rmb you mentioned - If you physically insulate a output transformer to allow operation at higher plate voltages, that may handle the primary to case problem, but what about the case of primary to secondary short where the secondary is floated and HT gets superimposed on speaker leads...that could be dangerous...
I totally agree with your point which is why insulating the core is a compromise solution at best. You run somewhat similar insulation risks as one has with Electrostatic drivers.
Typically small modulation transformers were insulated for 10 KV on the smaller ratings up to about a kw or so. Refer to the transformer article I placed the link for earlier in the post.
I know, just for an example, people getting zapped with 110 volts AC from the 12DC leads of some smart battery chargers. As the charger is switch mode and not using a power transformer, and has no earth wire on the mains - the DC output cables through inter-circuit capacitance float at half the 230 volt supply, rendering them in my opinion dangerous if one touches the cables and at the same time connects with a earth point. When connected to a 12 volt battery in a camper van it places the WHOLE wiring system in the van at half mains potential. I know of children getting small shocks off these chargers and parents wanting to sue for compensation. Yet because of international law the manufacturer does not have to do anything about it - as it's legally acceptable in their country to design equipment like this. So too - Audio Output transformers can exhibit similar insulation issues and still be legally allowed to proliferate the market place.
I totally agree with your point which is why insulating the core is a compromise solution at best. You run somewhat similar insulation risks as one has with Electrostatic drivers.
Typically small modulation transformers were insulated for 10 KV on the smaller ratings up to about a kw or so. Refer to the transformer article I placed the link for earlier in the post.
I know, just for an example, people getting zapped with 110 volts AC from the 12DC leads of some smart battery chargers. As the charger is switch mode and not using a power transformer, and has no earth wire on the mains - the DC output cables through inter-circuit capacitance float at half the 230 volt supply, rendering them in my opinion dangerous if one touches the cables and at the same time connects with a earth point. When connected to a 12 volt battery in a camper van it places the WHOLE wiring system in the van at half mains potential. I know of children getting small shocks off these chargers and parents wanting to sue for compensation. Yet because of international law the manufacturer does not have to do anything about it - as it's legally acceptable in their country to design equipment like this. So too - Audio Output transformers can exhibit similar insulation issues and still be legally allowed to proliferate the market place.
Gee ......
Frankly I am amazed about all this uncertainty/no-specs as to h.v. insulation properties.
All this suddenly left me nervous regarding own OPTs I wound. B+ is 600V, with associated maxima somewhere for p.p. use. I quickly went and rechecked; found two layers of mylar insulation between primaries and secondaries safe to at least 2,6kV (sigh of relief). The 1,5 mm nylon bobbin is rated as safe for 3kV, same 2 layers of mylar on top between winding and outer legs of core .....
As one does not seem to know what the Edcors use for insulation .... but it will have to be rather skimpy not to withstand some 2 kV. Anyway, I am just musing - maybe more the fool me for always accepting insulation to be at least 2 kV say - this still in the context of normal OPTs, not higher voltage modulation types. (Wish I still had access to the laboratory non-destructive insulation tester at work.)
Frankly I am amazed about all this uncertainty/no-specs as to h.v. insulation properties.
All this suddenly left me nervous regarding own OPTs I wound. B+ is 600V, with associated maxima somewhere for p.p. use. I quickly went and rechecked; found two layers of mylar insulation between primaries and secondaries safe to at least 2,6kV (sigh of relief). The 1,5 mm nylon bobbin is rated as safe for 3kV, same 2 layers of mylar on top between winding and outer legs of core .....
As one does not seem to know what the Edcors use for insulation .... but it will have to be rather skimpy not to withstand some 2 kV. Anyway, I am just musing - maybe more the fool me for always accepting insulation to be at least 2 kV say - this still in the context of normal OPTs, not higher voltage modulation types. (Wish I still had access to the laboratory non-destructive insulation tester at work.)
I've just had a couple of thoughts about all this. Experimenters ideas only here !
First one is - the secondary is often layered over the primary with insulation inbetween the two windings and that effectively places the two insulation layers in SERIES back to the core. You also have the wire varnish as an added insulation layer. Depending on what quality wire is used will determine how good this overall insulation is. [ I have seen some really cheap / budget Chinese ( power and minature audio ) transformers with NO Insulation paper what-so-ever between primary and secondary windings. These are simply reliant on the wire insulation only.]
If Electrostatic shielding is used, then the insulation breakdown risk between primary and secondary is reduced to some extent; but not between the highest voltage winding and ground.
Secondly - under traditional methods the chassis is grounded and so is the negative side of the power supply. BUT what if one grounds the positive B+ rail and makes that the chassis. Effectively the Audio Transformer is at ground potential even though its driven from the plate circuit. The positive and negative of the main power supply is effectively at the same potential as far as the ground signal path is concerned. The filter/bypass caps being the ac coupling. If the output tubes have top caps the top cap is only at the Ac primary swing potential above the chassis. A far safer arrangement than using a conventional wiring system.
The problem with this arrangement comes into its own with coupling between other modules ( preamp to power amp ) using a conventional power wiring system ( ie negative chassis return ). This is easily fixed with an inter-stage coupling transformer. Using any other coupling method without an input transformer, opens the way for high voltage potentials between the two chassis, or associated wiring. Fixing such a situation requires very careful thought and hence not generally popular. Though I favour this method ( output transformer wired at ground potential ), I have never tried it practically.
The bottom line with all this - is to safely utilise an Audio Output transformer with poor inter-winding insulation. Electrically it can be done, but in the end, for the cost of a bit more insulation paper, its best to stick with a conventional wiring arrangement.
First one is - the secondary is often layered over the primary with insulation inbetween the two windings and that effectively places the two insulation layers in SERIES back to the core. You also have the wire varnish as an added insulation layer. Depending on what quality wire is used will determine how good this overall insulation is. [ I have seen some really cheap / budget Chinese ( power and minature audio ) transformers with NO Insulation paper what-so-ever between primary and secondary windings. These are simply reliant on the wire insulation only.]
If Electrostatic shielding is used, then the insulation breakdown risk between primary and secondary is reduced to some extent; but not between the highest voltage winding and ground.
Secondly - under traditional methods the chassis is grounded and so is the negative side of the power supply. BUT what if one grounds the positive B+ rail and makes that the chassis. Effectively the Audio Transformer is at ground potential even though its driven from the plate circuit. The positive and negative of the main power supply is effectively at the same potential as far as the ground signal path is concerned. The filter/bypass caps being the ac coupling. If the output tubes have top caps the top cap is only at the Ac primary swing potential above the chassis. A far safer arrangement than using a conventional wiring system.
The problem with this arrangement comes into its own with coupling between other modules ( preamp to power amp ) using a conventional power wiring system ( ie negative chassis return ). This is easily fixed with an inter-stage coupling transformer. Using any other coupling method without an input transformer, opens the way for high voltage potentials between the two chassis, or associated wiring. Fixing such a situation requires very careful thought and hence not generally popular. Though I favour this method ( output transformer wired at ground potential ), I have never tried it practically.
The bottom line with all this - is to safely utilise an Audio Output transformer with poor inter-winding insulation. Electrically it can be done, but in the end, for the cost of a bit more insulation paper, its best to stick with a conventional wiring arrangement.
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Kimbal,
Indeed kind Sir! Haven't given such (common at positive supply voltage) a thought, but indeed food for thought. I sort of had the same notions regarding safety at high voltages, by putting the power filter choke in the power supply negative lead. That again gives zero potential between common and at least one choke side, which could then be the inside of the choke winding. In fact with a choke input filter I placed the choke input (a.c. live side) nearest the core (sufficient insulation then needed), which gave me an interference advantage regarding choke hot side interference being partly shielded by the core and the outer windings of the choke.
But off-topic there.
Yes, (also OT from Edcor), your other point is part of the reason why I started winding my own OPTs. I simply frown on non-layered winding (usually ending up quite random and hobbly in the end). No guaranteed specs. or insulation, space-wasting in the end (I calculate windings to occupy the whole winding window), only advantage probable economy.
Thanks for these inputs!
Indeed kind Sir! Haven't given such (common at positive supply voltage) a thought, but indeed food for thought. I sort of had the same notions regarding safety at high voltages, by putting the power filter choke in the power supply negative lead. That again gives zero potential between common and at least one choke side, which could then be the inside of the choke winding. In fact with a choke input filter I placed the choke input (a.c. live side) nearest the core (sufficient insulation then needed), which gave me an interference advantage regarding choke hot side interference being partly shielded by the core and the outer windings of the choke.
But off-topic there.
Yes, (also OT from Edcor), your other point is part of the reason why I started winding my own OPTs. I simply frown on non-layered winding (usually ending up quite random and hobbly in the end). No guaranteed specs. or insulation, space-wasting in the end (I calculate windings to occupy the whole winding window), only advantage probable economy.
Thanks for these inputs!
I habitually put the chokes in the negative return in supplies operating much above about 800V or so, works just fine and greatly reduces the insulation requirements and the potential for insulation break down in the choke. The one case where this is less true would be where a choke input is used in which case you have to deal with a fair bit of ripple on the input side of the choke.
In the case of a positive ground scenario as discussed in a previous post, I'd put the chokes on the positive side, and use an interstage transformer to drive the output stage, and power the driver on a conventional negative ground referenced supply. Note that this means bias supplies and filament supplies to that output stage would have to be referenced to the negative rail and insulated for those voltages, so that just shifts the onus to several other places in the circuit design - not sure it is a cost effective trade off. Something to think about..
In the case of a positive ground scenario as discussed in a previous post, I'd put the chokes on the positive side, and use an interstage transformer to drive the output stage, and power the driver on a conventional negative ground referenced supply. Note that this means bias supplies and filament supplies to that output stage would have to be referenced to the negative rail and insulated for those voltages, so that just shifts the onus to several other places in the circuit design - not sure it is a cost effective trade off. Something to think about..
I suggest for those less familiar with old time techniques, is to consult some of the old Amateur Radio Handbooks. ( ARRL Handbook from the 1930's-50's period. ) Using Chokes on the ground side of any high voltage supply, is an old technique of Amateurs/Hams.
Also with output transformers - use SPARK GAPS across the primary. These are an often forgotten device nowadays. Small gas type gaps used in telephone line modems for lightning flash over and rated for around 1000 volts work well. Neon bulbs also work well for small transformer circuits. Use a series resistor with them. Adjustable home made gaps, set with feeler gauges are far better, but need to be safely mounted. Older radio transmitter books speak of such devices. Getting the desired gap spacing requires some basic calculation and then trial and error adjustment. Just don't adjust them while the power supply is running otherwise you will soon know about it.
One will be amazed at how easy even a small 5 watt SE audio transformer can flash-over when the speaker load is removed. With small fine wire on the primary, these transformers can develop shorts and winding tracking inside the bobbin primary winding under the right conditions - ( dry salt air and or high humidity, with no, or too high a secondary load ). Oil filling the transformer winding seems to be the only simple way to suppress such flash-over events, with increased winding capacitance and poorer High frequency response.
Also with output transformers - use SPARK GAPS across the primary. These are an often forgotten device nowadays. Small gas type gaps used in telephone line modems for lightning flash over and rated for around 1000 volts work well. Neon bulbs also work well for small transformer circuits. Use a series resistor with them. Adjustable home made gaps, set with feeler gauges are far better, but need to be safely mounted. Older radio transmitter books speak of such devices. Getting the desired gap spacing requires some basic calculation and then trial and error adjustment. Just don't adjust them while the power supply is running otherwise you will soon know about it.
One will be amazed at how easy even a small 5 watt SE audio transformer can flash-over when the speaker load is removed. With small fine wire on the primary, these transformers can develop shorts and winding tracking inside the bobbin primary winding under the right conditions - ( dry salt air and or high humidity, with no, or too high a secondary load ). Oil filling the transformer winding seems to be the only simple way to suppress such flash-over events, with increased winding capacitance and poorer High frequency response.
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