Fig1.

This is an equivalent model of a PP output stage with a pair of class A UL 6550 tubes.
The OPT is considered without its centre tap and B+ supply because we are only interested here in the ac working.
So the OPT primary with CT can be considered here as one winding with one end grounded.
The Ra of 4k for each tube is summed to make Ra-a = 8k and this output anode resistance of the2 tubes
is shown in series between OPT primary winding input and the output of an imaginary voltage generator.
The model generator has an imaginary output = µ x grid input voltage which in this case is the summed grid to grid signal input voltage of 51.85Vrms
The µ of the tubes = 20, so the generator produces the imaginary Vout = 1,037Vrms.
We know there is 500Vrms across the the primary load and the load is 7.8k, so we have 64mA of load current
and this flows around the circuit through Ra-a to the OPT input, through the leakage inductance
which appears as a series inductance, then through the winding resistance which also acts as a series resistance,
and through the transformed secondary load which appears here as 7.8k at the primary winding input.
The actual two winding transformer is considered perfect with no losses, but the imperfections are added
as they behave in the real world and are depicted as lakage inductance, LL, = 5mH,
total Primary + Secondary winding Resistance, Rw, = 390 ohms, primary shunt inductance = 100H,
and

The Primary winding for anodes is 0.35 mm copper dia wire.
The overall dia including the coating of insulation will be 0.42 mm.
The traverse width of 62 mm of the bobbin will allow 145 turns per layer.
There are a total of 16 primary layers each with 145 turns, to make a total of 2,320 turns.
These layers are grouped into five sections, with two sections of two layers,
and three sections of four layers.
The HT is connected to a CT and UL screens can be connected to tappings at 37.5%
of the half primary windings to provide the UL screen feedback.
The central four layers are subdivided into four separately terminated windings to allow
tertiary cathode feedback use of either 12.5% or 25% of the total windings.

The Secondary winding for speaker connection consist of 6 windings in 4 layers using 1.0 mm copper dia wire.
Three have 57 turns each between each side of the bobbin.
The uppermost secondary layer in the wind up is divided into three sections of 19 turns each.
These four layers are alternately placed between the 5 main primary sections.
It can be seen that there are 5 primary sections, and four secondary
interleaved sections.
Thus 57, 76, and 114 secondary turns are available for matching purposes.

High temperature insulated best quality copper magnet winding wire must be used,
and neatly wound in even flat layers to fill the bobbin.

Insulation :-
Insulation can be Teflon, which is best, but it is very expensive and difficult to work with.
Polyester is quite OK and has sufficiently good temperature ratings and a dielectric constant of under 3, and very similar to Teflon so the effective capacitance is not increased too much. Don't use electrical paper. Nomex is another one which
is also OK. I have used polyethylene which gives a lower dielectric constant of less than 2, so capacitances are minimal
with polyethylene but the temperature rating is poor, so a short circuited tube may heat a primary winding and melt the insulation unless there is active protection against excessive tube currents.
Polyethylene, also known more commonly as polythene does not allow the use of electrical varnish and baking at 125C for 4 hours and the OPT must be impregnated by soaking in molten wax at less than 90C.
Alternatively use polyurethane two pack Estapol 7008 which is applied while you wind and which avoids the whole
process of varnish or wax or baking. See the page on winding transformers.

The OPT described will not run hot under normal conditions.
The insulation is placed as noted on the bobbin winding guide.
It should be pre cut from sheets to tightly fit between the cheeks of the bobbin, but not so tight to causes wrinkling of the sheet materials.
The arrangement allows for 0.05 mm thick insulation is placed between primary to primary layers with the same direct voltage potentials.
0.5 mm insulation is placed between primary and secondary layers, with the exception of placing it between primary to primary layers as shown on the winding guide within the four central layers
to allow the use of all these layers, or half of them for a tertiary cathode feedback winding.

All wires coming out of the wound bobbin through slots or holes are insulated with heat resistant sleeving cut to 50 mm long, so 25mm of the sleeving extends into the winding area. Outside the bobbin, this fine gauge sleeving extends 25 mm,
and allow for slightly larger sleeving to be fitted over the top between the bobbin
and the termination boards, following winding of the bobbin.

Fig 2.

The schematic for the OPT No1 is shown below with probably the most common and desirable load matching
for the majority of push pull amplifiers world wide.
Fig 3.

The Fig 3 above shows the OPT connected for standard Ultralinear with 37.5% screen taps.

Fig 4.

The Fig 4 above shows the use of 12.5% of the primary windings used for cathode feedback to the output tubes.
Its possible to use all primary layers from 6 to 13 ( see Fig 2 ) to have 25% of CFB.
My website page describing the integrated 8585 amplifier shows the details for using 12.5% CFB with
KT90, KT88, 6550, with Ea = 480V, screen suppl, Eg2 = 330V. This gave the best power output, lowest THD,
and biggest effective anode resistance reduction compared to any other way of setting up an output stage for multi-grid tubes. However the maximum grid driving voltage at each output tube is 56Vrms, and using 25% CFB increases this to
about 87Vrms, and the distortion of the drive stage begins to be a problem.

There are 3 main turn ratios.
(1)  2,320 primary turns to 57 secondary turns, with four paralleled 57 turn windings.
This gives a TR = 40.7:1, which gives an impedance ratio, ZR, = 1,656:1.

(2)  2,320 primary turns to 76 secondary turns, made up from three paralleled 57 turn windings, seriesed with three 19 turn windings in parallel.
This gives a TR = 30.5:1, which gives ZR = 930 to 1.

(3)  2,320 primary turns to 114 secondary turns, made up with two seriesed
pairs of 57 turn windings in parallel.
This gives a TR = 20.3:1, which gives ZR = 412:1.

Table 1.  Load matching and impedance ratios, OPT NO1.

The winding resistances are :-
Primary, 2,320 turns of 0.35 mm dia wire = 113 ohms.
Secondary, 4 x parallel 57 turn windings, 1.0mm dia wire = 0.085 ohms = 141 ohms reflected to the primary.
Total winding resistance looking into the primary = 113 + 141 = 254 ohms.

OPT Power Loss %  = 100 x Rw / ( Rw + RL ) %

Table 2. Recommended tubes for OPT No1 with approximate power output and winding losses :-

SPECIFICATIONS, OPT NO 1.

Low frequency behaviour.
The low frequency limit is due to core saturation which is voltage dependant, irrespective of load currents.
Frequency of saturation, Fsat = 13.2 Hz where Bmax = 1.8 Tesla at 547Vrms anode to anode, which
is 45 watts into 6.6k .
The primary inductance depends on the ac voltage and frequency applied and the permeability, µ, of the iron laminations.
At 50Hz maximum µ of the E&I GOSS laminations I purchase from Sankey in Newcastle, NSW, reaches 17,000 when the
lams are fully interleaved. The resulting maximum primary inductance = 1,077 Henrys.
At very low Va-a of 5Vrms, LP is greater than 100H and the inductance is entirely sufficient
so that with RL = 8ka-a and Ra-a also 8k, the -3dB point at LF at levels below saturation = lower than 6 Hz.

Power handling ability is only restricted by to whatever saturation frequency and losses are deemed acceptable, and limits on the DC input current. The primary wire size = 0.35mm dia so the maximum dc current at 3amps/sq.mm = 288mA,
or 577mA at the CT. This is much more that whatever may be used for any of the tube listed in the
recommended tube table 2 above.

110 watts into 1.65k is only 426Vrms a-a, and the ac signal current = 258mA.
This is approaching the safe continuous ac power but with music signals there is never a continuous maximum sine wave signal.

100 watts into 5k will be  707Vrms, and ac  signal current is well within the OPT safe working area.
However, as Va-a rises, the Fsat will rise, and at 800Va-a the Fsat = 19Hz, which is quite acceptable.

Winding losses rapidly increase as the RL is reduced. At RL = 5ka-a, Rw = 5% which is excellent considering the
OPT will most likely be used with loads above this figure where winding losses become negligible.

High frequency cut off.
The leakage inductance, LL,  referred to the primary = 4.2 mH. The Ra-a + RLa-a  = 8k + 8 k = 16k,
so the cut off due to LL = 606 kHz. This figure will never be seen since the shunt capacitance will cause the cut off
at a lower frequency. Cshunt looking into the end of each primary end with CT grounded and one end of the secondary
grounded will be approximately 1,000, so the equivalent Ca-a = 500pF.
The HF cut off is where the reactance of the shunt C = Ra-a and RLa-a in parallel = 4k.
Therefore HC cut off = 79.5 kHz, which is at a much lower F than than caused by the LL.

For much greater analysis of how to work out shunt capacitance see the calculations for OPT No1
in the page on 'Push Pull Output Transformer calculations'

The leakage inductance and shunt capacitance will form an initial series resonant circuit at 110 kHz when the response will
show a dip due the reduced load impedance due to the impedance of the series LC at resonance.
The dip in impedance can be reduced by a zobel network across the secondary of perhaps 8 ohms + 0.33uF.
The critical damping load required for a second order LC filter to prevent peaking in the response
= 1.41 x ZC or ZL at the resonant F. In this case, at 110kHz, 500pF has ZC = 2.9k so the damping resistance load
needed is 4k. Suppose we have the OPT set for 2,320 : 76 turns, giving a 6.6k : 7 ohms match then ZR = 932:1 so
the damping resistance theoretically should be 4k / 932 = 4.29 ohms. This is less than  the rated RL value of the amp
but 4.7 ohms could be tried with 0.47uF, thus loading the amp with a partially reactive load of 6.6 ohms at 72kHz,
and a mainly resistive load above 100kHz which should help prevent excessive response undulations.
One only needs just enough R&C for the zobel on the output to prevent excessive square wave overshoot.
The actual value of the zobel network across the secondary must be confirmed experimentally and perhaps 0.27uF and 8.2 ohms will be quite sufficient.
The phase shift and response sag at 20kHz will be negligible.

PP OUTPUT TRANSFORMER No2.

OPT 2 is larger transformer with an E&I wasteless core pattern with the same 44mm tongue dimension as with OPT No 1.
The size is simply increased by using a taller stack of laminations up to 100 mm.
The Primary turns are reduced to 2,080 total, but instead of 0.35mm dia wire, the wire diameter is 0.4 mm. The secondary wire diameter will remain 1.0 mm, and winding arrangements equal to OPT No 1. The insulation size of 0.5mm between P&S layers and else where may be reduced to 0.4mm and although this increases the Cshunt somewhat,
the RL likely to be used with such a transformer and the Ra-a will be lower so the capacitance will not reduce the
bandwidth greatly, and the slightly closer coupling distance between P&S windings will improve the leakage inductance.
All other sizes and arrangements are kept the same.
The TR available will be 36.5:1, 27.4:1, and 18.2:1, and another comprehensive array of impedance matches will be available
and most likely to suit a quad or six pack of output tubes.
My 8585 integrated amplifier design uses a 62mm stack of 44 tongue core material with 2,080 primary turns.
Four or more output tubes will be better suited to the No 2 OPT,
and another comprehensive set of impedance matches will be available.
The plate current input rating to the CT is 750mA, but 200 Ma would be
the normal working current for 4 tubes.

For both OPT1 and OPT2 the frequency performance is similarly pleasing; No 2 will go a couple of Hz lower than No1
if the stack height is 75 mm, and half an octave lower if the stack height is 100 mm.
100mm stacks of iron would be excessive imho, and begin to cause higher than wanted winding losses and
lower HF cut off.

  ---------------------------------------------------------------------------------------------------------------------------- 

SE OUTPUT TRANSFORMER NO 3

Single Ended Output transformers can be made using the same winding details as
No1 or No2, but there is an air gapped core instead of the usual maximally interleaved
laminations.
Let us consider what the OPT No1 design would be useful for were it used for an SE amplifier.

We will consider the design when used for SE to be OPT No3.

The 0.35mm dia wire will take dc current = 288 mA, at a rated 3 amps per sq.mm.
But 140 mA would be a safer figure to allow for the occasional "oops" factor when bias failure might occur.
If the B+ voltage was +500v, then power input would be 70 watts to the anode circuit. 
If Single Ended Ultralinear operation, SEUL, was called for, and maximum efficiency was 35%, then we could expect
70 x 0.35 = 24.5 watts be the maximum output power.
Load line analysis will confirm this to be the case. The load for 35% efficiency with SEUL will be one where at  clipping
the peak change in load current will be +/- approximately 0.9 x anode idle current, Iaq.

In this case it is 140mA x 0.9 = 126mA peak = 89mA rms.

ac Power in the load  = I squared x RL, where I =Amps rms.

We have PO = 24.5 watts = 0.089 x 0.089 x RL, therefore RL = 24.5  / ( 0.089 x 0.089 ) = 3,093 ohms.
The maximum load swing = 3,093 ohms x 0.089Arms = 275Vrms = + / -390V peak.

To accommodate 70watts of input power without causing undue heat stress in any output tube, and be able to operate at
Ea = 500V, we could use two KT88 or 6550 in parallel so that each was dissipating 35 watts at idle
which is well below the maximum Pda ratings of 42 watts. ( KT90 have Pda = 55 watts and would be a better choice )
The ac load seen by each tube would therefore be 2 x 3,093 = 6.19k, and even triode operation would be
quite excellent, although we would not have quite such high power maximum, but the 15 watts from triode
would be quite OK. In triode, the Ra of 2 x 6550 in triode = approx 500 ohms, so the damping factor
with RL = 3.1 = about 7, so global NFB need not be applied. UL operation will require global NFB.

With a maximum of 275Vrms signal voltage across the primary, the B at 14Hz = 0.85 Tesla.

This is a favourable result because it means that the iron could be magnetised by the dc so that the dc magnetisation
could be up to 0.75 Tesla, leaving some headroom for LF transients below 20Hz.
The Core will be able to be air gapped so that the µ is reduced from say 17,000 of the GOSS I am using to less than 1,000,
and the dc magnetization reduced.
The golden rule for SE amps is :- Reactance in ohms of the primary inductance, ZLp = RL at 20Hz or lower.
Since RL = 3.1k, Lp = 3,100 / ( 6.28 x 20 ) = 24.7H.

Experience tells me that the core could be gapped so that the measured Lp at 20 Hz = 24.7H
and the dc magnetization will not exceed 0.63Tesla.
The sum of ac and dc magnetization at 20 Hz at full power will be less than 1.63Tesla so there is plenty of headroom.
If the sum of ac and dc B < 1.6T at 14Hz, we are doing even better, and careful adjustment of the gap will
perhaps give this condition so that distortion of the LF audio signals is negligible, and bass performance is blameless.

SE OUTPUT TRANSFORMER No4.

The use of 2,080 primary turns of 0.4mm dia wire with a taller stack of core material
will allow a higher Ia current of about 180mA. With Ea = 500V, the Pda = 90 watts
from which we might obtain 31 watts of SEUL power to a load of about 2.5k.
So three x 6550, KT88, KT90 could be used at 30 watts Pda per tube,
or better still 4 output tubes at Pda = 22.5 watts each.
A quad of EL34, 6CA7, 6L6, 5881, KT66 could also be used but with Ea = 420Vmax.
Pda for the smaller tubes should be 20 watts max, so total = 80 watts, so
Ia could be about 190mA, load will be about 2k.  But as RL is reduced and power output increased,
winding losses increase.

The reasoning used for SE OPT No3 can all be applied.