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In Reply to: RE: Amazing JJ 2A3-40's posted by Tre' on March 12, 2015 at 07:54:16
Would the different filament voltage cause the variance between the two? A 300B running at 2.5V could be cooler than at 5V. A real 2A3 and a 300B both dissipate 6.25 watts at the correct voltage (2.5V @ 2.5A and 5V @ 1.25A). Is that the case with the 2A3-40?
Follow Ups:
The data sheet for the JJ 2a3-40 says 2.5v 2.5a
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre'
Of course it does, but my point was, I'm running it at 5v, sounds just fine - maybe a lot better than fine.
how much current are you drawing at 5V?
dave
With a JJ 300B in the socket, I get H = 5.0vdc (this is unreg by the way), I = 80ma, B+ = 450, Vpk = 370, Pd = 29.6w.
With the JJ 2A3-40 in I get H = 3.36v, I = 76ma, B+ = 450v, Vpk = 374, Pd = 28w.
I didn't measure heater current in either case, but clearly, higher with the 2A3.
The 5V supply dropping to 3.36V makes perfect sense. say it is drawing 3.25A and the extra 2A of current draw on the supply accounts for your lower supply voltage. I was trying to picture 5V across a 2A3 and how bright it would be drawing over 4A of current.
Thanks
dave
Almost forgot, the hum at output is 10mv with 2A3 and 5mv with 300b
So if the filaments are running at 3.36V they would be drawing 1.86A if the filament wattage is still 6.25W?
2.5V / 2.5A = 1 ohms with the standard 2A3 or with a 300B, 5V / 1.5A = 4 ohms
3.36V / 1.86A = 1.81 ohms with your 2A3-40
Is this correct? It doesn't seem right to me. Where did I mess up?
The filament supply can't handle the load so the voltage dropped. That throws everything off.
If the supply was up to the task then the 2a3 would have 5 volts across it's filament and be drawing 5 amps of current from the supply.
But that wouldn't happen because the "Hot" resistance of the 2a3 filament with 5 volts would be higher than the "Hot" resistance with the normal 2.5 volts.
So it would naturally end up some where in between.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It's clear now.
Right! As I said, the filament supply is unregulated.
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