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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

ratings of output transformer???

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I am looking at buying an Edcor XSE series output transformer for what will be my SimpleSE.

In order to gain maximum output watts and remain under my trafo's 175mA. I am thinking of running the B+ at around 420 - 450 VDC and either EL34's or KT88's at less than 150mA total for the pair.

George has a great page that lists simulations and output wattage through 5K and 3K OPTs.

The 3K OPTs give more output watts than the 5K OPTs. However, Edcor do not make a 3K OPT....BUT.....I contacted Edcor and they have a 3.5K to the same specs as XSE15-8-5K. However, the tech reckoned that the specs for the 5K OPT could not handle 450VDC at 80mA.

Yet, I have never seen maximum voltage / currents listed for OPTs.

Was the Edcor tech correct, or were they as confused as I was???


For SE, current handling is a function of the gap. Voltage tolerance is related to its insulation thickness. DC current doesn't care how much voltage is on either side of it, it makes magnetic flux based on the number of turns. 80 mA at 200V will saturate a core just as fast as it would at 450V.
An EL34 would not stand the same OP for long.

True, but due to the different characteristics an EL34 will draw about 55 to 60 mA when plugged into the same amp (SimpleSE at 450 volts with a 560 ohm cathode resistor). They smile, sound good, and don't even glow (22 to 24 watts dissipation).

I have found that most tubes will generate quite a bit more distortion when operated at 450 volts with a 3K ohm load. I prefer the sound of a 5K or higher load, and will live with the slightly lower power. I have been running my Industrial Amp with the 8 ohm speakers on the 4 ohm tap (3K Transcendar OPT) for a 6K ohm load because it sounds better. This is more noticeable with EL34's than KT88's.

Every SE OPT will have a current where it generates the lowest distortion due to the gap length. The XSE15-8-5K's that I have work best at 80 mA (measured at 75Hz). I have also had them running on supply voltages as high as 525 volts. There ARE some saturation effects at low frequencies, but these are to be expected in ANY OPT of this size when operated at power levels over a couple of watts. If you are using typical small (6 inch woofer) speakers you won't hear it because the speaker's impedance rises at the frequencies causing saturation. If you have 15 inch woofers and expect to rattle the house, you need bigger OPT's. The Hammond 1628SEA or the Edcor CXSE25-8-5K will shake my neighbors house with EH KT88's running at 100 mA each in a SimpleSE through a set of OB 15 inch speakers.
The only concerns an OPT has is frequency vs voltage vs inductance required to load the tubes complex impedance properly.

In SE the DC current generates a flux that limits the available flux for AC signal, at frequencies under 440 Hz. and really, only at frequencies under 100 Hz. In a typical M6 core SE, you will find a two to one ratio optimum. So, 10 kg for DC current load and 5 kg for AC swing voltage load.

In PP, only the imbalance of the primary DC resistance from CT, generates DC offset currents.

All OPT's suffer from a hump in the permeability. Typical is a perm of 350, for M6 core, at 1 volt 120Hz, to a peak of around 3500 at a flux density of about 3.5 kg. The backside of this curve sinks more slowly than the front side rises and typical designs vary in how they use this hump, for specific applications.

In PP I rarely utilize flux density above 10 kg at maximum AC signal voltage, which equals about 48% of the B+ DC plate voltage. In SE I never use AC signal flux density above 5 kg. These guide lines come from the inductance required for a given loss in db at a given frequency.

The simple equation of inductance X frequency X 2pi = impedance will provide about -3db at that frequency of interest, at the secondary leads. A multiplication factor of 3.63 will provide -0.5 db at this frequency.

These are some general rules bounding all OPT designs.

OK, I can't say that things are much clearer, but it is obvious that the Edcor OPTs will perform satisfactorily with around 450V. Actually, my models in PSUD indicate more like 430 than 450V and I have found PSUDII to be right one (+ or - 1V) when I modelled my Aikido.

So, I'll go with the 5K Edcor and design my chassis such that I can add physically larger sized OPTs when I upgrade.

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