Vacuum tube rectifier directly into choke okay?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by donnyjaguar, Dec 16, 2005.


  1. donnyjaguar

    donnyjaguar Member

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    Is it alright to run a GZ34 directly into a 10H choke without a 10uF capacitor to ground first? (I'm driving the output tubes after the choke). If I put a 10uF capacitor where it normally sits I end up with too much voltage on the output of the choke. Any thoughts appreciated.
     
  2. VacuumVoodoo

    VacuumVoodoo Member

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  3. bob-i

    bob-i Member

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    Ok... it'll work but if your goal is lower voltage there are better ways. The choke will be more effective at smoothing AFTER the first filter cap. Plus dropping voltage through an inductor will create sag, which may or may not be what you're after.

    I'd either replace the PT with the correct voltage or lower the voltage by using a zener diode on the center tap.
     
  4. donnyjaguar

    donnyjaguar Member

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    Thanks VacVoo.

    Roger on the voltage sag & possible hum. Its seems pretty solid though, but some sag is fine. I might try putting a low-value 1uF in there for good measure anyway once I get everything built up. This is a smaller amp and I'm hoping for that vintage warmed-over tone. I thought it would be cool to have no semiconductors in the amplifier this time. The power transformer and choke are from a 1956 piece of laboratory equipment so I thought it would be nifty to have an amplifier with some 50 year old old iron in it so its staying in. ;)
     
  5. Shea

    Shea Member

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    Donny, what you're trying to make is called a choke input filter. A choke input filter will result in lower DC voltage but better regulation than a capacitor input filter. But designing one properly is a little more involved than taking a choke you would use in a standard pi filter and putting it first. You have to make sure the choke can handle the current load for the whole amp. Plus, the amp has to be drawing a certain minimum amount of current at idle for the choke to work. I think generally you can get away with less henries in a choke input filter than you'd need with a cap input, pi filter.

    I've never fiddled with a choke input filter, so I can't tell you any more than that. But if you Google on "choke input filter," you'll probably find some good info. And forgive me if you already knew all this.

    Also, where I said "better regulation," that means less voltage sag. So you shouldn't expect to hear much if any rectifier sag if you go with a choke input filter.

    Shea
     
  6. hasserl

    hasserl Member

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    Yep, Shea is right (as usual!). Lower B+ voltage, but better regulation, less sag.
     
  7. bob-i

    bob-i Member

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    Hhmmm.... looks like I stand corrected, 2 against 1 :eek: . I always believe that choke first meant more resistance before the capacitance evened up the DC. Maybe it's undersized chokes that can cause this issue.
     
  8. Wakarusa

    Wakarusa Member

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    Choke first definitely brings the inductive reactance more into play against the ripple out of the rectifier, but the DC component is still just working against the wire resistance of the coil. The lower B+ (relative to a cap input filter) isn't so much from internal resistance (sag) like you find in the PT and rectifier, but more influenced by the current regulating characteristics of a coil - output voltage can never hit peak input voltage.
     
  9. bob-i

    bob-i Member

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    Thx, that makes since. My old brain can't remember much from EE skool
     

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