Randall RD45H

Discussion in 'Reviews' started by Tetanus, Nov 7, 2016.

  1. vonno

    vonno Member

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    I bought a 3 years old used RD45, maybe a tube replacement could be a good choice.
    Do you have any info from web or constructor about bias adjustment, if it is required? Did you do on your amp?
    Anyway the sound is still good, a little dark (I know it is characteristic of this amp). Do you think with 5881 have less low frequencies?
    Thanks!!!
     
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  2. Robstafarian

    Robstafarian The Good and Wise Call Me “Rufus”

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    5881 tubes have much the same low end as 6L6 tubes; the primary difference between them is that 5881 tubes compress earlier than 6L6 tubes.
     
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  3. Tetanus

    Tetanus Well-Known Member

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    I've done the biasing myself.
    There are 4 screws at the bottom of the amp, undo these and you can remove the chassis from the wooden case.
    Carefully though. Mine was so tight that it took me quite a force to remove it.

    Then i used a Bias probe. (This is the main reason i had to remove the chassis as the power tubes won't fit with the additional probes in)
    The Bias trim pots are on the top of the chassis, front if the power tubes.

    Regarding to the tone of 5881 vs 6L6:
    I found the 5881s sounds better for me. But there are not that big of a difference. It easily can be due to the different brand/manufacturer.
    I hear a bit tighter response, a touch more mids. And with the 5881s l can turn the volume up just a notch, which might not seems a big difference BUT it absolutely makes it easier for me to run the amp on quieter settings. (Valve amp, loves a cranked volume or run at least on 9-10 o'clock)

    What preamp tubes are you using in your rd45 ???
    I would like to experiment with these...



    Sent from my iPhone using Tapatalk
     
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  4. Tetanus

    Tetanus Well-Known Member

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    you can see the Bias adjuster trim pots on this photo:
    [​IMG]
     
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  5. Robstafarian

    Robstafarian The Good and Wise Call Me “Rufus”

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    I cannot see a photo.
     
  6. Tetanus

    Tetanus Well-Known Member

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    I've just linked the one from my original post... don't know why it isn't visible. I can see it on my side...
     
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  7. Tetanus

    Tetanus Well-Known Member

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    ohh and one more thing:
    be not afraid to turn the Mids and the Presence controls up a lot!

    as I hear it on this amp:
    more Mids are helping you cut through the mix of course but it seems the Mid on this amp raising some almost bass frequencies (acts like a lower-mid freq. control). Therefore I turn the Mids up (almost max) and dial back the bass to around 11 o'clock.
    Presence is for mid and high frequency feedback in the power-amp section. Therefore it can help you a lot to get a tight, mid heavy tone.
     
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  8. Tetanus

    Tetanus Well-Known Member

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    i saved the photo and marked the Bias trim pots. Hope it works this time:
    IMG_5584 bias trims.jpg
     
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  9. Robstafarian

    Robstafarian The Good and Wise Call Me “Rufus”

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    That works quite well!
     
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  10. texhex

    texhex New Member

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    What did you use for the bias settings? Where did you find the information as to how it should be set?
     
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  11. Robstafarian

    Robstafarian The Good and Wise Call Me “Rufus”

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    Welcome to the forum!
     
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  12. Tetanus

    Tetanus Well-Known Member

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    I can't remember for all the info by heart... but here are a few details:
    I read it on a forum that the plate voltage is 440V on the RD45. I also have the schematic for it somewhere but it is not stated on it...
    the bias for 5881 is around 30-31mA according to calculations at 440V Plate Voltage
    6L6 is around 40mA.

    you can use this calculator to get a guideline:
    https://robrobinette.com/Tube_Bias_Calculator.htm

    I also found somewhere the following:
    Bias Settings For Safe Plate Dissipation:
    70%=hot; 50%=cold; -> 60%=average/safe

    Calculation forms:

    Minimum value [mA] = (AV * 500) / AS
    Maximum value [mA] = (AV * 700) / AS
    (the 500 and 700 value above is coming from the 50% and 70% of Plate Dissipation)
    AV = plate dissipation in watts (6L6=30W; 5881=23W)
    AS = plate voltage in volts (RD45=~440V)
     
  13. texhex

    texhex New Member

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    I sent an email to Randall support and they said 35mA is the recommended setting for the RD45 when using 6L6. I just got a used 2017 RD45 and it has JJ tubes in it Randall tech support also confirmed it ships with Ruby. I’m going to check the bias on these JJs and verify 35mA.
     
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  14. Tetanus

    Tetanus Well-Known Member

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    Randall Customer Support services suck... in my experience.
    Also, Randall (and other brands) tend to bias the valves to a cold setting for "safety" purposes.
    35mA sounds right, but on the cold side.
    Also, a few mA difference from the "optimal" is not fatal. If you are in the right ballpark, then you are fine. With the RD45, with 6L6, anything between 33 - 45 mA should be fine.
    BUT !!!
    1.
    make sure you are double and triple checking to bias the 2 tubes as close to each other as possible. that is more important (IMO) then the mA value.
    2.
    use a proper Phase Inverter tube. And by that i mean a tube which was tested to use as a PI...

    Ruby tubes are great sounding but i heard many complaints about their reliability.
    I had some microphonic issues with them myself as well, but.. JJs should be good, perhaps a bit dark sounding.

    Let me know please what you think about the amp, its sound and about the tubes.


    P.S.:
    don't be shy, upload a photo of your rig !!! ;)
     
  15. Robstafarian

    Robstafarian The Good and Wise Call Me “Rufus”

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    I will add: it is better to be a little cold than a little hot if it is not possible to balance the pair at the ideal bias.
     
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