Wherein John Frum Goes Single Ended

Discussion in 'Tube Gear' started by John Frum, Sep 12, 2018.

  1. mhardy6647

    mhardy6647 SeƱor Member

    not that you're biased or anything...
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  2. Thermionics

    Thermionics Post Whore In Training

    Is there less voltage drop with a 6CA4? Granted it is a noval instead of 7-pin socket, and draws 400ma more current (but then again, you'll be down one signal tube most likely so YMMV with overall current load on the PT), but it might have less drop.
  3. John Frum

    John Frum Junior Member

    Looks like I could gain about 15V over the 6X4, putting B+ at 282V.

    The upgrade would probably cost me $40+ in additional tubes and sockets. Worth it?
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  4. Thermionics

    Thermionics Post Whore In Training

    Well, I'd say 282V is close enough to the 285V sweet-spot of the RH84 circuit you want to build. If it were me, I'd probably go that route. Just make sure the power transformers don't overheat with the additional load.
  5. John Frum

    John Frum Junior Member

    Looks like the 6X4 rectifier is fed of its own .6 A heater tap. Switching to a 6CA4 would involve moving the 12AT7 over to the .6A tap and running the 6CA4 and the EL84 together on the 1.8 A heater tap. That doesn't seem ideal. Worth all the added expense, trouble, and compromise to net +5.5% on my B+? Or is it a matter of 267V being only marginally able to do justice to the RH84 design?
    Last edited: Oct 12, 2018
  6. Thermionics

    Thermionics Post Whore In Training

    I'm not an RH84 expert. However, as Nate pointed out, 285v is the sweet spot for B+ on an RH84 circuit. And running your EL84 and 6CA4 on the same heater string isn't a big deal - Fisher did that all the time on their 30A PP monoblocks and those are very highly regarded amplifiers.

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  7. John Frum

    John Frum Junior Member


    I closely examined both amps together for the first time, and got a rude surprise. Let it be a lesson to all of you in getting too excited before you get your hands dirty.

    I knew that there were substantial differences in circuit between Left and Right amps, but I was dismayed to see that the choke on the Left amp is noticeably smaller. The printing on the power transformer is also more of a forest green than the blue printing on the Right amp, though I don't know if this means it differs internally.

    I've scrutinized photos of the M6, and the transformers and choke appear to be identical on both channels. I am unsure as to whether this is true for the earlier M5 I got these amps from, or whether the discrepancy is due to a running change in production and bad luck on my part.

    In any case, I think I'm going to mothball this project, in hopes of eventually putting together a pair with matching iron.

    Never fear, though. This copper-chassis Motorola single-ended EL84 amp showed up at my doorstep last week. Should be an easier project, being a stereo amp as opposed to dual mono, and already rocking a 315 V secondary and 6CA4 rectifier. I'm making plans for a wood frame, filter choke(s?), banana jacks for speakers, and a metal plate (Front Panel Express?) to be mounted on the forward section of the amp to hold 12AT7 socket, L & R RCA jacks, and dual DACT-type attenuators. Cost of those additional parts is mostly offset vs. the Akai amps in half as many power supply caps and no need for tag/turret boards. Hopefully, it won't take me long to get started, as I'm hot to make some progress.

    Motorola AMP.jpg

    The Akai OPTs still look a little beefier than the ones on the Motorola. I understand that both are 5K at 8 ohms. Wonder if I should "borrow" the outputs from the Akai amps. I'll almost certainly be stripping the Motorola down to the bare chassis before rebuilding anyway...
    Last edited: Oct 14, 2018
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  8. Thermionics

    Thermionics Post Whore In Training

    If the numbers on the PSU are the same, they should be the same. Do you have photos?
  9. John Frum

    John Frum Junior Member

    There are no numbers visible on any of the iron.

    20181014_172218.jpg 20181014_172027.jpg

    The power transformers appear to be the same, except for the color of the label. The content of the label itself is the same on both amps, giving voltage and current ratings for each winding - also, there's a tiny logo indicating they're made by Orion Electric - which is probably true of the other transformers, as well.

    250V-0-250V DC 60 mA
    5.7V 0.3A
    6.3V 0.6A
    6.3V 1.8A

    The choke on the Left amp has a black frame, and the choke on the Right amp has an unpainted frame. The Right choke is probably 1/3 larger, as well.

    Output transformers appear to be the same in all regards.
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  10. John Frum

    John Frum Junior Member

    On a side note, I love the font on the power transformers.
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  11. Thermionics

    Thermionics Post Whore In Training

    Let me go check my Roberts Amps. I'm wondering if one of those chokes was a replacement.

    [EDIT] The chokes look quite different on my Roberts - plastic bobbins, for example - but they are physically closer in size to the small choke with black frame in your above photos. I think you should be fine provided the resistance is the same between the two chokes.[/EDIT]
    Last edited: Oct 14, 2018

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