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,


On a separate page I have provided analysis of the compared frequency performance of my own designs and those produced by Mr Flanagan at
output transformer frequency behaviour.


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 monoblocks or a stereo amp.

There are a number of single stock transformers which may suit someone wanting to make only one amplifier channel.

Mounting brackets, terminals and leads for added tappings 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. 
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 5 times the idle current - for example, where you have 50mAdc at idle for a 6550, use a 250mA fuse.
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 5 times the the idle value for longer than 4 seconds. However, I cannot give a warranty where only active protection has been used.
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, with neatly wound layers of wire with 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 continuous 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 60 mAdc in a 6550 connected to a winding of 150 ohms wire resistance, the heat generated is 0.54 Watts. If the current increased due to bias failure to 600mA the heat generated increases to 54 Watts and the winding would soon over heat and damage insulation.
If there was a fuse of 250mA, the maximum amount of heat in the winding would be 9.4 Watts and the OPT winding may survive.

Active protection allows for turning off the amp if one or more output tubes increases its dc current to more than about 3 times the idle current for longer than 4 seconds.

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 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 and low torque to get lower rear wheel RPM but with higher torque. Output transformers allow a typical 6550 output tube to work ideally with an anode signal voltage of 220Vrms and signal current of 50mArms which if transformed becomes a speaker signal of 8.1Vrms at 1.35Arms which is in fact 10.9 watts in a speaker of 6 ohms. 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 0.7 and 7 times the nominal or average value. 
With speakers of "8 ohms" the impedance may vary between 5 ohms and 60 ohms at different frequencies. The lowest impedance value of your speaker should determine the OPT secondary winding load.
  
Few people know about resistance, impedance, sensitivity, voltage, current, reactance, 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 recomendations for amplifier power to satisfy 99% of listeners.
Most men enjoy maximum average levels of 88dB SPL and most females enjoy 84dB SPL from both 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.
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 power is needed than stated.
If the impedance dips to 5.6 ohms, the same 2.83Vrms is needed which is 1.42Watts from the amp.
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 1 kHz.
It is useless to make a tube amp produce its absolute maximum power with an 8 ohm load where your speakers have impedance which is 2 ohms at some part of the main audio band between 100Hz and 1kHz.
Some electrostatic speaker types have declining impedance well below the nominal value as frequency rises beyond 2kHz. For example Quad ESL57 impedance is 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.
Amplifiers are able to produce energy for many different frequencies in music simultaneously and into a load value which changes over a very wide range dynamically
from one milli-second to the next. 

Tube amps need to cope with modern low impedance and low sensitivity speakers. The absolute maximum power where THD begins to exceed 2% is only possible for one value of limpedance. Tube amps have rapidly falling power maximum abilities either side of the impedance value where the maximum power is produced. And at low speaker impedances, distortion is usually far worse than at high impedances.
But where the power levels remain at say below 1/10 of the maximum possible power at 2% THD, most good amps should make a constant voltage level of no more than +/- 20% maximum for all load values between 2 ohms and 32 ohms where the nominal ideal load is 4 ohms or greater.

I always try to make my PP amps with maximum class AB1 power when the load is 3 ohms if the nominal speaker is to be 8 ohms.
So if you have an Ultra Linear PP output stage with 2 x 6550 and Ea at =500V, you get about 60Watts absolute max at 3 ohms but only 34 Watts at 8 ohms.
Such an amp will cope well with most modern speakers if there is good load matching.
See my web pages on load matching and the 5050 stereo amp and see that a transformer with an impedance ratio of 8k:8 ohms makes perfect sense with Ea = +500V and a pair of 6550 in UL mode.
Some makers have 4 ohm and 8 ohm outlets which makes it easier to match a tube amp to a speaker. Such amplifiers usually produce maximum power when 4 ohms is the load connected to the "4 ohm" labelled outlet or 8 ohms is at the "8 ohm" outlet. Doing things my way there isn't any need for two lots of output terminals. Since only low power is actually needed for 90% of average listening, there will be enough power for 8 ohms speakers even though the power maximum of 60Watts is available at 3 ohms.
Amps with only a pair of EL34 or 6L6 may struggle.
 
SE amps operate always in pure class A and they are not able to produce more than their maximum class A output power as the load value reduces.
Therefore their maximum output power should be arranged to occur when the load is about 5 ohms, not 3 ohms, to suit most modern speakers. Even though an SE amp will have falling maximum output power below 5 ohms it is still all class A power and the SE amp has good tolerance of non ideal loads.
A 30 watt SE amp will usually perform just as well or better than a PP amp capable of 30 watts of AB power and with only 8 watts of class A power before it begins to work in class AB at above the 8 watt class A threshold. Most SE amps cop a lot of criticism about their poor technical performance. This is because so many SE amps are vastly under powered for what owners may expect of them with modern insensitive speakers. If you have normal modern speakers of 88dB/W/M sensitivity then don't expect one lone EL34 or 300B or with 5 to 8 watts to ever sound very good when turned up loud especially if there is "zero negative feedback".
If you have say 4 x EL34 or 3 x 300B for over 20watts and with moderate NFB, then you will begin to hear the glories of music via tube gear.
If you have horn loaded speakers of perhaps 100dB/W/M then the single EL34 or 300B is more than enough and a lone 2A3 or 45 can be just fine.
 
Nearly all tube amps operate in Push Pull mode where at least the first 1/4 of the total power maximum is what is called "pure class A" and is the most naturally clean undistorted power possible from vacuum tubes. So any typical PP amp with a capability of 32 watts AB maximum will make the first 8 watts or more in clean class A power and you want this amount of power to cover most of your listening levels with the remaining less clean "class AB power" used for the transient peaks of drum beats etc which are the noisy bits in music and which are little changed by the instantaneous rise in distortion.

Where an amplifier is classified as "Single Ended", ie, SE, it means that ALL of its power is type class A power. This also means that the distortion harmonics at all power levels may be higher than the PP amp but usually that the first 1/4 of the SE power has harmonics less displeasing to the ear than the lower artifacts of the PP amp.  This is not always the case, and I have made many SE amps which measure lower THD/IMD than a PP amp rated for the same power, but then I understand how to do this better than many makers.

The SE amplifier power is always pure class A, and if you want 32 watts maximum from an SE amp, you'll need to have at least 75 watts of anode power from the power supply at all times while the amp is turned on. So you'd need 3 x 6550 with each idling at 25 watts of anode dissipation. If you had a PP amp from which you wanted a maximum of 32 watts of pure class A then you also would need the same 75Watts of anode input power. With only 2 x 6550, you'd have Pda = 37.5W and this is far too high and uncomfortable and unreliable and likely to cause much reduced tube life. So you would have to use 4 x EL34 or 6550 perhaps with each having Pda = 19 Watts.
You could well have a PP class AB amp rated for a maximum of 20Watts of class A power but with the capability to make up to 60W AB into 3 ohms. The input power required at idle will be say 45W, so you may happily use just one pair of 6550 each idling with Pda = 22.5W. With Hi-Fi and music, the use of levels above the class A threshold for any load will be brief, and the anode power supply draw may only briefly rise to a maximum of say 80Watts only very occasionally for high volume levels. So when you build an amp using 2 x 6550, allow for the 80W of possible anode power and the excess capability means a cooler transformer.
The costs of building an SE amp rated for 32 watts is about the same as those for a PP AB amp of 60 watts. Hence you pay more for less watts with a class A SE amp compared to a class AB amp. If either PP or SE amp are rated to make the same class A power there should be little difference in cost. Using class AB operation lessens construction and operation costs without ruining music so it is why class AB has been used by so many for so long.

Some of my customers prefer a well made SE amp capable of only 1/2 the power of a class AB Push Pull amp using the same tubes. They say the SE amp sounds better and the higher cost of the pure class A SE amp is justified.

If you think that nothing is as good as the first watt from a 200W class A amp and you only ever use 5 watts, then you may not be happy with a 20 watt amp. But a 200W class A power amp is very heavy, wastes an enormous amount of heat, and very expensive. Anode input power must be 470 Watts which means 20 x 6550 are needed. Cathode heating power is 226W, and drive amp power about 20Watts with total power needed from the mains = 780 watts just for one channel,
With two channels the power is 1.56kW, and enough to be a nice heater in a room on a freezing winter evening.
I don't have transformers for sale for such a greenhouse un-friendly amplifier.
But I can say that a well made 20W pure class A amp can sound equally good as a 200W class A amp providing the 20W covers the peak power needed.

Tube amps cost per watt is very high for 4 Watts from a single 2A3 or EL34. But as the watts rise, the cost per watt rapidly falls, and a 300W class AB amp is not much more expensive than a 100W amp depending on the anode input power. 
                                                         __________________________________________________________


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.
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 where transformers produce most distortion at LF.
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 sustain negligible rise in temperature and offer lower distortion than ordinary non grain oriented E&I laminations. C-cores offer negligible benefit in guitar amps where harmonic distortion in the amplifier is much desired.
The transformers all have carefully layer wound wire with at least 0.15 mylar 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.
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.
Insulation between anode windings and earthy core and secondary windings is excellent.

Some listed transformers may be used for Push Pull or Single Ended projects.
The necessary clamp yokes to allow bolting 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 UltraLinear screen taps.
For PP operation, transformers will be supplied with C-cores clamped tight as they appear in images.
For SE operation, cores may be supplied loosely clamped to allow air gap adjustment by the amp constructor.
But usually I am able to set the air gap to suit the proposed use and tighten clamps before varnishing and painting the transformer.

Where I have stated the possible SE operation for what was originally designed to be a PP transformer, the maximum 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 a winding of 0.4mm dia wire R = 100 ohms, Idc allowable = 256mAdc and Pd = 6.5 Watts of heat. The size of the transformer would need to be fairly large.
Where one did have Idc = 256mA, the signal current maximum swing for class A operation could be +/- 256mA peak. If Ea was +330V then the anode signal voltage swing could be 300Vpk with a pentode or beam tetrode. The load is thus V / I = 300V / 0.256A = 1,172 ohms.
The total anode power = Va rms squared / RL = 212 x 212 / 1,172 = 38 watts.
The 100 ohms of winding resistance is effectively in series with the transformed speaker load.
Therefore the Primary winding power loss % = 100 / 1,172 = 8.5%, which does not include secondary winding losses which might be 5% and total power at the speaker terminals may be 38W less 13.5% total losses = 32 Watts.

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 tube amps give their lowest distortion and best damping factor when the speaker load is a high number of ohms. So when a 16 ohm speaker is connected to an amp with a 5 ohm rated output terminal, the music will be better than if the load was 5 ohms, as long as the amp does not operate near clipping levels.

The secondary windings of 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 will get a match for an anode load of perhaps 5k at the primary where a 3 ohm load is connected at the at secondary. If half all the secondaries are in series with the other half, you will get 5k loading for the anode at the primary only if the secondary load is 12 ohms.
It is fundamental to understanding output transformers that you know that the resistance load ratio of primary to secondary is the turn ratio squared.
Because these OPT have at least two loading possibilities, and because it is always possible to alter the Ea and Iadc conditions, variable but 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 25Hz at maximum mid band signal voltage. This means that the primary turns are 66% of a same sized transformer with Fsat = 17Hz 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 below 100kHz. So HF instability is not a problem when NFB is used carefully.  The output reactance of an amplifier with these transformers is more 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 behaviour.
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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.
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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.

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. 

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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