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6V6 Brown Super Build

Discussion in 'Shock Brother's DIY Amps' started by jmp81sc, Nov 23, 2020.

  1. jmp81sc

    jmp81sc Tele-Meister

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    HI Everyone,
    Been going through the amp. I cleaned up some of the wiring, and added some better shielded wire to the trem section signal wiring.

    Got all of my voltages pretty close to the schematic.

    Problem now is motorboating related to the trem circuit. When I turn on the amp it works, but no trem. After a few minutes if I max the intensity pot or play guitar through it, it starts motorboating, but the trem works.

    Trouble shooting:

    Double and triple checked wiring, caps and resistors. Re did the long trem signal wires with shielded cable. Checked and reflowed solder joints.

    Pulling the final tube in the trem circuit silences the problem. Pulling all other preamp tubes has not effect. Switched tubes around, no effect.

    Hooked up the preamp section for the Vibrato channel (stock 6G4A) past the trem circuit and the amp acts normally, but of course no tremolo.

    Isolated the Vibrato channel from the circuit, Normal channel works as it should.

    When motorboating occurs, the speed and intensity controls have some effect on it.

    Here is the strange part, if I touch my meter probe to the grid of third triode in the trem circuit it silences the motorboating, when I touch the probe to the plate it starts back up, or any other B+ source in the trem circuit. This is the tube in the circuit that only uses one triode (the middle tube in the 5 triode trem).

    If I touch my probe to the first grid of the phase inverter it starts motorboating. If I touch my probe to the second grid of the phase inverter it silences it.

    Posted below is a video.

    I don't really expect anyone to look at my build and try to find the problem, because the layout doesn't follow any conventional layout, but if y'all could look at the schematic posted above and watch the video, maybe there is a clue there.

    I knew this was risky cobbling together this amp, but I think I'm pretty close to having a working amp.

    Thanks
    John



    VC2.png
     
  2. Huddy

    Huddy Tele-Holic

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    This is such a bummer. Do you think it’s a layout thing? Tube shields do anything?
     
  3. NTC

    NTC Tele-Holic

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    Bad connection?
     
  4. jmp81sc

    jmp81sc Tele-Meister

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    Definitely could be bad layout or bad connection, but I can't find it. Chopsticking has no effect on the motorboating.

    My gut reaction is that the DC voltages from the tremolo are not playing well with the phase inverter, or preamp section. The original design was for 6L6 power tubes and 450 V of B+, I am using 6V6's and 360V B+. I tried to keep the voltages for the trem circuit close to the 6G4a schematic by only using a 100ohm 1 watt dropping resister before the first tremolo 20uf filter capacitor. I tried jumpering in higher values at this position even going up to 27K like the original, but it did not help the problem.

    I used my oscilloscope to try to trace out the signal. I can see the pulsing of the signal at the plate of the first triode of the preamp section, what I'm calling V3 pin 1. The wave form gets more erratic as i probe the signal towards the hi pass and low pass filter network where the preamp section joins the tremolo circuit.

    Can anyone walk me through which points I should be measuring with the oscilloscope to trace down the problem?

    The updated schematic is attached.

    Thanks
    John

    super 5g9 6g3 Corrected v2.gif.png
     
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  5. jmp81sc

    jmp81sc Tele-Meister

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    Been doing more poking around and testing.

    Using the oscilloscope, no input signal, I'm getting DC fluctuation/pulsing throughout the DC supply which is in rhythm with the motorboating sound. When I pull the final tube in the trem circuit, my DC supply is steady.

    I scoped the LFO circuit AC signal, no input signal, and got distortion on the AC wave that the LFO generates.

    I removed the DC supply to the first tube and the motorboating continued. I removed the DC supply to the phase inverter tube and the motorboating stopped, circled in red below.

    Super partial power x.png

    I lifted one side of the capacitors in the PI section, shown below, and tested with an ESR meter and a regular meter that has a capacitor mode. All capacitors tested fine, and all of the resister values are correct.

    Super partial.png

    Early on in this process I replaced this tube socket thinking that it may be bad, no change.

    Since the components and wiring "appear" to be ok, I'm thinking that my layout or the interaction between the trem circuit and the rest of the amp is causing the DC instability.

    I added 1500ohm grid stoppers to the power tubes. Should I try grid stoppers on the last tube in the trem circuit? I'm using shielded cable between the PI tube and the final trem tube.

    Where should I be looking, or what components can I add to stabilize the DC supply? Do I need to lower my DC voltage supply to the trem circuit? I tried to keep it close the original voltages.

    Thanks
    John
     
  6. mrriggs

    mrriggs Tele-Meister

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    I had a similar issue with my Brownface Harmonic Tremolo pre amp. Spent may hours trying to track it down but, unfortunately, never found the root cause. One thing I tried that worked for a while was to add another RC filter to the final mixer plate supply to lower the voltage and better isolate it from the rest of the circuit. The problem eventually came back and the only solution I found was to swap out final mixer tubes until the problem goes away.

    I suspect that it may be heater-cathode voltage related. My amp uses 6N2P tubes instead of 12AX7s. The 6N2Ps have a lower heater cathode rating so I elevated the heater supply to ~90 Volts to try and make it work with the cathode-follower. That puts the final mixer tube dangerously close to it's heater-cathode voltage limit. My guess at this point is that a fresh new tube works until the heater-cathode insulation breaks down and starts conducting.

    Your cathode follower is near the limits of it's heater-cathode voltage rating and is likely crossing the line as it swings up and down with the LFO signal. Try elevating your heater supply to 50 Volt or so and see if it makes a difference.
     
  7. jmp81sc

    jmp81sc Tele-Meister

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    I think I have found a solution. My voltages to the trem circuit are too high. Thanks to mrriggs I started looking at voltages again and I also reread a thread by user Snfoilhat who built the same preamp and trem on a cathode bias 6v6 build. I noticed the Snfoilhat used lower voltages than the original 6G4a super. I was always unsure of how the higher voltages from the 6G4a would interact with the lower voltages from my power supply that I copied from the 5G9 and the small output transformer.

    I rewired my power supply filter cap to feed off of the preamp power supply cap and ended up dropping the voltage to 285V to the trem circuit, vs 357 before. Amp seems to work much better now.

    The trem speed works great, the intensity seems a little weak, and I still hear some woofyness at max intensity, but it is pretty subtle. I am not getting that univibe type trem that I am looking for.

    I know that Snfoilhat played with his circuit to get it at the right place, so I will continue to study his mods and keep dialing it in. I also plan on trying to up the voltages to the trem circuit to see how that affects the trem intensity strength.

    I will not have much time to work on the amp until after the 1st but I will check in when I get it dialed in. And as suggested above I'll do a elevated heater supply.

    Thank you mrriggs for your reply and thanks also to Snfoilhat.

    John
     
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  8. jmp81sc

    jmp81sc Tele-Meister

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    Played around with the dropping resistor for the trem power supply and the amp is sounding pretty good now. I ended up about 20V lower than the original 6G4a for the trem power supply and the amp seams stable and the trem intensity is working better. I will probably do some tweaking of the trem circuit, I need to study what Snfoilhat did in his build.

    I played the amp for a little bit and it sounds pretty good. Very low floor noise. The tone is definitely brighter than the Tweeds and the tone stack sucks some volume. Bass control is kind of weak. But that is just messing around in the garage with lots of rattley stuff around. I will need to spend some time with it in a better test environment.

    The 6G3 channel is awesome. Set to the edge of breakup it reacts so well to pick attack. Hit it hard for some dirt and back off for clean with just some hair on it. I think I am learning that I like the simple one knob tone control.

    With all of the messing around I lost power to my relay switcher for my "gain" mod, so I need to dig in to that, then get my cascade mod worked up. It worked at first but I never got to really play it to see how it sounded.

    I won't have any more time to work on the amp until after the 1st. It's been a great learning experience so far.

    Take care, Happy Holidays
    John
     
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  9. Huddy

    Huddy Tele-Holic

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    Glad you’re finding the time to get it knocked out! Looking forward to some sounds clips when you get the trem sorted to your liking.
     
  10. jmp81sc

    jmp81sc Tele-Meister

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    Hello
    Finally got my build finished. I ended up going out of town for awhile.

    The Brown Super side is kind of quiet. The tone stack and trem circuit seem to suck a lot of volume. It makes for a great practice at home amp, but may not keep up with acoustic drums.
    Right now I have a Guitar Warehouse Veteran 10 and a 10" Rola that came out of the organ. A pair of Ragin Cajuns or other efficient speakers would probably make this very giggable.

    I'm not sure if I will bother with the master volume. Probably hook it up next round of mods.

    I think the 6G4a is probably better as a 6L6 amp with a GZ34 and higher voltages as originally designed. Leo might know have known what he was doing, ha ha.

    The 6G3 side is louder and sounds great. I am not happy with the gain mod I did. I was expecting more boost and grind by adding in a 12AX7 triode but it just seems to add some volume. I need to dig in to gain staging and modify that area.

    My presence control does not seem to have much impact so I need to mess with the NFB value and maybe do a NFB 3 way mod.

    The trem intensity control doesn't do much below halfway, but still has plenty of range, so I'll proably leave it alone. Otherwise the trem is awesome.

    The amp is pretty heavy, but I expected that. The limitations on the S6 organ transformers are that the PT is lower powered and huge, and the OT is small. I think they work better for the smaller 6V6 tweed or brown amps.

    Some photos below. I am missing one Cream knob(somewhere in my garage) so I just put an old brown one on the Master temporarily. I will attempt some recordings when I get a chance.
    Take Care
    John

    IMG_1022.JPG

    IMG_1026.JPG

    IMG_1027.JPG

    IMG_1031.JPG
     
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  11. dan40

    dan40 Friend of Leo's

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    Sweet looking combo... Nice job!
     
  12. jmp81sc

    jmp81sc Tele-Meister

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    Here are some quick demo's I did. I didn't spend too much time dialing in the sound. SM57 close micing the Warehouse Veteran 10 speaker, into a Focusrite Itrack Solo to Garageband on Ipad or Auria Pro. No effects except trem from amp. Guitar is a G&L ASAT Speacial with MFD pickups. Usual caveats, I am an amateur hack.





     
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  13. Huddy

    Huddy Tele-Holic

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    Bruh... that tremolo... Excellent work.
     
  14. The Ballzz

    The Ballzz Tele-Afflicted

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    A little off topic and out of curiosity, did that amp have a bunch of cool pre-amp tubes in it when you got it? On topic, it looks great! Is that wood mahogany?
    Just Askin'
    Gene
     
  15. jmp81sc

    jmp81sc Tele-Meister

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    Thanks Hubby.

    The S6 chord organs are full of 12AU7's, there are 2 12AX7's, some 12BH7's and two 6v6's. It uses a 5U4GB rectifier tube.

    I'm not sure what kind of wood it is. I salvaged the wood from the original organ case. It is a fairly thick veneer over really nice plywood. It can withstand some pretty aggressive sanding. I thought the wood was a lost cause, but when I stripped it, it cleaned up pretty nicely.
     
  16. mjcyates

    mjcyates Tele-Meister

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    Looks great! Sounds great!
     
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