I have recently completed a DR504 clone with use of the TT chassis as offered for the YAH100 clone. The bias PCB is used to provide individual bias adjustment for each EL34 tube. The DR504 has only 2 of these output tubes. No need for matched pairs of these anymore. I slightly changed the assembly of the bias PCB. The 2 pot.meters are soldered at the reverse side and provided with thumbweels. A small hole is now enough to make the neccessary adjustments. It also prevents to enter to deep into the chassis with the screwdriver. Spare holes for unused tube socket locations have been blinded with filler plates. The blue item on the output socket is a varistor. This(these) is added for additional protection on wiring from both output tubes to the OT primary connections

René

Hi Rene,

how do you connect the varistor?

Pin 3 - Varistor - GND ?

Yes, pin 3 to gnd on each tube. I have also added 3 each 300V varistors on the IEC entry socket for additional protection.

mfrgr,

René