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'Organ Donor' - Hammond AO-29 to 6V6 Plexi Conversion

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jan 3, 2017.

  1. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Kleydejong, we touched on this subject this early on. The amp as original had a 150ohm...iirc...resistor in that position. I haven't gone to el-34 world to see what results they have had and why they suggest 700ohms there. I have to think that the resistance there is tied to setting that B+ voltage to a more reasonable point. Are you using a 5Y3 rectifier?
     
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  2. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY

  3. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Sure. I got a little side tracked and am unsure if we ever figured out exactly what model I have and whether or not it used the field coil speaker and therefore the 700 ohm resistor is appropriate. Or if not and I had cited the correct schematic and the 180 ohm resistor is correct.

    Would it be a bad idea to just try the 700 ohm that I have on hand and see how it affects voltages?

    I currently have a 5U4 rectifier in.
     

  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    that is correct...180ohms...thanks for the correction, dtar.
     

  5. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Not correcting But just recalling the conversation. I would think Kley could use whatever value resistor needed to bring the VDC down to whatever is desired. I think 700r, if I had it already, why not try. I guess a little bit of math could get the answers but Im not that motivated right now.
     
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  6. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    D-tar...by correcting I am referring to my 150ohm "iirc" statement versus the schematic that you show. I agree with the try ti and see what occurs. I personally would be inclined to use the stock 180ohm resistor that came out of the amp with which Kleydejong started.....but it would be of interest to know why the EL-34 world people suggest 700 volts. There are two big resistors sitting there...unless they have been thrown away or unless their values have drifted. The values usually drift upwards in value, ime.
     
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  7. dan40

    dan40 Tele-Holic

    924
    Aug 19, 2015
    Richmond Va
    Are the voltages you posted Kley with all of the tubes installed and biased properly? Just checking to see if you may have posted the unloaded voltages. I'm not very familiar with the Hammond platform...how many tubes did the original circuit use?
     
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  8. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Good quesiton, Dan. I was wondering about the effect of current draw on the B+, as well; but I never thought about whether these voltages were taken with or without tubes in the circuit. Here is the original post with the tube compliment....

     
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  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Fwiw, the two original large resistors to which I referred are sitting to the right in the last picture...on the back panel of the chassis.
     

  10. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Those last voltages were with tubes installed. The cathodes of the two 6V6's have 1 ohm resistors to ground, and one of them blew out with the short. I haven't performed a new voltage check since fixing that blown resistor with a jumper.

    I did play some last night and it seemed like we're moving in the right direction, but something was off. Even on a clean setting there was some kind of low end muddy thing going on. I took a clip briefly here:



    The original AO-29 had 11 tubes. The tube compliment is as follows:

    v1 - 6AU6
    v2 - 6AU6
    v3 - 12AU7
    v4 - 12AX7
    V5 - 6V6
    V6 - 6V6
    V7 - 6C4
    V8 - 6C4
    V9 - 12AU7
    V10 - 68A6
    V11 - 5U4
     

  11. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I think the 700 ohm 50 watt resistor was suggested as a direct replacement for a field coil speaker. That builder had the original field coil speaker with the AO-29 amp. At some point in the process the builder decided against using the field coil speaker and instead using a typical magnet.

    Sluckey responded with "Now you can connect that CT directly to ground and your B+ will be about 100V higher than you will want. Or, you can connect a 700Ω 30 watt resistor between that wire and chassis to simulate the missing field coil."
     

  12. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I will check to see if I kept those. I did some cursory checking of the original components. I noticed a few caps that were bulging and used an ohmmeter on some resistors. They had drifted some. I felt as if I was getting pretty good value just using the chassis and transformers, so I was perhaps a little quick to discard.

    Thanks for the insight, you have been very helpful.
     

  13. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Okay, I went and read this thread at EL-34 world....or most of it so far. If all else is correct with your amp here, then I would take it that 'Sluckey' on that forum has the correct info on how to handle the high voltage of that PT. I am hoping adn thinking that my PT in my AO-29 has different HT output and will not necessitate that approach...but the future will inform neh?
    Both quotes from Sluckey....
    "I do not recommend using that 380-0-380vac PT from that AO-29. Your B+ will be way too high."

    "Now you can connect that CT directly to ground and your B+ will be about 100V higher than you will want. Or, you can connect a 700Ω 30 watt resistor between that wire and chassis to simulate the missing field coil."

    IF you could knock that 515vdc down by a 100 volts, that would be good. One might think that even a bit more would be advisable. Experimentation will illuminate...
     
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  14. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    There are two CT wires - a greenish one (previously marked as lead #6 in my diagram on pg 2) and a red/yellowish one (#2 on diagram). I was unsure which is the HV CT so I checked it with the ohmeter. The red ish CT measured half the resistance of the two red leads, so I infer that this is the HV CT. I ran that into the 700 ohm resistor I have and then grounded the other end. B+ at node A came down to 460vdc. I was on my lunch break so I didn't have time to check anything else.

    I bought a second 700 ohm 50 watt resistor. Would putting that in series continue to drop voltage? Any concerns in doing this? These resistors (link: http://www.mouser.com/ProductDetail/Vishay-Dale/RH050700R0FE02/?qs=%2b/1MMOhkBTd/VmVEoiJ4Ng==) are being mounted directly to the chassis using some thermal paste meant for a CPU in a computer buffering the connection to the chassis. Then bolted with a lockwasher.

    A second thought, what if I just use 6L6GC's? Or EL34's? Though I need to do more research to see if EL34's don't have a different pin layout.
     

  15. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Got a little side tracked. My wife and I actually had twin girls two weeks ago. Starting to get back into a 'normal' rhythm (haha).

    I ordered some new parts. I found a grab bag of 1/4 watt metal film resistors on Amazon for about $10. I bundled it with some other stuff to get free shipping. It has a 10 pack of 1 ohm resistors. They are listed at 5% tolerance. As these will be running from cathode to ground and used for bias measurement, is it pretty critical that they are very close with one another?

    And upon installing the 700 ohm resistor from the HT center tap to ground I can report updated B+ voltages:

    A - 443
    B - 442
    C - 321
    D - 274
    E - 255

    So we are down about 75 volts. I'm planning on throwing in a pair of 6L6GC's and seeing where we're at. Getting closer.
     

  16. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    A little! Ha.. Congratulations! It's been 5 years since my 4th born and crazy busy is the normal now!

    Voltages looking better.
     
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  17. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Congrats on the 'new normal' for you and your wife.
    what are your heater voltages....out of curiosity. IF they are high, then some sort of reduction in the AC input voltage would be advisable. Your B+, while acceptable, is still higher than the schematic calls for. That might be attributable to the higher wall voltage we see. I am going to have to pull my AO-29 out and see what the PT does....
     
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  18. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Thanks!

    Heaters are measuring at 3.3 volts.
     

  19. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Thanks for that info....
     

  20. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Over lunch I dropped in some 6L6GC's in the power section, a new 5U4 rectifier, and randomly swapped the preamp tubes around for fun. I was greeted with some crazy high pitched squealing upon startup. Confused as this was not happening before... Thinking I need to swap leads on the OT.

    I'm going to remove the NFB connection for the time being. Getting some weird stuff on my ohmmeter there. Will swap those leads and bias the 6L6's.

    Hoping things 'sound right' at that point. I'm also wondering if I may have some dodgey capacitors in the circuit. Before the babies came I was getting some very bleh low end or midrange stuff. Muddy or ratty sounding. Thinking I may have overheated some?

    I have a capacitor testor that a friend loaned me. This one: https://www.amazon.com/gp/product/B0036FQ3FW/ref=od_aui_detailpages00?ie=UTF8&psc=1. Going to go through the circuit with my ohmmeter and with this capacitor testor.
     

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