Power rating = 75W+ for RLa-a 4k0, and for winding loss % < 7%, 50W bandwidth

14Hz to at least 70kHz.

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Nominal Load Ratios 8k0 : 4r0, 8r0, 16r0 for 50W class AB1,

The higher 75W Po is available when 2r0, 4r0, 8r0 loads are used which makes

RLa-a = 4k0.

for OPT-1A.

Confirm max Po rating = 75W.

Calculate

NOTE. This formula was derived from a basic formula for Afe used for mains transformers,

This old formula is based on B being about 1 Tesla, or 10,000Gauss at 50Hz but for audio hi-fi,

Bac max should be less than 0.5 Tesla for an OPT at 50Hz for the same Vac across the coil.

After considerable trial I found Afe in sq.mm = 300 x sq.rt Po is a good guide for any PP audio OPT.

T = S = square root th Afe.

Afe = sq.rt 2,598sq.mm. A possible choice is standard wasteless E+I with T51mm x S51mm.

Moulded bobbins are made for this size of core. But there may not be C-cores to suit this

size.

T sizes commonly available for E&I laminations with relative dimensions shown above.

This pattern shape for E&I lams are known as the "wasteless" or "scrapless" pattern, which

means that there are no wasted off-cuts when the sheet metal is cut. E&I wasteless lamination

sizes are simply known by the Tongue size which for OPTs, chokes, and PT for amplifiers may

have sizes :-

20mm, 25mm, 28mm, 32mm, 38mm, 44mm, 51mm, 62.5mm.

These correspond to former inch sizes = 0.75", 1.0", 1.125", 1.25", 1.5", 1.75", 2.0".

OPTs may use GOSS C-cores arranged above as "double 0" and these will work very well for all OPTs.

But there are very many sizes available, and whatever is chosen MUST have sufficient Afe and a window

big enough to contain wire turns and insulation.

Most C-cores favour design for low iron weight and higher copper weight, giving less weight for any E+I

cored transformer. But this is only because very nearly all E+I transformers are wasteless pattern where

L x H / T ratio = 28.5. and if anyone wants a non standard E+I core pattern with higher window L x H / T

ratio, they will not find it easily. The Williamson OPT of 1947 used E+I with T32mm which had window

L75mm x H25mm so ratio = 58.6, very similar to C cores which might be now chosen for the same OPT.

But the rules for good OPT design insist that

A large range of C-cores is available from www.nicore.co.cn

From their table for standard 0.3mm thick GOSS strip, selections could be :-

Two x CD16x32x80 cores give Afe = T32mm x S32mm = 1,024 sq.mm, window H25mm x L80mm.

Va-a 400Vrms, Fsat 14Hz, Np = 4,000t x 0.315mm Cu dia wire, RwP 188r.

Four x CD16x32x80 cores ( stacked ) give Afe = T32mm x S64mm = 2,048 sq.mm, window H25mm x L80mm.

Va-a 566Vrms, Fsat 14Hz, Np = 2,828t x 0.375mm Cu dia wire, RwP = 124r.

OK for 40W to 8k0, weight 3.84g.

Va-a 632Vrms, Fsat, Np = 3,157t x 0.355mm Cu dia wire, RWP = 154r.

OK for 50W to 8k0, weight 3.84Kg

Notice that the L x H / T ratio = 62.5, which always means more copper turns than with lower ratio.

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There is no strict rule to have T = S for a square section for Afe with wasteless E+I.

I used T50mm x S110mm for

to try using a smaller T size than for square core, so S is higher. OPT-1A theoretical Afe = 2,598sq.mm,

so possible cores are T51mm x T51mm, T44mm x S59mm, 38mm x 68mm.

But only the square core suits a standard bobbin, and the latter two need stack increased to suit

bobbin sizes. This means increasing Afe beyond the minimum required, but that will allow lower Np and

less copper. The design of all transformers involves the balance of the weight of iron vs weight of copper.

Wasteless pattern E+I cores allow designers to use high iron weight with fewer copper turns, but C-cores

allow low iron weight with higher copper turns.

The choice of core shape to suit bobbins and with higher Afe sizes for smaller T are :-

T51mm x S51mm, T44 x S62mm, T38mm x S74mm or S85mm.

Afe = 44mm x 62mm =

and Po = 50W. Va-a = sq.rt ( RLa-a x Po ) = 632Vrms.

Th Np = 22.6 x Vrms x 10,000 / ( Afe x Fsat x Bac max ) where 22.6 and 10,000 are constants,

Afe in sq.mm, F in Hz, Bac in Tesla.

= 22.6 x 632V x 10,000 / ( 2,728sq.mm x 14Hz x 1.6T ) =

Wanted window

= overall dia squared.

= sq.rt ( 406.6sq.mm / 2,337t ) = 0.417mm.

Want oa dia not exceeding 0.417mm.

Try oa wire size = 0.414mm, Cu dia = 0.355mm.

= 22 x 0.355mm x 0.355 / 28 = 0.198Amps dc, say 200mAdc.

Idle Idc could be continuous 200mAdc in each 1/2 primary winding without wire getting

too hot or fusing, which would require much higher Idc. 6550 or KT88 would have Pda+g2

= 0.7 x Pda rating of 42W so Pda+g2 = 29W with Pda 26W and Pdg2 3W.

If Va-a = 632Vrms, then Va = 316Vrms = +/- 447Vpk, so idle Ea = 490Vdcand Iadc

= 26W / 490V = 53mAdc. KT120 could have idle Pda = 37W, so Iadc = 76mAdc, all OK.

Estimated Rw for primary could be 5% x 8k0 = 400r, with 200r for each 1/2 pri so wire heat

with 75mAdc = Rw x Idc squared = 1.13W = OK.

But if one KT120 has high Idc = 0.5Adc with bias failure, heat = 50W, and wire will get very

hot while enclosed between insulation layers and this may cause damage to insulation on

turns and to layers of insulation so the amp MUST HAVE some sort of active protection

circuitry to automatically turn off the amp if the Idc drawn by one or more tubes rises to more

than twice idle Idc for longer than 4 seconds. I used such measures in all amps I sold and

my customers were always happy that when their output tubes eventually wore out and

began to conduct far too much Idc before dying, the amp turned itself off and there was

no expensive repairs needed for OPT, PT, or to any other parts.

NOTE. Most moulded bobbins for OPTs between 10W and 200W have nominal cheek thickness

under 2mm. Therefore allow Bww = window L - 4mm. For OPT-1A,

NOTE. Some OPTs are wound using a rectangular tube of phenolic board 2mm thick without

cheek plates at each end of winding layers. Layers of insulation are same width as window L.

The wire layers begin 2mm in from edge of insulation layer and finish 2mm short of insulation

edge.

This manner of winding takes much more skill to keep layers neat and tidy and has the

advantage that the crepage distance between layers of wire is about 4.5mm, much more than

where all wire layers are brought to insides of bobbin cheeks where they exist on a moulded

standard bobbin. However, the cheekless bobbin favours high Vdc and high Vac over 1,000V,

where biggest peak Va + Ea difference could be 5,000Vpk. Cheekless winding is not needed

for most OPTs where idle Ea = +500Vdc, and highest Vac = +/- 500Vpk.

The Sec windings are at 0V potential but the P-S insulation is usually more than 0.4mm, and

with varnish available and the enamel used on Grade 2 wire, I never ever found there was any

arcing on the many OPTs I wound.

Th P tpl = 0.97 x Bww / oa dia

The 0.97 factor is used because of difficulty filling the layer with fine wire with oa dia < 0.5mm.

There should be an even number of P layers for equal layers each side of CT, and there should

NOT be any fractions of a layer, so for OPT-1A, use 16 P layers.

clipping Po = 50W, where Va-a = 632Vrms.

= 22.6 x 632V x 10,000 / ( 62 x 44 x 2,320 x 1.6 ) = 14.1Hz. So far, my formulas are

working OK.

this may be OK for a cheap guitar amp, but not for Hi-Fi. The highest forgivable Fsat for a 50W

amp should be 20Hz, reserved for those who just cannot accept the slight extra cost of best

practice, so therefore stack height of 62mm could be reduced to 44mm.

Using OPT1-A with lower Ea = 400V with higher Iadc for more class A Po will reduce maximum

Va-a from 632Vrms to about 525Vrms for 35W to 8k0, and this will give Fsat 16.4Hz = OK.

pye = 3.143, or 22 / 7.

TL = ( 3.143 x 22 ) + 2 x ( 44 + 62 ) =

where 44,000 is constant = 100,000mm / 2.26r for 100M x 1.0mm dia wire ),

and Pdia is the Cu dia from the wire tables.

P loss % = 100% x RwP / ( Ra-a + RwP ) = 100% x 118r / ( 8,000r + 118r ) = 1.45%.

dia wire. If NO, proceed to Step 23.

= 2.87% < 3% OK.

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