Building a Tube Amp for HiFi use, thoughts?

Discussion in 'Tube Audio' started by mbates14, Apr 15, 2018.

  1. mbates14

    mbates14 Well-Known Member

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    765
    I have had a Conn organ tube amplifier since around 2002 or so. With only a couple small circuit mods and a quad set of EL34s this thing honestly blows me out of the room. But, its fugly.

    The same amp another member has is here:
    http://audiokarma.org/forums/index.php?threads/what-did-i-buy.450964/

    Despite people saying the output transformers are mismatched, they measure physically identical and they appear to be equal in frequency response as far as I can tell. The only difference is the 6V6 stage is powered by a big dropping resistor. That is the modification I did was disconnect that, and powered both channels directly off of the 400V supply so both channels perform equally.

    The reason I am posting this thread is I like the sound so much, but I hate the chassis just as much, I want to transfer the iron at minimum over to a new chassis, MAYBE the circuit design as well, and that is where I ask for your experienced subjective opinions. The circuitry like i said sounds great to my ears, BUT, I am sure there is room for improvement so I was wondering what your thoughts are on the circuit? or if there can be an improvement sonically to the amplifier?

    I posted a copy of the reverse-engineered by me schematics of the amp. I cant find the original. Also, I cant really find the phase splitter topology anywhere either, at least not quite like that implementation.

    any thoughts?
     

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  2. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Well, you have 6L6's in one side, 6v6 in the other, big diff in power out. Other than b+, I didn't notice many other differences in component values or other obstacles to converting other side to 6L6's, other than to the hi-voltage b+ ,& opt to match the 6L6 side.. Looks doable to me, but I'm sure others are about to pipe up any minute & tell ya I'm full of beans! :)
     
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  3. kward

    kward AK Subscriber Subscriber

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    First thing I'd do is measure the output transformers. To have the hopes of a good stereo hi-fi amp, you'd want both transformers to measure with the same primary and secondary impedances, and have the same overall electrical characteristics. If they measure out the same, then there is hope to build something semi-decent out of them. This all assumes of course you'd use identical output tubes for both channels. Depending on what the primary impedance measures out as, will determine the best choice for output tubes.

    Personally the only condition under which I'd proceed is if the output transformers do actually measure the same and have the same electrical characteristics. If they measure differently, all is not lost though, since you might be able to find a mate on the cheap somewhere.
     
  4. kward

    kward AK Subscriber Subscriber

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    Assuming you get past that hurdle, there are a some circuit improvements I'd consider making. The two 12AU7 triodes resemble being wired in a paraphase configuration. This type of inverter is the bottom feeder of inverter designs. Since you have two triodes to play with within that 12AU7 envelope, might as well convert them over to a differential pair (also called long tail par) inverter topology. This will reduce gain as compared to the paraphase, but I think there is still plenty of gain available if you keep the first stage 12AX7. I'd run the differential pair with as much voltage as you can muster out of your power supply. I'd shoot for ~380V, which means the R3 and R5 (10K) in the power supply would likely need to drop down to 470 ohms or 1K or something around there, and R4 and R6 would need to be increased to 100K or so.

    If you change out the paraphase inverter for a differential pair, you will also need to change the feedback arrangement. Common practice is to bring the feedback tap back to the first stage 12AX7. It's not strictly necessary to do that, but if you are judicious this approach can work well.

    The 12AX7 offers a lot of gain which can be useful, but there are some drawbacks with it in that first position also. One drawback is the internal capacitance of high gain tubes used with large value volume pots can cause some unwanted rolloff of high frequencies. So largest volume pot I'd use with a first stage 12AX7 would be 50K.

    Power supply looks ordinary but workable. I'd move C2 to come after R1, and consider replacing R1 with a 0.5 or 1 Henry choke if you have room on the chassis to mount it, and then tap the 400V B+ for the output transformers after the choke. This will reduce ripple quite a bit over current power supply topology. I'd also add some small value screen resistors hanging one each off of each screen pin and connect the screen voltage tap to the other sides of those screen resistors.

    There are some additional tweaks you could do which are more involved, but the above changes are what I would start with. This "type" of project (salvaging donor parts and redesigning/rebuilding from scratch) is where I derive the most satisfaction. You will need an oscilloscope to really do this work right, as well as a decent digital meter and a sine/square wave generator would come in right handy too. If you have a little skill and some patience it can be very rewarding. Good luck with this project!
     
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  5. mbates14

    mbates14 Well-Known Member

    Messages:
    765
    Love the advice. Thanks, its just about what I was looking for.

    But let me re-iterate that I had already stuck 4 EL34s in this thing, and left the circuit mostly unmodified except moving the B+ tap for the 2nd channel to match that of the 1st channel. (plus a bias resistor change) and it sounds great to my ears on both channels. But that's subjective so I feel like the transformers might be ok. I posted the schematic on how the amplifier was originally/factory. Also note that when the amplifier is idling, its dead silent. no hum, no hiss or any anomalies which I find incredible given how "mediocre" the circuit is.

    However, to do it right, How do I measure the output transformers for consistency?

    Also as far as the choke, I am wanting to avoid it as I am not going to have room on the chassis for it, as it will be occupied by other "digital" circuitry for source input switching, speaker engage relay as well as I may possibly put in a B+ delay relay as well. I have heard debating stories about B+ before tube warmup.

    I am going to use a Vacuum Fluorescent display (to keep the tube theme) for the front panel VU meters and source indicator.

    I havent decided on how to build the source circuit. I want to keep the sound path as analog as possible. Maybe using JFETS to ground inputs that arnt selected. Not sure yet.

    Also, for asthetics, I thought about switching the tube compliment over to 6SN7/6SL7.
     
    Last edited: Apr 16, 2018
  6. Dandy

    Dandy Super Member

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    I don't have the engineering expertise of others, here, but I have usually followed this advice for measuring output transformer impedance: http://www.radioremembered.org/outimp.htm
    I note that the website places 6V6 and 6L6 tubes in the same column under Primary Load Impedance. (Near the bottom of the page.)
     

     

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  7. A27

    A27 Active Member

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    I have the 56525-3 chassis. Both outputs are the same number. I measured the outputs at 9K for a primary. 60 Hz with the variac. I am running some Tung Sol KT66's. I matched the 6V6 to thr 6L6 channel. Data sheet https://frank.pocnet.net/sheets/086/k/KT66.pdf. So about 30 watts or so.
     
  8. A27

    A27 Active Member

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    Location:
    Iowa
    6J5 6SN7 mono version here.
     

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  9. s-petersen

    s-petersen Scott Subscriber

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    I have been down this path. This is what I started with,Conn 7027, 7868 version 3 channel.
    conn top.jpg

    This it what I ended up with:
    P10110030.JPG P1010025 (2).JPG
     
    Last edited: Apr 17, 2018
  10. kward

    kward AK Subscriber Subscriber

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    That link Dandy provided is a handy reference, but really the only thing you need to remember is the reflected impedance is the square of the turns ratio, and the turns ratio is can be measured by measuring the voltage ratio.

    I.e., Zpri/Zsec = (Vpri/Vsec) squared

    rearanging terms:
    Zpri = (Vpri/Vsec)^2 * Zsec

    Here's the procedure I use:
    Put a proper resistive load on the secondary (16 ohm load on 16 ohm secondary for example). Unplug output tubes. Disconnect feedback. Set a variac to deliver about 100V output and connect the leads from the variac to the primary. Using your volt meter, measure the AC voltage delivered at the primary, then measure the AC voltage at the secondary across the 16 ohm load.

    Example if Vpri = 100.0 VAC, Vsec = 4.0 VAC, and Zsec = 16 ohms
    then Zpri = (100/4)^2 * 16 = 10K
     
  11. Dandy

    Dandy Super Member

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    Great reworking job Scott!
     

     

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  12. mbates14

    mbates14 Well-Known Member

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    765
    Do any of you have the wire color to impedance charts/labels for the Conn transformers?
     
  13. mbates14

    mbates14 Well-Known Member

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    765
  14. s-petersen

    s-petersen Scott Subscriber

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    Thank You!
     
  15. s-petersen

    s-petersen Scott Subscriber

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  16. A27

    A27 Active Member

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    137
    Location:
    Iowa
    Yellow - 4 ohm, Grey - 8 ohm, Green - 16 ohm. Black - common.
     

     

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