Volume Pot gets Gradually Louder, then gradually Lower.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by HaroldBrooks, Jan 14, 2018.


  1. HaroldBrooks

    HaroldBrooks Member

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    My amp Volume starts to gradually get louder as I turn the volume pot up clockwise, then at about 80% or so, further increases in the pot in the clockwise direction actually reduce the amp volume, gradually a bit.

    I cleaned the Pot, but not sure if that started the problem.
    Pot got quitter after cleaning, but still a bit noisy. Could have been there before.

    What would cause that, Just a bad pot, or something else ?

    Amp sounds great either way, just weird having the pot work that way, kinda backwards a bit, but not fully.

    Thank in advance for any help.
     
    Last edited: Jan 14, 2018
  2. pdf64

    pdf64 Member

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    More info about the amp, channel used, control settings, and whether any control setting, channel, input, instrument setting affects the unusual behaviour is needed.
    First thing that comes to mind is that the amp may be oscillating above audio frequency, so how about if treble/mid/presence/bright are minimised? Does the amp's tone go weird and bad when the vol goes high?
     
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  3. HaroldBrooks

    HaroldBrooks Member

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    Not the exact model, but same basic circuit (510-1) :

    http://www.oldfrets.com/Valco/Schematics/Supro_1600.pdf

    Only a volume Pot, no other controls whatsoever. Two inputs, one channel, Volume issue is Independent of : guitar volume / Tone settings, pluging it straight in, or using boxes in the chain before amp. Same behavior.

    It does however, appear to be temperature dependent. When the amp is stone cold, the volume pot behaves normally (gradual increase to max volume), when the amp warms up is when the trouble starts with the volume control going in reverse at the top 20% or so.

    Amp sounds ok at any setting, no weird sounds from high to low, no distortion unless driven. Distortion sounds great, cleans sound great.

    It's just the action of the volume pot when the amp warms. Nothing else seems out of the ordinary.

    I'm guessing it's perhaps a resistor getting hot and changing value ? Or maybe the Pot ? Or maybe a cap ? I checked the bias of all the tubes and plate voltages / voltage drops from the cathode resistors. All was well in that regard. Amp sounds fabulous, and I could just continue on, but something is off with the volume, for sure so I don't want to wait until it becomes a bigger issue.

    I've substituted all the tubes for other known good ones (I have a few Valco amps from the same period), same exact behavior with the 'new' tubes.

    I'm still a novice with this stuff, and Thanks for any help.
     
    Last edited: Jan 14, 2018
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  4. Avatar Tech

    Avatar Tech Member

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    How old are the filter caps? Is it possible that it's just compressing and sagging due to minimal filtering?
     
  5. HaroldBrooks

    HaroldBrooks Member

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    It could be, but even the background noise is attenuated by just raising the volume Pot once the amp warms up. I am going to take a look at the resistors in the circuit, but nothing seemed burnt on my first visual. If I can't figure it out on my own, or if no one has any suggestions, I might just replace the volume Pot to see how that works.

    Keep the ideas coming ! Thanks.
     
  6. jthomas666

    jthomas666 Member

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    Unlikely but: In building a copy of an amp that required an odd pot value I found that a resistor can be put in between the 2 outer lugs of a pot (I believe that I'm correct with that, but it's been awhile) to produce the odd R of the desired pot. Playing around with that at the time, it seemed that the maximum of the R of the pot was reached before the full travel of the knob was reached and that the R decreased a bit after that. It would seem unlikely that the amp was designed and built like that, but when you open up the amp, it would make sense to rule that out. Easy to do with visual inspection.
     
  7. HaroldBrooks

    HaroldBrooks Member

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    Wow, can't believe my Luck ! Got it, it's definatly the Volume Pot.

    I used a can of compressed air upside down to create a freeze spray (Thanks To suggestion from Rob Robinette !!!) and dropped the temp of the volume pot when it was warm and acting up. That fixed it (Temporarily) in a hurry when the temp dropped.

    Have to put a new one in, but 99% certain that is the issue.

    Thanks all for input, or even for taking a look and thinking about it for me !
     
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  8. HaroldBrooks

    HaroldBrooks Member

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    Thanks, it turned out to be the Pot itself, but I know about those resistors, and it could have definitely been that as well. Very good thought, and once again thank you for your help and input.
     
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  9. pdf64

    pdf64 Member

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    Is the vol pot 100k like the schematic?
    If so there may be more gain available if a higher value pot was used, eg 500K or 1M.

    Also, a possible cause of this symptom may be that the tube in the following stage has excessive grid current, eg bias voltage would increase with increasing grid circuit resistance.
     
    Last edited: Jan 15, 2018
  10. HaroldBrooks

    HaroldBrooks Member

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    The 6SC7 PI tube is right after the Volume pot, and I checked the cathode to plate voltage (134vdc) and the voltage drop across the 1503 ohm cathode resistor (1.57vdc), so the current draw for the plate (one of them) works out to 1.57/1503 = .001045 (1 ma), or about .14 watts.

    Should be ok with that as the spec sheet has 2 ma at 250v typical for the plate. http://tubedata.milbert.com/sheets/093/6/6SC7.pdf

    Not sure what the grid current is. How would I measure that ?

    Thanks for your help.
     
  11. pdf64

    pdf64 Member

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    In a simple common cathode stage, the Vdc at the grid should be close to 0.
    However, when the resistance between the grid and circuit common is high, grid current will create some Vdc at the grid.
    Much more than 0.1Vdc may tend to make controls scratchy and mess up the operating point as described.
    So perhaps try measuring grid Vdc at various volume control settings.
     
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  12. HaroldBrooks

    HaroldBrooks Member

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    Thanks ! I will do just that and let you know what I find.
     
  13. TimmyP

    TimmyP Member

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    The input impedance is exceedingly low for a guitar amp. If I am seeing it rightly, it is: 100k in parallel with 20k + 100k//20k = 20k + 16.6k = 36.6k. I would remove the jumper between the 20k resistors, and change the 100k on the top jack to 1M.
     
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  14. HaroldBrooks

    HaroldBrooks Member

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    What would the change you suggest yield, in terms of a specific improvement to the issue I described ?

    Thanks.
     
  15. TimmyP

    TimmyP Member

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    It should have no effect on your issue - I just threw it in as an observation.

    It may be that the unit starts to oscillate once the control is up to a certain point. Perhaps add a small cap across the 6J's plate resistor. If no joy, try one across V2A's plate resistor.
     
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  16. HaroldBrooks

    HaroldBrooks Member

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    I should add, this only happens when the amp is at full operating temp, about 1 hour or so after turned on. When it's cold, everything is ok.

    Thanks for your suggestions and help.
     
  17. pdf64

    pdf64 Member

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    What is 'this', eg changing the pot hasn't fixed the volume issue?
     
  18. Shiny_Beast

    Shiny_Beast Member

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    Sounds like a blocking distortion issue, too much signal overloading the grid of the tube, check grounds. Odd that the grid has a 1m grid leak with a 100k pot leading into it. Maybe I misread the schematic.
     
  19. pdf64

    pdf64 Member

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    In regard of the previously linked schematic, I think that the 1M resistor is the HT feed for the screen grid of the input stage 6J7 pentode.
     
  20. Shiny_Beast

    Shiny_Beast Member

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    t5hat's right I did notice it was going to a different pin, then forgot lol. I never got my head around tubes like that in the pre amp. My 3 watt harmony has some wiring I never bohered to try and understand.
     

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