Siemens 2754 (also found as 2735) tube amp as found. 50W power output from a pair of EL34 tubes driven by a pair of EF40s.

  This is a very nice made unit, which offers the possibility to connect a volt meter using built-in banana jacks to control the bias. Here you should read 0.5V.

  Unfortunately the output transformer only has a 200 Ohms ouput winding to connect a 100V lines system.

  So here I show how to install a Klangfilm OPT with 8 Ohms secondary winding to make a hifi amp of it. Great sound!

  Before any mods I recommend to ground the amp's chassis and to ground the input (connecting pin 4 and 5 on the input panel).

  Cut all cables from the transformer. Make sure you still have a connection from Pin 4 of the upper row to Pin 2 of the row close to the volume pot. Remove the old OPT and save the screws.  Now install the new OPT. It is easier if you remove the control plate before.

  Wiring of the new output transformer:

1 The feedback loop

1a solder the two leads with same color (green/yellow/red) together. Make sure they are well isolated. The other two wires should have a resistance of about 10Ohms now.

1b solder the red/yellow/black wire to the third connector

1c solder the red/yellow cable to the 9th connector

1d parallel the 20k resistor with a 5k resistor

2 Connect the center tap of the plate winding (two leads in one isolation) to the filter cap.

3 Connect the plate windings (the winding from plate to plate has a DC resistance of about 240 Ohms) to each Pin 3 of the EL34 sockets.

 The 2754 will work perfectly without any further modifications on the circuit. Parallelling the 20k resistor in the feedback loop has to be done as the new OPT provides a slightly lower feedback gain. If you only use the 20k resistor you have a very low feedback. Give this a try. Sometimes this sounds very nice!

Some test results:

50 watts sine wave into 8 Ohms

42 watts into 16 Ohms

38 watts into 4 Ohms