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LTSpice model for a commercial toroidal output transformer
LTSpice model for a commercial toroidal output transformer
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Old Today, 02:18 PM   #81
dch53 is offline dch53  Australia
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Default Leakage inductance

Thanks. That made a huge difference! THD < 1.2% up to 80W. the simulation even shows it dropping to 0.9% at 90W. Output tube dissipation at that point is still only 25W.

I'll read up on modelling the leakage inductance. I've read that toroidal transformers have lower leakage inductance than traditional EI cores. I'll also do deeper into Norman Koren's The Emperor's New Amplifier.
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Old Today, 04:57 PM   #82
euro21 is offline euro21  Hungary
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LTSpice model for a commercial toroidal output transformer
Quote:
Originally Posted by dch53 View Post
THD at the MOSFET sources deteriorates rapidly after 50W output due to clipping of the negative cycles (3rd attachment).
Before you changing anything, look at output tube G1 waveform in the simulator.
About +30V.... -150V swing.

Then look in 6550 datasheet the UL characteristic: 0V... -70V swing.
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Old Today, 08:17 PM   #83
45 is offline 45  Italy
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Quote:
Originally Posted by dch53 View Post
Any suggestions how I can improve the performance of the sim or the circuit?
Let's not call it improvement but another solution.

Disconnect the G2's from the UL tap and give them a separate regulated supply (that one you use for the 6CG7's). Remember that if the CFb is 24% then your actual UL ratio becomes 24+33=56% when CFb is on.
If you disconnect the G2's it will become 24% (modified) ultra-linear. Then disconnect the 27K anode resistors of the 6CG7's from the 505V supply and connect them to the UL tap in cross-coupled fashion (where you get 505V as well minus a few volts for the UL tap DC resistance). This way you will bootstrap the 27K anode resistors (i.e. will multiply their AC value = more swing with less distortion).

Because the UL ratio is less (i.e. closer to pentode) you might get even more power.
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Old Today, 08:56 PM   #84
45 is offline 45  Italy
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P.S.
I have just noticed that you are not using 24% CFb.
So the ratios will be according to the actual transformer you are easing but the idea is the same....
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Old Today, 10:29 PM   #85
45 is offline 45  Italy
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P.P.S.
If you are using the VDV4070 with 5 ohm load you can do the same with the Polish 4K when you use 24% CFb to get the same primary load.

In fact the VDV has 28.267:1 turn ratio without CFb. It becomes 30.437 in total when CFb at 7.1% is connected. So for 5R this is 4.63K.

The Polish OT is 22.36:1 without CFb. This becomes 29.431 with 24% CFb. That's about the same.....
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