Swapping 6L6 to 6V6 - change impedance ?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TheWayfarer84, Feb 20, 2012.

  1. TheWayfarer84

    TheWayfarer84 Supporting Member

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    I have a Silvertone 1483 that I replaced the 6L6's with 6V6's for a lower volume around the house. Do I need to change the impedance of the speaker? Stock the amp is a 2 6L6 30W, only running about 350V on the plates and an 8ohm speaker. I do not believe there is a second tap on the OT, but I'm not 100% positive as I am relatively new to reading schematics.

    Thoughts?
     
  2. Blue Strat

    Blue Strat Member

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    Ideally you'd want to double your speaker load (impedance) when going from 6L6s to 6V6s, but it's usually safe in this situation. Make sure the 6V6s are biased properly or they won't live very long.
     
  3. guitarcapo

    guitarcapo Senior Member

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    Usually I've found that using the same OT impedance, the amp sounds less bright when you plug in 6V6 tubes.
     
  4. TheWayfarer84

    TheWayfarer84 Supporting Member

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    That's exactly what's going on. It sounds a little dull.
     
  5. Blue Strat

    Blue Strat Member

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    Maybe you don't like the 6V6s are they're not biased properly. When you change multiple variables at the same time it's often impossible to figure out what's causing things to happen.
     
  6. Sweetfinger

    Sweetfinger Supporting Member

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    I ran my 1483s into 16 Ohm loads for years without trouble. When you look at the schematic, it looks like the amp was designed for 6V6s but then used 6L6s so that the tubes would probably never come back for warranty. The 1483 with its low voltage and tube rectifier is really easy on 6L6s.
     
  7. Blue Strat

    Blue Strat Member

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    A lot of people get away with lots of things. Is that reason enough to recommend it to someone else? (rhetorical)
     
  8. TheWayfarer84

    TheWayfarer84 Supporting Member

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    I have not checked the bias with 6v6s
     
  9. donnyjaguar

    donnyjaguar Member

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    I don't own one of these things but the output transformer is very small for the tube complement. I agree that running with 6L6 at low plate voltage will be quite easy on the tubes. What I don't understand is how it possibly could've been too loud in the first place. :)
     
  10. Tommy_G

    Tommy_G Member

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    This impedance thing makes my head spin. It should be so simple if it were laid out in a stepwise fashion. I think one of devils is in the transformer tap labels which creates a barrier to logical thinking. The other thing is that the corresponding relations: V=IR and Z=V/I aren't intuitive or helpful, but I=V/Z is far more intuitive because we're counting electrons flowing in some direction and we know that tubes produce electrons that end up causing the speaker to move. As an aside, Power = voltage x current (V x I)

    Thinking out loud: Lets start with something that should be basic to grasp...it has been said that when you pull a pair of tubes of a quad to run at "half power" you should mismatch your output to half the amp output impedance.

    So lets do some basic logic.

    Halving the power tubes is the same as halving the amount of current available to drive a speaker. Due to the inverse relation to current, its like doubling the primary impedance....ergo....you'd better have a speaker around with twice the impedance rating....oh but you have the same speaker as last time.....so must move to a tap that makes the tube think its seeing a speaker of twice the impedance....ergo....the 4 ohm tap with an 8 ohm speaker makes it appear to the tubes as if its a 16 ohm speaker. (and I think this last leap is where I get confused....gotta count primary:secondary windings or something because the tap labels are counterintuitive to what the goal is).

    How am I doing so far?
     
    Last edited: Feb 22, 2012
  11. Sweetfinger

    Sweetfinger Supporting Member

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    The 1483 with 6L6s has 350V on the plates with a 270 Ohm cathode resistor.
    It's "little brother", the 1482 with 6V6s has...wait for it.....
    350V on the plates with a 270 Ohm cathode resistor.
     
  12. SGspecial

    SGspecial Member

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    I have pondered the 6L6 to 6V6 swap in a Hot Rod Deluxe. I have read the JJ 6V6S will handle the plate voltage. Concerning impedance there seems to be at least a couple different ideas floating around however.

    Some say that a dummy plug needs to be inserted in the extension speaker jack. Others say it is not necessary. Who is right?
     
  13. donnyjaguar

    donnyjaguar Member

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    Very good! Output tubes like to see an "ideal load". If they don't get one, the output power is less than it could be. If the impedance they are working into is greater than optimum, they'll run out of voltage "swing" sooner. Hence, maximum output power will be lower. If the impedance they are working into is less than ideal, their internal resistance will be higher than the load and they won't be able to serve up the voltage "swing" because they are limiting their own current internally.

    So if your output transformer is 2k primary with four output tubes, each pair of tubes "sees" 4k. If you remove one set of tubes the remaining pair sees 2k. By "unloading" the secondary to the next impedance tap (16 to 8 or 8 to 4 etc) this will be reflected on the primary and you will double the primary impedance and hence provide the remaining output tube pair with the ideal output impedance once again.

    Clear as mud?
     
  14. TheWayfarer84

    TheWayfarer84 Supporting Member

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    Four to two tubes seems to muddy things up a lot more.

    I'll try throwing a 16 ohm speaker in the 1483 and see if that levels things out. It's not so much of only a treble loss, as just a loss of character or dimension.
     
  15. Tommy_G

    Tommy_G Member

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    "Impedance" within a current source isn't a particularly intuitive concept, which is why I tried building the story on "current" arguments alone. I can get that. My brain isn't big enough turn the basic reality of electrons flowing upside down and talk about impedance from tubes. I know the basic relationship between V & I from a tube will imply a theoretical impedance, but I just don't "get there" very quickly, at least can't ensure the accuracy of my logic at this point of my understanding.

    Yet I do understand impedance on the speaker side. Every analogy in physics suggests that to make something move you must first overcome resistance.
     

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