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In Reply to: RE: I was only worried by the biasing posted by Frihed89 on December 13, 2014 at 13:23:16
Just an aside,
There's not anything unusual about the operating point (275vdc, 40ma) but at that operating point you would cut the distortion by more than half with a 5k load.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Given the 2.5K OT, what would the ideal bias point be?
At 275vdc plate to cathode, about 63ma, but now the plate dissipation is 17 watts but the JJs can take that.
Otherwise 250 at 57ma.
Side note, The "classic" OP of 250 at 60ma really wants a load of 2.25k but tubes are forgiving.
But at 275vdc at 40ma into 2.5k, the HD at 3 watts output is high. About 13%
If you increase the load impedance to 5k, the HD at 3 watts drops to about 5.5%
The above is in line with the math and in line with the "Triode Loadline Simulator" and in line with Paul Joppa's formula.
The Triode Loadline Simulator is a little tricky to use. You have to start by reduce the yellow grid swing lines to get the OP* where you want it and at higher load impedance's you have to increase the Uamax voltage in the box below to make the higher load impedance's reachable on the graph.
Once you get your OP, you can then move the grid swing back out to match you intended use.
* note, it's not just the operating point, it's the operating condition.
Operating point usually means just the plate to cathode voltage and idle current.
Operating condition means plate to cathode voltage, idle current, load impedance and maximum intended grid swing under normal intended use.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
nt
Observe, before you think. Think before you open your yap. Act on the basis of experience.
that's why I made the suggestion that I made.
The impedance of that output transformer does not match your operating point.
You can use it as is or, if you want to greatly reduce the HD produced by the output stage, you could replace the OPT with one that does match your operating point or change the operating point of the output tube to match your OPT.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I trust the builder. He tried your OPs, but listened and liked the sound of the other ones better, even though he knew they were unusual.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
I recently tried a Sophia Electric 300B carbon plate 2.5v in place of my 2A3 DHT dac. The improvement is very clear and significant compare to EML mesh plates. Given JJ is 300b with 2.5v, I'm guessing you would notice a decent improvement. However I've heard they require large break in.
The break in for the JJ 2A3 is long but worth it. Mine are in a Bottlehead Stereomour and they are past that point. I keep coming back to them.
It's a good, well made tube. Based on the conservative operating points for this amp, it should provide a fair number of years of reliable service.
We shall see...
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