Stereo Amp build based on Hammond organ circuit

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

  1. gadget73

    gadget73 junk junkie Subscriber

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    I have something with a resistor straight off the rectifier. It has fairly significant voltage sag under load.

    Whats the AC voltage from center tap to plate on the rectifier?
     

     

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  2. grindfix

    grindfix Full time hobby Subscriber

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    With 5AR4 at 122vac wall each rectifier plate gets 428vac against center tap (ground)
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    hm, yeah that would come out a bit high with cap input, but shouldn't be horribly high. I'd figure about 460 vdc ? A lossier rectifier would drop the DC out. I wonder if this isn't a good application of active voltage regulation with solid state bits. It would get the supply voltage where you need it without a resistor to limit current. Unfortunately thats not something I've learned much about so I don't know the specifics of how to do that most effectively.
     
  4. grindfix

    grindfix Full time hobby Subscriber

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    I think I'm finally getting somewhere with this project. I installed series of 2 Hammond 194G chokes with 47uf cap to ground between them. 4.7uf Solen poly cap out of rectifier.
    Then I adapted D.Gillespie's EFB circuit with modifications to achieve correct voltages for my amplifier. I'm still waiting for Mouser order with correct resistors to replace test parts I currently have installed.

    B+ 459v
    Plates 455v
    Screens 385v
    at 37mA per tube at idle.

    Amplifier is reaching full power output of 13.8vac / 23watts per channel into 8ohm dummy loads with both channels driven. Scope shows around 0.9vac input signal. True RMS voltmeter shows 0.629vac input signal. Is it a bit too sensitive? I read that most amps are rated to reach full output at around 2.0V RMS.
    Another thing. Clipping is not even. Positive wave starts to clip sooner. Any suggestions?
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    Sounds like the phase inverter output isn't equal. The way this one is designed it would have slightly uneven balance. Its got a 47K on the plate and 47K + 2.7K on the cathode. If you want to add an AC adjustment, remove the 47K resistor on the cathode of the inverter and sub it for a 39K + 10K pot. The pot should allow adjustment for equal clipping.

    23 watts is a reasonable output power level, and thats not an unresonable input sensitivity for a tube amp either. A lot of them are a little under 1v.
     
  6. grindfix

    grindfix Full time hobby Subscriber

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    AC balance adjustment makes sense.
    I’ll see if I have 10k trimmer to use. How does imbalanced clipping affect the sound?

    Some vintage tube amps were not very quiet. I’m sure partially due to aging components. Wouldn’t signal to noise ratio be improved if Amplifier sensitivity is reduced though? Are there tests to suggest sensitivity level?
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    Higher distortion and uneven clipping mostly. Not sure how audible it really is, I've never fooled with the balance control while listening to see if I could hear any difference. The pot value isn't horrible critical, you just want pot + resistor to be at least 47K, and ideally have a little more. Too large a pot value will make it overly touchy. If its too touchy with 10K, a 43K + 5K may be a better choice. For initial adjustment, measure the value of the 47K plate resistor and adjust the pot so the total resistance from ground to cathode matches that. It should get you very close to ideal.

    A lot of the time noise comes from components and component layout. I've got stuff more sensitive that is quiet, and I've had stuff less sensitive that was not. What sort of noise are you getting?
     
  8. grindfix

    grindfix Full time hobby Subscriber

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    Component layout is not the greatest. If I knew that I'd have to use chokes I'm using now, I'd built bigger wider chassis.
    Chokes are placed closely between PT and OPTs. First choke out of rectifier cap is on the same side of the chassis as 120vac to power switch and CL-80 limiter.
    That side channel produces very faint hum with my test speakers if I put ear right against the woofer. Cornwalls pick up quite a bit more due to their efficiency.
    Top plate of the amp is anodized and makes grounding to be more creative. I think I will try to reverse choke connections so leading choke is on the opposite side of 120vac lines and see what happens. Also will make an aluminum shielding divider for 120vac lines.
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    if its audible only with your ear against the speaker, honestly thats not bad.

    Some shielding in the right place and a little wire re-routing may sort it.
    Twist the AC wiring if at all possible, it helps cancel hum.
     
  10. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

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    Do you have shielded input wires? that made a big difference in one of my single ended builds. And the shielded wire is only 2" long!
     
  11. grindfix

    grindfix Full time hobby Subscriber

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    Yes, input wires are shielded. I used Mogami W2965
    https://www.redco.com/Mogami-W2965.html

    I'm hoping to find time this weekend and make some changes in AC layout.
    EFB is finished with better quality resistors in place of test parts I had connected in series or parallel depending on what values I had on hand.
     

     

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  12. grindfix

    grindfix Full time hobby Subscriber

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    Almost like with meds, any improvements I make come with less then desirable side effects. Here are my latest developments:

    First, I completely re-arranged AC power input. Twisted pairs inside of grounded shielding sleeve. This made no improvements in hum but it looks better.

    Then I temporarily installed sheet aluminum grounded shielding divider. I expected this divider to shield leading choke inductance ripple from OPT outputs and NFB and ground wires.
    No improvement noted on the scope.

    Next, I doubled capacitance out of rectifier and powered up with variac. Output is a lot quieter through speakers. Hum is almost not there even with ear next to woofer, and looks twice cleaner on the scope. Before and after scope screenshots attached below. 5mv/div. leading choke side still shows higher ripple but not audible, so no concern, I guess.
    Obviously, additional capacitance bumped DC voltages. I'm achieving proper DC voltages at approximately 115vac input. My wall voltage varies between 120 and 122.5v.
    I already have two CL-80 limiters installed to control filament voltage. One limiter on Neutral and another one on Line.
    I tried increasing Drain dropping resistor in EFB but results are not significant within reasonable resistance increase. I tried up-to 2.5k from Dave's spec 1.8k

    Here are options I know to consider:
    1. Build bucking transformer. I don't know how to properly calculate what transformer I will need.
    2. Replace CL-80 (3A/47ohm) limiters with MS22-12104 (4A/120ohm)
    https://www.mouser.com/ProductDetai...=sGAEpiMZZMuqZeNK75brDxRgT1KUh2F4xWvLBVTVbw4=
    This will bring voltage down but will generate more heat inside chassis.
    3. Resistor out of rectifier. Probably the least desirable option due to heat generation and sag under load that was the reason to adapt EFB to avoid that.

    Are there any other ways to keep B+ no higher then 455v?
    Thanks
     

    Attached Files:

  13. gadget73

    gadget73 junk junkie Subscriber

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    Whats the B+ now? and what does your power supply look like at this point?
     
  14. grindfix

    grindfix Full time hobby Subscriber

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    B+ is over 460v out of trailing choke at variac 117vac input with 2 x CL-80 already installed.
    Voltages are fine at 122vac input with 4.7uf out of rectifier but there is a hum. Significant hum improvement starts with 8uf or higher. Voltages do not seem to change as much between 10uf - 15uf so I installed 12uf.


    PSUD2.JPG
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    Probably too much ripple for the chokes to handle with the small value cap.

    460vdc into a 7591 is actually still within acceptable limits. Datasheet gives 550 vdc max. As long as you keep the plate dissipation under control it should work OK.

    I don't suppose you know what the heater rating for the rectifier is? A 5U4GB would drop about 30 volts more than the 5AR4 would, but it wants an extra amp of heater to do it. Direct heat too, so the voltage will spike higher than a 5AR4. Not a big deal but your caps will need to be rated to handle it.
     
  16. grindfix

    grindfix Full time hobby Subscriber

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    I could probably do that but will have to replace both 47uf/500v caps with higher voltage ones. For some reason I remembered 500v to be the max plate voltage.
    Thanks!
     

     

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  17. grindfix

    grindfix Full time hobby Subscriber

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    Having troubles finding electrolytic capacitors rated over 500v. Solen Film caps are available but huge. Could someone recommend the source?
    Thanks
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    You can series two 350v caps with a 470K across them to end up with a 700v effective cap. Ends up at half the value of each cap though, so a pair of 100uf ends up as a 50uf.
     
  19. grindfix

    grindfix Full time hobby Subscriber

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    I got same series recommendation from Jim McShane. He did not mention 470k resistor though. How does it work?
    As I understand across means one end connected at the junction of two capacitors and another goes to ground. Am I correct?
    Thanks
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    It forms a voltage divider to make sure the voltage across each cap is equal. Basically it goes from + to - on each cap. If you don't do the resistors, the voltage across the cap will divide up however it works out based on internal leakage, and its very possible that one cap could end up with more voltage than it is rated to handle. With the divider each cap gets 1/2 B+, or as near as resistor tolerances work out. You might get away without it, but its cheap insurance.

    Also it gives you a bleeder for the power supply to slowly discharge itself, so thats not a terrible thing.
     

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