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wurlitzer 6420

Discussion in 'Tube Audio' started by Ndirish147, Aug 13, 2018.

  1. Ndirish147

    Ndirish147 Member

    Messages:
    56
    Location:
    Muncie, Indiana
    I recently picked up a wurlitzer model 40 tone cabinet. The cabinet was complete but in pretty rough shape. I have a couple questions. first off it has a pair of 6420 line amplifiers in it. What would it take to turn these into a pair of mono blocks and where would i even start with a project like that?

    Second is what to do with the field coil speakers? I dont want to throw them out but am not sure is there is much of a market for them? thank you for any help! IMG_20180812_224501.jpg IMG_20180813_062426.jpg IMG_20180813_062418.jpg IMG_20180812_224126.jpg IMG_20180813_062128.jpg
     

     

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  2. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    The field coil speakers are matched with those amplifiers. The field coil itself is part of the power supply which acts as a choke. Those amps can be modified to use permanent magnet speakers but in my experience with those, it's best just to leave them stock and use them with their factory amplifiers. You may or may not like how they sound with a CD player or tuner or such, but then again, you might! :cool:

    I suppose you can call them line amplifiers but we here call them power amplifiers. :D

    If you haven't heard, just simply replace the electrolytic and coupling capacitors and this will usually bring them back up and run safely for years to come. Also check for any out-of-spec resistors.

    Field coil speakers can actually sound quite good when the amps have been rebuilt. Some people might say that they are not high fidelity but if you think about it, the tone cabinets are used with organs which have to reproduce the bass pedals and the high flute and tibia sounds however, most of those field coil speakers are full range. I have a Wurlitzer 7020 organ amp with a 12 inch field coil speaker which I use as a dedicated mono set up with an old Magnavox mono AM/FM tuner preamp. Sounds excellent. :rockon:

    With their original speakers and amps, these things do make good mono blocks for what they are.

    Most Wurlitzer amps (and other brands) have their output transformers mounted under the chassis. Just in case you were looking for them. :thumbsup:
     
    Last edited: Aug 13, 2018
  3. knockbill

    knockbill Addicted Member

    Messages:
    7,782
    Location:
    SE PA
    Good post CT!!! I've never had the opportunity to work on, or hear an organ amp,, so I find threads about them very interesting... Just curious,,, to use them with regular PM speakers, would adding a choke to replace the field coil,, and adding an OPT(or would the organ OPT work?) to match the amp to a PM speaker be all that's needed to modify them for audio use? Seems they are already pretty full range...
     
  4. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    I have used a two henry choke coil with good results. I have heard of people using resistors in place of the choke coil but I have never used resistors. My 12 inch field coil is printed with 6000Ω on the paper wrap which makes it easy.

    I have used permanent magnet speakers with field coil amps replacing the field coil with a choke and all I really have to say is that it acts like any other amplifier and speaker system. You just have to find the right combo of speakers that will work well with those amps.
     
  5. knockbill

    knockbill Addicted Member

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    7,782
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    6K sounds just about right for PP 6L6s... What is the primary/secondary of the organ amp OPT? Think I better dig up a schematic and see how it works!
     
  6. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    Not sure what my primary is but it is the original field coil speaker and output transformer and amplifier. It's pretty much in a permanent location to get to to measure. I think I also have a schematic around here somewhere. I will look and see.

    The 6K is the field coil ohms.
     

     

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  7. knockbill

    knockbill Addicted Member

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    Got it, now... That's why resistors can be swapped in,,, I agree a choke would be a better fit...
    The organ OPT must have 6K or so primary, and whatever the speaker(s) secondaries are...
    I'll search for an organ amp schem... Don't want to get off topic here...
    Thanks...
     
    Last edited: Aug 13, 2018
  8. Ndirish147

    Ndirish147 Member

    Messages:
    56
    Location:
    Muncie, Indiana
    I am a totally new to all this so you will have to take it slow and pardon any dumb questions i might have. Would my fist step be to pull the amps and purchase/replace all the capacitors and resistors? As far as a choke goes is the purpose of it to only filter out ac? and the coil in the speakers was doing this before but if i do not use them then i would use a choke in their place?
     
  9. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    Yes.

    It smooths out the DC ripple current.

    Yes.
     
  10. mashaffer

    mashaffer Super Member

    Messages:
    3,341
    Location:
    North Central Indiana
    Depending on the quality FC speakers can be highly valued in the right communities. Very few are in current production and those that are tend to be fiendishly expensive. In some camps FC is considered the pinnacle of loudspeaker development. I would give them a try if they are serviceable.
     
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  11. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    I will also add that most amps and radios that use field coil speakers, employ a safety type of plug that when the speaker is unplugged, it disconnects the B+ so as not to damage the amp / radio. Some will simply not turn on without the speaker.

    So if you do decide to not use the field coil speakers, just be sure you have a choke in place and a speaker connected before you power up the amps.
     

     

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  12. knockbill

    knockbill Addicted Member

    Messages:
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  13. Ndirish147

    Ndirish147 Member

    Messages:
    56
    Location:
    Muncie, Indiana
    Thank you guys for the replies and the link to the schematic. Here is a shot from the bottom of the amp IMG_20180814_060919.jpg I will start getting a list together of some of the parts i will need to order.

    I have a pair of fairly high efficiency speakers i would like to use these on. Would it be smart for me to replace the opt on this amp or will the one that is in it work?

    How do i figure out what to look for in a choke other than it being 6k ohms.

    Last question for now is will any preamp work for this. I have a hk70 that i was going to use initially. will that work or do i need to modify the amp to make it work better?
     
  14. knockbill

    knockbill Addicted Member

    Messages:
    7,782
    Location:
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    I'd replace the can cap with separate caps, and the wax/paper caps first,,, also check the value of the resistors... Then I'd try it with original speakers and see how it sounds... You will have a base to compare to any modifications later on...
    Check the schematic to the values in the amp, to make sure its the right one...
     
  15. wlgrant6

    wlgrant6 AK Subscriber Subscriber

    Messages:
    523
    Location:
    Louisville Ky
    I used the power supply out of a Baldwin tone cabinet amp, and it also used 4 15" field coil speakers. But the voltage feeding the field coils was taken off after the B+ voltages, so it had no filtering benefits for the power supply. Removing the filed coils merely reduced the load, and raised the overall B+ voltages that were available. It used a filter capacitor followed by a very large choke after the rectifier tubes. I added RC networks to get the correct voltages that I needed. Ended up being very quiet.
    Bill
     
  16. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    I would leave the output transformers alone for now. Modify it later if you want to experiment or just have fun. They will probably drive any speaker fairly well and the these amps using push/pull 6L6GC tubes can be good for about 25 to 30 watts or so.

    But like I said previously, besides the original speakers, you will just have to experiment with different speakers to see how you like how they sound. Different speakers and efficiency will give different results on how soft or loud the amps will play.
    A 2 or 3 henry should be fine.
    The HK should work fine but like the speakers, you could experiment with other kinds of preamps, tube and solid state. The amps should have plenty of gain when used with a decent preamp, but you're not going to know until you get them running and report back.
     
    Last edited: Aug 14, 2018

     

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  17. Ndirish147

    Ndirish147 Member

    Messages:
    56
    Location:
    Muncie, Indiana
    ok another question about this choke. How do i figure out how much current it needs to handle? When i go to look at the current draw of a 6l6gc and a 6sn7 do i look at the peek current or idle current?
     
  18. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
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    If you already have one or two chokes on hand, don't worry about it. Just put them in. They are all made to handle the current unless you would have a possible short somewhere in the B+ circuit.

    Just my humble opinion but when you get this all done, when not using the original speakers, you might have to do more tweaks when using other chokes and coils and things to get the voltages just right. These amps were made for the original speakers so I would use them, but its yours to do with as you wish. I think I'll sit back and watch this thread as you continue.
     
    Last edited: Aug 16, 2018
  19. Ndirish147

    Ndirish147 Member

    Messages:
    56
    Location:
    Muncie, Indiana
    I guess where I am at is I would like to build a nice sounding amp If that means it would be better to start from scratch then that is what i would do Maybe I can use some of these parts for that? I know that I don't want to use the field coil speakers I'm open to suggestions thank you for the input
     
  20. century tek

    century tek Super Member

    Messages:
    3,340
    Location:
    The Evergreen State
    Send them to me! :D They'll shake my barn :thumbsup:
     

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