What are the functions of the 5z4 and 0d3 tubes.

Thanks Mlsstl for your reply. I do have a layout of the circuit that came with the pre. The 0d3 is the last to light up upon turn on but that may not mean anything. Why would this pre require two different voltage requirements. Mmmmm. It sounds really good!

The 5Z4 is a rectifier but the OD3 is a shunt voltage regulator. The 5z4 converts the incoming AC to DC which is then filtered to smooth it out. The output of this will drift up and down as the incoming line voltage drifts. The OD3 with it's associated circuitry is a regulator which will hold the DC level to a relatively constant 150V.

It lights up last because it doesn't turn on until the power supply before it builds up to a voltage that is somewhat above it's 150V regulation level. Once it gets up high enough the OD3 ionizes, lights up, and then holds the voltage at 150V.

Ah so that's why the glow is different than the rest of the tubes. Tell me Herman have you found any sonic differences between the various 0d3 tubes available. I did have a RCA and replaced it with a GE and found it sounding much better. I have not seen any euro tubes for this particular one and wondered if it was a US only made tube.

Sorry, I've only listened to one amp that used one and only briefly so I have no experience with comparisons.

Thanks Herman for all the info you provided. I'll probably check on other tubes and compare with what I have now. Good listening!