Modding champ/princeton tone control

Discussion in 'Shock Brother's DIY Amps' started by klobasa, Apr 14, 2019.

  1. klobasa

    klobasa Tele-Meister

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    I have an amp (not built by me) and I don't have a schematic, but it's obviously based on 5F1/5F2A type of circuit. Controls are tone, vol, master. Tons of gain, 2 x 12AX7, 1 x 6V6.

    The tone control is wired backwards: the higher the tone is the bassier / less bright it is. The tone pot caps were the typical value, don't remember exactly what they are.

    When playing loud, the tone control has very narrow useable area: tone at 8-10 is good, 1-7 too bright/thin/gainy. And this is with humbuckers!

    How to modify those tone pot caps to get more usefull sounds? Which one to make smaller value, which larger? The brightness can be reduced a lot and it would be cool option to have even darker/less gainier tones than now.

    With single coils the amp is way too bright.
    Thanks.
     
    Last edited: Apr 14, 2019
  2. BigDaddy23

    BigDaddy23 Tele-Meister

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    I'd suggest posting shots of the amp internals - Good closeups so cap values can be seen. It might be as simple as changing a coupling cap, V1 bias or the bright cap value (assuming there is one). Also, do you know what the Tone pot value is?
     
  3. klobasa

    klobasa Tele-Meister

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    I did previously take some pictures, but all of the caps were soldered so that the text was in the opposite side so the values cannot be seen. The values were identical to 5F2A (at the tone pot, coupling cap and cathode bypass caps).

    Since taking the picture, I have removed the first cathode bypass cap to also tame the gain a bit. Even less of gain would be good.
     

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    Last edited: Apr 14, 2019
  4. BigDaddy23

    BigDaddy23 Tele-Meister

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    If possible, could take a photo of the entire guts - it will be easier to trace the circuit.

    You could always try a 12AY7 in V1 - In anticipation of the pics, do you know whether there is Negative Feedback used as per the 5F2A? This would be the 22K resistor coming off the V2B cathode connected to the speaker jack?
     
  5. klobasa

    klobasa Tele-Meister

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    I have to open up the amp later again for more pictures. I think there is no NFB, if I remember correctly.

    How would it change the tone pot behaviour if the 500pf and .0047 cap would be changed to higher/lower value?
     
  6. BigDaddy23

    BigDaddy23 Tele-Meister

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    Ok. It will be an interactive combination of the V1 cathode resistor, slope resistor, coupling and bright caps. The former will have an effect on gain quite a bit. Not seeing a gut shot it is hard to say but it could also have something to do with the Tone pot value. Usually, reducing the value gives you a bit better sweep. See if u can determine the pot value.....u may have to undo the nuts and push em through to check that out.
     
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  7. klobasa

    klobasa Tele-Meister

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    Because the vol pot is mostly at 4-5, I guess the obvious thing to try is to take out the bright cap. Basically that should do the trick. I remember having a 5F2A before and when it's NFB was bypassed, I also flipped the switch that removed the bright cap. I remember that did the trick with that amp.

    Any idea what size coupling cap would be fine with 5F1/5F2A before the bass response goes too far/farty?

    I will solve the gain issue with 12AY7.
     
  8. BigDaddy23

    BigDaddy23 Tele-Meister

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    .0047uf would be a good medium. Is the V1 cathode bypassed? That will make a difference too. Check out RobRobs site for info under the 5e3 mods section. Without seeing a gut pic or a schematic, it’s all a shot in the dark.
     
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  9. klobasa

    klobasa Tele-Meister

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    OK, I took the amp apart and of course I forgot to take the pictures :p

    Well, anyways, I took the bright cap out and the result is that

    1. The bass levels are now pretty much spot on correct with all volume levels
    2. The highs are perfect with higher volume, but I could use a _tiny_ bit of more highs when playing with home levels (but since I use compressor I don't have to, because I know it will boost highs enough)
    3. The tone pot has to be linear taper. Tone doesn't change when turning tone pot between 1-6, slight gain change maybe. Tone changes between 6-10. Mostly between 8-10.
    4. I had the V1 cathode bypass cap uninstalled, but I gave it back. Theoretically there should be now more gain, but actually no. I would say that I can crank the volume more to get the same gain like before with tone/vol bright cap and without V1 cathode bypass cap.

    The bypass cap was 470pf. If I would one day want to add different value just to get a little bit of highs into the mix, what value to try (I don't have any components, I need to order)? If I understood correctly, with 470pf the preamp was pushed with too much of midds?
     
    Last edited: Apr 15, 2019
  10. klobasa

    klobasa Tele-Meister

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    300guitars site says about bright caps: "The smaller values will have less effect than larger values. Small values are in the 47pF range and have little effect. Larger values in the 100pf-180pF range will affect the upper treble frequencies. And even larger values in the 250pF-500pF range will affect a wider band of frequencies making the amp sound louder and have the lower treble/upper mids pronounced."

    So I guess I was right. I will try something between 50-180pf.
     
  11. BigDaddy23

    BigDaddy23 Tele-Meister

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    Good to hear you have it sounding better. FWIW, I used a 680pf bright cap in my 5e3 build. You can always get a selection of silver mica caps and try them by jumpering with alligator leads.......use your ears to make the choice
     
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  12. drew1d

    drew1d TDPRI Member

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    Please let me know how it goes.

    My cake pan amp was way to bright. It's kinda my own fault. I got a 500pf Sozo cap and talked myself into putting it in. Like ear piercing brightness. I mellowed it out a bit by putting a 25uf cathode bypass capacitor. Now I can turn it about a 1/3 of the way up and it sounds good. But it's not much usable range. I feel like I based my amp on an $8 part when one for 60cents would have been better. It's probably a great cap if used for good instead of evil. After reading your post, I'm going to go 100pf ceramic disk.
     
  13. klobasa

    klobasa Tele-Meister

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    I will. It will take me week or two, though.

    When I got the amp in a trade (traded without trying!) the amp was very much lacking bass (I had to swap speakers) and waaaay too bright. The overdrive was shrill and piercing.

    Now without the bright cap, the amp pretty much sounds like a hybrid between 5E3 and old Marshall. Just amazing thickness. Only drawback is that there is no room to get brighter sound at the moment. But removing the bright cap completely transformed the amp.
     
  14. klobasa

    klobasa Tele-Meister

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    I ordered the parts and I think the amp is now ready. I did the following:

    -I increased the bass by swapping the .022 coupling cap to .047 --> it was too much --> .033 was the correct value, bass levels are now great
    -replaced the linear tone pot to logaritmic
    -vol/tone bright cap was reduced from 470pf --> 10pf - 68pf didn't do almost anything, almost completely inaudible difference. I didn't order larger than 10-100pf caps and ended up using 68pf and 100pf in parallel. WIth this amp and my guitar (and my settings), the best value falls between 140pf and 180pf.

    I use the vol knob somewhere between 9 o'clock and 1 o'clock and with these modifications the bass levels are nice and the tone pot works really nicely everywhere between 1-10, the best area being between 3-9.
     
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  15. drew1d

    drew1d TDPRI Member

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    Nice!
     
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