From: "Saved by Internet Explorer 11" Subject: Output Transformers | Tubelab Date: Tue, 24 May 2016 15:44:13 -0700 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_0000_01D1B5D3.1F3CCBE0" X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 This is a multi-part message in MIME format. ------=_NextPart_000_0000_01D1B5D3.1F3CCBE0 Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: quoted-printable Content-Location: http://tubelab.com/articles/component-testing/output-transformers/ =EF=BB=BF =20 Output Transformers | Tubelab = =20 =20 =20 =20 =09 =20 =20

Tubelab

Dedicated to advancing the state of the art = in=20 affordable high end audio.

Articles =E2=80=94 Component = Testing =E2=80=94=20 Output Transformers

Beginner

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I built this test amp for quickly testing freshly assembled boards. = It also=20 comes in handy for quickly evaluating different transformers. They can = be=20 swapped in about 5 minutes with only a screwdriver.

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I decided to use the =E2=80=9CCoke Bottle=E2=80=9D shaped = 6L6GC=E2=80=99s made by Shuguang for this=20 testing (amp pictured has EL-34=E2=80=99s) for two reasons. I have = previously abused=20 these tubes to over 40 watts dissipation without failure, and they are = cheap ($4=20 each on Ebay). In order to subject the transformers to the most possible = power,=20 I cranked the bias up to 80+ mA for a dissipation of over 30 watts. The = actual=20 current and dissipation varies slightly since each transformer has a = different=20 DC resistance. Even though the tubes are running above the recommended = maximum=20 ratings, these seem to handle it well. This set has already served about = 4=20 months of tough duty in a Fender Bandmaster, The owner didn=E2=80=99t = like them because=20 they were =E2=80=9Ctoo clean=E2=80=9D. They have been sitting here = cooking away for hours at a=20 time while I make measurements, without issue. I can get more power from = a 6550,=20 but I can=E2=80=99t get 6550=E2=80=99s for $4, and I don=E2=80=99t want = to risk frying any of the ones=20 that I have.

I ran the usual frequency response curves because that is a common=20 measurement criteria for OPT=E2=80=99s. This however does not tell the = whole story.=20 There are several small transformers (like the little Edcor) that have = excellent=20 frequency response. Even at full power the low end response goes down = into the=20 20 Hz region. In listening tests it is obvious that the small = transformers can=E2=80=99t=20 deliver wall shaking bass like the big ones can. I spent some time = looking for=20 an electrical test that can quantify this ability.

I found that distortion VS power curves ran at several different = frequencies=20 can do it well. The 1KHz curves are the baseline. In the ideal world the = curves=20 at all other frequencies in the audio spectrum should be identical. In = the real=20 world the iron in most transformers will saturate if pushed hard at low=20 frequencies and high power levels. This will show up as distortion on = the low=20 frequency curves. Also the high frequencies may be distorted by = excessive=20 capacitance, and magnetic losses. Since the amplifier remains unchanged = from=20 transformer to transformer, you can learn about the relative efficiency = of each=20 transformer by comparing a common point, say how much power can I get at = 5%=20 distortion at 1KHz. The larger transformers eat up more of the valuable = power in=20 magnetic and resistive losses.

I tested several transformers from tiny to huge. Each was run in = triode mode=20 and UltraLinear mode if they had the tap. Each was tested with cathode = feedback,=20 and without CFB. All transformers are tested at 100 Hz, 1 KHz, and 10 = KHz. The=20 large transformers are tested at 44 Hz, this frequency was chosen = because it is=20 the lowest note on a bass guitar. The large Hammonds are the only ones = that=20 handle this frequency without issue.

Hammond 1628SE

The largest transformers tested were the Hammond 1628SE, 11 pounds of = iron=20 and copper here. These are the older 1628SE=E2=80=99s, NOT the recently = released=20 1628SEA. I have the new ones on order, but they haven=E2=80=99t arrived = yet.

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The Hammond 1628SE in UltraLinear mode at 5 watts without any = feedback. The=20 response is rolled off at the high end. It is 3.4 db down at 20 KHz. The = response is the same in triode mode.

3D"" Cathode=20 feedback improves this to 2.7 db.

Triode Mode

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This is a graph of distortion VS power at 4 different frequencies in = triode=20 mode with no feedback. The increased distortion at 10 KHz above 5 watts = should=20 not cause a problem with music. This transformer shows 4 curves that are = very=20 similar up to 5 watts.

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This is a graph of distortion VS power at 4 different frequencies in = triode=20 mode with cathode feedback. Cathode feedback improves the distortion = slightly=20 especially at low power levels. The distortion remains primarily second = harmonic=20 up until the onset of clipping (about 4.8 watts).

UltraLinear

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This is a graph of distortion VS power at 4 different frequencies in=20 UltraLinear mode with no feedback.

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This is a graph of distortion VS power at 4 different frequencies in=20 UltraLinear mode with cathode feedback. The distortion at 10KHz is not = helped=20 much by cathode feedback. The other 3 curves are lowered significantly = with=20 CFB.

Cathode Feedback

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This graph shows what cathode feedback does for this transformer at = two=20 frequencies. The blue and magenta curves are 1KHz. The yellow and cyan = curves=20 are at 44 Hz. This graph is made from the same data sets as the graphs = above. It=20 just graphically displays the improvements in distortion afforded by = CFB.

Edcor CSXE25-8-5K

These are almost the same size as the Hammond 1628SE=E2=80=99s, but = they don=E2=80=99t have=20 as many laminations so they are not as heavy.

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The -3DB response here is well centered over the audio band. This = plot was=20 taken in UltraLinear mode at 5 watts with no feedback. Triode mode = looked the=20 same.

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Here cathode feedback was applied. The frequency response was = improved=20 slightly.