Champ Reverb Build 6L6/EL34

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

  1. rdjones

    rdjones Tele-Meister

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    You should have some capacitance after the diodes to smooth out the rectified wave for a more accurate DC reading.
    Measure both AC and DC to see how much of the reading is ripple (AC).
     
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  2. bryankloos

    bryankloos Tele-Meister

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    Yep, you were correct. With the filter caps and choke I'm about 416 vdc with all tubes conducting other than the output tube.

    I still need to wire in the switchable cathode caps and resistors for the output tubes.

    What is a realistic plate dissipation for both the EL34 and the 6L6? I want to bias for both good "longevity" and also sound. Is 70% a good goal?

    Thanks.
    Bryan
     
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  3. 2L man

    2L man Tele-Holic

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    100% is safe A-class bias when power max is there where bias point meets the loadline and both down and up current the plate power gets lower.

    Use this following links Universal Loadline Calculator drawing exact loadlines and it is easier to adjust bias point. You get very good result without an oscilloscope measuring for guitar amp use. For HiFi amp using oscilloscope is needed.

    When A-class is biased to the middle of the loadline it produce most "good quality power". In practice when bias current flows thru OPT there comes energy which can double the operative voltage. EL34 plate voltage rating is 800V but I don't remember 6L6 so check it so that it is double the B+ you have.

    For cathode bias the anode voltage is B+ minus g1 and voltage loss what comes on OPT primary coil so you need to know your B+ and them can set anode voltage right. Also Screen voltage needs to be set because it effects to g1 voltage which then leads to carhode resistor value.

    Also Screen current sums to cathode current so carhode resistor value is bit smaller. But when actual tubes have tolerances calculators never produce exact results but it is possible to get very close. My last SE build bias came just right 22W for 5881 and when I installed 6L6A it was 18W.

    Loadline calculator is best tool for us tube amp builders so Giuseppe Amato has done excellent work.

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/
     
    Last edited: Jun 6, 2021
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  4. bryankloos

    bryankloos Tele-Meister

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    While I'm still waiting for shielded wire for input and vol pot, I did finish wiring up the amp and have initial voltages to report.

    V1 A
    158v
    1.327v
    V1 B
    155v
    1.35v

    V2
    272v
    2.66v

    V3 A
    163v
    1.34v
    V3 B
    163
    1.416

    V4
    342v Plate
    271v Screen
    18.01 Cathode (currently 300 ohm)

    Id like to have a few more volts on V1 and V3, but I'll wait for you all to chime in.
    Initial thoughts?

    Thanks,

    Bryan
     
  5. 2L man

    2L man Tele-Holic

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    I recall your OPT impedance is 4k so loadline/operation point comes this where I also used 6L6 which is 19W tube. Bias point is not center and this limits non distorted power to about 7W which reads on right as A-class power. If OPT can take more than 60mA you could increase cathode current to get more headroom. For distorted guitar sounds off bias point can be just fine.

    For higher power EL34 and 6L6GC tubes you don't need to make plate voltage lower but for 6L6 perhaps little so that bias power is not too much over 100%

    PHOTO-2021-06-07-16-06-35.jpg
     
  6. bryankloos

    bryankloos Tele-Meister

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    Thanks for the information and lesson. I have more to study.
    While the PT is able to offer more current, I'm not trying to achieve maximal clean headroom. That is, I would like the amp to break up/distort at a "reasonable" volume.
    I'm not sure if I want all power tube distortion or a combination of preamp and poweramp distortion. This is new to me in design and something I'm not familiar with...
     
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  7. 2L man

    2L man Tele-Holic

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    Obviously using that bias point your amp begins to produce 2nd harmonic distirtion which is called warm around 7W when other wave end flattens when current approach to zero. Then when drive is increased there comes more power up to 11W until other end meets max current but this needs current incease above 60mA. Possibly there comes even more power if signal turns more to a sort of square wave but depending output transformer saturation the current it can become a limiting factor.

    Single ended OPTs which can handle 80mA are quite heavy about 3lbs and up.
     
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Why would you want more voltage on V1 & V3? What are your goals?

    I don't think you are running near 100%MPD. Clip a 1.5K resistor in parallel with the 300R bias resistor and get new readings. The resistors in parallel should equal about 250R.
     
  9. bryankloos

    bryankloos Tele-Meister

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    Ya know, that's a good question. I recall from my other kit builds the B+ being a bit higher on the preamp tubes than I have here, but then again that was probably B+ and not plate voltage. I guess I'm in a decent area for V1 and V3, so I should test with the current volts.

    I am curious about V2 though... I tested with a 12AX7 and the design had a 12AT7/ECC81 in that position. I tested with the 83 as it was in front of me on the bench. I need to see where the 81 biases out with the current plate voltage and cathode values.

    The load line theory is still new to me and I'm trying to study up to better understand. I think the above reference is incorrect at I technically have the 6L6GC tube, not the metal 6L6. I think I'm closer to 65% MPD, correct?

    Previously I just aimed for common use parameters per tube data sheets. So, I'm in somewhat uncharted territory here, hence asking for help/guidance.

    My goals are for this amp to have some decent clean headroom, but also have a nice breakup as I get higher on the volume. Id like a mix of both clean and crunch. I'm not entirely sure how the preamp distortion versus power-amp distortion lends to my goals.

    I also want to learn how to use my new scope. Its been ages since I've used a scope, and I'd like to learn to measure and tweak the various stages with the use of the scope. I look at this as a learning opportunity, and hopefully with some guidance I can better educate myself and then not need to bother you all.

    Relative to goals, are there any more direct questions that I could answer to better guide guidance?

    Again, I appreciate everyone's time and effort.

    Thanks!
     
    Last edited: Jun 7, 2021
  10. 2L man

    2L man Tele-Holic

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    Yes I assumed you have 6L6GC tube when you also planned to use EL34.

    I believe there is some kind of consensus that biasing a single ended A-class amp only to 65% power does not produce what we are after on tube amps but I admit I have not bothered to test it myself.

    There are still 6L6G series available which are 19W stubby glass tube and coke bottles too but honestly I like 5881 reissue sound and price more.

    This link on bottom goes to loadline calculator which Giuseppe Amato has done and I think it is excellent because in a minute I can see what loadline and operating point looks. Just for interest I usually input data the other builders have measured on their builds.

    There are few helpful things: When tube is selected there usually come default values for A/B operation but tab is Single ended. When it is changed to Push Pull load impedance doubles.

    SE amps bias point comes good when A-class power window is about the same the Max power is and when loadline meets dotted max power line. Or goes bit above which is said to be OK as well.

    On cathode bias the designer must understand that B+ in actual amplifier is Anode voltage + Grid voltage when this calculator is used. Also good to know and test how Screen voltage effects to Grid voltage just like it does in amp circuit.

    Most tubes grid lines go up to zero volts but some to positive values which 6L6 does. I don't fully understand how it should be reacted when designing so I won't say more of it other than I just have copied resistor values from schematics where the same driver and power tube was used :)

    When Headroom is inputted (which for star you can use 50V less than Anode voltage is) there comes also distortion fields. On SE I set headroom so that 2nd and 3rd harmonics come about the same just that I can compare it to other tubes. For PP there has come only 3rd harmonics but it is still one variable.

    And when it comes a mess just select a tube and start from the default :)

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/
     
  11. bryankloos

    bryankloos Tele-Meister

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    Regarding MPD, I've been told (from a couple well know and regarded builders) to bias closer to 70% in an amp where the tube will be sitting upside down and close to the PT.
    I guess its a function of heat and longevity.

    Sonically, what are the pros/cons of biasing the tube hotter vs cooler? Is it simply headroom, distortion and power?
     
  12. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I went around the world trying things and experimenting with this amp, a 5f2a Princeton with bigger OT and 5881. You may find something in the thread that is useful. https://www.tdpri.com/threads/5881-tweed-princeton-2.1067115/

    And from the first one I built here. More "is this going to work?" on this thread. The other one I linked to first has a lot more "what can I do with it now?" trial and error involved.
     
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Speaking in generalities, increasing the voltage on the preamp tubes, increases clean performance. I remember it best by thinking of a Twin Reverb vs a Tweed Deluxe. The TR has high voltage on the preamp tubes and the TR is clean. The Tweed has low voltage on the preamp tubes and it is not so clean. There is more to it than that which can be explained with tube loadlines/operation points but as a general rule it is mostly valid.

    Using different type preamp tubes will result in different voltage readings on V1 as well. For instance my 5E3 with a 12AX7 pins 1 and 6 meter at 166V where with a 12AY7 it meters 130V.
     
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  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    It definitely makes a difference. And on that last 5881 Princeton I built, the biggest thing that finally pushed it over the edge to where I wanted it was changing the stock 22k dropping resistor to 10k - very noticeable difference. I think maybe that circuit has so much breakup & drive on its own that the higher preamp voltage is a winner there. Next one I build will probably just start with Blackface Champ values there - I think that's 1k & 10k vs 10k & 22k, but don't hold me to it without checking on it.
     
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  15. bryankloos

    bryankloos Tele-Meister

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    Thanks jsn619. I’ll read through your posts and see what I can gather.

    Good news is that the amp works. I wired up the volume and input and tried a few strums. Seems to work pretty well, other than some hiss from the test speaker (old POS from a mid 80s peavey classic). No hum, just some hiss.

    I still need to wire in the NFB and wire the reverb pan to test verb, but I’m hopeful that will work as well.

    Once I have a little time with it I’ll post back with questions and concerns. I’m sure I still have a fair bit of tweaking ahead of me to dial things in.
     
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  16. bryankloos

    bryankloos Tele-Meister

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    Hey guys,

    I hooked up the verb tonight and dialed in some waves. It works. This is good.

    while listening through my crap test speaker, the amp is clearly lacking some early clean tone. I’ve also got some hiss I’d like to sort, but no hum at all. In think the extra CLC before the plate was helpful. I’m going to go back and check for dry solder joints tomorrow.

    The amp moves into strongly distorted and floppy with little rotation of the volume knob. I dont have the actual knobs on but it’s hard for me to get any clean tones at all. With the raw pot at 0 I get the cleanest and times but still distorted. I’m guessing too much gain in the preamp stages.

    I also need to wire in the NFB and see what that does to the sound, but from the sound of things I’m
    going to have some work to do…

    Does anyone want to walk me through checking gains and waveforms with the scope and signal gen? I’ve clearly got a lot to learn.

    Suggestions on where to start?

    Thanks.
     
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  17. jsnwhite619

    jsnwhite619 Friend of Leo's

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    If you have NO nfb right now, a lack of any clean headroom is to be expected. I wouldn't try to change anything in that regard until you get the NFB finished.
     
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  18. 2L man

    2L man Tele-Holic

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    I measure amp power driving it to a power resistor load. I set potentiometers first to the middle and adjust signal generator 1kHz signal output so that output begins to flatten. Then I test if I can improve output adusting Gain/Volume pot and SG output because signal can begin to clip on pre amp before G/V pot. But when sine wave output begins to clip I take signal down a little and measure peak to peak and calculate power which should be very close the RMS

    If other end of a SE amp waveform clips first I connect probe to power tube anode and look it. That is interesting when peak to peak there can come almost double the B+ is and this is possible when bias current "pre loads" the output transformer and extends the voltage sweep. If other end clips much before other I have adjusted bias point if the OPT current rating has room to increase bias if bias needs to come hotter. Then possibly voltage needs to be lowered so that tube power stay around 100%.

    I also test what output comes driving 100Hz and 5k and 10k and test effect of Tone pots.

    I also look what output looks when driving it to very distorted. Some amps produce quite good looking square wave until it turns worse and square wave pp is not much more than the sine wave pp was but when it distorts bad there can come even higher peaks.

    So far I have changed only one pre amp stage operation point on my builds afterwards because usually I have used common schematics which works but principle is the same, looking which end clipping begins and then change cathode or anode resistor or both. When pre amp tubes have resistors the voltage sweep stay below B-voltage the stage has. If amp has two Volumes/Gains it should be possible to distort output using either while keeping SG signal the same and it looks like pre amp Volume cause more distortion than Master Volume.
     
    Last edited: Jun 10, 2021
  19. Lynxtrap

    Lynxtrap Tele-Afflicted

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    That doesn't seem normal IMO. Sure, wire in the NFB first, but that shouldn't have such a huge impact. This preamp does not have that much gain, due to the tone stack and the voltage divider formed by the 3.3M resistor and the paths to ground around the reverb pot.

    Do you get volume out of the speaker, like how loud does it get?
    Is the circuit around V3A (ie. the 220K resistor to ground and the 470K to the Mix pot) connected?
    My first thought was that you might be feeding V3B with full output from V1B, but that would also correspond to the amp being fairly loud with the volume set low.
     
  20. Mongo Park

    Mongo Park Tele-Afflicted

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    You could wire in a pot for the negative feedback and it can be used to dial in some clean. Which goes against the saying” you can add dirt but you can’t add in clean”. Or dial
    Ot in and replace the pot with the equivalent resistor So it would be a cool feature. Yes you will have to sort out which has to much overdrive preamp or power tube. From there you can dial it back to taste. And sure like by low sell high simple phrase, a little bit of work to follow as jsmwhite691 has documented, his thread might offer some insight where to start.
    If preamp bias tubes middle of the road will be a good start. Voltage divider before the power tune, common after drive stages in other amps, can help. 5f2a is a simple minimal parts amp so this was not needed on the original design. You have stepped away a bit form the original so this might be helpful to dial it back in. Also a resistor on the
    Screen grid of the power tube can change the nature of the tube overdrive. The screen grid should be a few volts Lowe than the plate. This is also lacking on the 5f2a the voltage comes from a lower node to the screen grid to give it a lower voltage but with a different power tube it might not be so. These voltage also swing and are not constant. If it is not you fix you are looking for it will Halle you to tweak the power tube tone overdrive “flavour”.
     
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