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Help troubleshooting my first SE OPT
Help troubleshooting my first SE OPT
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Old 4th February 2018, 06:05 PM   #1
Elerion is offline Elerion  Spain
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Default Help troubleshooting my first SE OPT

Hello everyone!

After making two successful push-pull transformers, I just finished my first 10W SE OPT to use with a fixed-biased KT88.

I now use a much better insulation, and my winding technique has improved.
I used a simple p-S-P-S-p sectionalization and 0.4mm insulation between windings.
The measurements seem good:

- DC resistance, primary = 93.62 ohm, secondary = 0.39 ohm.
- Leak inductance = 6.3 mH (1kHz)
- Pri/sec capacitance = 500 pF.
- Voltage ratio = 18.73
- Impedance ratio is 350.68, and reflected impedance is 2800 ohm for an 8 ohm speaker (I designed it for 2700 ohm, so it is close enough).
- UL tap is at 25%. I checked the ratio with AC multimeter and also an oscilloscope.
- Bandwidth, with 600 ohm signal generator, is 5.5Hz - 360 kHz. With this setup, the resonance is at 90 kHz, and the peak is not very high.

NOW, this is what I get (attached).
Connected to a test speaker, the sound is unbearable.

For the sake of completeness, I used .25mm wire for primary, and series secondary connection using 2x0.7mm wire, and 0.15mm gap.
I tested the amplifier with a commercial OPT, and works great.

Help is very much appreciated!!
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File Type: jpg SE_oscillation.JPG (155.3 KB, 345 views)
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Old 4th February 2018, 06:46 PM   #2
Lingwendil is offline Lingwendil  United States
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Help troubleshooting my first SE OPT
What did you do for an airgap? Often a piece (or stack) of paper is used...
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Old 4th February 2018, 06:55 PM   #3
Elerion is offline Elerion  Spain
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I'm using a 0.075 mm strip of mylar.
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Old 4th February 2018, 10:06 PM   #4
Elerion is offline Elerion  Spain
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I just found where the problem was.
It sounds nice, both in UL and, better still in triode mode.
Tomorrow I'll make some more test, for performance.
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Old 4th February 2018, 10:08 PM   #5
Lingwendil is offline Lingwendil  United States
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Help troubleshooting my first SE OPT
What was the issue?
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Old 5th February 2018, 09:36 AM   #6
Elerion is offline Elerion  Spain
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I was almost completely sure that I got the phase right, but the secondary wires were swapped, and this amplifier uses some GNFB. I just went over all the process again, and then found where the fault was.
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Old 5th February 2018, 07:58 PM   #7
Elerion is offline Elerion  Spain
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I've been taking some measurements of the amplifier/OPT, in search of core saturation, to validate my air gap. This is my first air gap, so I'm seeking for some feedback.

I've attached two scope captures.
One is with output tube working in UL (25%), the other one is triode strapped.
Cyan is input signal.
Yellow is output (to an 8 ohm dummy load).
Both are very close to full power.

I decreased frequency, in search of core saturation (to validate the chosen air gap), until I got this distorted waves.

Are they showing core saturation? Or just inductive loading on the output tube?
With triode, it happends at a lower frequency, so it makes me think this is just the primary inductance loading the stage.

Is there any other way I can check my air gap?
Attached Images
File Type: jpg Triode_sat.JPG (173.6 KB, 255 views)
File Type: jpg UL_sat.JPG (171.5 KB, 251 views)
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Old 7th February 2018, 02:06 PM   #8
Kay Pirinha is offline Kay Pirinha  Germany
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Which is the power and the plate current you wanted it to be designed for? Anyway, a 75 Ám air gap appears to be rather narrow. I'd expect somewhat in the tenths of millimeter's range. Try 0.5 mm.
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Old 7th February 2018, 02:44 PM   #9
Elerion is offline Elerion  Spain
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8W and 80 mA quiescent current.

It's a 0.15 mm aire gap. The sheet of insulation is 75 Ám.

It gets down to 20 Hz, full power (triode strapped).
But I'd like to learn how to measure the saturation frequency, or the low cut-off due to the inductive loading of the primary.

Attached, distortion for triode and ultralinear configuration, very close to clipping level.
Inside the amplifier, and using some 10 dB GNFB.
Attached Images
File Type: jpg Triode-dist.JPG (140.9 KB, 207 views)
File Type: jpg UL25-dist.JPG (142.2 KB, 202 views)
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Old 7th February 2018, 05:54 PM   #10
6A3sUMMER is offline 6A3sUMMER  United States
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Testing the transformer for things like bandwidth would require disconnecting the GNF.
If the transformer is the main limiter of bandwidth, that is the correct way.
(i.e. if bypass capacitors, coupling capacitors, and miller capacitance is not the limiter of the amp's bandwidth).

Here is a test for the aproximate primary inductance:
That does not include the effect of the operational (80mA) DC current in the primary.

Discharge all capacitors in the amp. Be sure they are discharged.
(and by the way, you should have bleeder resistor(s) across the B+).
But these take a long time to discharge, so check the voltage with a DMM
before jumping in with "hands and feet".

Remove the IEC power cord (and any other grounds that are connected to the amp).

Remove any and all loads from the transformer secondary, including the GNF resistor.

Then connect the 600 Ohm generator in Series with a 600 Ohms resistor, and connect that (1200 Ohm signal source) across the primary. Connect the scope across the primary (ground one end of the primary using the scope probe ground clip).
Set the generator to 1 kHz; measure the voltage across the primary on the scope, and record the voltage.
Set the generator to lower frequencies until the voltage is 0.707 x the 1 kHz voltage.
i.e. 10V at 1 kHz, 7.07V at a low frequency.
That is the -3 dB frequency (f) when the primary is driven from 1200 Ohms.

(1) 1200 Ohms = 2 x pi x f x L

(2) L = 1200 Ohms/(2 x pi x f). Solve for L.

Now, check the above answer by an independent measurement method:
Connect the 1200 Ohms signal source (600 Ohm generator in series with 600 Ohm resistor) across the primary.
Connect one channel's scope probe across the primary (the scope probe ground clip to one end of the primary).
Connect another scope channel's probe from the top of the 1200 Ohm source to the other scope probes ground clip (measuring across the series string of generator, 600 Ohm resistor, primary).
Starting at 1 kHz (the two channels may be fairly close to 0 degrees, but can be at another angle according to the self resonance of distributed C and primary L.

Lower the generator frequency until the phase angle difference in the 2 channels is
45 Degrees (1/8 of a complete sine wave).
That is where the primary inductive reactance is equal to 1200 Ohms.
That should be at or near the same frequency, f, that was calculated in (1) above.
Again solve for L by plugging in f into (2) above.

I estimate that the rp of the KT88 in ultra linear mode may be about 2000 to 3400 Ohms.
I estimate that the rp of the KT88 in triode mode may be about 1000 to 1700 Ohms.

Without GNF, that will result in the two different -3dB bandwidths you get in ultra linear
versus triode modes. It will also make a difference in the shapes of the low frequencies
versus UL and T modes, and the same shape will be at 2 different frequencies.

GNF will tend to bring the -3dB bandwidths, and shapes at low frequencies be a little closer to equal.

I hope that helps, and happy listening.

Last edited by 6A3sUMMER; 7th February 2018 at 06:00 PM.
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