15. Ten Over

    Ten Over Tele-Holic

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    Ditch the voltage divider, keep the 100K plate resistor, and use the 4K2 NFB resistor. It is a good compromise between the amount of NFB with a 6L6 and the amount of NFB with an EL34. The EL34 will have more NFB than the 6L6, but they are both within the range of typical guitar amps.
     
  16. Ten Over

    Ten Over Tele-Holic

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    Are you using a 5K primary on the OT?
     
  17. bryankloos

    bryankloos TDPRI Member

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    4K on the OT. Its the Allen T011S.

    I've ditched the divider, and now have a switchable bias configuration. I thought I may be able to better dial in each tube with the switchable bias configuration. I'll experiment with actual R values when the amp is in semi working form and I know my voltages...

    Thoughts?
     

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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The first 22uF cap in the PS. It depends on what you have decided for a ground scheme. If it will be a single ground to chassis at the input jack area, It would be on the ground bus between the first and third filter caps. If you will split the ground points, it would be with the *power transformer side* grounds of the power amp. The first 22uF cap is most closely associated with the red dot node.

    By electrical code, the mains ground should be bolted to the chassis on the power transformer side of the chassis. It should not be grounded with anything. Well, if there is a transformer shield, that wire would be with the mains ground.

    The high voltage CT should terminate with the negative of the reservoir cap to keep the PT/reservoir cap/CT circuit as compact as possible to reduce possibility of hum/buzz.

    You will notice some of the *ground triangles* of the schematic I did not mark with a color. The 1M on the input jack usually is on the jack. The ground of the secondaries of the reverb transformer and OT are isolated from the PS grounds due to them being separated from the primary side of the transformers. Due to the NFB, the OT ground could be thought of as part of the blue node. Most of what I have read indicates the OT ground should terminate with the input jack ground point. I have not found that necessary. OMMV.
     
  19. bryankloos

    bryankloos TDPRI Member

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    I’m planning on splitting the grounds.

    I’m good with A and B. For C and D, do each terminate at their own stars respectively with associated grounds as per your diagram?

    For the OT, I was thinking of grounding at the main speaker jack and for the Reverb Transformer at the reverb in jack. Is this okay?
     
  20. bryankloos

    bryankloos TDPRI Member

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    After some heavy reading on the subject of grounding schemes, I think I'm good with the grounds as advised above.
    Its amazing what one can learn with some surfing of the interwebs!

    I think I have a good starting block for the amp, and plan to finalize the layout in the next few days.

    Before I move on to the cabinet planning, lets talk speakers.

    I'm guessing a 10" for this amp. If it were you, what would you install into the open back cabinet?

    BK
     
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