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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 17th July 2018, 02:07 PM   #261
Tubelab_com is offline Tubelab_com  United States
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Sorry for the vanishing act, I have been out of town with only phone access to the internet for two weeks. I stopped by ESRC for a large box of tube sockets, but for the first time in 20 years I went there and came away without buying a single tube!

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It seems to me that 350V on the V2 anodes would be more appropriate.
It depends on what output tubes you are running and how much drive they need. I simply tuned my set up for minimum high order distortion and measured the voltages. I would assume a different setup with different tubes may achieve lowest distortion at a different voltage.

You are probably right that the voltage should be lower, particularly with a 450 volt or lower supply voltage, but driver tubes have 150 volts or so on their cathodes and need some voltage across them.

I am in the process of putting my test system back together so I can take some measurements with different output tubes and supply voltages.
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Old 17th July 2018, 03:27 PM   #262
dch53 is offline dch53  Australia
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Default Puzzling THD behaviour

Hi. Good to have you back!

It turns out that I've been able to duplicate in my sim exactly what you measured and I'm not completely sure why.

My LTSpice schematic is shown below with my current preferred cathode feedback output stage.

I set up the operating points for the LTPs by minimizing the THDs they generated on the assumption that the lower the THD arriving at the output stage the better.

I started wondering what would happen if I raised the operating point of the second LTP towards the voltage you'd recommended.

Sure enough, the THD from the second LTP was much higher (I put the 2 operating points on the 6CG7 plate curves and it's easy to see why). Gain was also reduced and again, easy to see why.

But what was really weird was that as the distortion produced by the second LTP increased, THD at the output decreased!!

Part of the reason has to be that the output stage cancels odd order harmonics but there has to be more to it.

I wondered if there was a sweet spot and plotted V2 Anode THD and Load THD vs V2 Anode Voltage (second screenshot for an input that gave me 40W out for a V2 Anode voltage of 342V).

Sure enough there was! At a V2 anode voltage of 420V.

As I said gain decreases as you increase the V2 Anode voltages and consequently the power out decreases. This is shown in the third image.

The final image shows the THD vs Power out for V2 Anode voltages of 340V and 420V. The 420V operating point provides a far lower THD up to 50W or so but is limited to 60W if you want < 1% THD. If you're prepared to put up with higher distortion there's good power out to 90W with the lower V2 Anode operating points.
Attached Images
File Type: png LTSpice schematic.png (217.5 KB, 65 views)
File Type: png THD vs V2 anode voltages.png (69.3 KB, 64 views)
File Type: png Power vs V2 Anode Voltages.png (38.3 KB, 64 views)
File Type: png THD vs Power.png (61.7 KB, 65 views)
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Old 17th July 2018, 04:36 PM   #263
Tubelab_com is offline Tubelab_com  United States
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Part of the reason lies in the fact that this is a class AB push pull amp. Each output tube contributes an equal share to the total output averaged over time, but not at the same time. The tube that is entering conduction begins to contribute some to the total output, as both tubes are conducting equally they contribute equally to the total output, and as that output tube reaches peak conduction it is contributing 100% of the output power because the other tube is cutoff.

Note that this is less true on a class A push pull amp since both output tubes are always conduction, but the share of total work is constantly changing.

The driver's contribution to total distortion is also not equally split between the two tubes. The tube that is driving a particular output tube into conduction contributes the most to the total amplifier power output and distortion at that moment.

As the driver's plates go lower in voltage the corresponding output tube is contributing less and less to the amps total power output and therefore distortion. I have seen drive signals with severe clipping on the bottom due to the mosfets hitting the negative rail but the amp's distortion doesn't change much.

I avoid clipped or severely distorted bottom halves due to limited negative rail voltage for another reason. The mosfets tend to emit a burst of high frequency energy when is goes into cutoff or comes out of cutoff, so I like to keep ample headroom across them. You can see this in a scope, hear it, and severe cases fry a tweeter with ultrasonic energy.

The THD of the driver stage can be higher than the amps total THD even without feedback. This baffled me for several years since I would always test and optimize the driver circuit before building the output stage, then wonder why the driver worked better with a different tune up once the output tubes were added. The top half of the driver's output is far more critical than the bottom half. This makes optimizing the driver board alone with distortion measurements rather pointless.
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Old Yesterday, 08:00 AM   #264
dch53 is offline dch53  Australia
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Thanks for the explanation. It certainly is counter-intuitive that increasing earlier stage THD should decrease output THD.

What you're saying is that the positive half of the waveform is amplified with less distortion than the negative half and as you raise the operating point of the second LTP the distortion introduced to the positive half reduces.

Pity about the associated loss of gain.

I'll play around with global negative feedback for the 341V and 420V 2nd LTP anode operating points.

Last edited by dch53; Yesterday at 08:00 AM. Reason: Remove extra blank lines.
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