Bucking Transformer For Vintage Receiver

I built this bucking transformer for the owner of the HH Scott 399 with Scott 355 FM multiplexer I am refurbing. A bucking transformer reduces AC line voltage. This tube gear was built in the early ‘60’s when AC line voltage was ~ 117VAC. The schematic specifies all voltages shown are at 117VAC line. Today we can have anywhere from 118 to 126VAC at the wall. 122 and higher is common for me.

When you supply higher input voltage to a step-up transformer, you get higher output voltages. This means the 6x4 and GZ34 rectifier see higher AC voltage and respond with higher DC output voltages. Tube heater voltage is increased, etc. Lots of voltages to verify if you want it reliable with long tube life. Or maybe I worry too much.

The Scott 399 uses 7189 output tubes. Max plate voltage spec is 400VDC. At 122 VAC line, depending on bias, I can exceed the 400VDC plate voltage. At 125VAC, even higher. I could use 7189A tubes which have a higher plate voltage rating, but now I’ll have to change resistors to get more negative DC bias voltage. I could also add drop down resistors after the transformers and change the value of DC drop down resistors. Some folks use variacs to lower AC voltage. To me, the best solution for this situation is to use a bucking transformer which reduces line voltage and not re-engineer the 399.

Voltage reduction and amperage depends on transformer used. A web search will bring up numerous sites and conversations about them. I chose a Hammond 166N6, a 4 amp transformer that can provide 3.7% and 7% reduction. 4 amps is ample for both the Scott 399 with 355 multiplexer.

I found the schematic pictured on the web so I can’t take credit for the design. I have made some changes to it. Simple bucking transformers have no AC volt meters or fuse, on/off switch. Some folks will split the duplex outlet with the top outlet at 3.7%, and the lower one at 7% reduction.

I wanted something fused with an on/off switch so the transformer isn’t drawing power when not used. I want to see input AC voltage and chosen output voltage. A selector switch offers line voltage, 3.7% and 7% reduction. I added a small toggle switch so he has the option to shut the AC volt meters off.

I received the incorrect size strain relief for power cord and the correct one is on the way. Connections under wire nuts are soldered. All wire terminals crimped and soldered.

The DVM's read line voltage and voltage from GZ34 before R172 step down resistor. Spec there at 117VAC is 380VDC. At 122.3AC line, I'm at 421VDC. IIRC I get 24VDC drop down after R172, or 396VDC. Spec there is 360VDC. GZ34 and R172 at top left of schematic.






Rocker switch is on/off. Left meter, line voltage. Right is output voltage of upper and lower outlets.





The toggle switch allows turning the AC voltmeters off.