do I need to upgrade my heater supply to add another tube?

[ultrachrome]

I have a scratch-built Rosenblit grounded grid with 3 12AU7's. I'd like to add a fourth 12AU7 for CCS and am wondering if my heater supply is up to the task.

The schematic specifies two 120V-50mA and one 12V-1A transformer. Being clever or stupid, I pushed outside my experience envelope (or so it seems in hindsight) and used an 80VA toroid kit on which I lovingly wound two 120V and one 12V secondary.

The 12V secondary is wound with 19 gauge magnet wire for 12 volts for an estimated capacity of 2.5A according to kit documentation. My supply circuit is comprised of a quartet 1N4007's, one 1000uF 25V, and LM340T-12 connected to pins 4 and 5 with all three tubes in parallel.

I'm assuming that it's able to provide 12V @ 1A. Am I correct to assume that each tube requires 160mA when powering with 12V? That seems to be less than 1A. Is this too much load for it to handle?

I thought about simply building a second heater supply identical to the first and trying to run both off the same secondary. Bad idea?

[nigelwright7557]

The 12au7's take 150ma at 12volts each so an amp is well inside.

[Tom Bavis]

19 gauge wire is good for 2.5A at the commonly used current density of 500 CM/amp or 450A/cm^2 (first CM is circular mils - a bastard unit commonly used for wire. Second cm is centimeters) RMS current in the secondary will be about 1A with a 600 mA DC load. Well within the transformer's capacity.

[ultrachrome]

Thanks for the responses.

How does one know if their heater supply is not up to the task? Lower than expected voltage under load?

[leadbelly]

Yes, and the transformer will run hot.

May I ask why you would want to use a 12AU7 as a CCS? It's not even the best triode for that service, let alone that a pentode or SS would do a much better job.

[ultrachrome]

Frankly because I don't know any better and I have the parts on hand.

I stumbled upon the schematic on the transcendent board while digging up information to help me troubleshoot a hum issue. It looked pretty simple to implement so I thought I would try.

[leadbelly]

Those are good reasons. If it doesn't work out start a new thread for recommendations.