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Robyn T-240D plate mod ? for Justin

Discussion in 'CB and Export Equipment and Accessories' started by yellow zonker, Aug 14, 2004.

  1. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Just got a Robyn T-240D and the plate modulation conversion sounds interesting. Does anyone have any information on this?

    IMG_20180906_195137235.jpg


     

  2. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    ARE YOU SURE YOU KNOW WHAT YOU ARE DOING!?

    This is an AC HV version of the RWOB mod others used...but for Bipolars...

    T240DSSModMod.jpg
    Enclosed in the Final TX strip of that radio so you can see how the bypass works...PLEASE BE CAREFUL and use HV coax cable - it can poke you and destroy tubes...

    T240DModMod.jpg

    I wish you the best of luck...
     
  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I know just enough to be dangerous. lol mainly I am just curious what it does and why it works. Would mini8 be HV or what would be needed? This doesn't look hard to do, what adjustments would be needed after doing this mod?
     
  4. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    It isn't the best mod for it.

    There are no real adjustments after this mod, for all the Modulator does, this mod bypass'es the chokehold they placed in it - and you have to ride the Mic gain to keep the thing from getting too excessive, in both modulation and plate dissipation...

    Referring to Grid and Plate issues.

    They "choke the mod" down, lower to allow the tube - both Modulator and the Final - to dissipate power for their size inside that box - else you'd need a larger tube and the associated power supply issues...

    Note the schematic and the mod itself - they use diodes to keep it "one way" towards the Final tube.

    (Man I'm not a tube guy so I can't believe I'm saying this...)

    When you broadband it, you're adding more audio -and adding an FM component similar to clipping on a Bipolar, only you're heating up the Final Tube and introducing another path for parasitic problems and one day it may just go SNAP and the tube is toast.

    In the Transistor logic view, you're placing Modulation on both the Base and Collector at the same time so what you do is add a compression effect - by Phase modulating the output of the Driver, into the Final - and the Final Tube itself - you hear unfiltered audio resonating with the Carrier the Driver sends to it, along with the Grids resonating with and offsetting, Plate modulation - so you're generating / making the all the grids in the tube work

    There's a neon light as modulation peak indicator - if it's on too long or stays on all the time, you have a squeal or oscillation you'll need to track down. Some people back in the day called it their on-air light - I'll let you figure that one out...

    The diodes' if they blow, may not blow open - they may blow shorted - think about the sharing of current...

    It's just a lot more work on that last stage tube...
     
    BJ radionut likes this.
  5. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Thanks for the explanation, I was able to turn up VR4 modulation and the mic amp. The MOD light still flickers, its just on more than it is off. Before it was off more than it was on.
     
  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    You just got it, don't blow it up before you get a chance to use it for real...

    The mod simply adds modulation to more of the tubes grids - not a big deal - but you're kinda playing with fire if you don't think thru the mod. You'd have better luck with as you discovered - the Mod limiter - and then just leave it at that - you don't have a lot of headroom in the Robyn for any big power swings and "volting" up finals - you're at 100+ volts now, and you have to think of the wattage at that voltage - it's a lot of heat to get rid of, inside that tube...
     
    Shadetree Mechanic likes this.
  7. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Chris/STM: I am with Andy here that rig is screen modulated...yea the Plate mod could make a difference, but you have to add the extra stress you will add to the tubes when doing so.
    When you could buy those 6BQ5A's for $.25 each for NOS RCA's or Sylvania's big deal! No longer the case, those and the EL84/6BQ5A (Audio Amps guys) are really in demand and have driven the pricing way up, even the Russian substitutes have risen much in the last few years.(last production dated's are in the mid 60's for RCA/GE/SYL's)
    So toss up! I would shop around at a good tube dealer(not some random Ebay'er)
    Tube Depot/ Antique radio supply/K5SVC...Electro Harmonics...etc. and buy a couple new ones, plug those in and see what see does. You want the 6BQ5(A) that is the Audio rated tube(IMHO) but good brand GE/ RCA's or Syl's 6BQ5's are good also.
    All the Best
    Gary
     
  8. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Couple other Notes: Best tubes(IMHO) for usage in the older "Tuber's...No specific order:
    RCA
    GE
    Sylvania
    Amprex (Holland made versions) Excellent but hard to find nowadays also.
    Philips/ECG...these are actually Sylvania's...When Sylvania got out of tube biz in late 70's/80's these production facilities were purchased and name re-branded.
    Svetlana/Winged "C"...Russian made substitutes...just make sure they are DIRECT sub's...That's where good dealer is important and know's the correct direct replacements.

    Some New Manufactured/Current production versions...
    Electro Harmonics/Mullard/Tesla...
    I know the Audio guys pay big bucks for some of these, but they are wanting TRUE Matched pairs/triplets...etc...for tuber mono/stereo/Guitar amps etc... where that is critical to rich sound quality.
    Tungsol ( I think is another re-branded) Some US/some Japan-China...the're are new and old versions around.
    So just some more info...for what it's worth
    All the Best
    Gary
     
  9. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    That's just awesome - you're my go-to guy for ideas and concepts on re-tubing.

    Thanks! That's some good stuff to know. It doesn't bring my ex-XYL back (GAWD maybe? Hope not) - but it tells me there were a lot of fans of tubes - back then - that are still around now.
     

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