Bottlehead Forum

Heater Current and Transformer Ratings

96.231.149.177

I'm embarking on designing an amp for the first time (Bottleneck 6V6 Contest) and I'm trying to learn some of the basics that I can't seem to find. How do I know if a transformer has enough current for the heaters of the tubes I want to use? Assuming I will be using AC heaters, is it as simple as adding up the heater current of all the tubes and if the value is less than the transformer is rated then it will work?
For example, will a transformer rated as 250V(125-0-125)@125mA CT & 6.3V@2A, be appropriate for an amp using two 6V6 and one 6SL7 tubes
0.45 + 0.45 + 0.3 = 1.2 amps
But the same transformer would not be appropriate for an amp using four 6V6 tubes and one 6SL7?
0.45 + 0.45 + 0.45 + 0.45 + 0.3 = 2.1 amps
Is there a rule of thumb for safety? That is, should the heater current of the tubes be a percentage of the rating (e.g., 75%). If I wanted to use the 5 tubes above, then I would need to have a transformer with a rating of at least 2.8 amps (0.75*2.8 = 2.1 amps), or is that unecessary - would a transformer rated for 2.5 amps be fine?

Edits: 05/04/09

Follow Ups:

Kirchoff's laws - Phil Sieg 05:49:25 05/05/09 (0)
Oversize it a bit. - VoltSecond 20:20:30 05/04/09 (0)
More than you wanted to know ... - Paul Joppa 19:34:55 05/04/09 (0)
RE: Heater Current and Transformer Ratings - Coca 19:30:37 05/04/09 (0)

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