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Stereo Amp build based on Hammond organ circuit

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

  1. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    You would get much better filtering if you removed the b+ wire from 6CA4, and connected to the 412v of the 5AR4 and changed the dropping resistor to get the desired voltage.
    You could test this by pulling the 6CA4 and using a jumper and resistor to the 412V connection, you may need yo decrease the resistor feeding the 412v.
    you could also raise the Filament winding by removing the center wire from the hum balance and connecting it to ~20-30V
     

     

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  2. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    I removed 6ca4 out of the equation, jumped to 400v connection. Noise is reduced but still there. Chokes are even less happy. The one that is connected to rectifier is getting warm.

    The organ place that I got this Hammond from, had a dismantled Wurlitzer sitting around. I pulled 2 chokes from it and tried them in place of ones from Hammond. They are larger but I could fit them inside the chassis if they end up working out. Noise has dropped some again, but transformer seems to buzz more now. I get about 15vac ripple from chokes. It is about the same as what comes out of the rectifier, but cleaner. 400v stage is perfectly clean after 520k resistor and cap. I find it strange that resistor is doing better job then chokes.
    What is the expected ripple from rectifier at 450V?

    Originally, this AO-63 chassis came with Chinese 5Z3P (5U4GC) rectifier installed. I looked it up and it seems as a close sub for 5AR4 so I tried it and my ripple at 450v B+ dropped below 10vac. I hear more noise from power transformer then from speakers, until I put my ear near the drivers.

    I think I should restore 6CA4 rectifier circuit and add one of the chokes to filter input and see if that reduces load from HV windings. Does it make sense?

    Scott, could you explain raising filament against 20-30 volt? Are you suggesting using bias supply negative voltage?
    Wouldn't it be better to rectify filament supply and adjust DC voltage with resistance to power all tube heaters? It shouldn't take much time or room inside the chassis.
    Bridge, 2 caps and a high wattage resistor, right?

    Any other ideas?

    Thanks
     
  3. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    You could try DC filaments, if you can get enough voltage. Raising the filaments with + 25v will reduce heater cathode voltage on the splitter, and reduce hum, you can use a high resistance divider from the last cap in the 6X4 supply. something like 470k connected to a 47k to ground, with the connection connected to the center of (2) 1k resistors across the filament wires. this will raise the filaments to 1/10 of the last cap. if that is too high, lower the 47k.
     
  4. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    That's pretty complicated to understand without schematic. Could you point to one? My filaments are at 6.4vac with all tubes in. Hum balance pot does not seem to have any effect on hum level but it does shift towards ground when turned.

    Any opinions on B+ 15vac ripple?
     
  5. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    15vac ripple seems awful high.

    humfix.png
     
    Last edited: Mar 24, 2018
  6. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    I got it nailed, I think.
    2 used chokes out of Wurlitzer. Single rectifier 5AR4 for all DC sources.
    I found and repaired bad ground. Hum at the speaker terminals is reduced to around 5mv. which is barely noticeable.
    I am considering raising filament voltage with DC per S-petersen recommendation to reduce it further.

    Those Wurlitzer chokes are much bigger then original Hammond ones and will not be a good choice, at least aesthetically.
    Where do I look for chokes and how to rate/select them?
    My current at idle is around 175ma out of rectifier if I measured it correctly with inductive pick-up.

    Thanks
     

     

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  7. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    Inductive pickup is usually used for AC current, it would be more accurate to sever the connection from the last choke and insert your meter in line, set to Ma in the proper range.
     
  8. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    My probe has both AC and DC current settings. It was sold to me by Snap-On tool dealer around 15 years ago. I use it to troubleshoot battery drains in cars when pulling fuses with meter in series "wakes up" computer circuits. It is pretty accurate in low voltage current draw. I like to see no higher then 20ma of parasitic draw after 30 minute key off timeout.

    I will take a direct measurement with meter in series to 450V point, but what is the choke selecting criteria? Are they voltage rated too or current is the only real parameter to pay attention to? mH is inductance rating, correct? How is that rated to circuit with known voltage and current requirements?

    Thanks,
     
  9. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    I think it is current and henries, I think most chokes are rated over 600v, But i am not positive.
     
  10. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    you can work out the idle current mostly with the cathode resistors on the output tubes plus whatever current is flowing through the resistor that feeds the driver stage. Just add them up. The vast bulk of it will be the output stage, if the driver is pulling more than about 5ma I'd be surprised. Keep in mind that the output stage current swings. You might see nearly double the idle current at max output.
     
  11. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    Scott
    What is the wattage of all resistors in your filament hum fix schematic?
     

     

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  12. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    1/2 watt
     
  13. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    My voltage out of (new) chokes is 520v
    Is there a better way to lower it to 450v then with a resistor?

    I used 330ohm 25watt sand block and its hot as iron. Added 22uf cap between chokes. Voltage went up some but resistor is much cooler now. If this is appropriate approach I can recalculate current draw and bump up resistor value to get needed voltage. Once I know resistor value that keeps voltage stable I can get chassis mounted resistor on the heat sink or on chassis.

    Added Scott's hum fix mod and can say that amp is quietest its ever been.
    There is a faint buzz in speakers I will have to come back to after I get all voltages where they need to be.
     
  14. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    Switch the 5AR4 with a 5U4GB there is a bit more voltage drop from a 5U4.
    I just saw that you tried that already.
    Did your inductive meter properly measure the current?
     
    Last edited: Mar 30, 2018
  15. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    I noticed voltage drop with 5u4 but wasn't sure it was normal or expected.
    It looks like power transformer is not original. It is a replacement "Trek" brand which is claimed to be a drop in replacement. I guess whoever installed it replaced 5AR4 with 5u4g to control voltage. I didn't think of that today.
    Will try switching it tomorrow and post results.
    Thank you!
     
  16. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    The 5U4 eats another amp of heater current vs a 5AR4, possibly an issue if the winding isn't rated for it. A 5V4 is a match heater-wise but it won't handle as much DC current. Voltage is less than a 5AR4, more than a 5U4.
     

     

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  17. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    My current probe shows 149ma draw out of chokes. Numbers add-up pretty close with outputs drawing 136mv plus 5mv each driver.

    5UGC (Chinese Ruby) installed. It still required a 207ohm (parallel added) resistor to drop to 450v under close to normal load with outputs drawing 34ma (.340v across 10ohm 1% cathode resistor) Looks like I can go even lower on resistance for tubes to draw higher.
    Is it ok to have a dropping resistor in first B+ supply?

    Gadget,
    Rectifier voltage is at 4.9v. Transformer is not getting hot. Does it look like it can handle 5UGC?
    I'll go through a stash of rectifiers I have at work and see if I have 5V4 there.
     
  18. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    5V4 won't work due to current limitations.
    5R4G variants might. Same cathode common filament connection. I read that it helps slowing down rectifier to let outputs get hot enough to conduct.
    Any other suggestions?
    Is dropping resistor EVER an appropriate solution in first B+ stage? I don't see it to be common for sure.
    All 3 voltage stages have 47/500 caps across. Would lowering them result in lower voltages? Or it will not be so significant at their lower currents?
    The problem is that transformer and choke specs are unknown. No real calculations are really possible, just hack and wack trial and error methods.
    I'm lacking experience to move forward.
    Any suggestions are welcome!
     
  19. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,572
    Location:
    Middletown,New York
    I have used a resistor in the first stage, it worked fine, but a small cap right after the rectifier helped with hum, much more
     
  20. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    545
    Location:
    New Albany, OH
    Voltage is way too high without a resistor after chokes.
    EDIT Hum is not an issue now.
    I haven't seen any choke input power supply designs with cap of rectifier.
    Nor I have seen any with resistor in first stage either. Either way is not traditional I guess.
    What do you think about 5R4G rectifier here?
    I asked Jim McShane for his opinion.
    Will post when he replies.
    It seems to be drop in upgrade.
    I could be wrong though.
     

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