Curcuit ideas for scratch build needed

Discussion in 'Tube Audio' started by Rex Everything, Dec 11, 2008.

  1. Rex Everything

    Rex Everything are you Shpongled? Subscriber

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    Not sure if you all have noticed but I recently came into a few Baldwin mono tube amps from a few organ cabs. In talking about these here on the forum Master Jay has suggested that it may be better to change the curcuit to something more suited for HiFi.

    This being said I come to the gurus here on the AK tube forum to ask for your suggestions on curcuits to build. The amps currently run 4-6l6's, 2-5U4's and a 6SL7 per amp. I am not married to this configuration. Infact someone had mentioned going to 6550's which sounded pretty good to me(Coming up with 8-6L6's would get pretty expensive).

    I am open to all of your ideas so draw 'em up, post 'em up or what ever. Let's see those ideas:thmbsp:

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  2. bricktop

    bricktop Couch woofer

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    What kind of test equipment do you have? It's hard to come up with something from scratch not knowing much about the transformer. Any designs using global feedback, (which I'd suggest for a pentode amp) will need tweeking with a square wave generator and an oscilloscope.

    You could do a front end like a pentode + triode cathodyne (much like an ST70 but with a cheaper, better tube). Also, i hear the bias is from a single shared R, you should split that up for best balance. A pair of 6550/KT88 is definitely doable with that iron I believe.

    Let's start backwards...what type of power tubes do you wanna use?
     
  3. shelly_d

    shelly_d Not An Audiophool

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    I would assume that you would want to keep the existing power transformer. That implies running in the same voltage / current range that the 6L6 does. for plate voltage and current the EL34 can opperate in the same envelope but it draws considerable bore heater current. It is a fine sounding tube and can put out similar amounts of power. You would probably need to add a supplemental transformer for the heaters to avoid overloading the existing power transformer.

    I think the output impedance of the EL34 is I think a bit lower then the 6L6 but that may prove a good thing using the same output transformers. It might provide a slightly better damping factor and slightly more control of the woofer.

    Just my $.02 worth. I'm sure that there are far more knowledgeable opinions out there.

    Good Luck
     
  4. GordonW

    GordonW Speakerfixer Subscriber

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    Well... Here's my thoughts, at the start:

    1) 6550/KT88 could be a good tube. The primary impedance of the output transformers should work with a single pair. It'll be a little low, but within accepted limits.

    2) The 6550/KT88 likes a bit more voltage than the 6L6... but that should be do-able. Replace the 5U4 rectifiers with 5AR4s... that should bump up B+ to levels that the 6550/KT88 could thrive on. Also, they are slow-start and lower heater demand... all good things.

    3) You didn't mention whether the output transformers are ultralinear (having 40% taps) or not. That would make a big difference in what I'd suggest for circuit types.

    Regards,
    Gordon.
     
  5. Rex Everything

    Rex Everything are you Shpongled? Subscriber

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    Yes, I'd like to stay with the same iron aside from that up to change/replace any and all components for this build. I'd like the best amps I can get from these units.

    As for test equipment all I have is a meter and an O-scope I haven't used yet. I'm hoping to get some help from a friend on these that has all the test equipment. I guess I should look inot a signal generator.

    As for tubes I'm not really wanting to use EL34's on this but I do have plenty if that's what works out best. I have a fresh matched quad of GE metal based 6550's that I wouldn't mind using. It could stay dual rectified w/ the 5U4's if that is best or I think I have a pair of 5AR4's or could get a pair.

    I am a noob to building and theory with tubes but I want to learn to build amps and thought this would be as good a place to staret as any. Some discussion on the transformer has already been done here:

    http://audiokarma.org/forums/showthread.php?t=197325

    Transformer was mentioned to be a 3K8

    Looking forward to all of your suggestions
     
  6. Rex Everything

    Rex Everything are you Shpongled? Subscriber

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    I don't know the answer to this question or how to find it but would be willing to find out with some guidance if possible.
     

     

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  7. jon_s

    jon_s AK Subscriber Subscriber

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    There are no ultralinear taps, so we are stuck with full pentode or triode. IIRC, there is already plenty of voltage (~450VDC), and swapping rectifiers will just mean that you need more expensive capacitors. I would not worry about that, esp if you already have a nice quad of 5U4s!

    Those 6550s would be awesome, but definitely use cathode bias to protect them from mistakes.

    I was gonna rip my Baldwin amps up for parts, but maybe I will play along and turn them into something "good" over the holiday ... lemme think about that!
     
  8. bricktop

    bricktop Couch woofer

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    a pair of 6CL6s driving a pair of 6550s would be the bees knees :thmbsp:

    Could do a 6SN7/6CG7/6DJ8 voltage amp to cathodyne to a pair a of 6CL6s driving the 6550s with a butt-load of plate feedback from the 6550s to the 6CL6s. The second stage cathodyne would remove the need to make a constant current source for the tail of the 6CL6 pair. This amp could rock :yes:

    Jon...new amp project?
     
  9. Kegger

    Kegger R.I.P. 1/12/1966 - 6/1/2017 Super Mod Subscriber

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    Push pull triode may be a good option, Loop feedback wouldn't be "needed" to make
    them perform well.
     
  10. bricktop

    bricktop Couch woofer

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    Certainly!...the 6SN7+6CL6 setup would have plenty of gain to drive triodes and a 6CL6 pair can make very large voltage swings at low distortion. To get some power back you could go class AB2 and use some FETs...getting crazy now :D
     
  11. Rex Everything

    Rex Everything are you Shpongled? Subscriber

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    You all are definitely talking way over my head.

    I'm sure hoping Master Jay will be willing to help this young Jedi in training :D
     

     

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  12. bricktop

    bricktop Couch woofer

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    I think I owe a few people some schematics. When I get some sit-down time at home I can scribble a starting point schematic for this too if ya want :thmbsp:
     
  13. jaymanaa

    jaymanaa AK Subscriber Subscriber

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    I'm digging out right now to clear a bench (just a couple to go).:thmbsp:
     
  14. Kegger

    Kegger R.I.P. 1/12/1966 - 6/1/2017 Super Mod Subscriber

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    "6SN7+6CL6 setup would have plenty of gain to drive triodes and a 6CL6 pair can make very large voltage
    swings at low distortion. To get some power back you could go class AB2 and use some FETs...getting crazy now"


    Now Brick that's something you or I might do, but should we keep it a bit simpler
    for the other people that aren't quite as crazy as us? :)

    Quick think was (we have 4 7AC pinots, 2 5U4 pinouts and an 8BD 6SL7 pinout)

    Easy says 2 5AR4, 2-4 6L6, KT88, and a dissimilar 8BD pin-out like a 6EM7, high gain
    triode direct coupled to the low mu then a concertina phase inverter driving outputs?
    Use the 6EM7 for the KT88, then "roll" in a 6SN7 for the 6L6 might be workable aye?

    I think something like that would have the drive. Just tossing it out there though.
     
  15. bricktop

    bricktop Couch woofer

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    Got a little excited there, but back under control now... :D
     
  16. Kegger

    Kegger R.I.P. 1/12/1966 - 6/1/2017 Super Mod Subscriber

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    Something along the lines as this maybe?

    Quite a few of the connections in the amps will allready be wired. :)
     

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  17. Rex Everything

    Rex Everything are you Shpongled? Subscriber

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    Thanks for the schematic. I'll have to wait for someone to interpret some of it for me :)

    I do want to get away from 6l6 right now as I don't have any 6l6 tubes. I have 6550's and EL34's for output tubes around here
     
  18. Kegger

    Kegger R.I.P. 1/12/1966 - 6/1/2017 Super Mod Subscriber

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    No biggie, sub the 6L6 with KT88/EL34, same wiring/layout. (6L6 is just there for looks) :)
     
  19. jon_s

    jon_s AK Subscriber Subscriber

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    BTW - If not using two of those sockets for output tubes, how about use them for octal driver tubes? :)

    Could easily do a 6CG7->6J5 Williamson. Or, use a pair of 6AC7 pentodes instead with plate feedback. Similar to the 6CL6 approach, but takes up more space and doesn't require new holes. I like the idea of using a cathodyne instead of a CCS - less work!
     
  20. Rex Everything

    Rex Everything are you Shpongled? Subscriber

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    Good stuff so far guys but how bout some more thought and ideas and a few more drawing?

    Jay, any feedback on any of these ideas? Your input is important if your gonna walk me though this :yes:


    :D
     

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