4p1l and djvu curves help please?

[Paul Barker]

Start with -5v on suppressor

[IslandPink]

Wow ! - that's really excellent info
All it takes is about +10V to straighten things up quite a lot in that region down towards 100V . The idea is it should trim a little 3rd-harmonic off the spectrum when it's being used as a driver.
Many thanks Paul .
Really great to see that - never seen plots of that behaviour before.

I'll give you some practical info on whether you can hear it, in the next week to 10 days, after one or two other mods I want to do first .

[Paul Barker]

If linear is the measure. Lower load required. 3k looks ok gain 16. +20v suppresser looks worth considering. Is 16 in the ball park?

[IslandPink]

In isolation that might be a good approach, Paul, but if you start working out the numbers for 465V supply, the +/- voltage swing you want for a 300B/GM70, the dissipation etc, you end up around where I am with 220V anode, 30mA and 7.8k load. This is fine, some 2nd harmonic in the 4P1L, a little bit of 3rd, but OK . It certainly sounds nice . Gain over 20 , too.
I did notice, however, that if ( on LTSpice ) you simulate a DH pentode with a sharper knee, for example 2E24, you get rid of the little bit of 3rd-harmonic that comes from straying into the curved bit .
So , this 'tweak' allows me to still use the 4P1L , but sharpen the knee and straighten the curves a bit, hopefully it should be really good .

Also, 16 gain is really not enough : 22 or whatever I have is JUST enough .

[JamesD]

Wow! Paul, thats terrific! Isn't it interesting that as G3 goes positive more and more tetrode behavior appears! And that makes perfect sense too as G3 becomes less and less effective as the suppressor grid...

But it really does show the effectiveness for straightening the pentode curves to more constant current like behavior and more power for an output valve or less distortion for a pre-amp or driver valve.

really brilliant to see the variation plotted like that

J

[Paul Barker]

In isolation that might be a good approach, Paul, but if you start working out the numbers for 465V supply, the +/- voltage swing you want for a 300B/GM70, the dissipation etc, you end up around where I am with 220V anode, 30mA and 7.8k load. This is fine, some 2nd harmonic in the 4P1L, a little bit of 3rd, but OK . It certainly sounds nice . Gain over 20 , too.
Also, 16 gain is really not enough : 22 or whatever I have is JUST enough .

Yes agreed it is about the need of the circuit, and also there is always a need for more gain than we can arrive at trying to meet other objectives.

I haven't blown it up and measured but at a casual glance with your load line and your voltage swing on the output (are you going for 150v peak to peak?) it looks better at higher current lower voltage. You mustt have it on the -9v line it may be less 2nd on the -7v line at say 150v without any danger of the input swing taking it to the knee, but places you further from the dangerous pinched distances which seem to start rapidly at -11v.

But you had a great sound at Owsten.

[Paul Barker]

Wow! Paul, thats terrific! Isn't it interesting that as G3 goes positive more and more tetrode behavior appears! And that makes perfect sense too as G3 becomes less and less effective as the suppressor grid...

But it really does show the effectiveness for straightening the pentode curves to more constant current like behavior and more power for an output valve or less distortion for a pre-amp or driver valve.

really brilliant to see the variation plotted like that

J

Thanks to you and Mark for alerting me to the possibilities.

We have talked about different uses of the individual grids

Keep coming with the ideas.