Distortion in PP vs SE

[bluefoxicy]

I'm looking through RC-30 and noticing some interesting things about THD in Class A1 SE vs Class A1 PP.

6BQ5/EL84

• Class A1 SE, 250V anode: 10%
• Class AB1 PP, 250V anode: 3%
• Class AB1 PP, 300V anode: 4%

Raise the plate voltage in PP, and THD goes up.


6V6

• Class A1 SE, 180V anode: 8%
• Class A1 SE, 250V anode: 8%
• Class A1 SE, 315V anode: 12%
• Class AB1 PP, 250V anode: 5%
• Class AB1 PP, 285V anode: 4%

Raise the plate voltage in SE and THD goes up. Lower the plate voltage and THD goes down.


6L6

• Class A1 SE, 250V anode: 10%
• Class A1 SE, 300V anode: 11%
• Class A1 SE, 350V anode: 15%
• Class AB1 PP, 360V anode: 2%
• Class AB1 PP, 450V anode: 1.8%

Raise the plate voltage in SE and THD goes up. Lower the plate voltage and THD goes down.


What's going on here? Does THD in an AB1 push-pull go up with plate voltage, or down, or is it a parabola or some other sort of curve with a peak, or just totally tube-dependent?

[Alex/Tubewonder]

If you only look at abbreviated data sheets you will only get confused.

The figures you quote should also include plate load, output power, biasing method and grid drive voltage. Otherwise it's like quoting a cars fuel consumption without saying how it was driven during test.

For EL84: full datasheet study all 15 pages of it, especially pp 10-15 where diagrams for distortion, both 2n & 3rd harmonic vs. output power, supply voltage, plate load etc are given. You will be surprised to find that class AB shows falling distortion level with raising output power.
If you draw load lines for each operating condition it will become clear how distortion depends on plate load and all the other parameters.

Therein lies explanation to differences in sound produced by cold vs hot biased amplifier. In short: everything's connected.

[bluefoxicy]

I don't understand Raa~ in that. In single ended mode, Raa~ is the load impedance of the output transformer (mine's 5k); in PP mode when it says 8k, I assume it means each tube itself sees 8k against the anode, not that the OT is 8k and two parallel tubes make it show each 4k?

In this case, if I used a lower impedance, would it produce more THD and more d2 than d3?

[Alex/Tubewonder]

I don't understand Raa~ in that. In single ended mode, Raa~ is the load impedance of the output transformer (mine's 5k); in PP mode when it says 8k, I assume it means each tube itself sees 8k against the anode, not that the OT is 8k and two parallel tubes make it show each 4k?

In this case, if I used a lower impedance, would it produce more THD and more d2 than d3?

No, each tube sees 1/4 of Raa. Raa=8k each tube sees 2k.
I need to ask if you are familiar with how load lines are constructed?

[bluefoxicy]

No, each tube sees 1/4 of Raa. Raa=8k each tube sees 2k.
I need to ask if you are familiar with how load lines are constructed?

No, enlighten me. There's 2 tubes btw, how the hell does that move 8k to 4k?

[Alex/Tubewonder]

No, enlighten me. There's 2 tubes btw, how the hell does that move 8k to 4k?

I'd like to suggest you devote some time to study the basics:

"Audio Classroom" by Norman Crowhurst Read, ponder, get more confused, read again until you wake up in middle of the night shouting "Eureka!"

[bluefoxicy]

I think I get it. It's 8K across the whole coil, but it's center tapped (so 4K to the tap). Two tubes in parallel, so that 4K becomes 2K. I'm not sure how or why.