You can get more power with a lower impedance. You are limited by the voltage you can apply at X Hz across the primary. The more current you can force through it the more power you will get. More turns on the secondary will do that - you draw more current from the primary. The primary inductance itsekf remains the same, but it has less effect if the real (in phase, resistive) part decreases.
wg_ski: That´s what I think too, voltage across the primary vs. frequency seems like the limiting factor rather than the core size.
In this aspect the 1,6k:8R connection seems like the best option as every single turn on the transformer effectively is a part of the primary winding.
In the end it comes down to choosing between building a modest ~5W amp using small to medium sized tubes (807, 6L6, 25E5 or perhaps 4P1L) and reasonably sized PSU parts OR to bring out the big bottles (40KG6/PL519) and the boat anchor mains transformers and aim for ~20W.
In this aspect the 1,6k:8R connection seems like the best option as every single turn on the transformer effectively is a part of the primary winding.
In the end it comes down to choosing between building a modest ~5W amp using small to medium sized tubes (807, 6L6, 25E5 or perhaps 4P1L) and reasonably sized PSU parts OR to bring out the big bottles (40KG6/PL519) and the boat anchor mains transformers and aim for ~20W.
Yes, EL86, PL84 etc. (= xCW5).
Anyway, EL86 is not a redesigned EL84, but a UL84 with the EL84's 6.3 V heater instead. UL84 and EL84 were designed by the same time in about 1953. The heater is quite remarkable, as heater power is the same for EL84 and EL86, but cathode current rating is 100 mA for EL86 vs. 70 mA for EL84.
Best regards!
Anyway, EL86 is not a redesigned EL84, but a UL84 with the EL84's 6.3 V heater instead. UL84 and EL84 were designed by the same time in about 1953. The heater is quite remarkable, as heater power is the same for EL84 and EL86, but cathode current rating is 100 mA for EL86 vs. 70 mA for EL84.
Best regards!
The power achievable is related to what you do with the secondaries. If the transformer is good for 250Vrms at 50 Hz (a decent 230V transformer will have a 10% margin) then it is good for 150Vrms at 30 Hz. Fuling is adding the "18V" windings to the primary so should be good for about 175 V rms at 30 Hz. With secondaries in parallel and an 8 ohm load, primary impedance is 5.6k. 175Vrms into 5.6k is 5.5W. With secondaries in series and an 8 ohm load, primary impedance is 1.4k. 175rms into 1.4k is 22W. Assuming the VA rating is high enough to supply the increased current.
These transformers are designed for 50/60hz and no amount of fancy juggling of windings will get round this simple fact. This limit is defined by primary inductance and core saturation. Power handling will drop off sharply below 50hz - but will still be usable down to 20hz if you overspecify the transformer.
Shoog
This is universal , for any audio or not audio transformer the max power at 20Hz is related to primary inductance and core saturation ( core size and number of turns ) , not how the secondaries are connected and reflected load ...
Otherwise it would be like a free lunch taken from nothing
Otherwise it would be like a free lunch taken from nothing
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Really you shouldn't be asking your valve power amp to handle below 50hz as that means you have build much bigger and more expensive amps for marginal gain (a valve amp will never be good at these frequencies even if it can do it). This is plate amp subwoofer territory and can be done for a fraction of what it would cost to build a suitable valve amp.
Shoog
Shoog
I was messing with a 230V/12V/105VA toroidal I have laying around, and here's what gives:
The DUT is driven at about 1V with a small amp (lo Zo) connected to the 12V side, and the 240V side is loaded with a 300pF capacitor and fed to the differential mic input of a Focusrite, through a couple of 2.2M resistors.
My goal is to find an alternative way to drive my electrostatic headphones, hence the 300oF.
The DUT is driven at about 1V with a small amp (lo Zo) connected to the 12V side, and the 240V side is loaded with a 300pF capacitor and fed to the differential mic input of a Focusrite, through a couple of 2.2M resistors.
My goal is to find an alternative way to drive my electrostatic headphones, hence the 300oF.
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Shoog has a good point here: If the ambition was to build a tube subwoofer amp, then using undersized mains transformers as OPTs would be a terrible choice 
Still, 5W at 30Hz isn´t bad for someone like me who has high efficiency speakers and a preference for class A triode amps.
What really makes these transformers (and the big ones in the thread start) interesting is the CFB windings.
Still, 5W at 30Hz isn´t bad for someone like me who has high efficiency speakers and a preference for class A triode amps. What really makes these transformers (and the big ones in the thread start) interesting is the CFB windings.
But it IS dependent on the reflected load. Simply parallel another speaker on the output. With the same voltage applied at 20 Hz, you now draw twice the power. Assuming of course the tubes driving it can supply the current. Usually not without adjustment, but you could with a different set of tubes. Reconfiguring the secondary windings does the same thing.This is universal , for any audio or not audio transformer the max power at 20Hz is related to primary inductance and core saturation ( core size and number of turns ) , not how the secondaries are connected and reflected load ...
Otherwise it would be like a free lunch taken from nothing
it is NOT a free lunch. When you halve or quarter the reflected impedance, the series resistance and leakage inductance now have twice or four times the effect. Real world losses go up. HF response wont go out as far. Push it too far and now the wires can’t handle the current. This must be considered if the plan was to “run big sweep tubes low and hot in class A”. The thin wire on the primary has to handle all that current - quiescent current which does not contribute to output power. What you usually end up doing is using a mains trafo at far less audio watts than its VA rating. Not usually a problem because power trafo watts are cheaper than “output trafo“ watts. If watts cost the same we wouldn’t bother with this exercise.
Good point there, wg_ski, the copper losses and leakage inductance might become overwhelming if the transformer is used in a low Z configuration. Primary Rdc end to end is 36 ohms, which is lower than most OPTs I´ve measured, but the two 9V windings i series gives Rdc= 0,8-09 ohms which is a bit high for an 8 ohms load.
When the day comes to build a big class A tube amp the bigger pair of toroids would probably be a better choice with their slightly higher turns ratio, better power handling and much lower copper losses.
This leaves us with the 5,6k:8R option left to play with. From the top of my head I think a pair of PL36 (25E5, 6CM5, EL36) per channel might be a good choice here, being something of a "poor mans 2A3" in triode connection The only drawback I can think of is that they are low Mu, making the CFB less effective. OTOH, low Mu low Rp triodes usually don´t need much negative feedback.
When the day comes to build a big class A tube amp the bigger pair of toroids would probably be a better choice with their slightly higher turns ratio, better power handling and much lower copper losses.
This leaves us with the 5,6k:8R option left to play with. From the top of my head I think a pair of PL36 (25E5, 6CM5, EL36) per channel might be a good choice here, being something of a "poor mans 2A3" in triode connection
The only drawback I can think of is that they are low Mu, making the CFB less effective. OTOH, low Mu low Rp triodes usually don´t need much negative feedback.
A mains transformer will handle amperes of inrush current. I wouldn't worry about too much tube current on the primary at all. Even a 50VA transformer draws ~700mA under load... My Stupendous amp uses two 50VA coils interleaved and will handle full power (~10W) at 15Hz. They will also pass 5Hz
If you put a current probe and scope on the secondary lead, while testing the secondary with a Variac, you will be shocked at how bad the magnetizing current spikes are when approaching the rated secondary voltage. Power xfmrs are V rated right up to the saturation level setting in. If you back off the Variac V until the spikes go away, your 100 Watt toroidal OT will be good for only a few Watts. Good audio xfmrs only use 50% to 70% of that max magnetization level, which translates to 1/4 to 1/2 the expected Watts, -AFTER- the freq. derating of voltage, which is already pretty severe from 60 Hz.
There is a scheme to remove the magnetizing current effects by using positive current Fdbk from the tube cathodes back to the driver stage to effectively remove the primary resistance and plate resistance, hence removing the magnetizing current spill thru.
For a subwoofer amp, why not get rid of the OT altogether. I saw these SIC "Triode" curves in another thread. They look better than most $$$ tubes:
(so linear, you'll have to add back in the "tube sound" in an earlier stage though. )
There is a scheme to remove the magnetizing current effects by using positive current Fdbk from the tube cathodes back to the driver stage to effectively remove the primary resistance and plate resistance, hence removing the magnetizing current spill thru.
For a subwoofer amp, why not get rid of the OT altogether. I saw these SIC "Triode" curves in another thread. They look better than most $$$ tubes:
(so linear, you'll have to add back in the "tube sound" in an earlier stage though. )
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The saturation limit is dictated by the core properties AND the number of turns. If you add more turns from other windings in series, you will increase the allowable voltage before saturation or lower the low-frequency cut-off.
Adding turns will also increase the inductance, and it will ease matters for the amplifier, but if sufficient GNFB is used, it will not make a huge difference.
Saturation is different: it is a hard limit, fixed by the core area, properties and the voltage and frequency applied
I can't take the credit for that - MaxHifi suggested it and I tried it. Works extremely well. My monoblocs will make 113WRMS @ 30Hz with a pair of 250VA coils.kodabmx: I´ve read about your way to use dual toroids with interleaved windings. Very clever indeed, but personally I would be too paranoid about insulation breakdown between the windings every time Jaco Pastorius or Ian Kilmister hits the low E on their bass guitars
There's no insulation breakdown with 240VAC on them. That's 640V peak to peak. I don't see any issue with breakdown.
I've even has a bad tube arc over but the transformers didn't care... It just kept playing...
I have no doubt there will be no insulation breakdown under gentle hi-fi use. But what happens when you go whacking on it with a hammer when it’s under that kind of voltage stress? Those are the kind of forces that go on inside of transformers when they are under a heavy load. Ever listen to a 5kW solid state PA amp when it’s driving a dummy load under heavy clipping? The transformer is not exactly silent (and neither are the output transistors). You just can’t hear it over the music. So the jury will be out a while until one of us tries it and runs the amp for a couple of years. Under PA/guitar conditions. My thought for high power using mains toriods is to use a very high voltage (1600 to 1900 VCT) primary on the tube side, and run the speaker off the 120 volt winding. Since I’m looking at 40 to 60 volts RMS at the speaker, it should run at fairly low flux density giving magnetic headroom for 30 Hz subwoofer use. And the “primary” will have been designed to run with a kilovolt or two end to end, when running at operating temp, with several hundred or 1kVA running through it continuously. Lifetime of the transformer would be no worse than under the intended use. It’s not an el-cheapo solution, just a lot less than having someone like Heyboer make me a 300 to 1000 watt custom OPT. If I’m going for el-cheapo, I’ll just keep the supply under 250 and use a single $28 mains trafo. You can find $5 sweep tubes that can push 800 mA of peak current. Try getting that out of an EL34.