Loadline Tools

[Paul Barker]

cool

[simon]

There is a calculator on this site that calculates distortion https://www.vtadiy.com/loadline-calcula ... alculator/
This can be used in conjunction with the calculator Paul has illustrated above. You can find a suitable op point and self bias resistor with the first calculator, then open another tab, transfer the figures into the other calculator. You input the max swing available and the software will fill in the distortion figures for 2nd, 3rd, and 4th harmonic below.

Just had a play with this and checked what it thinks of my GM70 SE OP:

For V=530V, Vg=-39V, Ia=90mA, Z=5k, P=5.3W, with an output headroom of 230V it's
H2=1.06%, H3=0.14%, H4=0.01%, THD=1.07%

Is this any good?

The only time I've had it near full volume was at Owston last year, normally it's much less and rarely more than half volume. An output headroom of say 130V gives:
H2=0.58, H3=0.04, H4= 0.00, THD=0.58

Higher Z OPTs help of course, but I've got 5k and that's what I'm using!

[Cressy Snr]

Looks alright to me.
it’s better than a triode strapped pentode like mine.

[JamesD]

Generally an SE triode with no feedback that achieves 1% THD at full power is excellent - normally they are rated at 2-3% THD at full power and sometimes 5% of course domestic usage of an SE triode should normally be at 1W or less and aiming at 0.1% THD or less at 1W for excellent performance. These are not critical numbers and assumes that at least 70% of the distortion is generated in the power stage.

It should be said also that spice modelling of power stages at full power is also subject to error generally less than 10% but some triode models are some way off...

[izzy wizzy]

There is a calculator on this site that calculates distortion https://www.vtadiy.com/loadline-calcula ... alculator/
This can be used in conjunction with the calculator Paul has illustrated above. You can find a suitable op point and self bias resistor with the first calculator, then open another tab, transfer the figures into the other calculator. You input the max swing available and the software will fill in the distortion figures for 2nd, 3rd, and 4th harmonic below.

Just had a play with this and checked what it thinks of my GM70 SE OP:

For V=530V, Vg=-39V, Ia=90mA, Z=5k, P=5.3W, with an output headroom of 230V it's
H2=1.06%, H3=0.14%, H4=0.01%, THD=1.07%

Is this any good?

The only time I've had it near full volume was at Owston last year, normally it's much less and rarely more than half volume. An output headroom of say 130V gives:
H2=0.58, H3=0.04, H4= 0.00, THD=0.58

Higher Z OPTs help of course, but I've got 5k and that's what I'm using!

Like you, I popped in some values I'm using for the GM70 and it caused a lot of head scratching. Had to go do some reading to fill in what seems to be the many gaps in what I thought I knew. The figures for PP don't make much sense ie bugger all to 0 distortion.

But what it did show is I should possibly increase the a-a Z that they see which I'll play with by reconfiguring the secondary.

[simon]

Thanks James, not bad then. Might have a bit more of a play later.

[simon]

I thought it would be interesting to model the classic WE 300B and RCA 2A3 SE OPs, and then see what the distortion would be at 1W. I did GM70 as well - the data sheet doesn't offer an OP so used the default values in the programme, as well as what I currently have.

Looks like <0.1% THD @ 1W might be a little demanding.

[IslandPink]

Cor ! - that's interesting, Simon.
Makes a great case for GM70 copper with a 600V supply, if you have sensitive speakers.
My experiences with GK-71 from 460 supply at only 3Wpc channel were so positive, this is very encouraging.

[simon]

Well I guess, and that's all it is, that higher powered valves used at low power should have lower distortion than lower powered valves at higher power, for similar distortion profiles. It's just a shame the GM70 burns a regulated 100W to heat them.

(My understanding of distortion is that it's a consequence of unequally spaced grid lines?)

[JamesD]

Double checking the 1W distortion for SE power triodes - and you are right - I was too optimistic at 0.1% and it should be less than 0.5% for good and less than 0.3% for excellent.

My excuse - I have been doing some extensive investigation of Sakuma style SE with PT15 driving PT15 (for instance) and there it is easy to get the 0.1% at 1W as this schema has good distortion cancellation at 1W. In fact its relatively straight forward with a lot of SE designs to use 2nd Harmonic distortion cancellation targeted at 1W output - rather harder to get it to track through all power outputs and particularly above 25% of maximum power out.

[Nick]

In fact its relatively straight forward with a lot of SE designs to use 2nd Harmonic distortion cancellation targeted at 1W output

Especially if you assume that speakers are perfect and resistive.

[JamesD]

Thats where the Sakuma style design comes in as to a large degree one makes the driving stage mimic the output stage so that the driver stage sees an elliptical load line - thats been part of the area I've been working on to try to get the cancellation to work over a range of output levels - the other is the tracking of non-linearity with increasing signal levels ... two aspects of the same problem

[Nick]

What you need is some way of comparing the output with what the output would be if the load was ideal, than applying the changes that would make the non ideal output mimic the ideal one.

[JamesD]

That made me laugh out loud!!!

Harold Black would be proud of you!

[simon]

Thats where the Sakuma style design comes in as to a large degree one makes the driving stage mimic the output stage so that the driver stage sees an elliptical load line - thats been part of the area I've been working on to try to get the cancellation to work over a range of output levels - the other is the tracking of non-linearity with increasing signal levels ... two aspects of the same problem

That's a bit of a leap for me. I thought the driver mimicking the output stage would exacerbate the harmonic distortion profile? But maybe I'm not understanding mimic in this context and using too literal a meaning?

It's years ago I read about eliptical load lines and sadly don't recall much now - is it the AC component???