Dynaco MK III with 6146A tubes.

[Duo]

An interesting modification out of necissity happened today...

I had my old Mark III out on the bench today after ten years of being moved about and not rebuilt.

It was missing the output tubes, power cap and choke, and some other tidbits.

I finally got the desire to make it work again, except I had no proper parts to fit the original specification.

The best thing on hand? 6146A VHF radio transmitter tubes as outputs, no choke, and bigger caps.

With some rewiring, I used old computer SMPS caps in the PSU, lots in series with resistor dividers to share the voltage drop.

With thi 6146, triode or ultralinear operation requires really really high bias in order to get the plate current down to the necessary level. In my case, about -200V to reach the proper plate current.

Instead of redoing the bias, I removed the U/L configuration and added a regulated 150V supply for the screens. This consists of an OA2 tube with a cascoded pair of SMPS power transistors to handle the necessary 450V drop and boost current.

The new topology sounds very good, with little modification to the actual dynaco circuit.

A picture or two: