Tubes and circuit ideas for a vintage speech amp driver

Discussion in 'Amplitude Modulation - AM Fans' started by KM1H, Aug 4, 2018.

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  1. KM1H

    KM1H Ham Member QRZ Page

    Something in the pre octal era that will sound good without excessive work. Mod tubes undecided but likely 100TH's, HK'354's, maybe even a pair of NOS gas free VT4A/ 211's. Ive reached the 860 RF output stage but still looking for a very good 861 or comparable tetrode for the final.

    I do have original QST's if anyone can suggest at least a year or article name/author.

  2. W7UUU

    W7UUU QRZ Lifetime Member #133 Volunteer Moderator Life Member Platinum Subscriber QRZ Page

  3. AC0OB

    AC0OB Subscriber QRZ Page

    Will this need to be single ended or push-pull, and how much power or much voltage is needed.

    Do you want a simple ClaasA speech amplifier to drive a Push-pull ClassB?

    "pre-octal." Well that leaves me out. All of my circuits are for octal and miniature tubes.

    Last edited: Aug 4, 2018
  4. N2DTS

    N2DTS Ham Member QRZ Page

    Whats wrong with octal tubes?
    They are my favorite!
  5. W7UUU

    W7UUU QRZ Lifetime Member #133 Volunteer Moderator Life Member Platinum Subscriber QRZ Page

    Different strokes :)

    I too have an affinity for the "pre-octal" tubes myself even though they significantly predate me. The shapes of so many of them are very different, very unique. Lots of really cool "old time radio" there

    WZ5Q likes this.
  6. K4KYV

    K4KYV Subscriber QRZ Page

    Not all pre-octal tubes ever had octal equivalents manufactured. Not all octal tubes ever had miniature equivalents manufactured.

    For example, one of the most desirable tubes for class-B driver service is the 2A3. There is an octal equivalent, the 6B4G. But there is no miniature equivalent, nor is there a miniature tube type that can equal its performance.
    N2EY likes this.
  7. K5UJ

    K5UJ Ham Member QRZ Page

    A couple of places to look for ideas might be pre-WW2 tube manuals like the RCA TT-3 -- it has speech amps and modulator circuits. Transformer companies like Thordarson had project sheets also.

    note page 17 here:

    sift through american radio history site using google and appropriate search terms example: thordarson

    Here's another one that's still out there:

    Also RCA Ham Tips
    GE Ham News

    Two problems: Getting distracted and lost in all the stuff on line and losing focus. Finding tubes in the RCA 8XX type series that are in good shape.
  8. AC0OB

    AC0OB Subscriber QRZ Page

    I did pick up an Amperex A-21 today (Graphite Plate) at a Hamfest.

    As soon as I can find a spec sheet/base diagram I'll check for filament continuity.

  9. W7TFO

    W7TFO Subscriber QRZ Page

    Maybe build it after the audio stages of the SP-200. It used PP 6F6's, just sub 42's.

    Drive the 1st stage with a 76.

    That worked well in the SP-10 & 100.

    KC8YLT likes this.
  10. AC0OB

    AC0OB Subscriber QRZ Page

    The A-21 4-pin tube has good filament. Plate is ~ 2.25" Long by 1.5" wide. Total length is 9.25".

    It looks similar to an 813 that's been lengthened considerably.

    Apparently it was an industrial heating tube. I still don't have any specs on it as yet.



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