Novice question about heater windings for Princeton Reverb

[deuce42]

Hi guys

I'm building a Princeton Reverb head but with a long tail PI and a PPIMV, housed in a 19" rack case.

As the amp will be 15 watts and because I'm also trying to add a little computer fan (to keep the case cool) which uses a little DC converter tapped from the 6.3V heater wires, I need to consider how much current my power transformer can take.

I'm guessing that between the 2 x 6V6 plus the 4 x 9pin tubes, plus the fan (fan rated 12v at 110ma), I would need a bit in excess of 2 Amps in heater current?

I note my power transformer has two secondary 6.3v windings. One is labelled "green wire/green wire 6.3v @ 2A. The other is labelled blue wire/white wire/blue wire 6.3v-0v-6.3v @ 1A. Assuming it is the case that I need a bit more than 2A, I'm thinking I'd try to use both the 2A and 1A windings. If I want to do this, my questions are therefore:

1) Can I simply connect each green and blue together to parallel them to make a higher amperage?
2) If I do, is there any issue with which green and blue to connect? I realise it's AC so it doesn't have polarity, but is there is any phasing issue or similar concept with connecting either wire?
3) As I already installed the 2x100r centre tap resistors on my board, Im guessing I simply leave out the white wire?

I'm sorry these are basic questions but I want to be sure of safety before I zap anyone near me:)

[martin manning]

1) Yes, you can run windings in parallel, but 2) also yes, they must be connected in-phase.
3) No, the Blue-White-Blue winding is 12.6VAC with center tap (White), so you would only use half of it (one Blue and the White) in parallel with the 6.3VAC winding.
That said, I wouldn't recommend it. There may be a difference in the voltages, which would cause some internal current flow. I would use the 2A, 6.3VAC for the filaments (exactly what you need), keeping the 2x 100R faux CT, and use the 6.3-0-6.3 for the fan.

[deuce42]

Thanks Martin for your response.

Can I clarify, if I use the 2a 6.3v without the fan, would that be enough heater current for the circuit? I'm not sure why but I seemed to have calculated more than 2a required. Am I missing something?

[martin manning]

Can I clarify, if I use the 2a 6.3v without the fan, would that be enough heater current for the circuit? I'm not sure why but I seemed to have calculated more than 2a required. Am I missing something?

6V6 are 0.4A, and 12A_7 are 0.3A, so you have current for 2x 6V6 (0.8A) + 4x 12A_7 (1.2A) = 2A.

[deuce42]

Ah perfect thanks Martin!