• Re: Interstage Transformer Nomenclature - KevinC 13:35:30 01/15/05 (2)
  In Reply to: Interstage Transformer Nomenclature posted by Sidewinder on January 15, 2005 at 09:20:18:

  There are two basic ways to express transformer primary/secondary relationships: as a turns (voltage) ratio or an impedance ratio. The impedance ratio is the mathematical square of of the turns ratio. When one refers to a 1:2 transformer, this means that there are twice as many turns of wire on the secondary side of the transformer, therefore, if a 1VAC signal is present at the primary, a 2VAC signal can be measured at the secondary. If the source impedance of the generator of the signal is 1000 ohms, the impedance measured at the secondary would be 4000 ohms (2 squared). A transformer listed as 1+1:2+2 simply has two separate primary windings and two separate secondary windings. The primaries can be connected in series or in parallel (as can the secondaries), so that the 1+1:2+2 transformer can be used to obtain many different resulting ratios.

  For a SE amp the selection of an IT is fairly straightforward. Normally a ratio of anywhere from 1:2 to 2:1 is used. The step-up 1:2 case is useful to get more "voltage drive" from the driver tube, but presents a higher source impedance to the grid of the output tube due to the impedance step-up (2 squared). Often a 1:1 ratio is chosen, assuming that the driver has enough voltage swing to drive the grid of the output tube to nearly 0 volts. Occasionally, when a low driving impedance is needed a step-down IT is chosen (2:1), but it necessitates a large driver tube voltage swing to overcome the voltage step-down.

  I hope that this basic stuff helps get you started.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  
  

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  • Re: Interstage Transformer Nomenclature - Sidewinder 14:33:43 01/15/05 (1)
    In Reply to: Re: Interstage Transformer Nomenclature posted by KevinC on January 15, 2005 at 13:35:30:

    Thanks Kevin for the jump start reply. If a 1:1 IT is called for, then what are the actual impedances specifications needed? Would they be 5k, 2.5k, or somethink else? If this depends on the driver tube and output tube, what are the rules in determining the impedances?
    Regards,
    David
    
    

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    • Re: Interstage Transformer Nomenclature - KevinC 16:14:44 01/15/05 (0)
      In Reply to: Re: Interstage Transformer Nomenclature posted by Sidewinder on January 15, 2005 at 14:33:43:

      Often the "nominal" impedance spec is given for the transformer to indicate its intended application. Interstage transformers are typically designed to be used with source impedances in the range of 10K-50Kohms. Lower impedance tubes can be used, but will usually result in resonances above 30KHz, although I have never found these to affect the sound substantially. Lundahl interstage transformers have an upper plate impedance limit specified for the connection chosen to indicate whether a particular tube choice might be the best for the application. Take a look at the datasheet for an LL1660, for example. The upper limit for connection alternative S is specified as 15K ohms. A tube with a plate impedance of this value or less will meet the frequency response specifications if used appropriately.

      
      Kevin Carter
      K&K Audio
      www.kandkaudio.com
      
      

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