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OPT resonance Q factor in the HF region - your opinions?
OPT resonance Q factor in the HF region - your opinions?
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Old 16th July 2018, 08:03 PM   #11
6A3sUMMER is offline 6A3sUMMER  United States
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Quote:
Originally Posted by 50AE View Post
Opinions are welcome for both cases - NFB available or not. Speaker and cable loading have a significance as well. I would probably strive to the optimal solution for music listening, compared to resistive load measurements. Cable capacitance is one evil and it's in parallel with the secondary inductance.
Answer: Of course, I strive for optimal performance for music (and therefore with loudspeaker loads). Testing the amp with load resistors is great, it tells me lots about what I did right and/or wrong. THEN I connect the amp to loudspeakers. Note: I would only use Negative Feedback for Ultra Linear, and Pentode modes (for either single ended or push pull).

I do Not use negative feedback for Triode, Triode wired Pentode, and Triode wired Beam Power output tubes. I design what primary impedance to use, so that "Triode" rp gives reasonable damping factor and reasonable distortion (but tradeoff power).

Quote:
Both transformers will have the exact same core, number of winding, labour, hence price. There will be a slight alteration in insulation quantity and type, changing Cp vs Ls.
Answer: If turns and core are the same, and only the insulation changes (i.e. dielectric constant, and thickness) how did you get:
Ls = 0.69mH (transformer #1)
and
Ls = 1.13mH (transformer #2)

You must have used thicker insulation on the transformer that has Ls = 1.13mH. That would give slightly less coupling from winding layer to winding layer. Right?
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Old 17th July 2018, 10:35 AM   #12
SemperFi is offline SemperFi  Wake Island
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They both seem like good transformers.

Isn't Q dependent on L,C, and Rs? If the windings are the same, just different insulation, how can the Q (and thus Rs) be so different?

Anyways, for a class-AB PP design I understand you want the lowest possible leakage inductance so that the tube shutting off in the class-B portion of the signal doesn't produce horrible spikes. Leakage inductance is worse than leakage capacitance in most cases I'd think. Unless we're talking very high impedances (low current high voltage which case capacitance is worse).

My vote goes to #1.
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Old 17th July 2018, 08:59 PM   #13
6A3sUMMER is offline 6A3sUMMER  United States
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SemperFi,

Yes. Not just the factors that I said in post # 11. You also caught what I did not think about . . . the possibility of the different Rs. In order for Rs to be different if the number of turns are the same, the wire size has to be different. But different wire sizes will change the capacitance too, smaller wire size will pack closer, but have less surface area from top to bottom that is next to the other surface area from top to bottom. It will also change the number of turns across the full width winding, since smaller wire size will pack tighter. That will change the number of layers required to get a specific inductance (total primary inductance, I do not mean Ls).

Last edited by 6A3sUMMER; 17th July 2018 at 09:02 PM.
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Old 18th July 2018, 01:03 AM   #14
PRR is offline PRR  United States
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> different wire sizes will change the capacitance too

Not much. Each turn counts for less, but you typically put on more turns. You can usually estimate C simply by the area of the winding, not the wire size.

That's for audio. If you are working high frequencies (and no bass) or high voltages, you wind with appreciable space between conductors (perhaps by laying a thread between turns). If only half your area is copper, C is about half.
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Old 18th July 2018, 08:42 AM   #15
6A3sUMMER is offline 6A3sUMMER  United States
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PRR,

The stipulations were that Rs was changing, and the turns count was the same.
Only two things to change Rs in that case:
Wire size
Wire alloy
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Old 18th July 2018, 12:54 PM   #16
50AE is offline 50AE  Bulgaria
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OPT resonance Q factor in the HF region - your opinions?
Quote:
Originally Posted by 6A3sUMMER View Post
Answer: If turns and core are the same, and only the insulation changes (i.e. dielectric constant, and thickness) how did you get:
Ls = 0.69mH (transformer #1)
and
Ls = 1.13mH (transformer #2)

You must have used thicker insulation on the transformer that has Ls = 1.13mH. That would give slightly less coupling from winding layer to winding layer. Right?
You answered to the question yourself. Yes, by doubling the distance between the layers, you decrease the capacitance by half, but Ls doesn't double. You can find an optimum if you play with the insulation parameter. Second comes the dielectric constant. I have some experiments in mind considering a hybrid air-nomex insulator.


Quote:
Originally Posted by SemperFi View Post
They both seem like good transformers.

Isn't Q dependent on L,C, and Rs? If the windings are the same, just different insulation, how can the Q (and thus Rs) be so different?
The "problematic Q" is a series RLC circuit and is dependent of Ls (leakage), overall capacitance (Cp) and Rs. Different insulation (type and thickness) alter Ls due to coil thickness increase/decrease and Cp due to distance and/or dielectric constant change).

There is a resonance before this one, dependent of Cp and the main inductance L being parallel to it, but its Q is naturally so low that this resonance is practically insignificant and can be hard to measure. Its nature is more problematic in inductors with a low main inductance, like air coils for speakers.
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Old 18th July 2018, 06:05 PM   #17
45 is offline 45  Italy
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By the way, I have done once the output transformers for a 60W Dynaco Mark III kit (DIY clone) with KT88's and they had some 3 dB peak at the resonance around 77 KHz. This was really easy to deal with. In fact the original Dynaco design already had a small 12 pF between B+ end of the anode resistor and grid of the phase splitter and another 390 pF cap in fdbk loop. Both these caps are there to reduce gain above 40 KHz and get the correct phase. Just needed to adjust those values.

The EI core transformers had 77KHz resonance due to 5.3 mH Ls and 0.8 Cp. As the UL output stage with KT88 has a Zout of about 6.5K and the Zaa is 4.5K and it had to be affordable there wasn't much choice. But it was as good as it gets in the audio range. In fact it could deal with 60W @25Hz with just 0.8T induction and over 400H primary inductance for very low distortion.
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Old 18th July 2018, 06:54 PM   #18
6A3sUMMER is offline 6A3sUMMER  United States
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50AE,

I used to have access at work (after hours) to a German $60,000 4GHz Vector Network Analyzer (VNA) that could make precision measurements as low 10Hz. It used 'external' mode, and a T connector so it could get down to 10Hz (classical Return Loss Bridges need not Apply).

The very high dynamic range, external mode, T connector, and precision cal kit (open, short, load) with high order math in the software, gave it the capability to make very good tests on Output transformers, and Interstage transformers (as well as on amplifiers).

The many output transformers I tested had primary resonances (L and Distributed-C) that were between 500Hz - 2kHz. These were high impedance at resonance, and yes as you noted are swamped out by the drive impedance (especially by triode output tubes).

Swamping this resonance out with an un-fedback Pentode or Beam Power tube does not happen, the only swamping that occurs is from the secondary load (and local or global negative feedback).

The vector network analyzer, plus some external resistors for driving the primary, and for loading the secondary allowed the following tests: Bandwidth I/O Phase Power Loss Primary Inductance Primary Distributed C Leakage Reactance Primary to Secondary C Damping Factor at the secondary versus Frequency (When the primary was shunted with a resistor that simulates the driving plate impedance).

I also tested amplifier's frequency response, gain, and phase versus frequency. I used load resistors, and also loudspeaker loads, and compared the results. The amps tested had triode output stages, and did not have negative feedback.

Perhaps there were other tests that I forgot that I did with that VNA years ago.

I miss having access to that VNA.
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