In one of the Sticky's on the first page of the Forum there is a thread about running your Dynaco ST-70, Mark II, Mark III or Mark IV in the triode mode. There is a basic diagram of a switching system that may be used to go back and forth from triode to pentode. The diagram below gives a little more detail. All you really need is a pair of DPDT switches that can handle 400+ VDC for the ST-70 and one DPDT switch for each of the Mark II, III and IV mono amps. You will also need two 100 ohm 1 watt resistors for each switch. The thing you have to decide is where to put the switches? If you have upgraded your stock driver board to the VTA driver board the choice is easy. Pull the original bias pots and you have a nice 3/8" hole to install the switches. I have seen amps that holes have been drilled on the back or top surface of the chassis. It really doesn't matter where you put the switches but if you do, consider easy access to them as one criteria for placement. Use good quality wire to wire them up. In general what you will be doing is pulling the green or green/white wires off pin #4 of each output tube and run them to the switch instead. What I like to do, since the bias diode is on the VTA board, is to install two 2 lug terminal strips to use as junction points for one leg of each 100 ohm 1 watt resistor. If you have the original driver board and have replaced the selenium rectifier with a diode, then just run a wire from lugs 3 and 6 on each switch back to pin 3 of each output tube and put the 100 ohm resistor at the end of the wire and connect the other end of the 100 ohm resistor to pin # 3 of each output tube.
Bob
The diagram below is for an ST-70. On a Mark II, III and IV you would just use one switch for each amp.
Bob
The diagram below is for an ST-70. On a Mark II, III and IV you would just use one switch for each amp.
