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ALL NEW 6SL7 Champ build

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Feb 24, 2021.

  1. Jewellworks

    Jewellworks Tele-Meister

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    some of you have been working with me on recent threads and know ive been struggling with this "Vintage Amp" ive been trying to build, using oddball tubes. turns out it worked exactly as designed, with the tubes i chose, and it sounds terrible. then i switched to different tubes, and it sounded "better" but still not very good. then it was suggested i try a 6SL7 and use the first triode as a cathode follower to run my Bone Ray tone stack, and that sorta worked, but still didnt give me the results i was hoping for. During all this, my layout and grounding scheme was a disaster, and made all kinds of hummz and bzzzezz...
    so im rethinking this entirely. again. after doing some research on this forum, i came across this thread:

    https://www.tdpri.com/threads/5c1-5c3-5f1-5e3-concurrent-builds.335898/page-7#post-4724778

    from 2013 with a design by @motor_city_tele dubbed the "F15". -a take on a Fender 5F1 champ, but using an Octal 6SL7 instead of a 12AX7. i like it alot.

    https://i979.photobucket.com/albums/ae273/brzys/F15_schematic.jpg

    but of course, i had to modify it just a little bit, and i added a simple 1 knob tone control from a Gibson GA-30, which gets the treble boost from a partially bypassed cathode. thought it was a neat idea.
    im also adding a built in 8 ohm, 15 watt attenuator, so i can drive it hard, and still turn it down for bedroom noodling, while the wife watches TV in the other room. ive been wanting to try that.

    attached is my schematic

    6SL7CHAMP.jpg
    and the layout i intend to use. i tried to match a 5F1 layout as much as possible.

    6SL7CHAMP LAYOUT.jpg

    please advise, especially the grounding scheme.

    thanks again
     
  2. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The OT secondaries need to enter the chassis as close to the output jacks as possible. You don't want any of those high current output wires traversing across the chassis. Also, the output secondary ground should ALWAYS be connected directly to the load (speaker jack), and then to the chassis. In other words, the output load current should never ever flow through the chassis. The OT primary should also enter close to the plate and main filter cap connections.

    All of the power supply ripple current flows between the main filter cap ground and the PT CT. So, it's best to connect those two directly at one point, and then ground them to the chassis. Then, ideally, the screen node cap and cathode ground can then connect to that same chassis ground point. The preamp ground buss should connect to the chassis at the input jack.

    Trying to do a fancy ground scheme can get you into trouble if you don't know exactly what you're doing. The main thing that's important is that you want to keep current out of the chassis as much as possible, especially the high current sources. Another important thing is that the input needs to ground to the chassis at the input to keep RF and other EMI out of the input.

    I've built plenty of amps that have had a single power supply and power amp ground point (can cap with a single ground), and a separate preamp ground buss. Those amps have been all been more than acceptably quiet, especially compared to their vintage cousins.
     
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  3. Nickfl

    Nickfl Friend of Leo's

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    If you are doing a split ground scheme, you need to reference booth buses to the chassis. Your layout has the power amp ground bus floating, you need to either reference it to the chassis or connect it to the preamp ground bus or the preamp filter caps have no way to return current to the PT center tap.
     
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  4. Jewellworks

    Jewellworks Tele-Meister

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    @Nickfl , yeah, i saw that last night. along with a few other mistakes on my layout. i had my 150k plate load resistors going to ground instead of the B+. that wont work...
    i actually built this up last night. i tied the PT C/T (at the 16uf cap) to the chassis. fixed the above mentioned plate resistors and adjusted my audio grounds accordingly, made a change or two to the 8ohm attenuator, and fired it up.
    MUCH better results than my previous attempts! much more drive than ever before when cranked. tone control sounds pretty good, but overall, it has a very warm tone. i need to look into voicing a little low end out of it. clean up some mud. and i need to rethink my attenuator. as in, maybe i shouldnt have made the last minute changes... i just kept it turned up all the way, took voltage readings everywhere and put them on my load line chart, and both sides of the 6SL7 are pretty much center biased, maybe a tad warm, but not by much. 6L6 is running a little hot at 19.3 watts.
    that said, im getting some strange nasty distortion when i turn the volume/gain down. i get to a certain point and i can hear a sudden "clunk" in the background hiss or hum, and when i play guitar right at that point, its like the overdrive is in the background behind the cleaner tones, but if i hit it hard, i get a really nasty distortion. i get nearly the same thing when adjusting the tone control, although not as pronounced.
    im wondering if my volume and tone pots are cheap and the taper is weird. or if maybe i have the volume/gain in the wrong place? and when i turn it down, it throws off the grid leak? maybe move it to after the tone control? but keep the 220k on the 6L6 as a safety grid leak?
     
  5. Jewellworks

    Jewellworks Tele-Meister

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    this is something i didnt think of. when i made this chassis, i made it as symmetrical as possible, because i had a certain "look" i was going for. but if it makes for a bad amp design, then im going back to the drawing board, keeping my PT and OT on the same side, put the OT as close to the output jack as possible, and move the tubes to the other side, keeping V1 as far from all that as possible. and keep my zig zagging wires to a bare minimum.
    i had planned on making another chassis for this amp anyway, as my original design has changed so much, i actually lost a tube in the process. so ive got this extra hole and socket i dont need in this current chassis. now that i know my entire chassis design is part of my noise issues, ... -all the more reason to start a new chassis

    I saw that i had the PT C/T floating, with no reference to the chassis. i made that change.

    isnt that what i have? the input jack itself mounts to the chassis, and thats chassis ground, isnt it? then i ran a wire to the preamp/audio ground buss.

    one thing thats always confused me is what to do with the output jack ground? yes, the 2ndary OT ground ties directly to the sleave of the output jack, and the jack is connected directly to the chassis, but does it ALSO need to go to the audio/preamp buss? isnt it already connected through the chassis ground at the input jack, which is tied to the buss?
     
  6. 2L man

    2L man Tele-Meister

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    I build my amps using a single ground bus and connect it to chassis near input jack. The other wire I connect to chassis is mains cable earth wire. All other circuits are isolated. Then no operative current runs thru chassis and chassis acts as a shield against electromagnetic noise and protection against electric malfunction.
     
  7. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    @Jewellworks you may not need to physically move your transformer, just run the wires outside the chassis to hole closer to their destination.
     
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  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    You're talking about the ground return for the NFB? It should be fine going through the chassis. You would only run a separate ground wire for it if you're were going to isolate the output jack(s).
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am not sure the following is included in your build but it is on the schematic...

    The two resistors in the red circle are in series. They total 174K of plate resistance which is ok but FYI the 24K has nothing to do with the B+ filtering.

    The green line depicts a local negative feedback path within the purple circle. I am far from an expert when it comes to local negative feedback but ime this would allow a huge amount of signal to cancel. The values are not near what I am use to seeing for a local negative feedback loop. (This loop is not to be confused with the NFB from the OT.)

    Inked6SL7CHAMP_LI.jpg
     
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  10. Jewellworks

    Jewellworks Tele-Meister

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    @Lowerleftcoast , that 24k/2w resistor was a goof on my part. its not in the circuit. it was a part of my earlier designs, but when i dropped a filter cap, i forgot to delete it from the schematic. but its not there. just the 150k plate load.
    as for the purple circle and green lines, i took this tone control stack directly from "https://chasingtone.com/guide-to-single-knob-tone-controls/". its the Gibson GA-30 tone control.

    GA30 tone.jpg

    now, it did say its sounds 'warm" overall, so maybe this local negative feedback loop is whats causing the problem. i dont know if the Gibson tone control is any good or not, but i kinda liked the idea of using a partially bypassed cathode for the highs. its easy enough to switch to a regular fender tweed tone control.
     
  11. Jewellworks

    Jewellworks Tele-Meister

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    so i came up with a Plan B for a whole new chassis layout, not just a component layout. take a look, tell me what you think

    dECO cHAMP PLAN B.jpg

    PT and OT are off to one side, along with the power cord and fuse. rectifier is close to the PT for short runs, and very close to the filter caps it needs to feed. output jacks are right next to the OT with the 8ohm attenuator on the same side, so all the high power, high current stuff is on 1 side of the chassis and away from the more sensitive preamp components. (i suppose i could lose the RCA jack and just put a right angle plug on the speaker leads, and plug it into the 8ohm jack.)
    input jack is far away from all the high power stuff, and very close to the 6SL7, so i can run a grid stop resistor directly from the jack to grid pin 1. and work my way right to left on the turret board. i havent corrected my ground jumpers on this dwg yet, but its on the dwg for reference.

    i haven built this yet, but before i dive in all head-strong and make a ton of mistakes like i did the last time, i thought id share my plans and get ideas first...;)
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Looking at the GA-30 tone control, I noticed this design is only connected from anode to the cathode. I mistakenly associated it with the anode connecting to the control grid. So for my post #9 I will have to put on my best Emily Litella impersonation and say "Never mind".

    :oops::oops::oops:
     
  13. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    @Lowerleftcoast @Jewellworks

    Well first, the .22uF is a partial bypass that would normally boost the gain of the stage, above 483Hz I believe, when the pot is turned up. At the same time, the same frequencies from the NFB signal will also be reduced, which will also affect the gain of those frequencies. If the gain stage were fully bypassed, the NFB wouldn't work at all. This is why you see an extra 'tail' resistor between the bias resistor (and cathode cap) and ground on the blackface Champ and Princeton Reverb circuits.

    Putting tone controls inside the global feedback loop is a bad idea anyway, for a number of reasons.
     
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  14. Jewellworks

    Jewellworks Tele-Meister

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    Noise noise noise.
    I have redesigned this amp more than 8 times. Changed the order of things, changed all the tubes, and I keep getting the exact same noise.
    I have attached links to 2 sound files. I hope you can hear them and tell me what it is.
    The Amp Noise file is when I turn it up all the way, I get this hiss. The hiss is there when I turn down the tone pot (meaning, I roll off the highs). I also get a nasty distortion. Near the end of the file, I turn the volume/gain pot down, and you can hear it clunk and drop.
    The Noise Pots file is how it sounds when have the volume/gain at about 3/4 up, and it's noisy, hissy and the distortion is horrible.
    Attached is the latest schematic, where I moved the tone stack and volume/gain to the front, and V1B hits the 6L6 without any controls in between.
    I'm also attaching the layout, for reference.
    The only variation is, I DONT have the NFB installed, and I DONT have the 8 ohm attenuator installed. The 2ndary of the OT hits the 8ohm jack directly.
    I also have the ground of the 8ohm jack going to the ground lug of the RCA, and not relying on the chassis to carry that signal.
    Otherwise, this is it.
    I've had this exact same issue from the very beginning. Even with different, known good tubes. In the sound files I'm using a 5Y3 rectifier, which drops the voltage from the 5Z4. The 6L6 is running at 16.7W. the other tubes are 165 V-ish, and center biased.
    What is this!? It's driving me nuts.

    20210226_201935_1614389747618.jpg

    20210226_201917_1614389989817.jpg

    Sound files:.
    https://drive.google.com/file/d/1dqQpu9GwGrCuuQ3PPkqtnocTPeoFShLS/view?usp=drivesdk

    https://drive.google.com/file/d/1dfpvtU132KQsCB9rBGen2EDw0rGpZVb9/view?usp=drivesdk

    HELP!!!!
     
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    What 6L6 is in this puppy? 6L6G? 6L6GC? Vintage or new production?

    Give us the bias numbers. Plate to cathode voltage on the 6L6. Voltage drop across the bias resistor. How many Ohms does the bias resistor measure?

    How about a voltage chart for the 6L6 and the 6SL7?

    What is the heater voltage for the 6.3VAC and 5VAC?

    What is your wall voltage?
     
  16. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Oscillation issues. Hang a grid stopper directly off the grid of each tube, at least temporarily, to see if that helps.
     
  17. Jewellworks

    Jewellworks Tele-Meister

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    vintage 6L6G. Big bottle

    Plate to ground is 330v
    Voltage drop is 23.87v
    Bias resistor measures 476
    Comes out to 16.7 Watts. Max is 19

    I drew up the load lines for both triodes of the 6SL7, and I'm at 1.7mA, with a supply voltage of 263. W/a 2k cathode resistor I'm nearly dead center biased w/my plates at 147 and 152 volts.
    I really don't know how to do a chart for the Pentode. I know robrob has a how-to on his website, but it confuses me.
    330v at the plate
    303 at the screen
    353 at the supply before the OT
    23.87 drop at the cathode


    6.26
    4.93

    120v without it on
    112.8 with it on
     
    Last edited: Feb 27, 2021
  18. Jewellworks

    Jewellworks Tele-Meister

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    One thing I discovered is I wiggled a wire and the noise came and went. I think there might be a hairline crack in my V1 tube socket. I just happened to have an extra socket and when I moved everything over, most , if not all, the exploding noises and hiss went away.
    HOWEVER...
    I STILL get the nasty distortion when I turn the volume down, from full up. Rolling off the treble doesn't seem to help much. It sounds pretty good when cranked, but anything between 1/3 up and FULL up sounds pretty terrible, no matter what I do with the tone control.
    And I tried it in a different speaker cabinet, and it's not the speaker. It's the amp.
    I've been through this so many times... I can't imagine what it is.
    I've got a 47k grid stop on the V1a grid, but nowhere else.
    Why would the nasty distortion be volume dependant, but only to a certain point? Uuuugh
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    A cracked socket could give odd sounds.

    Using Rob's bias calculator
    Plate to cathode voltage = 306
    The calculator estimates a Plate dissipation of 14.5W or 76.3%PD

    To my ear the distortion could be blocking distortion. The hissing and other noise might be the cracked socket but could be a cracked resistor or even the octal 6SL7. They are not as reliable as a 12a_7 for microphonics and noise.

    The 6SL7 has a mu of 70 so it should push the 6L6G hard enough to get blocking distortion.
    I am with you about turning down and getting more distortion though. That is a head scratcher.
    The tone pot when adding highs is increasing gain.
    It is possible the lack of global NFB could cause some trouble and the global NFB inserted on the same cathode with that tone pot arrangement might be causing fits.
    __________________________________________________
    So maybe the 6L6 PD should be increased because SE amps sound/act wrong when they are too cool. Clip in a 1k to 1k5 1W resistor parallel to the 470R bias resistor to raise the Plate to cathode voltage and have the PD rise.

    Try a 470K grid stopper on the 6L6. (A pot could be tried here to dial in a grid stopper resistance.) This would help with blocking distortion.

    Maybe clip in a 47K to 220K resistor across the 1M resistor at the input to reduce input signal. This would reduce gain throughout the signal chain.

    It is also odd to have the wall voltage drop that much with just the amp turned *on*.
     
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  20. Jewellworks

    Jewellworks Tele-Meister

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    it's a real buzzy distortion. Not an overdrive, pleasant distortion. It's hard to judge on a pot, but it's really bad about halfway. My on/off switch is also the volume pot, so in order to measure the pot value, I have to set it where it's the most horrible sounding, and unplug it. Then, because of the way it's mounted, it's really hard to access with meter probes, so I have to remove it, then measure. Last time I did that, it was about 840k out of 1M.

    Not so convinced on that one... I built a 5F2A variant last year with an NFB on/off switch, and I never had any trouble like this.
    And besides, the NFB on the same cathode as the tone may have been the case earlier in the week, but I changed all that and moved everything up front. See the latest schematic.
    And... I changed the entire tone stack AGAIN last night, and I'm now using the fender tweed tone control, and not the GA30 off the cathode. So it V1A plate, volume, fender tweed tone, V1B. I've got both A/B cathodes w a bypass cap. Sounds frikkin great cranked, but pretty horrible everywhere between 1/3 and nearly full.
    I keep wondering if it's a problem w grid leak? It changes with the volume control? Grid block may be it too, I dunno...
    I've tried different 6L6's, and a known good 6V6 (bias and voltage was WAY off when I plugged it in) but I generally got the same results...

    My Power Transformer is a new Hammond 290AX, referring to the voltage drop.

    I'll try all the above mentioned remedies and hope for the best.

    I'm really tempted to buy all new components from Mouser. Most of what I have is already from Mouser, or a local surplus store, but all new, specific for this amp is tempting...
     
    Last edited: Feb 27, 2021
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