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Understanding load on triode and impedance matching
Understanding load on triode and impedance matching
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Old 15th June 2019, 08:44 AM   #61
itsikhefez is offline itsikhefez  United States
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Understanding load on triode and impedance matching
Parts have been ordered so..... "doing" will get started soon.
Thanks for the push
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Old 17th June 2019, 09:55 AM   #62
johnsurnamerobinson is offline johnsurnamerobinson
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Go for it !!!!
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Old 10th September 2019, 04:57 PM   #63
itsikhefez is offline itsikhefez  United States
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Understanding load on triode and impedance matching
tl;dr thread until now- I was planning to build a 45 based amp for headphones and was unsure which OPT to buy (to optimize for 64-300 headphones), and whether load resistors are required. ----

Fast forward few months, I ended up buying 5K:32 OPT's and my build is now almost complete. I made a few measurements in ARTA. (I'm new to using this system so please correct me if I did anything wrong).

I measured the with 33R, 300R (no load resistor) and 300R (w/ 75R load resistor, effective load 60R). ARTA generated a -3dbFS signal and I set the volume to match as close as possible.

Distortion was best on the 300R load, followed by 60R and then 33R. FR was pretty similar, maybe slightly more lower end roll-off on the unloaded 300R.

I am surprised that the 300R load performed better without any load resistors.
Attached Images
File Type: png FFT 33R.png (75.1 KB, 148 views)
File Type: png FFT 300R (75R).png (76.9 KB, 147 views)
File Type: png FFT 330R Unloaded.png (76.4 KB, 147 views)
File Type: png STEPS_FR_300R_0R.png (58.5 KB, 147 views)
File Type: png STEPS_DIST_300R_0R.png (57.8 KB, 146 views)
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Old 10th September 2019, 11:50 PM   #64
PRR is offline PRR  United States
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Quote:
Originally Posted by itsikhefez View Post
....I am surprised that the 300R load performed better without any load resistors.
Simple amplifiers make lowest distortion with "least load". 32r is heavy load, 300r is light load, for a "32r" tap.

The flip side is that a 300r load would typically need more *voltage* to get the same loudness. For equal-power, the 300r should be driven 3X the Voltage (+10dB) of a 32r. However I have observed that the hi-Z phones tend to be more sensitive, the 32r phones are often indifferent about efficiency, so maybe not a big deal. Especially since you seem to have 10X the power any phone needs to blow your brains out your nose.
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Old 11th September 2019, 09:19 PM   #65
johnsurnamerobinson is offline johnsurnamerobinson
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Re:> itshhefez.Your test results seem about right,and pretty much what I would expect to see as at high impedance you are reducing the influence of the magnetics (amongst other things)
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Old 12th September 2019, 03:07 AM   #66
Sadface is offline Sadface  New Zealand
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Hi Guys,

I have a similar query regarding the output on a VTA m125 -parallel push pull with kt88s.

The amp is designed for 4x kt88 but it turns out I don't need remotely that level of power so I have only ever ran these with a pair of kt88s in each.

The OPT is an 8k and I am using the 4ohm taps to power some dynaudio audience 42 speakers (nominal 4 ohm).

If I am correct in my thinking. What I am looking at here is the inverse of the itsikhefez was dealing with.

My thinking is as follows: if the amp were using 4x kt88 per channel, that 8k OPT is being shared by the 2 kt88s on either side of the pushpull. Therefore, by only running 1 kt88 per side of the push pull, each kt88 is dealing with a higher resistance load.

I don't yet have any equipment for measuring such things. But assuming my thinking is correct: as currently used with a single pair of kt88s per channel, the amps are likely putting out slightly less power but at a lower distortion due to driving an easier load?
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Old 12th September 2019, 04:41 AM   #67
Wavebourn is offline Wavebourn  United States
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Understanding load on triode and impedance matching
People get confused by THD graphs for power pentodes that show increase of distortions with increase of load resistance after the tube starts saturating, such ^ - shaped curves. You should interpret that curves properly. If to keep the anode voltage swing below saturation distortions with increase of load resistance decrease, and only decrease.
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Old 12th September 2019, 07:45 AM   #68
johnsurnamerobinson is offline johnsurnamerobinson
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Re Sadface i think you might be right (first thoughts on the matter)
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Old 12th September 2019, 09:26 PM   #69
PRR is offline PRR  United States
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> each kt88 is dealing with a higher resistance load.

What Wavebourn said. The tube-charts are drawn for *constant* grid drive and often extend into clipping.

In real life, if it distorts, you Turn It Down.

For any tube amp: the lowest THD is at infinite load. However this is also zero power output. So we use the highest load which gives the required power without getting into clipping. But commercially, if you paid for two 6L6, you must extract "ALL" the 40 Watts you have paid for, so you lower the load accordingly.

Your quad 6550 amp would *probably* show lower test THD with all four tube working. It doubles the load impedance on each tube, working more constant-current. However if you are so far from quad-6550 levels that you could consider removing a pair, it probably don't matter a bit.
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Old 13th September 2019, 05:59 AM   #70
disco is offline disco  Netherlands
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For push pull the distortion line will flatten out but this is grossly the idea. You are shifting the load to the Rl=rp point:
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