Increase filter cap value on screen supply or leave it stock?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Jul 22, 2008.

  1. brad347

    brad347 Member

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    I opened my Deluxe today and to my horror realized that in my early days of tinkering (about 2 years ago), I seem to have replaced all four filter caps with 30µf caps! The stock values are 16µf for all.

    From what I understand, increasing the values on anything other than the mains filter supply is bad for the sound; from what I understand, upping the value for the preamp supply is especially a no-no.

    So I was going to leave the first stage at 30µf and replace the others with 16µf.

    My question is, is it only that very first cap that gets increased, or do you increase the screen supply also?

    Thanks a lot!!
     
  2. SatelliteAmps

    SatelliteAmps Member

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    It's not necessarily bad in filtering, but certain rectifiers will only work correctly with certain values. Upping them will sometimes make an amp have a better bass response. In a 5E3 using 30uf's isn't going to hurt anything, but it will make the amp bassier than normal.

    Upping values on preamp supply will change things as well, and there are some limiting values on what should be in there.

    The answer to your question is that you can just increase the first cap to get the bulk of the difference.
     
  3. brad347

    brad347 Member

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    I should've clarified, it's an AB763 non-reverb Deluxe.
     
  4. jay42

    jay42 Member

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    I think you have to stop at 40uF for the 5Y3. Otherwise, experiment with it.
     
  5. brad347

    brad347 Member

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    There is no 5Y3 in an AB763 Deluxe.
     
  6. brad347

    brad347 Member

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    I figured it out. In Gerald Weber's third book, he says that increasing the filtering on the screen supply will provide more clean headroom.

    He says the reason to avoid increasing filtering on the preamp stages has something to do with the preamp section being RC coupled.
     
  7. jay42

    jay42 Member

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    I think there's some decreasing return vs. investment past 20uF per node.

    ...and yes, GZ-34 or 5U4GB...hard to remember it all. Those have limits too...just don't remember them offhand.
     
  8. 1guitarslinger

    1guitarslinger Member

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    Gerald is right. When you increase the preamp filter it seriously effects the tone and response for the worse. The bass response is greatly reduced.

    Installing 16uFs in the preamp and PI filters will make a big improvement.

    You could stick with the 30uF on the main filter. I would reduce the screen filter to 16 - 20uF. But that's just me. All of this stuff is highly subjective, and YMMV.

    Experiment (within safe parameters) and see what you think. The styles of music you play with the amp, and the volume levels you tend to play at are factors for consideration.

    I have seen on here where some don't like Sprague Atoms. I tend to not like the sound of anything else. In particular I have had bass ghost note issues with F&T, though their smaller size makes them attractive. But again, YMMV.
     
    Last edited: Jul 23, 2008
  9. darkbluemurder

    darkbluemurder Member

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    He probably meant to say that the cap together with the power supply resistor form a time constant. I can imagine that a larger filter cap in the preamp could therefore slow down the attack on the amp. If so this can certainly be compensated by making the resistor smaller - at the expense of a higher voltage to the tubes which may or may not be what you are looking for. But this certainly has to be considered as part of the whole system. Or was Marshall wrong to use 50uF caps in the preamp throughout? And Mesa used 33uF caps in the Mark series amps and nobody complained.

    Bottom line: if it sounds good to you it is right.

    Have fun
    Stephan
     
  10. Blue Strat

    Blue Strat Member

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    Everything else is negotiable and subjective but observe the max on the B+ specs as listed for whichever rectifier tube you're using. Google "[tube number] data" to find the maximum capacitance value for the cathode of your rectifier tube.

    Note that the first two caps are 2-16s in parallel for a total of 32 uf.
     
  11. SatelliteAmps

    SatelliteAmps Member

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    5AR4 limits for cathode input are 60uf or under. Using a 100uf is not a great idea if you are still using a tube rectifier. Only for a solid state rectifier.
     
  12. brad347

    brad347 Member

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    So there are 4 caps in my amp... Blue Strat, you're saying the first two are in parallel for the B+?

    It doesn't look like that to me on the schematic, but I'm not as slick as some of you when it comes to knowing what the hell I'm talking about.

    From the schematic, the first cap (on the other side of the standby switch) connects to the plates and primary of the OT. The second cap looks like it connects to the screens. The third one connects to the junction of the mismatched resistors that balance the LTP phase inverter (100k and 82k) and the last one looks to supply 230v to the plates of the preamp tubes.

    So where is the other cap that's in parallel with the first one?
     
  13. jamison162

    jamison162 Supporting Member

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    If the 16uF caps are indeed parrallel that would be 8uF not 32uF.
     
  14. Rosewood

    Rosewood Member

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    Parallel doubles the value, series halves the value in capacitance.
     
  15. tmac

    tmac Goldmember Gold Supporting Member

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    Nope, Blue Strat is correct - on capacitance in parallel two 16uf would be 32uf. Series would be 8uf.
     
  16. darkbluemurder

    darkbluemurder Member

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    Sorry, no. Caps in parallel are like resistors in series and vice versa.
     
  17. 1guitarslinger

    1guitarslinger Member

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    Thanks for catching my mistake guys. You're right, 100uF would be seriously pushing things. I was thinking of two 100uF in series for 50uF on the rectifier side of the standby switch, but was foggy an just confusing matters.

    "Or was Marshall wrong to use 50uF caps in the preamp throughout? And Mesa used 33uF caps in the Mark series amps and nobody complained."

    An AB763 Deluxe is not a Marshall or a Mesa...

    Want to experiment? Take a 5F6A, and replace the schematic value 8uF preamp filter cap, with a 20uF or 30uF (first reduce the filtering elsewhere to compensate for the rectifier's sake) and check out the tonal differences.

    Again, it's all subjective, and what is great for one amp may be not-so-great for another.

    "Bottom line: if it sounds good to you it is right"

    Exactly (within reason)
     
    Last edited: Jul 23, 2008
  18. Blue Strat

    Blue Strat Member

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    Parallel caps add, parallel resistors (of the same value) halve. And vice versa.
     
  19. Blue Strat

    Blue Strat Member

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    Yes, that was what I WAS saying until I double checked.

    In the Deluxe (no reverb) there's only one B+ cap. In Deluxe Reverbs and several other Fenders the first 2 caps are in parallel.
     

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