Stereo Amp build based on Hammond organ circuit

Discussion in 'Tube Audio' started by grindfix, Mar 4, 2018.

  1. grindfix

    grindfix Full time hobby Subscriber

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    Several of you helped me make some decisions regarding building this amplifier. I still don't know if it will sound and perform well as designed or will need modifications. I'm prepared to make some changes but afraid that some of my layout decisions may backfire with noise. We will see.

    Below are some pictures of my progress.
    Chassis base was made out of Conn organ power supply box. Top plate was made by Front Panel Express.
    It took me a while to figure out their CAD interface but it came out nice and exactly I have envisioned it.
    I'm still waiting on 2.5mm SS Torx screws from McMaster Carr, otherwise tube sockets would be installed too.
    I decided to put all capacitors inside rather then adding a can on the top.

    More to come this week!
     

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    RUmad likes this.

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    6bq5 tubes? If so those are pretty stout transformers, should perform well.

    usually I use a #4-40x1/4 or 3/8 for mounting tube sockets.
     
  3. crispycircuit

    crispycircuit AK Subscriber Subscriber

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    Great project.... I've had good luck with organ amp parts...Let us know how it turns out....I'll miss the day when we run out of old tube organs... Fun stuff..........
     
  4. grindfix

    grindfix Full time hobby Subscriber

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    Location:
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    No
    7591 outputs
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    still good size iron for that power level.
     
  6. s-petersen

    s-petersen Scott Subscriber

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    What output transformers are you using?
    It wouldn't be hard to change to the Russian 6l6 equivalent (6P3S 6P3S-E), to save some money, and be able to roll 6l6 variants, including EL-34's
    The project looks pretty good for a start.
     
    Last edited: Mar 5, 2018

     

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  7. grindfix

    grindfix Full time hobby Subscriber

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    Scott,

    All 3 transformers and chokes are out of E-100 organ.
    Amplifier Chassis # A0-63-1
    Output Transformer # A0-30072-1

    Original schematic is attached as a PDF. I will be using both rectifiers. 6CA4 will power pair of 7247 tubes and 5AR4 will power 7591 quad. Bias will be individual.

    What changes will it require to convert to 6L6 tubes and what is the benefit besides lower tube cost?

    Thanks,
    Dmitriy
     

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  8. s-petersen

    s-petersen Scott Subscriber

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    The 6l6gc can produce more power if the opt can handle it, One change is that the Bias would have to be able to go to at least -40 to bias the tube properly, the 6l6 also needs a little more drive, but may work fine with the circuit as is.
     
  9. grindfix

    grindfix Full time hobby Subscriber

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    Location:
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    My bias supply is 33v out of transformer. I guess I can use full wave rectifier to bump that up. I think, I will stick to the plan to build it as originally designed. I have all tubes to start.
    I might consider modifying after taking output and power consumption measurements. I'm afraid Power Transformer will be maxed out.
    Thanks for ideas!
     
  10. s-petersen

    s-petersen Scott Subscriber

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    No problem, you also could go voltage doubler later if you didn't get enough voltage on it.
     
  11. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    415
    Location:
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    I'm finished putting together power supply section and decided to check my voltages before I go further.
    Variac to 150watt DBT to Amp. No load + no bulb glow = good.
    Highest B+ I need is 450v. I reached 450v at 95vac. I bumped it to 110vac and got 530v at B+. Filaments are at around 6v at that point.
    How much higher B+ voltages usually are without any tube load on them?

    Thanks,
    Dmitriy
     

     

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  12. gadget73

    gadget73 junk junkie Subscriber

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    A fair bit, but it depends on the transformer. The more overkill the transformer, the less the voltage drops with load.
     
  13. grindfix

    grindfix Full time hobby Subscriber

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    Amp is together with some not so desirable results.

    With my wall voltage at 122.6vac and CL-80 in-line I got 452vdc out of choke. Right on the money by schematic.
    Pins 4 & 8 of 7591 tubes are powered from same 452vdc B+, lowered by 800ohm resistor and get 446vdc. Why is this voltage so high? I'm obviously miscalculating current somehow.
    4500ohm resistor dropped voltage to 416vdc.

    Tubes are drawing around 32ma (320mv across 10ohm cathode resistor) and there is a hum through speakers increasing as current is increasing. It sounds like this hum is coming from power transformer, but mostly from 2 series connected chokes. They are buzzing and getting warm. Not hot but warmer then all other components.
    Is this a sign of low capability of power transformer or possibly weak rectifier tube? 5AR4 in this case.

    7247 driver tubes are powered by 6CA4 rectifier through 64ohm resistance and getting 360vdc. (spec 350)
    All other voltages check out very close to what is posted on the original Hammond AO-63 schematic.

    All drivers and output tubes are new from Jim McShane. Rectifiers are original Hammond tubes.

    Any ideas?

    Thanks,
    Dmitriy
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    What exactly is your power supply configuration now? If there is nothing downstream of that 800 ohm resistor it will not drop very much. This would be a good spot for a regulated supply, a big resistor will swing voltage a lot under load and that creates extra distortion. Even a resistor divider to provide a more stable drop would help, something that duplicates the original tube load if they aren't all there would be sufficient. A problem for another time though.

    The chokes buzzing sounds like they're not really rated for what you're asking of them. Are these the stock chokes used in the original amp circuit? If so I'd expect them to be OK with all this.
     
  15. grindfix

    grindfix Full time hobby Subscriber

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    Location:
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    Gadget,

    I'll re-draw PS and will post later today.
    Chokes are original but they are powering 2 channels now vs. single channel in original configuration.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    Ah, that may be the difference then. If the originals were only current rated for one channel they probably aren't going to appreciate two channels of draw. Might be able to work around that somewhat with a cap right off the rectifier, though that will bump your voltages a fair bit vs a choke input supply. The other option is to replace the two chokes with one larger one to handle the load.
     

     

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  17. grindfix

    grindfix Full time hobby Subscriber

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    Large one will be tough. No room. What about 2 in series but larger?. How do I rate those?
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    Location:
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    Its the current you have to worry about. For a given value, a high current choke will be physically larger than a low current one. Similarly, for a given physical size you're going to get less inductance and usually less resistance as the current rating goes up. Basically its a matter of fatter wire, as the gauge goes up less fits on the form.

    Honestly choke input supplies aren't something I've got a lot of experience with so I don't quite know how those are sized. If the two chokes it has are identical, you could make the power supply more complicated and split it. One choke per channel feeding into the appropriate caps. That would get each choke handling the same current as it would have originally, though its at half the original inductance.
     
  19. grindfix

    grindfix Full time hobby Subscriber

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    415
    Location:
    New Albany, OH
    Here is my power supply setup
     

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  20. grindfix

    grindfix Full time hobby Subscriber

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    415
    Location:
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    So I ran couple of tests.

    1. I soldered a jumper wire across choke wires to cancel it and raised variac until I got to 450v. Variac output was somewhere around 100vac at this point and hum was still there. Less but still there.

    2. Then I removed jumper and one lead of the choke and soldered one large choke I have. This one is huge, about 3/4 the size of OPT.
    At 122vac I got 377vdc out of the choke and hum is not as bad as with two small chokes but still there. Power Transformer is obviously humming. I can feel it and hear it through screwdriver to my ear (3/8'' drive extension makes best stethoscope to diagnose wheel bearing noise while car is on the lift)

    Could weak rectifier tubes cause that or my Power Tranny is simply out of steam? Or incorrectly selected choke can overload and cause transformer to buzz?
    It has original 2amp mains fuse and it holds up.
     

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