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In Reply to: RE: 2A3 Actuals posted by maxhifi on November 26, 2014 at 13:43:20
"operating point" is voltage and current, operating condition is voltage, current and load impedance.
At 42ma 250vdc the load would need to be 3.9k.
While a 5k load will work at 42ma 250vdc, it's not optimum.
Thankfully tubes are pretty forgiving.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
I orginally bought these transformers for a single ended EL84 project. I put this amplifier together using things I already had on hand, to get a taste of what real triodes are capable of. If doing it from scratch, there's no way I would pick a 5k impedance transformer but at this point it's no longer a variable. I am just trying to optimize what I have rather than spend money on things like new transformers, as my longer term project is a 300B amplifier and I would prefer to put funds in that direction.
I hate to say this, but convert it to a 45 amp? Just throwing that out there.
I love your driver stage. It's what I would want.
Jamie
Big speakers and little amps blew my mind!
I actually did have that in mind when I first built it in 2006, the driver stage was inspired by a Japanese 45 amp. I went with the 2A3 becasuse they are available new. With the JJs on the way I am more or less committed to the 2A3 for the time being. To be honest this little amplifier really sounds good as is, I am just in the process of optimizing it. The operating point may well not be ideal, but it has as much power as a 45 would make and it is extremely enjoyable to listen to. That said it may have a future application driving a tweeter or mid horn in which case the 45 option will be revisited.
"With the JJs on the way I am more or less committed to the 2A3 for the time being."
No your'e not. If you can design and build your own amps you can do anything you want. $120 mistakes are cheap in the high-end world.
Jamie
Big speakers and little amps blew my mind!
Is the secondary tapped?
A 8 ohm speaker on a 16 ohm tap gives you a 2500 ohm primary.
Just thinking out loud.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
There is the extra primary inductance and better HF but the disadvantage are the copper losses of a 5k primary and the 16 ohm secondary tap which would be much higher than a 2k5:8 transformer . Sowter's 5K SE TX already has high copper losses , especially on the primary .
Al
Yes, I certainly could give that a try. The only reason I haven't gone down that path, is the transformer will be more lossy that way. Us engineers can tend to hit a mental road block when entering the "use outside of design intent" territory, I am happy for this suggestion, especially since it should have been an obvious one.
This extra transformer loss may be more than compensated for though by the tube being loaded properly and making more power. In either case this is sure easy to test.. will try it tonight and see what happens. I suspect I will get better power output and a slightly worse damping factor.
What am I missing?
Dennis/ Jeff have been criticised for suggesting a 2k5 load was suitable for the ~ 42mA 250V operating point... and that was into a pair of speakers that likely did not need a low impedance drive. Now 2k5 is considered a more suitable load than 5k?
Well, it will net you higher power... at the cost of distortion and lower damping factor, which I imagine could be an issue with your speakers (it is with most speakers). OTOH, I guess it is "proven" is Dennis' amps.
I think I need a break...
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
When I said "A 8 ohm speaker on a 16 ohm tap gives you a 2500 ohm primary."
I was just saying he wasn't stuck with 5k. He could go for 250vdc 60ma. 2500 ohms for instance.
But at 250 42ma, 5k is too high (but might be fine assuming the power output is enough for his situation) and 2.5k is too low leading to high HD at all power levels.
I really wasn't trying to confuse anyone.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I am just a little (lot!) sleep deprived and therefore have pulled my cranky pants on.
Sorry, I did not mean to hassle.
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
H Tre, can you please clarify/ confirm:
> > > While a 5k load will work at 42ma 250vdc, it's not optimum. < < <
I thought the increasing the load would lower distortion, considerably. I am guessing that by "optimum" you mean that more power can be achieved at clipping or similar?
I need about a Watt for my speakers in my room at my loud listening levels; would a 5k load be more ideal than ~4k?
Thanks Tre.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
Optimum (in terms of loading a power tube) is not anything that is cast in stone.
Normally, a designer would want to pick an operating condition that would give a reasonable amount of power at reasonably low HD.
A operating condition (idle current, voltage and load impedance) that gives the straightest transfer curve (lowest HD) wouldn't be the one that gives the most power before clipping.
Only requiring 1 watt should make it easier to achieve low HD.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
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