Yeah, they can move amperes, but you‘re usually listening to milliamperes with an occasional amp or two thrown in there. The jury is out on how long the trafos will take it running 1/3 power, pink noise. Don’t think anybody has ever tried this yet for longer than it takes to make a couple of measurements.
I think you will fail, adding the sort of primary you are speaking about is technically very difficult and practically almost impossible given the fact that the cores will not offer enough room for the many thousands of loops you will need to add to the transformers.
I think you are fundamentally missing the point of what people are trying to achieve with these "cheap" transformers.
Shoog
What part of my arguement do you disagree with? The voltage applied to the primary has been reduced to allow operation at 30 Hz instead of 50Hz, it's the ratio of voltage to frequency that is important. I've measured these (I know you have too) and seen the core saturation. With a low enough primary impedance and voltage you can get a decent amount of power out of them at 30 Hz before they saturate. A 1.4 k load is probably the lowest practical (unless you go to 6C33), so 20W at 30 Hz is doable. Probably need at least 50VA core (I've seen 40 watts at 30 Hz with a 120 VA core). All this under the assumption that triodes (or triode connected pentodes) are used to keep the driving impedance low and swamp parasitics.
Shoog: The way I understood it, wg_ski talks about taking a transformer with a 110V primary and a 1900VCT secondary and then use it backwards as an OPT?
This makes me think of some old mains transformers in my junkbox, especially one monstrous piece from an old cinema amp. 2x460V HV winding and 10VCT filament for dual 5U4 rectifiers and 12,6VCT for a quad of 6L6s and some small signa tubes. 13,2k/8R, remind me of that one the day I want to build a transmitter tube subwoofer amp...
This makes me think of some old mains transformers in my junkbox, especially one monstrous piece from an old cinema amp. 2x460V HV winding and 10VCT filament for dual 5U4 rectifiers and 12,6VCT for a quad of 6L6s and some small signa tubes. 13,2k/8R, remind me of that one the day I want to build a transmitter tube subwoofer amp...
I think the level of derating to get decent power at 30hz would be more like 1/4 VA rating to final power in Watts. In my estimating you would need to use 50VA to get 12Watts at 30hz. To achieve 30watts at that frequency I wouldn't suggest at least 120VA. Throw in any sort of current imbalance and you can kiss that all goodbye. Primary inductance however tends to be inversly proportional to the VA rating so simply going bigger will not produce the goods beyond a critical balance point.
The problem is that these value have to be determined by empirical measurement since manufactureers never publish this data on their power transformers (why would they).
None of this is a problem though since 50VA transformers are small and relatively cheap.
Shoog
Do such things exist ? What is the application for such a mythical transformer, if they do exist I would imagine that they would be more expensive than a custom valve power transformer due to their huge size and limited market.Shoog: The way I understood it, wg_ski talks about taking a transformer with a 110V primary and a 1900VCT secondary and then use it backwards as an OPT?
This makes me think of some old mains transformers in my junkbox, especially one monstrous piece from an old cinema amp. 2x460V HV winding and 10VCT filament for dual 5U4 rectifiers and 12,6VCT for a quad of 6L6s and some small signa tubes. 13,2k/8R, remind me of that one the day I want to build a transmitter tube subwoofer amp...
An old Fashioned monster EI transformer might well make a good valve subwoofer OT as they barely make 1K output (I tried it way back) - but it would still be a very poor subwoofer amp since it would not have the necessary damping factor to control your woofer cone.
Shoog
Shoog: I think Antek makes (or at least used to make) mains toroids with some rather hefty HV secondaries, I vaguely remember seeing one with 2x750V windings on their homepage years ago. Probably quite espensive though. Ham radio transmitters also use very high voltages, that´s where I would look first for transformers north of a kilovolt.
The tube subwoofer amp part was mostly a joke, although I have a pair of bass horns that will need amplification some day. My new ported midbass horns made the bass horns more or less obsolete but a lite extra below 50Hz or so wouldn´t hurt. No need for any monster amps though, the bass horns are loaded with 16 ohm 8" midrange drivers with very low Xmax.
The tube subwoofer amp part was mostly a joke, although I have a pair of bass horns that will need amplification some day. My new ported midbass horns made the bass horns more or less obsolete but a lite extra below 50Hz or so wouldn´t hurt. No need for any monster amps though, the bass horns are loaded with 16 ohm 8" midrange drivers with very low Xmax.
That's exactly what I said earlier: there is a strict proportionality between frequency and voltage handling capability. Simple arithmetics, no more.
A 50Hz transformer can handle 1Hz signals, if you take into account the voltage limitation, and the less favourable 2*Pi*F*L/R ratio.
The lowest practical output valves such as the 6080 family have voltage ratings around 100V Plate to Cathode which means that the theoretical ability to grab low frequency extension by running lower plate voltages has reached its limit before you even begin.
The only solution I have ever seen proposed which actually works to extend the potential of these transformers is a pair of series connected transformers. One primary winding of the first transformer connected in series with the same primary winding on the second transformer, and the same for the other side. You get 460V CT and can series/parallel connect your secondaries for any number of interesting combinations.
Shoog
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I once had a boat anchor transformer capable of producing over 1KV and I built an amp out of it and briefly ran it - but it genuinely scared me to death and I got rid of it as soon as I could - gave it to a radio ham friend.
Shoog
I believe that is what kodabmx does in his amps. The Crowhurst Twin Coupled amp with one transformer on the plates and another one on the cathodes is another option. The obvious drawback here is that you need four transformers for a stereo amp, unless they can be found cheap there´s no real point in using mains toroids instead of "real" output transformers.
"it genuinely scared me to death"
I know the feeling... Years ago I spent big money on parts for a GM70/813 SET project that never happened, the 1500VCT power transformer had the same vibe as a loaded gun... Still have those 300 dollars/each Lundahl OPTs in a box somewhere.
I know the feeling... Years ago I spent big money on parts for a GM70/813 SET project that never happened, the 1500VCT power transformer had the same vibe as a loaded gun...
Still have those 300 dollars/each Lundahl OPTs in a box somewhere.
To my mind the most optimal use for these transformers (@50VA) is a 6080 PP amp run at B+130V, 100V across each triode passing 100mA. IXCY 10M45S CCS in each cathode (settling at about 30V each). Driven at their simplest by a LTP ECC88 or something with a bit more gain to allow for feedback. Simple cheap 4 valve class A amplifier. You get about 7Watts per channel.
I built various different versions of this arrangement until my ideas got to fancy and I ran into very high frequency oscillation issues when the CCS started to fight with the high transconductance of the 6080's and the plate to plate feedback arrangement.
Shelved it but am toying with going back to the very first and simplest iteration which was liked by everyone who heard it - very sweet tube sound.
Shoog
I built various different versions of this arrangement until my ideas got to fancy and I ran into very high frequency oscillation issues when the CCS started to fight with the high transconductance of the 6080's and the plate to plate feedback arrangement.
Shelved it but am toying with going back to the very first and simplest iteration which was liked by everyone who heard it - very sweet tube sound.
Shoog
Thanks for your input here, Shoog (and everyone else of course!)
No 6080s or 6AS7s in my tube stash but I have a box full of russian 6S19P, roughly equivalent to 1/2 6080. Judging by the plate curves it doesn´t seem to be much room for voltage swing at 100V 100mA though?
I don´t think it would be necessary to go to any extremes when choosing tubes to go with these transformers, a pair of triode wired 6L6s worked surprisingly well and I believe pair of trioded PL36 should work even better:
No 6080s or 6AS7s in my tube stash but I have a box full of russian 6S19P, roughly equivalent to 1/2 6080. Judging by the plate curves it doesn´t seem to be much room for voltage swing at 100V 100mA though?
I don´t think it would be necessary to go to any extremes when choosing tubes to go with these transformers, a pair of triode wired 6L6s worked surprisingly well and I believe pair of trioded PL36 should work even better:
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They do exist, at reasonable prices. I have a pair of 15T950’s and a pair of 8T800’s - destined to be used as output transformers when I get around to building the big sons of bitches. They were supposed to be used as ham radio linear amp power supply trafos. The kind people are afraid of. The big one is 1k:4 at around a kilowatt, the smaller one 1.4K:8 at around 350 watts. Going to make a hell of a tube PA.
With enough global NFB the damping factor gets plenty high. I have these wonderful 200 watt Williamsons that have bass that sounds like a Crown Macrotech. I may not (probably won’t) get as nice a HF response using the toroids as the open loop pole will probably need to drop to make it stable, but they’ll be driving speakers that might make 13kHz. Very loud speakers with 2” compression drivers. The big ones will run a couple of my 32 cubic foot horn subs. If I can get the DF up to 15-20 it will be dominated by the cables and connectors anyway.