VT25, VT25A Push Pull amplifier Part 1

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The 10 is a very old triode and probably one of the first mass production power tube. It's linearity and sound qualities brought this tube a favorite among many DIY's. Dozens of amp and preamp using this tube and it's relatives (10Y, 801A, VT25) can be found on the net. The only and main drawback is it's flea power.
Long time ago I decided to build an amp that could drive speakers with 95/98 dB efficiency and the logical solution was push pull operation.
In that way I began modeling an AB1 operation push pull to reach 2 goals, higher output power (about 4/5 watt) and better amplitude distortion figure ( Push pull operation helps to compensate characteristics curvature). For that purpose I had on hand a pair of Tango FX50/16G, one of the finest transformers of Mr Hirata company.



Sylvania VT25 in "modern" ST shape

To get the very best of these tubes a good driver with high output voltage and low distortion is mandatory. Furthermore the perfect way to split signal in two is by inter-stage transformer. The main difficulty is to find one that accepts some primary current with large bandwidth, I finally selected the Tango NC14 for its very good reputation and its current handling capability.



To provide enough voltage a two stage driver was necessary. After several tests with different dual triodes my choice remained on the 5687 tube.
However due to driver limitations and NC14 step down voltage ratio (1/0,7) I can't go very deep in AB. Operating point is defined by plate voltage ( 415V ) and cathode resistor (3,3 Kohm) giving a steady plate current of 12 mA @ Vg -38V. The load (8 Kohm) of each tube give me the maximum plate voltage swing, here 275V peak.



5687 operating points have to be carefully chosen to give a large voltage with minimum distortion. It is especially critical for the 2nd stage as anode current must remain in the 15/18mA to keep good bandwidth in low frequencies from the NC14. To drive the VT25 full power we need to swing grid ± 38V peak, it means ± 55 V anode peak for the 5687 due to transformer voltage ratio.



Amplifier circuit



Power supply circuit
Two independent 7.5V supplies for VT25/VT25A tubes (just one shown on schematic)



Construction details

To check power tubes bias voltage I used a 6AF6 magic eye, although not accurate it gives an original look to the amplifier.



There is a bonus, I can use VT25A tubes without any change in amplifier circuit.

Sylvania VT25A big plate triode and oxide coated filament. Power rating seems to be the same but gives a bit more output mainly because of its lower internal resistance.