Using a choke vs just using filter caps

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Sep 4, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    Some amps use a choke coming out of the PT secondaries to help filter the rectified power. Others use only large caps connected to ground. Why use one strategy vs the other? Functionally can a choke be obviated just by simply adding more filter caps to ground, and resistors in series?
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  3. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    Choke!
    Unless it's a small amp.
    I think the choke is replaced by a big resistor, not a cap, right?
     
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  4. danlad

    danlad Tele-Meister

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    Sounds like you mean choke input, as opposed to capacitor input.

    You get a way lower voltage by having the PT straight into a choke, but a better regulated one. And the choke has to be able to take the whole amp"s power needs going through it. So a big, expensive choke right after a bigger power transformer to make up for the lower voltage.

    Which tells you straight off why it doesn't crop up in guitar amps much (ever?) ;)
     
  5. RLee77

    RLee77 Friend of Leo's

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    For your perusal and enlightenment, a delightful article from ‘Cathode Ray’, Wireless World, December 1957 (well, *I* thought it was cool anyway, finding an article on this right around my birth year):
    http://www.r-type.org/articles/art-144.htm

    Anyway, choke input supplies are rather non-trivial to design properly, with some “gotchas”, and were only really done for high-power class b type amps.
     
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  6. Gunny

    Gunny Tele-Holic

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    Guess I'm just stuck in 'old school'. I believe in chokes for tube amps. I built one of those kits for a tweed champ. As expected, it had hum. I had a choke from another old amp that I knew could handle the total current load so I fitted it on the outside of the chassis, fed the wires through a hole + grommet, connected it... and that was the end of hum. I do understand that amp builders try to save money by omitting the choke and subbing in a resistor which is cheaper. I hate hum, so that's why I'm willing to invest in a choke.
     
  7. HaroldBKNY

    HaroldBKNY TDPRI Member

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    What about using a choke between 1st and second filter cap, vs using just a resistor between first and second filter cap?
     
  8. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Choke input is a choke before the first cap, and not really a guitar amp thing. I'll attach the Hammond rectifier cheat sheet and you can see how the voltages and currents work out.

    Choke as first smoothing stage (LC filter) like Fender Deluxe Reverb. If you want you B+1 and B+2 to be as high as the power transformer can possibly get you, then use a choke here. You get close to your target of low-pass filtering down to 2Hz or whatever without the huge voltage drop that a resistor carrying all the screen current and preamp current would drop with the same smoothing as a choke.

    Resistor as first smoothing stage (RC filter) like Fender Princeton Reverb. Inexpensive compromise on clean headroom and 'perfect' screen voltage. In hindsight, this is going to sound better to modern ears because screen compression is something some people like the sound of, and a single 3W resistor is way cheaper and takes up way less space than a small choke. Merlin makes this point somewhere in his documentation.

    All of this is said with the assumption that the capacitance is constant, which was a pretty reasonable assumption back in the day, when caps were bigger and doghouses were already pretty full. Instead of higher R or L you could always go bigger on C if you had the space or the budget for bigger caps.
    Hammond_Rectifier_Guide.png
     
    Last edited: Sep 4, 2019
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  9. danlad

    danlad Tele-Meister

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    You'll get a smoother supply for your screens and preamp for less of a drop in voltage with a choke. Hopefully meaning a bit more clean power

    Or, depending on your goal, you turn it round and say you get a more dynamic degree of screen voltage drop with a resistor, hopefully translating into some kind of pleasing response.

    Obviously all depends on the resistor value, choke value etc etc.
     
  10. Nickfl

    Nickfl Tele-Afflicted

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    The simple version is that a choke gives you better power supply filtering with less voltage sag as current draw increases. But there are a lot of variations in how a choke can be used.

    It's important to understand that there is a difference between a choke input power supply and a power supply that uses a choke. A choke input has the rectified voltage running through the choke before it sees a filter cap, this gives you a very well regulated and much lower B+ voltage. You will probably never see an true choke input filter in a guitar amp. What you will see instead is a capacitor input filter that uses a choke, which is a fundamentally different thing.

    There is also the question of whether to use the choke for the whole power supply or just the supply after the power tube plates. Most of the time chokes in guitar amps only filter the power supply after the power tube plates because this puts less current through the choke and allows the amp to use a much smaller and cheaper choke while still getting the benefit of extra filtering and less sag. Some amps do use a choke for the whole supply, including the power tube plates, for example the 5e8a. However, even in this case, where the choke sees the current for the while amp, this is not a choke input filter as there is a reservoir capacitor in front of the choke.
     
  11. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    I think this is the typical guitar amp use of a choke, always used in 18w/ 45/50w /100w Marshalls up until they cheaped out in the JCM900 series, Fender in the amps over 12w, and the Vox AC15 and AC30.
    Not much good reason to consider not using a choke in a bigger amp unless to save money?

    I didn't know this about the 5e8a.
    I assume the choke is quite a bit bigger in that application?
    Can't find many pics of original 5e8a chassis' to see the physical size of the choke.
    I'd guess it's a sort of anachronism from a time when guitar amps that powerful were a new thing and the designs were evolving.
     
  12. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Interestingly, in my building of "Turbo Princetons " over the years. (6L6 power tubes, large transformers up to Bassman/Super Reverb size) I never used a choke, but did vary filter cap size. Frankly it seemed to work fine, I'm really not hearing anything bad. I've often wondered what the tradeoffs are.
    Using the Princeton circuit with NOT upgrading to a long tailed PI seems to produce a bit of old school grit in the tone. Just a hair of "rasp". At least I've attributed it to the Princeton PI, but maybe it has something to do with no choke also? Maybe for this use more "clean power" is not what is wanted sometimes.
     
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  13. peteb

    peteb Friend of Leo's

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    I would agree with that.



    Location wise, a resistor is seen to replace a choke, probably for voltage drop, but functionally, it would be a cap.




    Look at the 5F2. The choke supplies the plate.


    Then look at the -A, the choke is gone and one more filter cap is added in its place.


    A resistor does not filter, in my opinion.



    9BBEDDA4-B2B1-4BD7-B11A-D20C2C6BBB4A.gif BDCA5D88-32E2-4B0C-8881-A1DF12BB15CB.gif
     
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  14. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    A resistor, a capacitor and a choke all do not filter on their own.

    You need any combination of 2 of those in series and tap the output signal from the middle. Then all combinations 'filter'. Two of the same type only have a fixed amplitude effect (voltage divider), though; as the capacitor and the choke have a different frequency dependent impedance and the resistor's is not frequency dependent, a mix of any types can make a frequency dependent voltage divider: a true filter.
     
    Last edited: Sep 5, 2019
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  15. peteb

    peteb Friend of Leo's

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    Thanks Tom, I don’t disagree with any of that.



    A cap passes AC and blocks DC.

    A choke passes DC and impedes AC.


    Passing something and blocking something is what a filter does.

    Resistance acts on AC and DC the same.






    I see what you mean that resistance is crucial to filtering a specific frequency.
     
  16. peteb

    peteb Friend of Leo's

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    One benefit of a choke is that it adds filtering with little voltage drop in the B+.

    Adding resistance or capacitance will lower the available B+. Added resistance would drop the voltage more than a choke, I’m not sure How added capacitance compares with the added inductance of a choke as far as affecting the B+.
     
  17. gusfinley

    gusfinley Tele-Holic

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    We talked about the function of the capacitor in another thread about tube rectifiers and their limits on capacitance.

    A capacitor stores energy in an electric field. It acts as a short until it is charged, then it acts as an open circuit.

    A choke ( inductor ) stores energy in a magnetic field. It does not allow current to flow until the magnetic field is fully charged. So it acts like an open circuit first and then acts as a short circuit.

    The combination of resistors, capacitors and inductors create filters which control charge/discharge rates and resonant frequencies.

    There's a lot of goofy math involved,

    If you want to play around with types of power supply and see what the effects are check out Duncan's Power Supply Designer.
     
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  18. David Barnett

    David Barnett Doctor of Teleocity

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    But they've also decoupled the screen grid with an RC section.




    Not by itself. But an RC section does filter and decouple.
     
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