SE 12AX7 and 6BQ5 schematics please

Discussion in 'Tube Audio' started by Dandy, Oct 2, 2013.

  1. Dandy

    Dandy Super Member

    I'm having another look at the little SE 6BQ5 amp I got from a scrapped record player. The original schematic is attached to this thread.

    The feedback went via the tone and volume pots. Another feature is that the output transformer has a tapped primary and the B+ at the reservoir capacitor (C67) immediately after the rectifier goes to the tapping on the transformer at a point on the secondary so that the ripple current flowing in the output valve anode cancels with the ripple in the B+ current flowing to the rest of the amp including the smoothing capacitor (C50). This makes it nice and quiet in operation.

    When I removed the tone controls and made it a power amp to use with a 50k pot, I used the values of the attached schematic. It works, but now I don't know where/how to apply feedback! Without feedback there is some high frequency distortion. If I apply the 22K feedback, either to cathodes as in the 'new' circuit, or the grids as in the original one, it oscillates with HF.

    So, I'm thinking of starting again. Any favourite schematics?

    Attached Files:

  2. Pio1980

    Pio1980 AK Member Subscriber

    For a stereo amp using those bubs, a twin of this example of German boilerplate is about as good as it gets.
    Last edited: Oct 2, 2013
  3. Tom Bavis

    Tom Bavis Audiophool Subscriber

    Try reversing the primary leads on the output transformer - if feedback phase is reversed, it's POSITIVE feedback, and will oscillate. If you're using the primary B+ tap, reverse the secondary - either one will do...
  4. Dandy

    Dandy Super Member

    Thanks Tom. It's a while since I worked on this, and I thought I'd tried reversing those leads, but the point of asking for help is to look again! Will do.

    By the way, please can someone explain to me the symbol (capacitor inside a circle with a dot) between the 330k resistor and ground at the end of the power supply in my attachment?
  5. Tom Bavis

    Tom Bavis Audiophool Subscriber

    Neon bulb - pilot light.
  6. Dandy

    Dandy Super Member

    Thanks again Tom. Wired like that to show the rising B+, I guess.

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