Preamp idle voltages radically different with OD channel engaged

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, May 7, 2021.

  1. NSB_Chris

    NSB_Chris Tele-Holic

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    This has me stumped...
    I built an amp inspired by SLO: https://www.tdpri.com/threads/slo-inspired.1057751/
    The amp works fine. I recently hooked up the tube driven FX loop and I was taking voltages to determine if I was going to change any power supply dropping resistors when I noticed a huge differences in the V2 voltages depending on if the channel was engaged or not. This is with volume and master volumes at zero.

    Specifically: see preamp schematic below. S3 is the OD channel switch and it services both ends of V2.
    For instance, the V2A plate pin 1 voltage reading is 235V with the OD channel engaged and 180V with the OD channel switched out. Any idea how and why that would happen? Up to now I never payed any attention to what channel was engaged when taking idle voltage readings. I am stumped!!!

    SLO Preamp V03.jpg
     
  2. dan40

    dan40 Friend of Leo's

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    Is the switch a activated by a relay or just a simple toggle switch on the front panel.
     
  3. sds1

    sds1 Tele-Afflicted

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    What the frick
     
  4. Nickfl

    Nickfl Friend of Leo's

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    Maybe an oscillation that is only occurring with that channel engaged? Could be ultrasonic and you wouldn't hear it, but it could be drawing enough current to cause the power supply to sag.

    Edit:. I notice the od channel has a snubber cap across the plate of v2B which might be killing the oscillation when that channel is switched in, but the other channel doesn't have a snubber, so that might be why it would oscillate while the OD channel doesn't.
     
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  5. NSB_Chris

    NSB_Chris Tele-Holic

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    It is just a simple mini-toggle switch.

    Exactly! I was about to roll a new tube into that spot when I by chance tried the switch and watched it jump. I checked the pin 2 and pin 7 locations thinking maybe I fried a capacitor and was having DC bleeding where it shouldnt, but everything seemed good there.

    I considered removing that snubber cap because of a comment from @Badside. I ran extra shielded signal runs to quiet the amp and he mentioned that the shielded runs act like little capacitors and in that case he removes the snubbers. Maybe I will do that and see if it makes a difference with this voltage swing.


    Anybody have this circuit built and able to see if this is normal?
    There is nothing really new or novel going on here. The amp sounds great to me so I am not motivated to change it, but I don't like the feeling of not knowing what is going on.
     
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  6. sds1

    sds1 Tele-Afflicted

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    The trouble indicates some amount of unknown current flowing... no I wouldn't feel good about not knowing either.

    Plus, it's super interesting! (?)
     
  7. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Can you make a voltage chart of both situation? And also show a schematic of the power supply with voltages?
    That way we could deduce some more, e.g. where the current is flowing and where not.
     
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  8. NSB_Chris

    NSB_Chris Tele-Holic

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    I have a voltage chart and calculation of currents for the power supply but not apples to apples comparison. I will take the voltages now and post them.
     
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  9. Wally

    Wally Telefied Ad Free Member

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    Subscribed...
     
  10. NSB_Chris

    NSB_Chris Tele-Holic

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    So here are the voltages and my calculations of resulting mA feeding into the supply.

    I took the voltages for the power tubes and the PI but left them out of the tables below to make them more readable.

    Screen Shot 2021-05-08 at 11.07.24 AM.png Screen Shot 2021-05-08 at 11.15.08 AM.png
     
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  11. sds1

    sds1 Tele-Afflicted

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    Hmm

    It's also puzzling that v2b cathode current doesn't seem to vary with changes seen at the plate..

    Isn't it?
     
  12. NSB_Chris

    NSB_Chris Tele-Holic

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    It is puzzling!! That is why I initially implicated the tube and was going to exchange it. I thought one of the triodes was just not doing well. Has to be DC getting in there somehow right?
     
  13. printer2

    printer2 Poster Extraordinaire

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    Put a snubber cap across the plate, heck even a large one, easiest thing to perform at the moment. We only want to see the dc characteristics when switching. If it still does the same thing you know it isn't oscillating. Eliminate possibilities that we do not have the answer to.
     
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  14. NSB_Chris

    NSB_Chris Tele-Holic

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    Got it!!!!!
    When I killed the normal channel switch earlier, I had to reroute some ground leads that landed on the damaged switch and then went on from there. The wire that supposed to connect a S3A terminal to ground was not connected to ground. Was really hard to see as it was routed behind other components. When the OD channel was switched off, V2A pin 2 and R11 were left hanging and flapping in the breeze. That was enough to change the current through V2A.

    Connected it up correctly and it is solid now! Thanks for the help and support everyone!
     
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  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Comment: The schematic above has no ability to switch off the other channel. Implementing switch 3 will parallel the channels and the signals will be out of phase.
     
  16. NSB_Chris

    NSB_Chris Tele-Holic

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    Correct, that is the way the SLO is designed. I originally did this amp so that I could switch both channels in and out independently. After testing, I realized that the switch on the crunch channel was not needed. When the OD channel is active, it is so dominant that the crunch channel has no impact. I detailed that in the build thread and put in a demo. When I went to quiet the amp and shield some of the long signal runs, I accidentally killed S2 with too much heat desoldering one of the pole connections. R19 (2M2) in front of V3 makes more sense if you look at the original circuit. Didn't bother taking it out because it is in there like a tic and I did not want to risk damaging anything else. o_O Although it is now unnecessary, it does no harm.
     
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  17. sds1

    sds1 Tele-Afflicted

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    Hah, I wanted to suggest it was almost like the DC reference was missing, and I looked at those grid leaks as part of the switching... but you said the amp was working well and I wasn't ready to suggest errors.

    Does the amp sound better now though? Missing DC reference at grid will cause some audible issues usually.
     
  18. NSB_Chris

    NSB_Chris Tele-Holic

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    Ding, ding, ding, we have a winner!
    Have not gotten it put back together and on a proper speaker. I think because the only time V2 voltages were out of whack, it was separated from the signal path and the only thing it was doing was affecting the B+ voltages.
     
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