Balanced Push-Pull Mic Amp / Driver

Discussion in 'Technical' started by W5HRO, Apr 8, 2014.

  1. W5HRO

    W5HRO Administrator

    Here is a balanced push-pull mic amp / driver design for use with a D104 or other balanced mics to drive tubes such as 810's, 811's etc. It also features AVC control. The neat thing is it has full push-pull voltage swing via the first stage and that eliminates problems when using single ended stages because you don't need any interstage transformers. The 2nd stage also acts as a compression / AVC control stage. That means you can have the exact same peak-to-peak output voltage level at the final modulator tube grids with low level voice tones and high level voice tones both :surprised:


    Anyway, I will be modifying my current speech amp driver to the circuit above in my HB transmitter. Most of the values in the drawing should be correct or pretty close and I will post an updated drawing when finished if anything changes.
  2. K4TQF

    K4TQF Member

    Balanced indeed ! :icon_thumbup:
  3. W5HRO

    W5HRO Administrator

    The only issue some people might run into is not having the driver transformer with around the 2:1 needed step-down ratio. It couples the 2A3/6A3/6B4G plates to the 803/805/810/811 grids.

    Either the old UTC-9 or the Stancor A-4761 will work. The stancor's are the hardest ones to find, but UTC-9's pop up on the web all of the time like on ebay.

  4. K4TQF

    K4TQF Member

    Brian: Did you whip that out on pspice? That looks like a great design. and...speaking of grid bias... one of my favorite audio amps of all time is the Fischer AZ-80. It and the Brook you posted both use self-bias as does the one in the diagram above. This goes to the same thing you were addressing about my PP250TH needing a little bias swing on the grids. It's all coming back to me now... this is much better than working crossword puzzles ! :icon_thumbup:
  5. W5HRO

    W5HRO Administrator

    I used LTspice and that's the company I work for. Been there almost 10 years now. High performance analog devices.

    Anyway, the Brook amp design uses the bias shift scheme on the 6B4G grids to get more power out. That's not really the same thing as my push-pull circuit here. The 6SK7 is a remote cutoff pentode that was designed for AVC control and to handle the large voltage swing from the 6SJ7. Early receivers often used 6SJ7's for the RF amp stages then 6SK7's for the IF stages and AVC control. That's the same concept I'm using here, but the 6SJ7's are the mic amp stage and the 6SK7's are the compression stage with AVC control.
  6. K4TQF

    K4TQF Member

    Grid resistors to ground... tune for minimum smoke... Got it ! ::)
  7. W5HRO

    W5HRO Administrator

    Not really... ::) ... :mrgreen:

    Grid resistors to ground and using cathode bias instead isn't always the best solution. One example would be with the 2A3/6A3/6B4G. Using fixed grid bias is really better, but in my case I have a KW1 driver transformer designed for 5K plate-to-plate. If I went to fixed grid bias it would change it to 3K plate-to-plate and throw my ratio off. However, it doesn't erase the fact that I'd get about 5 more watts out and less distortion using fixed grid bias instead. Every situation is different.

    The Brook amp attached below varies the 6B4G grid bias to gain more power out. It's fixed bias and not cathode bias, but they auto vary that fixed grid bias voltage to get about 30 watts out :surprised: It's actually a good idea, but more complicated.

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