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Saturday, December 21, 2013

Interstage Transformer



Interstage Transformer

If you noticed, we like to use inter-stage transformers on our design. Actually interstage transformers are not something new. It was available in very old designs. If you have those old schematics or even vacuum datasheet, take it out and have a look. You will find a lot of them being mentioned everywhere.

It is being used lesser now with the availability, cost, quality improvement and advancement of capacitor technology. Furthermore, interstage transformer is huge, heavy and expensive, especially for wide bandwidth high quality types. 

Interstage transformer is still not as common capacitors but popularity is gaining, and choices are limited to only a few famous manufacturers, not to forget that the price is expensive! Design of a good interstage transformer is not easy. To get a 1:1 ratio or even 1:2 ratio, the windings or turns on the interstage transformer will be significant as it would need the same or more windings for primary versus secondary. In ordinary output transformer, usually secondary will require less winding as it needs to drive low impedance speakers, and will be easier to manage. 

Due to such limitation, we would need to set a few criteria before we move on to determine what kind of interstage transformers are more feasible to be designed to obtain wide bandwidth and good sound. 


  • Use low Rp (plate impedance) tubes – low Rp tubes will require low transformer primary impedance and therefore cut down the number of windings needed. 
  • Use high trans-conductance tubes or tubes capable of >0.5W output power – required for high bandwidth and sufficient drive strength, especially important for 1:2 or 1:4 step-up interstage transformer. 


What are those tubes, you may ask. We can recommend: 5842, 6C45PI, 5687, 6BX7, 8233, EC8010, 6BL7, WE437, E55L, and similar tubes. 

This way, we can limit the primary impedance of the interstage transformer to be ~5K and the secondary could be at 5K, 10K or 20K, depending on requirements. It is still not too bad to squeeze all those wires into the interstage transformer. 

Benefits of interstage transformer, I will not touch here since there are plenty of articles out there. One thing I must bring up is the use of interstage transformer on class A2 design. Take 801A for example. In class A1 design, the output power will be very low even if we drive it to 600V or so. If we drive the 801A into class A2, the power could be doubled to ~7W, not mentioning the linearity of the operating region and that will require interstage transformer. Of course, a powerful driver tube is required to produce power / current as at A2, the 801A or similar transmitting will be drawing grid current. 

Now that really brings out the need of having an interstage transformer - double the output power with just using the same tube. Now that's what we call magic!

J&K Audio Design
21/12/2013

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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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