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previous: rise amp gurus,please comment on... -- 9/12/1999 4:09 AM View Thread

Re: Gibson Tranny Driver ??

9/13/1999 5:48 PM
J Epstein
Re: Gibson Tranny Driver ??
Well, the final preamp tube (V5A) is loaded with the primary of the driver tranny, instead of using a resistive load. The primary side is just like any single ended power output stage, see the tweed Champ schematic for an example, or the usual Fender reverb driver using a transformer, as another example. (You can hook up a speaker to that transformer BTW, it is a -quiet- 8 ohm output!)  
This primary is magnetically coupled to the centertapped transformer secondary (because both coils are wound on the same transformer core,) which does the phase splitting duty and feeds the grids of the pushpull output tubes.  
This transformer may step up or step down the drive voltage or it may keep it the same, it depends on how it is wound, but anyway by being center tapped it replaces a phase splitting tube or tube network. (Actually, the transformer phase splitter probably came first historically.)  
DC conditions in the output section are governed by the bias (-20.5VDC) which is applied at the center tap, the output tubes' cathodes are grounded so bias will be -20.5V between the grid and the cathode.  
It's an older way of doing it but it's actually simpler and probably should sound pretty good. Check that bias voltage for a start? This circuit would be called Push-Pull Interstage Transformer Coupling with Fixed Bias.  
Note that you *could* ground the center tap of the IT (interstage transformer) and put self-bias on the output tubes by adding cathode resistors between cathode and ground, as long as there is that 20.5V between grid and cathode you will get more or less the correct operating point. You could even do a combo : pick cathode R's that give you 10V between cathode and ground, and apply 10.5V at the center tap, etc.  

Farrow Also, if your tech guy doesn't want... -- 9/14/1999 9:47 PM