Output Transformer Notes, March 2011.

In this page below I have updated some useful notes of 2008.....
Sale conditions for transformers.
Speakers, amplifiers and the output transformer,
Description of output transformers for sale,
Output Transformer Design Parameters,


I have provided a comparison of the frequency performance of my own

designs and those produced by Mr Flanagan which
I now have for-sale. See
output transformer frequency behavior
-------------------------------------------------------------------------------------------------------------------

It is a fact that although I have hundreds of power and output transformers and chokes
in my stocks, most are widely different to each other, and the chance that something
I have might suit a given project some DIY person says he wants is remote. And after
supplying about 25 items from my stocks of hundreds during the 4 years to about 6
customers, only 1 has completed work on an audio amp project.

So I will NOT spend a pile of time sorting out what you want unless you can convince
me you are serious about building something.


You need to tell me about your project, and give all details of schematic of the amp
and PSU.
Please try to send me a schematic which is well drawn and less than 200kB in file size,
.pdf or .gif, .jpg,
Or send me a link to a URL.
--------------------------------------------------------------------------------------------------------------------


Sale conditions for transformers.
Prices are in Australian dollars excluding freight and packaging.

Payments for transformers can be via direct deposit into my bank account or
money order or cheque or cash and always after email negotiation with me,
Patrick Turner, info@turneraudio.com.au

I mostly have pairs of matched Output Transformers and they must
be purchased as pairs for dual monoblocs or a stereo amp.


There are a number of transformers with only one in stock

which may suit someone wanting to make only one amplifier channel.


Mounting brackets, terminals and leads for added taps will be
supplied
with all information.
There are usually multiple connections available on the primary winding
to suit local cathode feedback, or various % of screen taps for UL use.


Warranty period is 12 months from purchase.
There is no warranty on transformers which have been damaged after
being subject to excessive current flows, dropping, corrosion, moisture, or fire. 


Fuses.
To prevent damage from excessive current, users must connect suitable
fuses between each output tube cathode and 0V.

Fuses should be slow blow types with current rating of 3 times the idle current,

based on Idle current = 0.67 x rated max Pda / Ea.
For 6550 Idle current could be 0.67 x 42 / 470V = 60mA.

3 times this idle current = 180mA, so use a fuse = 200mA.
Never ever use one fuse to protect more than one tube.

Active protection.

A better way to prevent transformer damage is to fit an active protection board

which turns off amp automatically if the average cathode dc or ac current rises
to more than twice the the idle value as calculated above for longer than 4 seconds.
If Ia in a 6550 were to rise from 60mA to 120mA, Pda = 56Watts and the tube
anode would begin to glow red hot if no audio power was being produced.
But in class AB1, with a very low RLa-a, or a short circuited speaker lead, Ia
can easily reach 3 times the idle value, and active protection will turn the amp
off, saving the OPT. The above is the least sensitive basis for active protection
and in fact I prefer to have active protection turn off the amp when Iadc reaches

only 1.5 x idle value based on the idle current formula above.

Therefore a 6550 with Ea at 470V could sustain a rise in Ia idle current
from 60mA to 90mA, with Pda reaching 42 Watts. In hi-fi applications,
such sensitive protection works just fine, because often cathode biasing is used,
and because normal loadings are seldom as low as used in PA or guitar amp
applications. Protection schematics are available elsewhere at this website.

I have no control over how anyone might use the transformers being offered

and cannot offer a warranty unless you can easily prove there is a defect in
the transformer winding workmanship by returning the transformer to me for
inspection.
The winding method used is inherently reliable and I have closely examined all
OPT and found all have with neatly wound layers of wire with Nomex or
polyester sheet insulation between every consecutive layer of wire.
Buyers should carefully design their amp schematics with regard for anode
supply voltages and anode load values.
The design should allow for some flexibility for the anode supply voltage.
 
To easily calculate current ability of any wire size based on 2 Amps per

square millimeter, Maximum average current allowed in wire = diameter squared  x 1.6
where the diameter is the bare copper dia and measured in millimetres.

EXAMPLE :- 0.4mm dia wire has a current ability = 0.4 x 0.4 x 1.6 =  0.256Amps,

or 256mA. See my website for deeper analysis of transformer design parameters.

If you had a normal idling current of 60mAdc in a 6550 connected to a winding

of 150 ohms wire resistance, the heat generated is 0.54 Watts.
Pd wire = Amps squared x R in ohms.

If the current increases due to bias failure to 600mA, the heat generated
increases to 54 Watts and the winding would soon over heat and damage
insulation, and the transformer is ruined beyond repair.
If there was a fuse of 250mA, the maximum amount of heat in the winding
would be 9.4 Watts and the OPT winding would survive.

For PP operation, C-cores will be supplied tightly clamped as they appear in

images and with fixed support brackets ready for bolting to a chassis.

For SE operation, C-cores may be supplied loosely clamped allowing final

assembly and air gap adjustment by the amp constructor unless prior
arrangements are made with me to set a gap to suit the proposed use.
---------------------------------------------------------------------------------------------------

Speakers, amplifiers and the output transformer.

Tube amps you want to build probably will produce much less power than
most solid state amps and many other commercially available tube amps.
Lower power is entirely acceptable when the tube amp is well matched to
the impedance and sensitivity of speakers.

Output transformers are like a form of electronic gear box
to convert
the high signal voltages and low signal current of tubes to low voltages and
high currents needed for loudspeakers.
It is very much like a car gearbox to match the high engine RPM with low
torque to get lower rear wheel RPM but with higher torque.
Output transformers allow a typical 6550 output tube to produce 220Vrms
at anode at 50mA rms which is 11Watts of audio power to an anode
load of 4k4.
At the OPT secondary, this power is transformed to become 8.1Vrms
at 1.3A rms which is in fact 10.5 Watts in a speaker of 6 ohms.
Notice than the OPT gives a slight reduction of power due to the winding
resistance losses which vary between 3% and 7% in most well designed OPTs.
Good music depends on the right choice of OPT impedance and turn ratios.

You need to have an overall plan for your sound system
, know what you
want in engineering terms, why you want it, and realize you have to spend
some real money and a lot of time to make a tube amp. You should know
the technical specs for your speakers you have now or of those you
wish to buy. 

Most people have dynamic speakers ( with coned and domed drivers )
with "nominal impedance 8 ohms, sensitivity of 87dB SPL" at 1metre
with one watt of input power. But in fact most speaker impedance varies
between about 0.7 and 7 times the nominal or average value. 

Therefore speakers of "8 ohms" may vary between 5 ohms and 60 ohms at

different frequencies. For frequencies between 100Hz and 500Hz,
for PP amps, the lowest impedance value in this band should determine the
secondary winding load.
For SE amps, the average Z in this band should determine the secondary
load value.
  
Few people know about resistance, impedance, sensitivity, voltage, current,
reactance, LCR theory, phase shift, Ohm's Law, or anything technical.

If you don't know much about technical issues, I will try to give you an

OPT which handles the power needed to create 105 dB SPL from both
channels with speaker rated for 1.0 watt giving 87dB/W/M.

The table below gives my recommendations for amplifier power to satisfy

99% of listeners.
Most men enjoy maximum average levels of 85 dB SPL from each of 2
speakers, and and most tolerate up to 81 dB SPL from each of 2 speakers.

Everyone I know with speakers rated for 87dB/W/M is satisfied with

32 Watts per channel which is the clipping power with 0.7 x nominal
speaker impedance.
So if your speaker is "8 ohms", then an amp must make at least 32 watts
with 5.6 ohms.
A suitable amp would therefore have a pair of 6550/KT88 in push pull,
like that shown at Integrated 5050 


Speaker sensitivity, SPL at 1Watt and at 1Metre, dB.
Amp power max for one channel giving 102dB SPL.
( 105dB SPL, both channels used )
Push-Pull Tube type recommended 
Single Ended Tube type recommended,
Multigrids used as tetrodes/pentodes.
87dB
32 Watts
2 x 6550, KT88, KT90.
2 x EL34, 6L6, KT66, 807.
4 x EL84, 6V6. 
1 x 13E1,
4 x EL34, 6L6, KT66, 807, 300B  parallel
3 x 6550, KT88, KT90 parallel.
90dB
16 Watts
2 x 6550, KT88, KT90, 300B.
2 x EL34, 6L6, KT66,
2 x EL84, 6V6
4 x 2A3
1 x 13E1, 1 x 845, 211.
2 x EL34, 6L6, KT66, 807, 300B  parallel
2 x 6550, KT88, KT90 parallel.
4 x EL84, 6V6 parallel.
93dB
8 Watts
2 x 300B, 2A3.
2 x EL34, 6L6, KT66,
2 x EL84, 6V6, 6BM8, 6GW8

1 x 845, 211, 300B.
2 x 2A3  parallel
1 x 6550, KT88, KT90, EL34, KT66, 6L6, 807.
3 x EL84, 6V6 parallel
96dB
4 Watts
2 x 2A3, 45.
2 x EL84, 6V6, 6BM8, 6GW8

1 x 2A3, 6550, KT88, KT90, EL34, KT66, 6L6, 807, EL84, 6V6.
99dB
2 Watts
2 x 2A3, 45.
2 x EL84, 6V6, 6BM8, 6GW8
1 x 2A3, 45.
1 x EL34, KT66, 6L6, 807, EL84, 6V6, 6BM8, 6GW8.
102dB
1 Watt 2 x 2A3, 45.
2 x EL84, 6V6, 6BM8, 6GW8
1 x 45.
1 x EL84, 6V6, 6BM8, 6GW8.
 

Loudspeakers have  varying impedances at different frequencies bands
between say 5 ohms and 60 ohms for what may be a nominal "8 ohm speaker".
For something nominally 4 ohms, it is common to find an impedance dip to
2.8 ohms. The frequency band where low impedance exists is often right
between between bass and midrange where most audio energy is located, ie,
between 100Hz and 500Hz where a good quality speaker has 3 drivers,
bass, midrange and treble. 

The manufacturers usually give the SPL generated at 1kHz at 1 Watt of
power at 1 metre distance and for the nominal impedance stated.
If 1 watt is needed for 8 ohms it means 2.83Vrms is needed at the
speaker. But where the impedance dips to a lower value than nominal,
the same voltage is needed for the same SPL but more current flows with
lower Z and the power is needed is more than stated.
If sensitivity is 87dB SPL for 1 Watt at 1kHz, with 2.83Vrms needed for
the 8 ohms, but the impedance dips to 5.6 ohms at 250Hz, the same
2.83Vrms is needed to keep the SPL response flat, but power needed is
1.42Watts. Thus at 250Hz the sensitivity has fallen from 87dB to 85.6dB.
Makers are often very vague about their specifications because they
often have so much to hide.
Beware of speaker makers stating a nice high sensitivity SPL for 2.83Vrms
for what is a nominal 4 ohm speaker. If the impedance dips to 2.8 ohms,
then 2.83 watts is needed for the stated sensitivity, not 1 watt.

You don't want the variations in your loudspeaker impedance to cause high
amplifier distortion in the main large energy band of frequencies between
100Hz and 500Hz.
It is stupid to make a tube amp produce its absolute maximum power with
an 8 ohm load where speakers have impedance which is 2 ohms at some
part of the main audio band between 100Hz and 500Hz.
Some electrostatic speaker types have declining impedance well below
the nominal value as frequency rises beyond 2kHz. For example Quad
is 30ohms below 100Hz, 8 ohms at 1kHz, then only 1.6 ohms at 18kHz.
However, at such a high frequency, there is a very small amount of
audio energy so very little power is needed. Nearly all amplifiers can
easily cope with such speakers even where the ideal load for the amp
is 16 ohms. In fact, ESL57 have average Z of about 15 ohms between
100Hz and 1kHz, and are very easy to drive. Amplifiers are able to
produce energy for many different frequencies and load values which
change over a very wide range dynamically from one millisecond to
the next. 

Tube amps need to cope
with modern low impedance and low
sensitivity speakers. The absolute maximum possible power where THD
begins to exceed 2% is only possible for one value of load impedance.
Tube amps have rapidly falling power maximum abilities either side of this
load impedance, and for very low speaker impedances, distortion is
usually far worse than at high impedances, even though the max PO
possible is equal. For example, many amp makers will have 3 output
terminals, labelled Com, 4 and 8. This often means maximum possible
PO of say 35 Watts is possible when a nominal 8 ohm speaker is
connected between Com and 8. But if the speaker has a Z dip to say
5 ohms then PO becomes maybe 25 Watts with twice the THD
for 8 ohms. And where Z = say 12 ohms, PO will also fall to 25 Watts,
but THD may be 1/2 that of the 8 ohm load. This is all because of the
tube loading where the lower the load is, the more non-linear the tubes
become, and their voltage gain reduces, so the amount of applied NFB
is also reduced so the sound gets worse. In this example, if the "8 ohm"
speaker is moved to the 4 ohm outlet then one might get 30 Watts for
5 ohms, with THD 1/2 where the speaker is connected to the 8 ohm
terminal, and for 8 ohms there would be 25W, and for 12 ohms there
may be 20 Watts, and the THD is much less than if the speaker is
powered from the 8 ohm terminal. But one must accept the power
reduction. If there is enough power and nothing clips, the 4 ohm
terminal is the one you should use.

For most PP and SE amps, the maximum PO should be developed at
5.6 ohms for "8 ohm" terminals or OPT sec winding configuration,
or for 2.8 ohms for "4 ohm" terminals or OPT sec configuration.

For where only one load match is possible with just two output
terminals, say Com and 6, PP max PO should be higher, say 55 Watts
from a pair of KT88, and to occur with a 3 ohm load.
This allows any load above 3 ohms with low THD, as in my
5050 stereo amp
With SE amps with only one pair of terminals, max PO should be with
5 ohms, and all loads above 3 ohms will be OK, one reason being that the
PO with SE is always all class A1.

I have often given OPT impedance ratios of an OPT for sale of say
5k0 : 4 ohms and this means you could have 6k3 : 5 ohms, or 7k5 : 6 ohms.
Possibly one might move down to 3k8 : 3 ohms, but then a given
transformer's winding resistance is fixed, and as loads become lower, winding
resistance losses rise inversely. If wind losses are 7% with a 4 ohm load, with
2 ohms the losses become 14%.

I hope that all helps people choose from what is available.

For more info on choosing load matches, see my pages on load matching
to SE and PP tetrodes, pentodes and triodes.
--------------------------------------------------------------------------------------------------------
Description of output transformers for sale.


Most transformers have have grain oriented silicon steel double C-cores,
made by AEM in Sth Australia before about 1999 when AEM ceased
manufacturing C-cores. Some have even better C-cores made in Sth Africa.
The maximum permeability of the C-cores, µ-max, with no air gap is above
4,500 and entirely adequate. Music sounds excellent with these cores.
These GOSS cores give excellent technical performance with low iron related
distortion artifacts which are much lower than harmonic artifacts produced in
the vacuum tubes even at high levels and down to 50Hz or lower where
transformers produce most of their distortion.
They are most suitable for tube amp DIY projects or perhaps repair/upgrade
replacements. Makers such as McIntosh used C-cored transformers.

The low loss low distortion C-cores in output transformers offer lower distortion

than ordinary non grain oriented E&I laminations. Core heating is negligible
in all OPTs because the magnetic flux density, Bac, is much lower than for
power transformers. But C-cores offer negligible benefit in guitar amps
where harmonic distortion in the amplifier is much desired to add to the
tones produced in strings. 
The transformers all have carefully layer wound wire with at least 0.15mm
polyester insulation between every layer of wire. Leakage inductance is
extremely low due the extensive amount of primary and secondary interleaving,
resulting in very wide potential bandwidth if the driving source resistance is
low enough. Many OPTs have what I think is an excessive amount of
interleaving, with an equal high number of both P and S sections with
one layer of wire in each. So winding losses are usually very low.
The bobbins have a 3mm base wall thickness with ends of wire layers all
kept back 3mm from the edge of the insulation to maximize creepage distance.
This was the most common old fashioned and best way to wind a bobbin,
but it required much more practiced skill. Mr Flanagan was a master
winder before he retired. Insulation between anode windings and earthy
core and secondary windings is excellent.

Some listed transformers may be used either Push Pull or Single Ended

projects. A transformer meant originally for PP with no air gap may be
changed to having an air gap by loosening C core clamps, removing
C-cores, inserting gap material to suit the intended use, and re-assembling
and re-clamping cores, and then soaking in epoxy casting resin to prevent
any future core movement.
The brackets to allow bolting C-core transformers to a chassis will be
provided with each transformer as demand requires. Some extra
terminations are required for provision for local CFB tertiary windings
and/or ultralinear screen taps.

Where an OPT originally meant for PP operation has been listed as

suitable for possible SE operation, the maximum SE primary winding dc
idle current has not been allowed to to exceed 2 Amps dc per square
millimetre of copper wire section area.

So if the wire size is 0.4mm dia, you may only safely have Idc = 256mAdc.

If the idle current were to rise to say twice this if there was bias failure, the
current density rises to 4A / sq.mm, and the heat in the winding rises four fold.
So do not exceed the 2A/sq.mm rule. PP use of the same transformer usually
involves less Idc to each side of the PP circuit because there are two tubes and
two Idc paths in each 1/2 primary, and bias failure may not cause so much
heating of the winding.

In nearly all these transformers for sale the secondary winding losses are much

lower than the primary losses because there are so many paralleled secondary
windings with so much interleaving.
Where I have listed the SE operation conditions for anode supply voltage
and currents, I have aimed to have maximum power produced when
the load to be driven as 5 ohms.
This means the SE amp can drive any speaker impedance above 3 ohms.

Most of these OPT have only two useful ways of connecting the secondaries

to vary the load matching. Where there are say 8 secondary windings using
one layer of wire across the bobbin, one can only have all of them in parallel
or two lots of 4 windings in parallel in series.
With all secondaries in parallel, you might get a turn ratio of 28.68 : 1 and
impedance ratio of 833 : 1 giving 5k0 : 6 ohms.
But with half the secondaries in series with the other half will give TR = 14.34 : 1,
giving ZR = 206 : 1, giving 5k0 : 24 ohms, and such a load match is useless
because nobody has 24 ohm speakers.

It is fundamental to understanding output transformers that you know that

the impedance load ratio, ZR of primary to secondary is the turn ratio, TR,
squared.
Because these OPT have at least two loading possibilities, and because it is
always possible to design the amp to have Ea and Iadc conditions to best suit
an available OPT, so that useful load matches can be found.

In general the total winding losses in C-cored transformers are usually low

because most have a large winding window area ratio to central iron core
area compared to wasteless E&I laminations so wire diameter is generous.
The designer has selected saturation frequency to be not lower than 30Hz
at maximum mid band signal voltage. This means that the primary turns
are about 2/3 those in a same sized transformer with Fsat = 20Hz at the
same signal voltage. Thus primary wire dia is large for PP operation when
primary and secondary winding losses will be lowest.

SE use always results in higher winding losses. The interleaving between

primaries and secondaries is what I would call excessive, and in most OPT
the windings have alternative single layers of primary and secondary from
the bottom of the bobbin to the top. Thus these C-cored output transformers
have extraordinarily low leakage inductance and far below what is regarded
as low by the experts of 1953 who wrote the Radiotron Designer's Handbook.
However, there is higher shunt capacitance than what is regarded as low by
the same experts, but for most applications where the source driving resistance
is at least one pair of high transconductance beam or pentode output tubes
or triodes, the capacitance does not cause excessive open loop gain reduction
or phase shift beyond 90 degrees and below 100kHz. So HF instability is not
a problem when NFB is used carefully. 
The output reactance of an amplifier with these transformers is basically
capacitive because it is capacitance which causes the frequency attenuation
above 20kHz rather than the presence of leakage inductance.
I have prepared a page for deeper design comparison at
output transformer frequency behavior.

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

Output Transformer Design Parameters.
For OP1 to OP16, there is the following listed information and allows
anyone to calculate all other transformer design parameters :-

Weight  in Kilograms; 1Kg = 2.25 pounds.
Overall size as shown, Length horizontally across the two C-cores,
Width across the external windings, Height of C-cores including
clamping straps.
These dimensions are taken with the transformers oriented as they appear
in the pictures. Dimensions all in millimetres, 25.4mm = 1.0 inches.
Anode Load to Speaker Load ratios for Push-Pull and Single Ended use.
Clipping Power, Class AB1 PP or SE class A1.
The amount of class A1 within PP amps is a variable depending on
the Idc at the OPT CT.
Ea is the supply dc voltage applied between cathodes and transformer
input excluding cathode biasing if used. Eg2 is the screen voltage used
with nominated tubes if beam tetrode or pentode is employed;
Ea = Eg2 for triode and ultralinear.
Core dimensions are usually given as T x S x L x H for
E&I laminations and C-cores.
T = Tongue = width of the central leg of the E&I iron penetrating the
winding, or twice the build up of wound strips in C-cores.
S = Stack = height of the E&I lamination stack penetrating
the winding, or the width of the wound strips in C-cores.
L = Length of winding window for both E&I lams and C-cores,
H = Height of winding window for both E&I lams and C-cores.
Dimensions are all in millimetres.
Np is the number of Primary turns. The turns per layer and wire
Cu diameter is given where possible.
Ia maximum dc current is at 2 Amps per sq.mm.
Ns is the number of Secondary turns. The turns per layer and wire
Cu diameter is given where possible.
The Interleaving pattern is the arrangement of Primary and
Secondary winding sections.
Ips is the insulation between primary and secondary windings.
----------------------------------------------------------------------------------------------


Below are calculated results for OP1,
worked out to be optimal for OP1 only but other options could be
calculated depending on tube RL, B+, and idle currents.

Keen prospective buyers will do the necessary loadline analysis
for the use of the transformers.

At other pages at my website the methods for designing output
transformers are outlined. The winding losses and bandwidth
performance can be calculated from the listed information supplied
with each transformer.

Please do not expect me to carry out YOUR job of working out
what you
want in complete detail. But I usually do offer advice
and schematic after purchase. 


If you design any output transformer, you should be able to copy the tables
below and fill in the right hand column with relevant figures for the PP or
SE design you propose. When you have all 54 listed items filled in, you
should be able to calculate the shunt capacitance, the leakage inductance,
and the winding resistance losses to confirm that your design will give low
distortion, enough power, and wide bandwidth.

The 2 tables have the same 54 items listed and some are Not Applicable
depending on whether you are working something out for either PP or SE.
Feel free to copy and paste a table to a folder where you can delete all
the right hand numbers, then print a copy of the blank template you
will have so that your calculations on paper in your exercise book may
be written down, then copied for a PC folder storage.
I doubt you will come up with better suggestions than I have included
for the 16 varieties of OPT listed in high detail.
Other additional OPTs are listed at
for-sale-OP17-OP58-output-transformers.html

Table 1 for OP1 Push Pull use.
1
Output transformer number,
OP1
2
Push Pull or Single Ended ? PP
3
Weight in Kg 7.4
4
Overall size, length x width x height, mm 176 x 132 x 138
5
Turn ratio, TR, Pri to Sec 23.7 : 1
6
Impedance ratio, Pri to Sec, ohms
562 : 1
7
Primary load x secondary nominal centre load value, ohms.
2,845 x 5.0
8
Primary turns, NP, and wire size, Cu dia, mm 1,496 x 0.45
9
Primary layers x turns per layer 11 at 136t
10
Primary section number, all in series,
11
11
Secondary turns, NS
63
12 Secondary layers x turns per layer x wire size, Cu dia, mm 12 x 63 x 1.0
13
Number of parallel Secondary sections x Secondary turns per section 12 x 63
14
Grain oriented steel core with max µ > 4,500, description double C-cores
15
Afe, Tongue x Stack, or total strip build up x strip width, T x S, mm x mm
52 x 52
16
Window size, L x H, mm x mm
76 x 28
17
Iron path Magnetic Length, ML, mm 296
18
Insulation, Nomex or other, P to S, mm
0.22
19
Average turn length, TL, mm 300
20
Primary winding resistance, RwP, ohms 50
21
Secondary winding resistance, PwS, ohms
0.036
22
Total winding resistance RwP + RwS at primary, ohms
70
23
Total winding losses, %
2.5
24
Maximum allowable current density in any winding at idle, Amps dc per sq.mm 2
25
Maximum continuous allowable dc or ac current, primary wire, Amps
0.48
26
Maximum design value for dc current, primary wire, for SE operation, Amps NA
27
Load values for tests, primary anode load x secondary load, ohms.
2,845 x 5.0
28
Nominal Anode Source resistance ( between highest and lowest Ra possible ) for tests, ohms
2,845
29
Maximum primary inductance, PP operation, Lpmax, no air gap, µ > 4,500, Henrys, H 128
30
Minimum Primary inductance, PP operation, Lpmin, approximate, no air gap, µ = 1,000 at low Vo, Henrys, H 28.4
31
Effective permeability, µe, with air gap for SE operation, Bdc = 0.75Tesla NA
32
Air gap calculated for reducing maximum iron µ to above wanted µe, mm, ( confirmed by experiment in amp )
NA
33
Non magnetic material thickness used to gap cores on both breaks in magnetic loops, mm
NA
34 Primary inductance, Lp, for SE operation, for µe, Henrys, H NA
35 Leakage Inductance, at primary input, LL, mH 0.24
36 Shunt capacitance, at each anode primary input/s, pF
3,000
37 Maximum audio power at anodes for THD < 2%, at 1kHz, at rated RL loaded, Watts
135.0
38 Maximum anode signal voltage across primary at THD < 2%, Vrms
620
39 Maximum class AB1 power, PP operation, at rated RL, Watts 135
40 Maximum class A1 power at rated RL, Watts
30
41 Frequency of core saturation at max audio power signal across primary at Bac + Bdc = 1.5 Tesla, Hz 22.3
42 Low frequency cut off at 0.1 x max Vo, source resistance = 1/2 anode RL, Hz
8
43 High frequency cut off with Secondary loaded. Primary source resistance = primary RL, kHz 75
44 Tubes usable, type numbers
6 x KT88
45 Tube configuration, Ultralinear, UL, Cathode Feedback, CFB, Pentode, P, Beam Tetrode BT, Triode, T. UL, CFB
46 Maximum Idle dc anode and screen dissipation power setting for each tube, Pda, Watts
25
47 Maximum total tube idle dissipation power Pda, Watts 150
48 Anode to cathode dc supply voltage ( excluding possible cathode biasing voltage ), Ea, Vdc 500
49 Total Idle condition anode and screen supply dc current for all output tubes, Idc, mAdc
300
50 Anode plus screen Idc per tube, mAdc
50
51
Actual anode plus screen dissipation in each output tube ,Watts
25
52
Maximum Idc needed for maximum output power for stated load, approximate, mAdc
500
53
PP tolerated loads with the tubes above, Primary load : Secondary load, ohms 1,686 : 3.0
2,248 : 4.0
2,800 : 5.0
3,372 : 6.0
4,496 : 8.0
54
SE tolerated loads with the tubes above, Primary load : Secondary load, ohms NA

Table 2 for OP1 Single Ended use.
1
Output transformer number,
OP1
2
Push Pull or Single Ended ?
SE
3
Weight in Kg 7.4
4
Overall size, length x width x height, mm 176 x 132 x 138
5
Turn ratio, TR, Pri to Sec 11.85 : 1
6
Impedance ratio, Pri to Sec, ohms
140.5 : 1
7
Primary load x secondary nominal centre load value, ohms.
711 x 5.0
8
Primary turns, NP, and wire size, Cu dia, mm 1,496 x 0.45
9
Primary layers x turns per layer 11 at 136t
10
Primary section number, all in series,
11
11
Secondary turns, NS 126
12 Secondary layers x turns per layer x wire size, Cu dia, mm 12 x 63 x 1.0
13
Number of parallel Secondary sections x Secondary turns per section 6 x 126
14
Grain oriented steel core with max µ > 4,500, description double C-cores
15
Afe, Tongue x Stack, or total strip build up x strip width, T x S, mm x mm
52 x 52
16
Window size, L x H, mm x mm
76 x 28
17
Iron path Magnetic Length, ML, mm 296
18
Insulation, Nomex or other, P to S, mm
0.22
19
Average turn length, TL, mm 300
20
Primary winding resistance, RwP, ohms 50
21
Secondary winding resistance for NS, RwS, ohms
0.144
22
Total winding resistance RwP + RwS appearing at primary input, ohms
70
23
Total winding losses, %
9.0
24
Maximum allowable current density in any winding at idle, Amps dc per sq.mm 2
25
Maximum continuous allowable dc or ac current, primary wire, at 2Amps/sq.mm Cu,
0.324
26
Maximum design value for dc current, primary wire, for SE operation, Amps
0.324
27
Load values for tests, primary anode load x secondary load, ohms.
711 x 5.0
28
Nominal Anode Source resistance ( between highest and lowest Ra possible ) for tests, ohms
711
29
Maximum primary inductance, PP operation, Lpmax, no air gap, µ > 4,500, Henrys, H
NA
30
Minimum Primary inductance, PP operation, Lpmin, approximate, no air gap, µ = 1,000 at low Vo, Henrys, H NA
31
Effective permeability, µe, with air gap for SE operation, Bdc = 0.75Tesla
278
32
Air gap calculated for reducing maximum iron µ to above wanted µe, mm, ( confirm by experiment in amp! )
1.0
33
Non magnetic material thickness used to gap cores on both breaks in magnetic loops, mm
0.5
34 Primary inductance, Lp, for SE operation, for µe, Henrys, H 7.9
35 Leakage Inductance, at primary input, LL, mH 0.3
36 Shunt capacitance, at each primary input/s, pF
5,000
37 Maximum audio power at anodes for THD < 2%, at 1kHz, at anode rated RL loaded, Watts
36.0
38 Maximum anode signal voltage across primary at THD < 2%, 
160
39 Maximum class AB1 power, PP operation, at rated RL, Watts
NA
40 Maximum class A1 power at rated RL, Watts
36
41 Frequency of core saturation at max audio power signal across primary at Bac + Bdc = 1.5 Tesla, Hz 14.4
42 Low frequency cut off at 0.1 x max Vo, source resistance = 1/2 anode RL, Hz 7
43 High frequency cut off with Secondary loaded. Primary source resistance = primary RL, kHz 75
44 Tubes usable, type numbers
4 x KT88,6550
45 Tube configuration, Ultralinear, UL, Cathode Feedback, CFB, Pentode, P, Beam Tetrode BT, Triode, T. SEUL, SECFB
46 Maximum Idle dc anode and screen dissipation power setting for each tube, Pda, Watts
25
47 Maximum total tube idle dissipation power Pda, Watts
150
48 Anode to cathode dc supply voltage ( excluding possible cathode biasing voltage ), Ea, Vdc 295
49 Total Idle condition anode and screen supply dc current for all output tubes, Idc, mAdc
350
50 Anode plus screen Idc per tube, mAdc
86
51
Actual anode plus screen dissipation in each output tube ,Watts
27
52
Maximum anode Idc needed for maximum output power for stated load, approximate, mAdc
330
53
PP tolerated loads with the tubes above, Primary load : Secondary load, ohms
NA
54
SE tolerated loads with the tubes above, Primary load : Secondary load, ohms
423 : 3.0
564 : 4.0
711 : 5.0
846 : 6.0
1,128 : 8.0

Good luck!

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