Geezer wrote:I have an OEM ST70 PCB I'm using in a "why not" build using an Antek toroid PT and additional small trannies for the rectifier and bias...the toroid is 360V@ 550mA x2, 6.3V@4A x2 (more than enough). I have a 5V/4A Hammond that should serve the 5AR4. I can't find info on bias current required in the ST70, and the stock PA-60 55V tap appears to be high amp. I have a 36V/140mA Hammond, is this adequate for the bias supply of the VTA70? If not, what transformer rating do i need?
The Dynaco bias tap is 55VAC and after it is rectified it is -75 roughly.
It doesn't take much current wise for the bias supply. Your transformer in my opinion will work better if you make a negative voltage doubler circuit in order to get the supply higher in voltage. This will mean that you will have to play with the resistor values a little to get a nice compromise. My 70 has a switch that I installed so I can run KT88's and bias them correctly. I built a bias supply for a pair of Wurlitzer 6420's that I own. I used a voltage doubler in that circuit and it works great as I have individual tube bias and can run a number of different output tubes as a result.
A while back I built a Dynaco with an HP transformer that didn't have a bias tap. As a result I mounted a small transformer under the chassis and made my own doubler circuit.
I think there is a schematic on my site of the bias circuit I made.