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Valve Amps
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Output transformers
The output transformer is the heart of a valve amplifier. The = right=20 pic shows a stereo Leak amplifier. The two output trannys are = rotated=20 90deg to the power mains tranny to avoid picking up induced hum from the = mains=20 tranny external magnetic field. Output trannys contribute most of = the=20 distortion in a valve amp and have a limited bandwidth.
Valve amp manufacturers purchase trannys from transformer winding = companies.=20 Transformer winding is an extension of the fitting and turning = industry.=20 Few people working in transformer winding companies have a = knowledge of=20 electronics or valve amps. Most valve amp manufacturers have a = basic=20 academic understanding of output trannys, but often have a limited or = incorrect=20 understanding of the physics of transformers or how they are made.
Academic formulas for calculating the = design of=20 output trannys is available in many text books and web sites. = There is no=20 need for this page to repeat academic information that is readily = available.=20 This page gives an overview of the physics that governs the = performance=20 of output trannys and helps give a perspective that academic text often=20 omit.
Output trannys are the highest cost and most labour intensive part of = a valve=20 amp to make. Each output tranny, depending on size, takes approx = 1 to 2=20 hours to wind, assemble with laminations, heated in an oven, dipped in = resin and=20 baked overnight, cleaned, painted, external leads connected, dress caps = fitted,=20 packaged and sent to a valve amp manufacturer. A tedious task to = say the=20 least. The right pic is a simple analogy of an output = tranny.
B+ historically represents battery. = B+=20 is the supply Voltage to the CT Center Tap of the primary windings of = the output=20 tranny. 560V DC supply is connected to the CT Center Tap of the = Primary=20 winding. The amplified audio from the 2 output valves, push pull = is in=20 opposite phase 180deg. 300VAC + 300VAC =3D 600V AC is across the = primary=20 winding.
Depending on the tranny design there may be 2,000 turns of wire in = the=20 Primary and 90 turns for the Secondary winding. 22:1 step down = ratio.=20 600V AC / 22 =3D 27V AC into an 8&Omega speaker is 90 = Watts.
The primary and secondary windings are isolated. The iron core =
is=20
magnetised by the AC current in the primary winding and the AC magnetism =
is=20
passed into the secondary winding generating an output Voltage to drive =
the=20
speaker. 
Copper mass The total mass of copper = wire is=20 divided 50% 50% approx between primary and secondary. Often the = primary=20 has slightly more mass. In the example tranny the primary has = 2,000 turns=20 of thin wire, whereas the secondary has 90 turns of thick wire.
E I and C core. The majority of = power and=20 output trannys in valve amps are E I construction. The right pic = shows=20 the thin E I mild steel laminations that are stacked together to = assemble the=20 core. The tranny next to the E I is a C core which can be used = for the=20 same size bobbin. Bobbins can be open or have a center division. = =20 The C core is approx 20% more efficient compared to the best quality = grain=20 orientated E I laminations. The square shape of the E I = laminations is=20 not an efficient conductor of magnetism. A % of the induced = magnetism is=20 spewed outside of the core and lost. A further % is lost as eddy = currents=20 within the laminations which causes the laminations to get hot. E = I mains=20 power trannys are noticeably hot when touched. C cores are an = efficient=20 conductor of magnetism, but C cores do not have an aesthetic square = shape and=20 are more difficult to mount onto a chassis. C cores are mostly = used in=20 industrial electronics and were approx twice the cost of E I = laminations. =20 Today, C cores are only slightly more expensive than E I laminations and = it=20 makes no sense not to use the superior quality C core in all output = trannys,=20 including power trannys.
It is essential to fill the window space with copper wire. = Once the=20 number of turns is calculated, then the thickest wire is selected to = fill the=20 window space. Any space not filled with copper wire causes = magnetic=20 energy being lost, described as leakage Inductance, resulting in reduced = performance.
Mains power tranny The primary is = wound on the=20 bobbin first, then the secondaries are wound on the outside of the = primary.=20 There are required safety standards for the minimum thickness of=20 insulation placed between the primary and secondary windings. A = mains=20 tranny only functions at a fixed primary Voltage and frequency (120V AC = 60Hz) or=20 (240V AC 50Hz) depending on the standard of the country.
Output trannys function over a wide range = of=20 frequency and Voltage. The primary and secondary windings must be = split=20 up and inter-leaved. Secondary - Primary - Secondary - Primary = etc.=20 Interleaving the primary and secondary enables the tranny to = achieve a=20 high frequency response. As a general rule, the greater the = numbers of=20 primary and secondary interleaving the better the high frequency = response.=20 A physicaly large transformer has increased capacitance between = windings=20 which causess phase shift and restricts the high frequency response. = A=20 smaller tranny with less turns favours the high frequency response.
Total turns Increasing the total = number of=20 turns in an output tranny increases the Inductance (magnetic efficiency) = which=20 improves the bass response. But, increasing total turns requires = a=20 smaller wire gauge which increases DC resistance and reduces the power = output of=20 the tranny. Calculating turns ratio, maximum number of turns and=20 interleaving is a balancing act.
Traditional approach is to minimise total = number of=20 turns and core mass, restricting the bass response in favour of = improving high=20 frequency response. The argument for justifying the approach of=20 minimising core size and total number of turns also minimises cross = capacitance=20 keeping phase shift to a minimum which allows a greater amount of = Negative=20 feedback to be applied hereby improving the pseudo academic figurers of = lower=20 distortion. Valve amps were part of the vinyl era where deep bass = caused=20 the needle to jump off the record. Most HiFi valve amps had a = rumble=20 filter to reduce sub-bass. The majority of speakers were approx = 6dB to=20 10dB more efficient and larger than most speakers boxes today. A = few=20 Watts was very loud.
Inductance. The real reason for = minimising the=20 output tranny size was/is to minimise the cost of making the tranny. = An=20 inductance of 20 Henries is often insufficient. 40 Henries or = higher is=20 required for an output tranny to deliver full power below 40Hz. = The=20 mathematical Inductance can be academically calculated, but in the real = world,=20 the physics of Inductance in output trannys is not a fixed value. = For a=20 full range output tranny, the aim is to get as much Inductance as = possible=20 regardless of what the academic number ends up being.
At bass frequencies the speaker Fs (Fundamental resonance) raises to = approx=20 20 to 30Ω. Valve amps naturally have a high output Impedance = and the=20 output Voltage will automatically rise in an attempt to maintain = constant power=20 to the speaker. As the output Voltage rises, especially at high = power, a=20 tranny without sufficient Inductance at bass frequencies can easily = saturate=20 creating intense distortion. To prevent core saturation, the = primary=20 turns should be the maximum number and the core mass must be as large as = possible for a given chassis size.
In the past when only valve technology existed, materials were = expensive in=20 comparison to labour. An output tranny for a 100 Watt valve amp = was=20 approx $10 to $20 in quantity. $5 for materials, $5 for labour = and little=20 profit. The majority of output trannys were the minimum size, = made from=20 the least expensive materials. Only a few Hi-end valve amps had = output=20 trannys that were made with the best quality materials.
Paper layering Early trannys for = valve amps=20 mostly used the less expensive, lower Voltage rating enameled copper = wire and=20 thick impregnated paper insulation between each layer of winding. = Paper=20 layering enabled the tranny to be wound at high speed by automated = machines,=20 reducing labour and keeping costs under control. Poor insulation = of the=20 paper and poor enamel coating on the wire enabled flashover to occur = between=20 windings.
Extreme high Voltages are generated across the primary winding if the = speaker=20 is disconnected while music is playing. Flashover between = windings was a=20 major problem with valve guitar amps.
A disproportional % of window space is consumed by paper layer = insulation=20 which limits the total number of turns to enable the transformer have = sufficient=20 inductance to obtain a good bass response. With a restricted = window space=20 the wire has to be thinner which causes further losses in the DC = resistance of=20 the wire. The space occupied by thick paper insulation causes = leakage=20 inductance, reducing efficiency of the tranny.
Modern materials High Voltage thin = Polyester=20 mylar, Kapton and Nomax insulation including high Voltage hardened = enamel wire=20 (used in electric motors) was readily available from the mid 1960s. = Using=20 these higher quality insulating materials does not require paper = layering=20 between each layer of winding. The above pics of modern mains and = output=20 tranny windings shows the thin high Voltage insulation is only used = between=20 primary and secondary windings, minimising the space occupied by the = insulation,=20 enabling the maximum mass of copper wire to fill the window space.
But very few valve amp manufacturers were/are aware of the physics of = output=20 tranny design, therefore the higher quality insulations and high Voltage = enamel=20 coated wire including C cores were rarely used. Output trannys = were made=20 to a budget and when solid-state technology arrived the era of valve = technology=20 came to a close.
Today, an output tranny for a 100 Watt valve amp is approx $50 to = $100 for=20 materials, $100+ labour, 100%+ profit. End price approx $200 to = $600. =20 But because of alchemistic beliefs in imagined magical = perceptions of=20 bygone brand names and components, many output trannys today are still = made=20 using the same methods and low cost materials as in the past.
Output Tranny types
In most output trannys the windings are layer wound across the = bobbin. =20 Interleaving the primary and secondary windings enables the tranny to = achieve a=20 high frequency response. The primary windings are connected in = series.=20 The secondary windings are mostly connected in parallel. = The=20 secondary windings often have taps to adjust the output Impedance for = 8Ω or=20 4&Omega speakers.
A cheaply made output tranny, used in many tube guitar amps, will =
have 2=20
primaries with a single secondary between the primaries (P1, S, P2,) or =
have 3=20
secondaries interleaved between the 2 primaries
(S, P1, S, P2, S,) =
as shown=20
in the right pic.
A better output tranny will have 4 primaries with 3 secondaries =
interleaved=20
between the primaries
(P1, S, P2, S, P3, S, P4,) as shown in the =
right=20
pic.
Some high quality output trannys may have as many as 8 primaries = interleaved=20 with 7 to 9 secondaries. The greater the number of primary and = secondary=20 windings, the better will be the high frequency response, but the longer = the=20 time it will take to wind the bobbin.
The right pic is a circuit showing the series connections of 4 = primary=20 windings. P1 is the first winding on the inside of the bobbin = whereas P4=20 is the last winding on the outside of the bobbin. P1 and P4 are = referred=20 to as outer windings, because the other windings are in-between.
Cross connection The primary = windings are=20 alternately cross connected in series around the secondaries (outer P1 - = inner=20 P3 - inner P2 - outer P4). Alternate cross connecting the = primary=20 windings enables the physics of the tranny to be closely balanced and = the=20 induced magnetic current to be distributed equally into the secondary=20 windings.
The aim is to balance (as close as possible) the primary secondary = windings=20 (equally) on both sides of the Center Tap CT. However, balancing = can not=20 be perfect, because the inner and outer diameters (circumference) of the = windings are different. Wire length and DC resistance of P4 is = greater=20 than P1. Increasing the number of primary and secondary = interleaving has=20 been the traditional approach to improve the balance and performance of = an=20 output tranny.
(P1, P2, CT, P3, P4).
Very few output trannys are constructed using this lateral approach.
Why this lateral approach of using a split bobbin to achieve a fully=20 ballanced output tranny was/is not seen by the majority of transformer = winding=20 companies is a mystery. The answer is possibly tradition and = conformity=20 and that very few electronic designers, especially audio valve amps, = directly=20 participate in the design and construction of transformers.
27kg bass tranny. The right pic is a = 27kg=20 (60lb) bass output tranny, fully balanced split bobbin. Yes you = read=20 correctly 27kg is possibly the largest output tranny ever constructed. = =20 This output tranny was constructed for experimental reasons. The=20 Inductance was higher than the test equipment could accurately measure. = =20 The tranny is capable of delivering 1,000 Watts below 10Hz. = However, the=20 practical application is for 4 KT88s in push pull parallel 200 Watts. = =20 These large output trannys can be made available to valve enthusiasts = who wish=20 to have the ultimate active sub-bass valve amplifier.
Pi windings The Greek letter Π = is the symbol=20 for transformer windings that are rotated 90deg in a bobbin. = Π windings=20 are sandwich assembled. The below pic is an experimental valve = amp with=20 Pi wound output trannys. The final design had 8 Primaries = sandwiched=20 between 9 Secondaries. Pi wound trannys are 100% symmetrically = balanced=20 and have a high frequency response x 2 greater than a conventional layer = wound=20 output tranny.
Pi winding is commonly used in ferrite core trannys that transfer = energy at=20 very high frequencies, as seen in switch mode power supplies and Rf = amplifiers.=20 Pi winding is also the perfect method to construct an audio = output=20 tranny. Pi winding output trannys are not known to have been = used in=20 audio valve amps. A possible reason is the high tooling and = assembly=20 costs. Also, the marketing of audiophile products, particularly = tube=20 amps, is driven by brand names, romantic nostalgia and reviews, = not by=20 engineering or performance.
Toroidal trannys have superior magnetic coupling between windings. = =20 But, a toroidal core can easily saturate at low frequencies creating = intolerable=20 distortion. Therefore, the mass of the toroidal core has to be = larger=20 than a conventional E I tranny (for the same power) to avoid any chance = of=20 saturating the core. Toroidal cores are un-forgiving of any DC or = AC=20 audio imbalance in the primary windings. Both output valves must = be=20 perfectly biased with exactly the same current and have exactly the same = gain=20 when driven. Providing these conditions are met, a toroidal = tranny will=20 outperform a conventional EI tranny.
4 way active valve tower. The below = pic is a 4=20 way active valve tower using toroidal output trannys as in the above = pic. =20 There are 5 100 Watt Ultra linear amps using KT88s. The 2 amps at = the=20 bottom of the chassis are paralleled to give 200 Watts. The = chassis is=20 divided down the center. One side of the chassis (left side of = pic) is=20 the power supply. The power supply also uses toroidal core = trannys.=20 The other side of the chassis (right side of pic) are the = toroidal output=20 trannys.
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Right pic shows output trannys and active crossover → Hi-frequency tranny (5kHz - 50kHz)→ The 3 small toroid chokes mounted above the large bass output =
tranny=20
are for filtering the B+ ↑ The small top toroid power tranny (+-20V and +-100V) is = for the=20 electronic crossover and driver circuits of KT88 output = valves. Below the top small power toroid is the power toroid and toroid = choke=20 for the 12.6V DC filaments heater supply. ← The 2 large toroid trannys create the 560V B+ supply. = The B+=20 supply is constructed from 3 X 188V supplies in series. ↓ Power turn on management. Total mass 60Kg approx. |
 |
The 4 way active crossover and driver stages are solid state to = insure a=20 perfect distortion free symmetrical signal is delivered to the output = valves.=20 Each KT88 has dual LED monitoring to enable the quiescent current = through=20 the KT88s to be calibrated.
Output Tranny application
Valve amps require perfectly matched output valves in order to give = the best=20 performance. This is similar to having balanced pistons in a = vehicle=20 engine. It is essential that the quiescent current through the = output=20 valves be calibrated exactly. A 10R resistor is placed in series = with=20 each Cathode. The -bias Voltage to the Grids is adjusted so 500mV = appears=20 across each 10R resistor. 500mV across 10R is 50mA. The = valves are=20 then driven to full power and the current through the valves will = increase to=20 approx 150mA. Both valves must be perfectly equal in both = quiescent and=20 full power condition. Any imbalance of current between the valves = will=20 result in the iron core of the tranny being partially DC magnetised. = The=20 Inductance will be reduced, the bass response will be diminished and the = core=20 will readily saturate at bass frequencies.
Tetrode Many early HiFi valve amps = and almost=20 all guitar valve amps are configured in Tetrode operation. In = tetrode=20 operation the Screens of the output valves are connected to a second B+ = filtered=20 supply Voltage from the power supply.
The primary of the output tranny is connected to the Anodes of the = power=20 valves. Tranny designed for Tetrode operation only need 3 wires = from the=20 primary. CT Center Tap to the B+ supply and 2 wires to the = Anodes. =20 Trannys designed for tetrode operation (guitar amps) are often made as = cheaply=20 as possible. However there are exceptions.
Ultra linear Output trannys designed = for Ultra=20 linear application are often made from the highest quality materials and = have=20 multiple primary secondary interleaving. The screens of the = output valves=20 are connected to the primary windings. Ultra linear provides the = same=20 high power as Tetrode configuration with superior power and = inter-modulation=20 performance than a Triode configuration. The power supply for = Ultra=20 linear amps must be of high regulation, smooth and ripple free.
The original academic text stated that the screens should be = connected to a=20 42% tap position of the primary winding, to achieve maximum power. = The=20 original text only refers to graphs and did not take into consideration = how an=20 output tranny is wound. It is difficult to mechanically achieve = perfect=20 symmetry with 42% tap positions on both sides of the Center tap. = However,=20 50% can easily be achieved. If a trade off has to be made, the = mechanical=20 symmetry of the primary windings, must come first. Also, the = theoretical=20 difference between 42% and 50% is too small for any auditory or measured = difference to be detected.
Output Tranny parameters
The distortion figurers and frequency response of Valve amps are = often=20 specified at 1 Watt. Most output trannys can easily achieve a = bandwidth=20 of 20Hz to 20kHz at 1 Watt. But this bandwidth is rarely achieved = at full=20 power. The low frequency response at full power is directly = proportional=20 to the number of primary turns and mass of the iron core.
Above 2kHz the iron core has little effect. The high = frequencies=20 response is directly dependant on the interleaving of the primary and = secondary=20 windings. However if the tranny has a large core and a large = number of=20 primary turns enabling it to achieve sub-bass at full power, then the = large area=20 of the windings causes increased capacitance between primary and = secondaries=20 which restricts the high frequency response. Therefore a small = core with=20 less turns favours the high frequency response.
There is no one way to wind the perfect tranny. There are a = few highly=20 skilled people who wind trannys and will have various techniques for = achieving=20 the objective. The inductance of the tranny will vary depending = on core=20 mass and quality, and number of primary windings. The aim is for = the=20 inductance to be as high as possible. The below pic is a complied = list of=20 basic formulae for those who wish make output trannys.
Academic formulas for calculating output = tranny=20 design are available in many text books and web sites. Those who = make=20 output trannys will already be overwhelmed by the math and there is no = need for=20 this page to repeat academic information that is readily available. = This=20 page is intended to give an overview of the physics that governs the = performance=20 of output trannys, from a perspective that academic text often omit.
Impedance ratio
Understanding Impedance ratio between primary and secondary is = difficult for=20 those who are technical and almost impossible to understand for those = who are=20 not. Hopefully this explanation will make it easier for anyone = who is=20 interested.
Academic text states that a pair of KT88s in push pull Class AB, from = a B+=20 560V supply Voltage will deliver 100 Watts. Under this condition = the=20 output tranny primary Impedance is stated as 4k5Ω. The Impedance of = 4k5Ω is an=20 academic figure and represents the highest plate to plate Impedance that = will=20 enable 100 Watts to be obtained. (Plate to Plate simply means = Anode to=20 Anode). This academic figure of 4k5Ω assumes 100% = transformer efficiency=20 with zero losses.
A pair of KT88s will rarely achieve 100 Watts in the real world. The = average=20 is 80 to 90 Watts. Also the plate to plate Impedance can vary = between 3kΩ=20 to 5kΩ with little change to the available power.
Our sample output tranny has 4 primary windings (600 turns per = winding) a=20 total of 2,400 turns.
4k5Ω plate to plate / 8Ω speaker =3D 562.5. =
√562.5 =3D 23.7 turns=20
ratio.
2,400 primary turns / 23.7 =3D 101 secondary turns.
This academic process requires the formula to be repeated over and = over for=20 each change in calculation. Another way to view this process is = through=20 the math related to the output tranny physics. The Voltage ratio = between=20 primary and secondary is the same as the turns ratio.
A general rule for push pull Class AB valve amplifiers. The = RMS=20 Voltage across the Primary winding is 1.1 x B+ supply = Voltage=20 (at full power).
1.1 x 560V DC =3D 616V AC
100 Watts into an 8Ω speaker =3D =
28V RMS
616 /=20
28 =3D 22 turns ratio.
2,400 primary turns / 22 =3D 109 secondary =
turns.
There is a small difference of 8 secondary turns between the two=20 calculations. This second formula does not look at the plate to = plate=20 Impedance. Therefore, which calculation is more practical to use = ? =20 By reversing the last calculation of a 22:1 turns ratio, by the first = formula,=20 we can see what the reflected plate to plate Impedance has been reduced = to -
22 x 22 x 8Ω =3D 3,872Ω plate to plate Impedance.
A difference in turns ratio (23.7:1) (22:1) results in a small = difference of=20 8 secondary turns. This small difference causes a large change to = the=20 academic reflected plate to plate Impedance.
Output trannys are not 100% efficient. Core losses, = leakage=20 Inductance, DC resistance of the copper wire, all contribute to = increasing the=20 overall reflected plate to plate Impedance, higher than the academic = figure.=20 Therefore the second formula closely represents real world = conditions.=20 However it is wise to be conversant with both approaches and = cross check=20 each method.
Valve Amps
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