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In Reply to: Re: Load Lines posted by danlaudionut on May 8, 2007 at 06:56:00:
The curves for the tubes I've measured mirror my own findings. Note how much better tubes work at higher current ... higher gm, lower rp, greater linearity. Note also how important a more horizontal load line is for some tubes, especially if high voltage swing is required. But all of this has to be taken in the context of the load presented by the next stage / OP TX etc and the drive of the preceding stage. I think this is where some draw, not wrong conclusions but incomplete ones when they describe the sound of a particular tube in comparison to another or at particular operating points.
Follow Ups:
NazYes they look so much better at higher currents.
But there is a point they start sounding strained.
I was just wondering whether the current and
thereby the power dissipation causes the strain
OR the higher voltages causes it.
Maybe the higher impedance OPT used on the lower
powered amps for a flatter load line causes
the more relaxed sound.
Jeff says the high voltage and power dissipation
are the source of the strain.
Most things in life function better
when not pushed to the limit.
So I think I will try a 300B
at 400V/50mA/5K load.
I think everything is a variable so you have to suck it and see how it works in your system.IME higher current has always worked better in SE OP stages of any kind including 300B but I always stay well within the limits. The sound is fuller than at higher current and perhaps it's just that the thinner sound of lower current is providing a better balance with the rest of the system which may be perceived as producing greater detail.
In my book higher HT voltages generally provide greater opportunity to design a more linear stage, especially where the plate load is a simple R. However there is a limit in OP stages with an OP TX, which are another thing again. While a higher impedance will always be more linear the trade-off may come in the form of additional leakage C caused by more windings and resulting in lower BW. It costs considerably more to alleviate this problem.
I have never found less linear operating points producing greater distortion to sound better UNLESS the particular stage is cancelling some distortion up or downstream. It's the total that counts. And, I'm not sure what the definition of "strained" really means. Is it describing noticeably higher distortion or merely excessive weight?
So back to your suggested operating points ... 400 - 450V is fine for a 300B but personally, I'd favour 60 - 70 mA, the higher the voltage the lower the current. Distortion jumps appreciably at currents that are too low and to my mind 50mA is bordering that point. If you really want to idle the tube at low power consider lower plate voltage with higher current, it's more linear.
NazI agree that flatter loadlines are preferred.
I like my 5K OPT on a KT88 vs a 3K OPT.
On driver stages I like really high
henry chokes with low rp tubes.
I have a MQ EX0-99 150H on my 12B4
and a MQ EXO-01 100H on my 6J5.So you are saying my original OP
of 450V/60mA/5K is better?
I stress that this is only my opinion but I'd favour something closer to 450V / 70mA than lower but if you feel that 30W dissipation is too high for you then keep the current up and drop the voltage rather than the other way around. Also, 5K impedance is great for linearity (and still OK for good BW) if you 8W is all you need.Now here's something that is not intuitive, you may achieve better results with lower current in the driver stage, even more so than varying the current in the OP stage IMHO. Worth playing with anyway and I'd be interested in whether your findings approximate mine.
NazIf I thought I could get away with lower power
I would definitely try the "deep class A" route.
With 93dB efficient speakers, I need 8-10 watts.
At 450V/60mA/5K, I can get 10 watts.
At 400V/50mA/5K, I can get 8 watts.
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