Partridge style OT 8K or3.5K taps

Discussion in 'Building the Classics' started by Rotorcraft230, Jan 1, 2020.

  1. Rotorcraft230

    Rotorcraft230 Active Member

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    F1DE91D0-C6FB-48C1-85FB-4376BCF86812.jpeg F5EECD44-B6D6-48B0-B4D5-1C8B02C6D869.jpeg Amp is a Trinity Triwatt HIWATT DR103 cline

    So I’m getting ready to hookup the OT and it has 2 sets of inputs. The manual says to use the 8K taps but it also has 3.5k taps as well.
    My question is what would be the difference between the 2. Here si a few pics of the build.
     
  2. South Park

    South Park Well-Known Member

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    It is how much resistance the the speaker has over the power tubes.
     
  3. neikeel

    neikeel Well-Known Member

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    The important thing is the ratio between primaries and secondaries and the type of tubes you are going to use. A normal Hiwatt uses EL34/KT77/6CA7 etc. and would use the 3.5k option.

    The kit you have bought has probably got 6V6 and in that case use the 8k option.

    These are the reflected impedances that the output tubes 'see' and 8k works best with 6V6.
     
    Last edited: Jan 1, 2020
  4. Rotorcraft230

    Rotorcraft230 Active Member

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    Thanks for the answers. Neikeel, I forgot to mention we are putting KT66 in. The amp is designed for both, actually I can run
    6V6
    KT66
    Or EL34s
    There is a switch for 6V6/KT66 tubes
    If I choose EL34 I have to change a resistor. But this amp is gonna get KT66. Would I still use the 8K tap for the KT66 tubes.
     
  5. Rotorcraft230

    Rotorcraft230 Active Member

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  6. ampmadscientist

    ampmadscientist Well-Known Member

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    The 3.5K taps go to the screen grids. The 8K go to the plates.
    It's an ultra-linear transformer. (which is an advantage actually)
    [​IMG]

    The ultra linear configuration allows the bias to be set cooler, but maintains the lower distortion of a hotter bias.

    But more like this actually:

    ultra-linear output.png

    So if your'e running it push pull instead of ultra-linear, you just use the 2X 8K outer taps alone.
     
    Last edited: Jan 1, 2020
  7. Rotorcraft230

    Rotorcraft230 Active Member

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    Excellent answer!! Gonna use the 8K taps.

    I didn’t know how much fun it was building amps and how much I’m learning.
     
  8. neikeel

    neikeel Well-Known Member

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    Unusual to have UL with 43% taps in a Hiwatt clone but useful nevertheless. 8k will be fine with KT66 and 6V6 although you would have to switch taps or switch down to half your speaker value on amp selector for EL34s.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    The ultra linear operation IMHO is much better.
    You can get lower distortion with less heat and save some wear and tear on the output tubes.
     
  10. neikeel

    neikeel Well-Known Member

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    Yes, great for a bass or hifi amp, can't think of many guitar amps that sound great with UL OTs, but I am prepared to be educated?
     
  11. Rotorcraft230

    Rotorcraft230 Active Member

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    Neikeel, I just herd back from the designer of the amp and that is exactly what he said about the impedance. The previous OT he was using didn’t have the 3.5k taps and said if EL34s are used you have to cut 1/2 the impedance. Now that the taps are there use the 3.5k with EL34s and just match up the impedance selector to the cabinet no more cutting in half.
    Ampmadscientist So on the other hand if I’m using KT66 tubes I can use the 3.5K taps to pin 4 and 8K taps to pin 3. That gives me less distortion and. Cooler bias? Not quite understanding the benefits of it!
    Thanks Mark
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Please do not confuse his build.

    The manual/builder's guide tells him exctly what to do and use. It does not use ultra-linear and has a B+ node for the power tube screen supply along with screen resistors.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Is somebody confused?
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I would say he will soon be thanks to your modification implementations to his kit build mentioned above.
     
  15. MickeyJ

    MickeyJ Active Member

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    Using that 3.5k tap like that, pinning them to pins 4 ( and running the 8k taps to pins 3) will have the amplifier running in what's called ' ultralinear ' mode, or ' UL'. UL usually has taps at 43 percent for screens. UL is good for monitor systems, home stereo and any application that doesn't require or would be offended by the associated distortion/compression.

    They run cleaner with more headroom.

    Also, you don't need a choke.

    If I was you , I would decide if I wanted some grit, or not.

    People bag out UL in guitar amps, but they've never heard a late silverface super reverb in full flight, my god what a tone monster ( however they're not at 43 percent, only 12 ) ( you may note that 3.5k is around 43 percent of 8k)

    My hunch is that you'll prefer no UL connections ( you just tape them off and run a choke and hook up pin 4 as ' normal'.

    You can very easily try the amp one way, and then the other and then just decide, or just do what mickeydg5 implies and just build it exactly as the instructions want you to.

    if you want to run kt66 without UL, Just use the 8k taps and tape off the 3.5k taps.

    if you want to run kt66 in UL, hook up 8k taps to pins 3, and 3.5k taps to pins 4.

    if you want to run el34, just run the 3.5k taps and tape off the 8k taps.

    you may try the el34 using the 8k taps ( and tape off the 3.5 k taps ) you may like what you hear.

    similarly, you may want to run the kt66 using the 3.5k taps ( and tape off the 8k taps)
     
    Last edited: Jan 1, 2020
  16. Rotorcraft230

    Rotorcraft230 Active Member

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    Not confused
    Well saidMickeyj, I am doing what the Designer calls out for which is
    6V6 or KT66 use 8K taps only
    EL34 use 3.5K taps only.
    Lots to learn thanks guys for elaborating on all your extensive knowledge I’m having a blast.
    Mark
     
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  17. Rotorcraft230

    Rotorcraft230 Active Member

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    Oh there isn’t a choke on this build. Any reason why?
    Pros?
    Cons?
     
  18. MickeyJ

    MickeyJ Active Member

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    beautiful wiring job btw, be proud of yourself, that's military precision.
    for what it's worth, I'd advise to stay away from metalized films for this build, try and use or consider film foil. Metalized film signal caps have a boldness to them that would accentuate that bulky iron, film foil are a little ' soft' sounding and would even it out a bit, make it more warm.
     
  19. MickeyJ

    MickeyJ Active Member

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    no choke means they want you to use a resistor, resistors are good, nice and quiet.
    and more sag.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    Sounds like they are using a 100W 4 tube ultra linear output transformer for a 50 watt amp.
    That is kind of weird that they would use that particular transformer...
    Well you can use a choke if you want to but sometimes causes a low frequency resonance. So it could be a good thing it could be a bad thing.
     

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