Replacing 7199 with two octal tubes...

Discussion in 'Tube Audio' started by knockbill, Sep 13, 2017.

  1. knockbill

    knockbill Addicted Member

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    I was looking at the 30W PP amp schematic in the back of RCA manuals... it uses a pair of 6L6GC,, and a 7199 pre amp/inverter tube... I have a pair of RCA NOS 6L6GC black plates,,, and a single suitable 40W output tx...

    The thought behind this question is I have a Challenger PA amp chassis which I built into a Bogen 6L6 mono amp, with the funky tone controls (remember Mellow and Crisp?!!)...
    I was never satisfied with the tone controls, and since I never had either of the original TXs,,, Figure I'd try to build a hi-fi amp on the chassis... There are 6 octal sockets holes... So,,,, IF I can find an separate octal for each half of the 7199,, use the two 6L6s, a 5V rectifier,, and the "Bass and Treble tone control stage" (6EU7 replaced with 6SL7) also in the RCA manual, I can fill all the octal sockets...

    Not sure if there are any direct octal replacements to each 7199 section, which would match the schematic,,, if not, I would scrap the project...
    Thanks...
     
  2. battradio

    battradio Electron trainer Subscriber

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  3. knockbill

    knockbill Addicted Member

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    Thanks for replying,,, They are the two I was looking at, but I didn't think they matched the 7199 sections close enough,,, will they work with the cap/resistor values the 7199 sections use?
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    probably not, I'd expect to have to fiddle with at least the screen resistor to get the voltage right on the pentode. If its direct coupled that becomes even more important to get the inverter balance correct.

    A 6J5 is also an option for the triode. Its half a 6SN7.

    Related to that, a Williamson front end with a pair of 6SN7 driving the pair of 6L6GC wouldn't be out of place.
     
  5. knockbill

    knockbill Addicted Member

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    Thanks for replying,,, I have an OPT I believe would work with the RCA schem... Williamson amps use pretty high end, special OPTs, if I'm not mistaken... Here's a link to the RCA manual schem... Modifying the value of the preamp-inverter tubes would be a little above my pay grade, I think! Then there's the FB mods to deal with...
    Tried to load the page the schematic is on, but it loaded the whole manual...
     

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    Last edited: Sep 13, 2017
  6. gadget73

    gadget73 junk junkie Subscriber

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    yeah, they want good iron. Its all the feedback they ran, poor trafo characteristics tends to make them very unstable.

    7199 appears direct coupled, pentode plate straight to triode grid.

    What you want there is for the voltage drop across the triode plate resistor to match the drop across the triode cathode resistor. The resistors should be the same value, so the plate voltage of the pentode is what has to change. Messing with the screen resistor is the easy way to get there.
     
  7. knockbill

    knockbill Addicted Member

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    So you think its possible? I kinda like the Challenger/Bogen mix,, but that tone stack just sux,,,, I think you saw that amp while it was in its 1st iteration (6V6),,, I don't have the original OPT for either the Challenger or Bogen schem, so I thought I'd start over,
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    Don't see why it couldn't work. The stuff in the RCA manual wasn't exotic but it worked reasonably well. They did make the tubes, seems reasonable they could figure out how to make them do something useful.
     
  9. knockbill

    knockbill Addicted Member

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    Agreed,,, I have had good luck with RCA schems,,, The phono pre amps work pretty well,,, figured this 6L6 amp looked simple enough, and I'd add their Bass -Treble stage just to see how it works, and fill all the chassis holes!!!
    Collecting winter project ideas...
    Hope to find some 1625 sockets at K'town, that would be the next project... Need any parts or PTs? I can bring em with me...
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    I actually just picked up a pair of output trafos to get something assembled and out of my way. Think I'm fairly OK otherwise. I appreciate the offer though.
     
  11. knockbill

    knockbill Addicted Member

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    OK,,, just a thought... figured I'd try to make room for whatever is following me home!
     
  12. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The RCA schematic shows DC feedback from triode cathode to pentode screen, so this circuit is likely to settle in closer to the intended operating point than most of its type, even with slightly different tubes.
     
  13. knockbill

    knockbill Addicted Member

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    Thanks for replying,,, are there any cap/resistor changes needed to use 6SF5 and 6SJ7? I can easily adjust the extra heater current by changing the PT if necessary, the Bogen uses the same tubes except for swapping one 6SF5 to the 6SJ7... Just want to get the pre amp and inverter right...
     
  14. Blast

    Blast I Wanna Rock Subscriber

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    Also, be sure the filament current from the extra tubes doesn't out do that of what the PT can put out. Sounds like a cool project!
    Edit: Oops- missed your last post, knock bill.
     
  15. dewdude

    dewdude I fix stuff.

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    Quite a number of jukeboxes used 7199s, and often there isn't room to put an octal in it's place. One such amp that comes to mind are most of the 6973 based amps.

    We've had decent luck using the 6U8 to 7199 adapter TubeDepot sells.
     
  16. arts

    arts Super Member

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    You could always try a 6N7 as the phase splitter,just turn it into a single triode by tying the dual grids and plates together as recommended by the RCA manual.Just suggesting this as I have a ton of these and have been wanting to try this myself:)
     
  17. 604man

    604man Active Member

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  18. knockbill

    knockbill Addicted Member

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    No need for adapters,,, this is an octal tube project,,, using what I got!!!
     
  19. knockbill

    knockbill Addicted Member

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    I used a 6N7 in the 6AS7PP amp I built, it really sounds good, uses a pair of Pioneer OPTs I salvaged,,, man those 6n7s get hot!
     
  20. arts

    arts Super Member

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    Are your's the metal ones? I have heard that they were also once made in glass,but have never seen any. For such a small tube,it can certainly output a fair amount of wattage as a class B amplifier too.
     

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