Triad R-115A: Good enough?

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kenzen
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Triad R-115A: Good enough?

Post by kenzen »

Hi all! New to the forum and tube amps, but have an EE and was a military tech way-back-when. I've been building "frankenstrats" for a long time, and have an old yellowed copy of Craig Anderton's "Electronic Projects for Musicians" where I built projects in my teens.

I'm looking to build an 18W. I have a pair of NOS (still in the box with paper stuffing) Triad R-115A PT's - here's the specs:
Pri: 115VAC / 60Hz
Sec:
680V with CT @ 70ma
6.3v @ 2.5A
5v @ 2A

Q: Will 70ma cover the swing of PP EL84's (or 6V6's)?

Thanks!
Ken
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Bieworm
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Re: Triad R-115A: Good enough?

Post by Bieworm »

If you use those you're theoretically going to have 480V B+ unloaded. Typical 18 watts have 550 to 600VCT. You want to end up with 345V on the plates.. with that transformer you will have at least 50 to 80V more than that. It might be managable with bringing down the voltage with zener diodes.. but my gut feeling tells you to stay away from it.

To be sure you want to know where that 70mA comes from. Likely it will be a point on the curve of the loadline. This doesn't per se mean It's only got 70mA available. I think it is 70mA @ 680V. So it will probably have less voltage when you draw more current from it.
The question is how they relate. You need the actual transformer spec curves to determine that...
With the 2.5A filament current you could light up 2 EL84's and 3 12AX7 tubes...
You could also put in an octave rectifier of some type.. that will bring your B+ down too.. but your beautiful 18W will be sagging the crap out of it...
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kenzen
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Re: Triad R-115A: Good enough?

Post by kenzen »

Bieworm - thanks for the reply. I was thinking of putting a 5Y3 in to drop B+ by ~60v (I think that's about what is expected, and I have the 5v secondary) to get me closer to the more ideal plate voltage range. With respect to the 70ma @ 680v, yes I'd need the PT curves to figure how fast B+ would be dropping as I pulled more than that out of the windings. I just have no feeling/experience how much north of 70ma the 18 will pull in "normal" use.
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Re: Triad R-115A: Good enough?

Post by Bieworm »

That's the question. For example, a hammond 290PAZ 18 watt PTx will have 140mA @ 580VCT. I'm not sure your triad will double it's HT current at that voltage.. we need the specs for that. I'm not an expert on that, but again.. I doubt that will happen. If you don't manage to get a hold on those docs there will be more experienced people here who can help you to determine that in a lab experiment with all the math and such
Ohms law... I wonder if it's applicable. But that would mean you have all you need to know when you measure the resistance of the HT winding.
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kenzen
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Re: Triad R-115A: Good enough?

Post by kenzen »

I never went hunting for the data sheet on this PT since the specs are printed on the box and the PT itself. I'll do some hunting to see if I can find the load curves. Thanks!
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