6L6PP 20W amp

[Helmholtz]

It is not "the simulation". It is the .op analysis, which regards idle state. And indeed, i am also frustrated not to see any voltage drop across the G2 stoppers - while i do see the effect of the G2 currents in the voltage drop of R2.

Consistently, not only in this simulation.

The voltage drop across R2 seems to be produced by R26, not screen current.

See my Edit above.

[bea]

Yes You' right. A few minutes ago i have switched to the Ayumi model for one of the 6L6GC. It shows clear differences to "JJ6L6GC" which is Koren model modified by results of µ-tracer measurements by a user from the tube town forum. With the Ayumi model there is indeed a voltage drop through the G2 stopper in the .OP analysis, and of course also a reduced idle current. Unfortunately, the Ayumi model is computationally so expensive that it is necessary to use a faster model except for "fine tuning".

Even with the simpler model, .meas does show G2 currents, and their behavior looks plausible in the log file of the .tran analysis.

Please not that the actual voltages are significantly smaller (308V at the anodes of the 6L6). So there are clearly shortcomings in the simulation, and they are apparently mode severe than the incomplete tube model.

Upper tube: Ayumi, lower tube: modified Koren:

[Helmholtz]

As mentioned, I cannot comment on tube models. But I would consider a pentode/tetrode model that doesn't emulate screen current useless. Actual screen voltage influences tube gain, impedance and max. output power.

Why not focus on real amp? If you are looking for advice on optimizing, please post measured voltages at idle and full output.

[nickb]

Yes You' right. A few minutes ago i have switched to the Ayumi model for one of the 6L6GC. It shows clear differences to "JJ6L6GC" which is Koren model modified by results of µ-tracer measurements by a user from the tube town forum. With the Ayumi model there is indeed a voltage drop through the G2 stopper in the .OP analysis, and of course also a reduced idle current. Unfortunately, the Ayumi model is computationally so expensive that it is necessary to use a faster model except for "fine tuning".

Even with the simpler model, .meas does show G2 currents, and their behavior looks plausible in the log file of the .tran analysis.

Please not that the actual voltages are significantly smaller (308V at the anodes of the 6L6). So there are clearly shortcomings in the simulation, and they are apparently mode severe than the incomplete tube model.

Upper tube: Ayumi, lower tube: modified Koren:

[ATTACH=CONFIG]52086[/ATTACH]

If you want a good model for beam tetrodes then take a look at Derk Reefman's. He developed new models for beam tetrodes and also improved the screen current models for all types.

Here is his white paper.
Theory.pdf


Here's the copy of his models ( might not be the latest)
TubeLib.zip


6l6GC sample:

[bea]

Works. In the past i had some convergence issues...

... next project shall run with either KT77 or GU50 under 750V ... i am currently checking the old transformer set for 2 EL34.

[nickb]

So a screen shot - NFB loop is open.
Yellow: at load resistance
Blue: one leg of the PI through a coupling capacitor.

The resonance is caused by the transformer, isn't it?
I would learn how to compensate for it. Damping on the primary of the PI? Using a a cap in series with the resistor in order not to affect the lower frequencies? How to estimate its values? (i recognize the frequency of the resonance can be read from the screen shots).

And i would like to understand how it is mapped backward into the circuit and whats the reason for the doubled frequency?

[ATTACH=CONFIG]51245[/ATTACH]

and the schematics:

[ATTACH=CONFIG]51246[/ATTACH]

It looks to me like the ringing is about 20KHz in which case it's inaudible so no need to compensate. It hard to say where it's coming from, but interesting to know!

My first place to look would be the primary leakage inductance and self capacitance of the transformer.

I see you're really getting to grips with SPICE - well done

[bea]

This is the old version, before i changed the design. Now i am having the usual topic that the stability margin is reached when NFB becomes too strong. Not a problem: i used a trim pot and and adjusted it that it remains in the safe margin under ALL circumstances.

BTW: i am going to take up the LoV project again pretty soon... now that the amp is working and the transformers seem to have survived the hazard of a tube meltdown at 750V (not by me...)

[mikepukmel]

Thanks for posting all your work Bea. I only wish I knew enough electrical engineering to understand it better.