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Old 21st May 2013, 09:10 PM   #1
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Default Is this transformer saturation? (pictures inside)

Trying to drive 4 KT88's connected to a Hammond 1650T to the max (120 Wrms rated), I saw this on my oscilloscoop. The 12AT7 LTP phase splitter has signal swing to spare (blue line), but the 4 ohm transformer tap shows the clipping yellow line, connected to a 4 ohm resistive load. Is this the saturation effect? Or is something else going on.
screen_21-5-2013_22.22.04.png

Last edited by funk1980; 21st May 2013 at 09:13 PM.
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Old 21st May 2013, 09:43 PM   #2
DF96 is offline DF96  England
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It looks like clipping. The output stage is running out of either voltage or current.
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Old 21st May 2013, 10:00 PM   #3
MelB is offline MelB  Canada
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What is your B+ voltage?
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Old 21st May 2013, 10:08 PM   #4
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Quote:
Originally Posted by MelB View Post
What is your B+ voltage?
It's 420 volts. The reason I think it's saturation, is because the Vpp is around 120W into 4 ohm. Would be too much of a coincidence. And a clipping LTP looks dufferent AFAIK. But I'm not shure.
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Old 21st May 2013, 10:52 PM   #5
DF96 is offline DF96  England
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The LTP is not clipping. The output stage is clipping.
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Old 21st May 2013, 11:06 PM   #6
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The positive peak of the driver signal is flattening a bit. My guess is that you hit grid current on the output valves there. With mosfet source followers you could probably go into AB2 and get some more watts if you want.
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Old 22nd May 2013, 04:52 AM   #7
Yvesm is offline Yvesm  France
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At such frequency (1Khz) the OPT CANNOT saturate !
It would be different below, say, 50 or 30 Hz.

As said above, tubes are probably running out of current or voltage.
I'm curious to see what happen in slghtly changing the load, f.e. from 3 to 5 Ohms.

Yves.
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Old 22nd May 2013, 08:30 AM   #8
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Flux density decreases as frequency increases. At 1000Hz, I wouldn't expect that to occur, at least not unless you hit insane volts/turns ratios somehow. Besides, saturation looks more like square waves.

It also looks like your driver doesn't have enough drive capability. The positive peaks are flattening, and that's what grid current looks like when you have inadequate current sourcing. You've taken the Vgk of the finals as far as you can go. Any farther, and you're in Class AB2.
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Old 22nd May 2013, 09:19 AM   #9
DF96 is offline DF96  England
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Yes, the LTP is flattening a bit on one peak but that is not the cause of the output clipping. However, any more signal and the LTP would be the limit even if the output stage could cope.

For maximum possible output you want one side of the output to be hitting a voltage limit (bottoming) while the other side is at the current limit (cut-off) - they swap over at the opposite peak. This requires the right bias and right impedance load. For smoother clipping you want to avoid this, but at the expense of slightly less power.
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Old 22nd May 2013, 10:21 AM   #10
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But could it really be the output tubes/stage?? I mean, it's 420V into 4 KT88's with a 1900 ohm output transformer. Although the OPT is rated at 120Wrms, the KT88's should be capable of putting out way more than 120W!

Tonight, I'll replace the 12AT7 with an ECC99 I've got lying around. It's able to source more current than the AT7.
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