Converting a 240VAC set to 32V DC operation is really doing things backwards these days. After all, how many people have a 32V DC supply available to power such a set? However, as well as checking out the performance differences after the conversion, I also wanted to determine which valves worked best with low HT voltages, particularly in the audio section.

A redundant 240VAC mantel set was chosen as the guinea pig for this task. It proved to be an extremely interesting project, with some unusual challenges in design. A set with only 32V HT is not as "tame" as many might think. It ended up being a really hot performer – better, in fact, than the original set and still with only five valves.

Ho-hum circuitry

The design of typical 4/5 valve mains-operated receivers is rather "ho-hum" to most restorers. Typically, they include a 6BE6/6AN7 converter, 6BA6/6N8 IF amplifiers, a 6AV6/6BD7 detector/first audio stage, a 6AQ5/6M5 audio output stage and a 6X4/6V4 rectifier. The circuitry is all fairly predictable, with 200-250V HT on the plates and about 85V on the screens of the RF valves. The bias is usually obtained from a back-bias network.

Of course, there can be quite a lot of variations in the circuitry and valves used but it’s usually nothing out of the ordinary.