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Thread: Pulling tubes and impedance change questions.

  1. #1
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    Pulling tubes and impedance change questions.

    Hello all,

    With the help of this forum I modified my Peavey Mace a while back to have an adjustable bias and the results are awesome.

    my question is regarding the tubes. the amp runs off 6 - 6L6GC but i pulled the two outside tubes and only run it on 4. I also swapped the 100ohm screen grid resistors for 470ohm ones which are more common in 6L6 based amps. plus i blew one and needed to replace them anyway. would all these changes have an effect on the amps output and impedance?

    the amp is decently old and I don't want to be stressing the transformers too much with an impedance mismatch. i run it into 2 - 8ohm 80watt speakers, wired in parallel for a 4ohm load that i plug into the amp's 4ohm output. Is there any way to calculate any changes that might have occurred? I would imagine it's not running anywhere close to the original 160watts and i don't know what the 4ohm and 8ohm speaker outputs are actually putting out.

    Any thoughts?

    Thanks.

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    Output Transformer

    The purpose of the output transformer is impedance matching.
    The most efficient transfer of power is achieved by matching the load to the generator.
    By dropping out tube sets, you have changed the generator.
    So you have to change the load.
    I would recommend using the 8 ohm output tap.
    Impedance matching - Wikipedia, the free encyclopedia

    I stand corrected. Use the 4 ohm tap.
    See Robert's explanation below.
    Last edited by Jazz P Bass; 02-24-2010 at 02:43 PM.

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    Quote Originally Posted by iwantmypie View Post
    Hello all,

    i pulled the two outside tubes and only run it on 4....would all these changes have an effect on the amps output and impedance?

    the amp is decently old and I don't want to be stressing the transformers too much with an impedance mismatch. i run it into 2 - 8ohm 80watt speakers, wired in parallel for a 4ohm load that i plug into the amp's 4ohm output. Is there any way to calculate any changes that might have occurred?

    Any thoughts?

    Thanks.
    Hi,
    The formula is pretty straightforward,

    R=Sqrroot(Zin/Zout), or R^2=Zin/Zout

    Where : R = Ratio
    Zin = Impedance "seen" by the OT's primary
    Zout = Load impedance

    The output impedance of six 6L6s in parallel push-pull operation is in the 1.7 KOhm range, and, since your OT has two taps ( 4 and 8 Ohms ) we can calculate the transformer's ratios.

    R(4) = Sqrroot(1700/4) -----> 20.6

    R(8) = Sqrroot(1700/8) -----> 14.6

    By removing two tubes you raised the impedance "seen" by the primary winding ( remember OT don't have an inherent impedance of their own, they have ratios ) to about 2.5 KOhms, so, by reversing the formula to solve for the output impedance we have (for the 4 Ohms tap):

    Zout(4) = Zin/(R^2) -----> 2500/425 = 5.8 Ohms

    While, for the 8 Ohms tap we have :

    Zout(8) = Zin/(R^2) ----> 2500/213 = 11.7 Ohms

    So, while it's true that the power transferred from the primary to the secondary will not be at its maximum due to the slight mismatch, it's still advisable to use the 4 Ohms tap IMHO ( connecting the 8 (now 11.7) Ohms tap to a 4 Ohms load would result in worsening the mismatch ).

    Hope this helps

    Best regards

    Bob
    Hoc unum scio: me nihil scire.

  4. #4
    Supporting Member Jazz P Bass's Avatar
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    Impedance

    Thanks for the clarification Robert. (and correction)
    Had the math wrong.

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    Quote Originally Posted by Robert M. Martinelli View Post
    Hi,
    The formula is pretty straightforward,

    R=Sqrroot(Zin/Zout), or R^2=Zin/Zout

    Where : R = Ratio
    Zin = Impedance "seen" by the OT's primary
    Zout = Load impedance

    The output impedance of six 6L6s in parallel push-pull operation is in the 1.7 KOhm range, and, since your OT has two taps ( 4 and 8 Ohms ) we can calculate the transformer's ratios.

    R(4) = Sqrroot(1700/4) -----> 20.6

    R(8) = Sqrroot(1700/8) -----> 14.6

    By removing two tubes you raised the impedance "seen" by the primary winding ( remember OT don't have an inherent impedance of their own, they have ratios ) to about 2.5 KOhms, so, by reversing the formula to solve for the output impedance we have (for the 4 Ohms tap):

    Zout(4) = Zin/(R^2) -----> 2500/425 = 5.8 Ohms

    While, for the 8 Ohms tap we have :

    Zout(8) = Zin/(R^2) ----> 2500/213 = 11.7 Ohms

    So, while it's true that the power transferred from the primary to the secondary will not be at its maximum due to the slight mismatch, it's still advisable to use the 4 Ohms tap IMHO ( connecting the 8 (now 11.7) Ohms tap to a 4 Ohms load would result in worsening the mismatch ).

    Hope this helps

    Best regards

    Bob
    Wow Bob, this is some great info. I took me a few glances to get the formulas straight but I think I understand now. Biggest reason I was asking originally was because I'd like to purchase an atteuner for this bad boy and I wanted to make sure I was matching properly.

    Also, would it better if I wire my 8ohm speakers in series for a 16ohm load and plug into the 8ohm tap (now that it's 11.7)? I've heard that if you have to mismatch it's better to have the OT go into a larger load value than the tap's value instead of a smaller load value if there aren't other options.

    Thanks again!

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    'I've heard that if you have to mismatch it's better to have the OT go into a larger load value than the tap's value instead of a smaller load value if there aren't other options'

    No, that's true for solid state amps but it's the other way around for tube (not that you've got much of a mismatch anyway).

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    Quote Originally Posted by pdf64 View Post
    'I've heard that if you have to mismatch it's better to have the OT go into a larger load value than the tap's value instead of a smaller load value if there aren't other options'

    No, that's true for solid state amps but it's the other way around for tube (not that you've got much of a mismatch anyway).
    Really? I thought it wasn't like that.

    Just thinking about it without much knowledge, it seems to make sense that if we use an amp's 8ohm output with only a 4ohm speaker load, the OT will overload because they can't get it all out. But if it was the other way around with the amp's 4ohm output into a 8ohm speaker load than it can properly operating without overloading.

    Any other inputs?

  8. #8
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    'Just thinking about it without much knowledge, it seems to make sense that if we use an amp's 8ohm output with only a 4ohm speaker load, the OT will overload because they can't get it all out'

    See
    http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch

  9. #9
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    Great read! thank you for the clarification.

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    With doing the bias pot mod don`t you run the risk of over powering the tubes? I have a Mace VT and ask out of caution, I would like to find some mods to make the amps tone a little brighter on the high end, am running into a 412 cab with 4 texas heat 16 ohm speakers, it has a bright overall sound but the highs are lacking. The les paul I have I put a Seymour Duncan Custom P/U in and it has boosted mids and highs already, it seems like the amp itself on the eq for the highs needs about 2-3 more notches above 10 to get better highs. Thanks for any input.

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