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  1. #1

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    Hey You amp techs! My pal, a professional bassist/guitarist, has a notorious Polytone II with a Sonic Circuit in it. From late 1990's, I don't know. 12" speaker.

    He has used it with guitar but once he took it to a double bass gig. After the gig amp started to act funny: it has too much bass! Even turning the bass pot fully counterwise does not reduce the bass enough.

    I took it to my home to test and when I compare it to my felt era Mini Brute (late '80s/'90s) IV with a 15" speaker it is true: when the Sonic Circuit PT has bass pot in zero (meaning -5, fully counterwise it is) the sound is more bassy than my Mini Brute IV with bass pot in halfway. (With all the other parameters same, in the name of Science!)

    Bass pot is working, no scratching or anything. The parts side of the PCB is towards the amp's inside so I can't see it without disassembling the combo but I took a picture from it with my cell phone and there seems not to be any apparently burnt parts.

    The amp and all the other pots and features work normally. No even scratches from any of the pots.

    Has anybody an idea where to seek a fix this? Some capacitor has tired?

    I was surprised how goods this modern era Polytone sounds. It has THE Sound in it although it has more knobs than the classic era amps in it. Stop the bad rap about them! (Although I don't like the black metal grille...)


    Too much bass from a 'Sonic Circuit' Polytone II?-poramopolytonepcb-jpg

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  3. #2

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    Is it too much bass both with the sonic circuit turned off and on?
    _____________________________________________
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  4. #3

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    The trick is NEVER use the sonic circuit. THE sound is in the regular channel. It's a really good amp.

  5. #4

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    Quote Originally Posted by Stringswinger View Post
    Is it too much bass both with the sonic circuit turned off and on?
    At least there is too much bass without Sonic Circuit, with the Channel 1.

    I have never before used the SC so I try to avoid it.

  6. #5

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    Quote Originally Posted by Greentone View Post
    The trick is NEVER use the sonic circuit. THE sound is in the regular channel. It's a really good amp.
    Yeh. But in this case the bass muchness is just there, in the regular channel.

  7. #6

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    Ah, that's a horse of a different color

  8. #7

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    Quote Originally Posted by Herbie View Post
    Hey You amp techs! My pal, a professional bassist/guitarist, has a notorious Polytone II with a Sonic Circuit in it. From late 1990's, I don't know. 12" speaker.

    He has used it with guitar but once he took it to a double bass gig. After the gig amp started to act funny: it has too much bass! Even turning the bass pot fully counterwise does not reduce the bass enough.

    I took it to my home to test and when I compare it to my felt era Mini Brute (late '80s/'90s) IV with a 15" speaker it is true: when the Sonic Circuit PT has bass pot in zero (meaning -5, fully counterwise it is) the sound is more bassy than my Mini Brute IV with bass pot in halfway. (With all the other parameters same, in the name of Science!)

    Bass pot is working, no scratching or anything. The parts side of the PCB is towards the amp's inside so I can't see it without disassembling the combo but I took a picture from it with my cell phone and there seems not to be any apparently burnt parts.

    The amp and all the other pots and features work normally. No even scratches from any of the pots.

    Has anybody an idea where to seek a fix this? Some capacitor has tired?

    I was surprised how goods this modern era Polytone sounds. It has THE Sound in it although it has more knobs than the classic era amps in it. Stop the bad rap about them! (Although I don't like the black metal grille...)


    Too much bass from a 'Sonic Circuit' Polytone II?-poramopolytonepcb-jpg
    Pots work? so Bass control still cut/boost? dark/brite/normal switch works, everything else works and nothing - switches/ pots interferes with the excessive bass?
    The marked areas look like hack jobby mods/repairs etc. If the soldering was as poor as the parts arrangement/finish, low freq. rumble from bass playing could easily rattle a poor joint loose, just guessing. Id start by poking the caps/resistors i outlined, theres only ca. 15V DC powering the preamp section but be carefull anyway with a open amp.
    Too much bass from a 'Sonic Circuit' Polytone II?-skærmbillede-2019-08-15-kl-22-20-45-png

  9. #8

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    Quote Originally Posted by jazzmus View Post
    Pots work?Too much bass from a 'Sonic Circuit' Polytone II?-skærmbillede-2019-08-15-kl-22-20-45-png
    Yes.

    Quote Originally Posted by jazzmus View Post
    so Bass control still cut/boost?Too much bass from a 'Sonic Circuit' Polytone II?-skærmbillede-2019-08-15-kl-22-20-45-png
    Yes, but does not cut enough nor nearly as much as my Mini Brute IV.

    Quote Originally Posted by jazzmus View Post
    dark/brite/normal switch works, everything else works and nothing - switches/ pots interferes with the excessive bass?Too much bass from a 'Sonic Circuit' Polytone II?-skærmbillede-2019-08-15-kl-22-20-45-png
    No.

    Quote Originally Posted by jazzmus View Post
    The marked areas look like hack jobby mods/repairs etc. If the soldering was as poor as the parts arrangement/finish, low freq. rumble from bass playing could easily rattle a poor joint loose, just guessing. Id start by poking the caps/resistors i outlined,
    You have good eyes! I noticed them too, and thought that seem like a repair or a mod. But my pal bought this as new from Thomann!

    Quote Originally Posted by jazzmus View Post
    theres only ca. 15V DC powering the preamp section but be carefull anyway with a open amp. Too much bass from a 'Sonic Circuit' Polytone II?-skærmbillede-2019-08-15-kl-22-20-45-png
    Thanks for warning, I am! I have built one tube amp (so far) so I try always to be über-careful.

    As You see from the picture the PCB has not all the components where there is a white figure for one. So I thought that maybe this is a default Polytone PCB for all models and all components are not included in all models? This is only a supposing, not a fact.

    Now when I think of this the point is simply that the bass knob does not cut bass enough. I found a diagram for Polytones:

    http://www.modernguitarist.com/wp-co...schematic1.pdf

    Could the problem be with this part of the circuit? Which one of the capacitors is a bass cutter? C7 or C8? They might be same as You marked.

    Too much bass from a 'Sonic Circuit' Polytone II?-polytdiagrbasspot-jpg

    (Sorry about all those quoted pictures! I tried to get a rid of them but couldn't.)

  10. #9

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    Herbie, the schematic diagram is pre-sonic circuit, best practice would be to find the exact schematic.
    The baxendall eq is such a integral part of all era polytones so i doubt they added vital parts last minute like the blops we both noticed.
    Above bass on the schematic is says “cw” = clockwise, so boost is cw cut is ccw. Missing parts/the silkscreen is no biggie, some polytone pcps are generic so a bass treble mid volume amp is built on the same pcb as a amp with dist. Effects loop, Reverb etc etc.
    I dont have a sonic circuit era amp schematic try and see if you can find one, then its much much easier to locate the essential components and trouble shoot.
    edit. The voicing switch ( Dark norm. Brite) its working?

  11. #10

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    Have you tried turning the bass pot all the way to the right, which would normally be max, but could be min depending on the wiring?

  12. #11

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    Did you try giving it a dose of piccolo flute? (I mean, you know, if playing a double bass through it increased the bass response, maybe the correct medecine would be acting on the opposite direction...) Sorry, just a (very bad) joke.

  13. #12

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    Quote Originally Posted by jazzmus View Post
    Herbie, the schematic diagram is pre-sonic circuit, best practice would be to find the exact schematic.
    The baxendall eq is such a integral part of all era polytones so i doubt they added vital parts last minute like the blops we both noticed.
    Above bass on the schematic is says “cw” = clockwise, so boost is cw cut is ccw. Missing parts/the silkscreen is no biggie, some polytone pcps are generic so a bass treble mid volume amp is built on the same pcb as a amp with dist. Effects loop, Reverb etc etc.
    I dont have a sonic circuit era amp schematic try and see if you can find one, then its much much easier to locate the essential components and trouble shoot.
    edit. The voicing switch ( Dark norm. Brite) its working?
    Ok. I'll search the schematics.

    Has anybody in this forum a clue where to try?

    Yes, the voicing switch works as it should.


    Quote Originally Posted by sgosnell View Post
    Have you tried turning the bass pot all the way to the right, which would normally be max, but could be min depending on the wiring?
    Yes. It works as it should work, it boosts the bass.


    Quote Originally Posted by Pierrot View Post
    Did you try giving it a dose of piccolo flute? (I mean, you know, if playing a double bass through it increased the bass response, maybe the correct medecine would be acting on the opposite direction...) Sorry, just a (very bad) joke.
    Hah! Maybe a bad joke but made me laugh!

  14. #13

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    I believe that I have a schematic with the sonic circuit but I am away from my computer. I will check when I get home.

  15. #14

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    Just to close the show: problem solved!

    After some advice from always helpful jazzmus and my amp tweaking friend showed me the way to a site which explained "How the Baxandall Circuit Works" a bit more detailed way:

    Amplifier Tone Control

    There I learned what happens when the Baxandall bass pot cuts bass.

    Then I was helped to a site where one can tweak the circuit virtually and test new values:

    TSC in the web

    I found out that decreasing the R9 of the Polytone circuit deepens the cut curve. So I soldered a parallel 4700 ohm resistor with it and voilá: trick was done! No more too much bass!

    Thanks to everybody involved, again!

  16. #15

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    Resistors can change value, and have a rather wide range of tolerance. Perhaps the original resistor in the circuit has the wrong value. It doesn't really matter, though, as long as the resistance in the circuit now gives you an acceptable sound. Congratulations on finding and applying a fix.