Tube DIY Asylum

Posted by Ray Moth (A) on October 07, 2005 at 08:23:20

Does anyone know why Radford used a 6U8A triode-pentode as a long-tail pair phase splitter? It occurs in the STA25, for instance, and it seems a very strange arrangement to me but perhaps I'm missing something?

• Re: Odd splitter in Radford amps - fourhooker 13:33:39 10/08/05 (1)
  In Reply to: Odd splitter in Radford amps posted by Ray Moth on October 7, 2005 at 08:23:20:

  Hi Ray,

  This circuit was described by A. R. Bailey in a three page article, entitled "New Phase-Splitter" appearing in the September 1962 issue of Wireless World. The author addresses the high-frequency roll-off displayed by the ECC83 in the conventional long-tail-pair phase-splitter (28db gain, but 2db down ~ 12khz). He achieves 24db gain with the ECF82 tube, but the high-frequency roll-off does not commence until 40khz, and does not fall 2db until after 100khz....

  I am happy to respond to E-mail..

  Graeme
  
  

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  • Re: Odd splitter in Radford amps - Ray Moth 19:50:40 10/08/05 (0)
    In Reply to: Re: Odd splitter in Radford amps posted by fourhooker on October 8, 2005 at 13:33:39:

    Thanks, Graeme, that makes sense, especially if the input stage to the splitter has a high OP impedance. The additional loading on the first stage of an ECC83 splitter's Miller capacitance could also cause distortion, I suppose. Still, I wonder how Radford achieved balance with such an unlikely arrangement and why no other designers ever used this idea?
    
    

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• Re: Odd splitter in Radford amps - Eli Duttman 18:17:09 10/07/05 (1)
  In Reply to: Odd splitter in Radford amps posted by Ray Moth on October 7, 2005 at 08:23:20:

  Are you sure it's a LTP? Scott used pentode/triodes for gain and splitting in a paraphase circuit.
  
  Eli D.
  
  

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  • Re: Odd splitter in Radford amps - Ray Moth 19:55:12 10/08/05 (0)
    In Reply to: Re: Odd splitter in Radford amps posted by Eli Duttman on October 7, 2005 at 18:17:09:

    Yes, I checked for that. The second (triode) section of the splitter has a grounded grid and is cathode-coupled to ther first )pentode) section. It looks like a classic LTP, except for the fact that one half is a pentode and the other half is a triode.

    I've never seen this design anywhere else. Radford could have used a triode-hexode frequency changer tube for this purpose, I imagine!
    
    

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