Darren Holdstock
- Joined Feb 10, 2009
- 262
Bill and Bertus are quite right, the heater supply voltage can be either 6.3 V DC or 6.3 V AC rms. Traditionally AC is used, but only because it's the cheapest method as the heaters were fed directly from one of the secondaries on the power supply transformer. It has the drawback of introducing mains hum into the signal path, to a greater or lesser degree depending on circuit and layout. Twisted pair wires to the heater supplies are a proven method of keeping the hum field down, but I think for the purposes of this project it would be best to use DC, with the heaters in parallel. Series connections can result in power-hogging if the heaters aren't well matched, but generally it works well if both heaters are in the same valve, and it does work out a little cheaper to implement.
Valves tend to like a soft start on the anode supply, which they used to get with the saggy drive from a valve rectifier circuit. The sudden step input of a contemporary PSU can be a bit of a shock. Supply sequencing can matter too. I'll dig out some old books and see if I can't find some decent cites here.
Valves tend to like a soft start on the anode supply, which they used to get with the saggy drive from a valve rectifier circuit. The sudden step input of a contemporary PSU can be a bit of a shock. Supply sequencing can matter too. I'll dig out some old books and see if I can't find some decent cites here.