Building a JTM 45/100 (Malcolm Young/KT66)

Discussion in 'Building the Classics' started by _Steve, Nov 23, 2020.

  1. _Steve

    _Steve Active Member

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    Investigating the Dropping Resistor

    I've been giving some thought to the dropping resistor that was apparently installed in Malcolm's 45/100.

    Here's what was written about it in https://solodallas.com/the-evolution-of-the-100w-circuit-from-jtms-to-jmp-superleads :

    Malcolm Young of AC/DC is reported (by Marshall Law magazine) to prefer a 100W plexi with white back panel (alongside a Superbass). According to Ritchie Flieger, the amp Malcolm used on Ballbreaker had 625V on the plates of the KT66s (suggesting it is a #3B or possibly a #4A). However, the choke is rumoured to have been replaced with a big resistor in order to reduce screens voltage and increase tube life.

    So first off, I think the 625V "on the plates" is most likely a slight exaggeration. The Classictone PT I got is supposedly to vintage spec with a 560VDC B+ in their spec sheet. When I fired it up I actually got a much higher secondary voltage (440VAC) that would amount to around 625VDC when rectified an UNLOADED. When I used this online calculator: Calculate Tube Amp Power Transformer Current (thesubjectmatter.com) to work out what the loaded B+ would be it works out to be the expected 560VDC. Im guessing this is not a coincidence and the quoted 625V was the unloaded voltage at the pin. And so far I haven't read or heard about a JTM with a higher voltage than the 560V.

    So I'm going to guess this was just a regular 66/67 JTM that he had from the 70's but he was finding that modern tubes in the mid 90's (when the article was written) could no longer hack the high 560VDC B+.

    So I did some rough estimates of what that resistor might be. I estimated that the total current draw from the screens and downstream triodes would be about 21mA at idle and around 79mA mid-way through Whole Lotta Rosie (I used a static 3mA for each of the 12AX7s and 3mA for each KT66 screen at idle and 17.5mA maxed out - please let me know if thats wrong??).

    So I worked out the following potential dropping resistors and their effect on the screen B+ node:
    1. 1Kohm would drop idle to 539V and 481V when driven (6 watts)
    2. 1.5Kohm would drop idle to 530V and 450V when driven (9 watts)
    3. 2Kohm would drop idle to 518V and 402V when driven (12 watts)
    4. 3Kohm would drop idle to 497V and then 323V when driven (20 watts)
    What's getting me is that in these scenarios, to bring the voltage down at idle means introducing a massive drop when its driven which im guessing is the same thing as sag. This does not take into account the upstream sag alredy caused by the plates being driven. Malcolm's sound really does not sound very saggy at all to me?

    I posted this really just to dump my thought process. There's people in here much more knowledgeable and experienced than me. I would appreciate any thoughts and corrections anyone may have? Maybe Im going about this all wrong?

    p.s. I totally understand that running KT66s at these voltages is a bad idea. This is more of a learning experiment :)
     
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  2. roef

    roef New Member

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    Mal had several 45/100s. I've seen pics of the el34 ones (with the 1202-119) but he probably had the kt66 one too. And he is reported to have a big power resistor which reduced screens voltage and current

    I'm having good luck running the new russian kt66s at 535v with the RS dlx choke, with a 690 dc resistance (not the 110 dc resistance of the 3h drake and dagnall chokes). If you use the latter, try adding a 470R or higher reading power resistor after the choke before the screens
     
    Last edited: Jan 14, 2021
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  3. _Steve

    _Steve Active Member

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    Thanks for the info its really interesting and useful. In your view do you think it was more likely he was using an EL34 version (version 4a in the Evolution article) for the Ballbreaker album? Guessing the likely dropping resistor he used (for EL34s) might be a bit easier than the KT66s. I'd guess he would have been aiming for 450V max at the screens?
     
  4. roef

    roef New Member

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    reportedly, he used kt66s for Ballbreaker, but it could have been an amp/transformer intended for el34s (1202-119). We just do not know really. I have 4 of these amps (and have had a few others at my bench) and do recommend the 1202-119/132 for Malcolm style tones - it is just much more powerful and punchy than the 1202-84 (in the kt66 amps). For el34s/1202-119, 500v works well.

    Ballbreaker sounds like the original 32/33uf mains filtering. other recordings sound like 50uf, which is much better for hard rock. 33uf is better for blues, psycedelica, incl. Cream and Free

    The screens voltage will be way higher than 450v at idle. but it needs to drop greatly unless you want to destroy the valves.
     
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  5. _Steve

    _Steve Active Member

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    Thank you sir!

    I read in another article that it was Rick St Pierre from Wizard that did the dropping resistor mod. I'm wondering if anyone here has a line to him? Apparently hes a really nice guy.
     
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  6. _Steve

    _Steve Active Member

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    Bolt of the PT sheared right off as I was fastening the grounding connections to it!

    20210116_172654.jpg
     
  7. adew1

    adew1 Active Member

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    That’s a pain when that happens. I managed to do the same thing in both of my builds. :rolleyes:
     
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  8. Chris-in-LA

    Chris-in-LA Well-Known Member

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    That’s messed up!
     
  9. neikeel

    neikeel Well-Known Member

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    The nylocs don't need to be that tight!

    In general I would suggest dedicated ground lug and bolt with serrated lockwashers. YMMV.
     
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  10. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Dedicated ground, yes. PT bolts are not good for grounding.
     
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  11. _Steve

    _Steve Active Member

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    Getting close to finished. Its been a very stop and start build.

    I've found my power board sits pretty much directly over some of the power tube socket pins - particularly one of the reservoir cap + terminals sits less than a 1/4 inch directly above the control grid of V7 (it looks further in the photo). I've compared to other builds online and it does seem mine is a bit closer than others.

    I should point out that im using a 12 series chassis with 45/100 boards which could be the cause of the issue - ive already had to rejig the caps to accommodate different standoffs positioning. I cant move it in the other direction because the power switches block it. I've seen a couple of builds online where they cut out bits of the board around the power switches to move it over that way.

    Should I be worried with this? I guess I could move the turrets around at a pinch.

    20210121_170955.jpg
     
  12. neikeel

    neikeel Well-Known Member

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    that is ok.
    Even on a true 45/100 chassis it is tight. Mine is very close to the switches. If you ar worried you can make the stand off slightly longer (assuming your filter caps are not too fat!)
     
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