Service manual for an Australian transformer set

[regenfreak]

This Australia Radiola set have components directly soldered on chassis RF earth that eliminates the need of a RF ground wire, probably another advantages of having a transformer. I use copper cladded boards forming the chassis box and I probably may follow the same.

[Argus25]

You could regard a hot chassis set as being safe if it was in a completely insulated enclosure with no metal parts projecting to the exterior.

Many modern appliances in the home, kitchen etc with switch-mode PSU's in them are like this and therefore thought of as safe and you won't see an earth pin on their plugs either. Many hair dryers have a motor and elements supplied directly by P & N. And USB wall warts etc with plastic bodies and no earth pin.

However, a valve radio chassis, especially home made is a completely different cup of tea, because its almost certain that the chassis will be touched when its running and or other test instruments attached to it, RF generators, scopes etc will need to be connected to it and many valve radios sit in cases with open backs for ventilation.

So sooner or later the owner or someone else will touch the chassis and perhaps a control shaft when a knob falls off.

For all these reasons it is important that the radio has a power transformer and a properly earthed chassis and is not made in the fashion of an AC/DC set with the vain hope that the chassis is probably at neutral potential. It could easily accidentally be phase with a mis-wired plug or extension cord or even an improperly wired GPO.

[regenfreak]

However, a valve radio chassis, especially home made is a completely different cup of tea, because its almost certain that the chassis will be touched when its running and or other test instruments attached to it, RF generators, scopes etc will need to be connected to it and many valve radios sit in cases with open backs for ventilation.

So sooner or later the owner or someone else will touch the chassis and perhaps a control shaft when a knob falls off.

For all these reasons it is important that the radio has a power transformer and a properly earthed chassis and is not made in the fashion of an AC/DC set with the vain hope that the chassis is probably at neutral potential. It could easily accidentally be phase with a mis-wired plug or extension cord or even an improperly wired GPO.

Thanks for the food for thoughts. I dislike homebrewing by a simple copy of the All American Five designs with series filaments and hot chassis because the design was fundamentally optimised for cheapness instead of safety. Having parallel filaments make it a lot easier to debug the stages as well.

Safety comes first

[usradcoll1]

Originally Posted by regenfreak

While I am looking for information on the suppression techniques for oscillation, instability and motorboating, I have come across this schematic for the transformer equivalent set of the All American Five, possibly Aussie radio?

http://www.thebakeliteradio.com/page...5/page105.html

Anyone knows the model number for this schematic or similar radios (Aussie or American) with the similar valves line-up; 6A8G, 6SK7GT, 6V6GT and 6X5GT? I would like to find out the component values of the audio and rectifier stages in the schematic from the service manual. 6A8G is just an old version of 6SA7GT.

I am in the middle of homebrewing my own version with the following valves line-up:

6SB7Y, 6SK7GT, 6SQ7GT, 6DG6GT and maybe 6X5GT.

6SB7Y is equivalent to 6SA7GT on steroids. 6DG6GT is a lower power alternative of 6V6. While I am clear about the changer, IF amp, detector and audio stages, I am not sure about the best way of building the rectifier stage. So I need to study the existing examples.

I used the 6SB7Y in the Hallicrafters multi-band receivers, instead of the 6SA7, a marked improvement.
The 6DG6GT is a low HT output valve used in many US TV sets as an audio output and a voltage divider, using 280 volts on the anode and screen and furnishing around 120 volts on the cathode. It also draws 1.2amps @ 6.3 volts heater current. The newer design 6X5GT rectifiers are considerably better.

[usradcoll1]

Originally Posted by regenfreak

Thanks for the food for thoughts. I dislike homebrewing by a simple copy of the All American Five designs with series filaments and hot chassis because the design was fundamentally optimised for cheapness instead of safety. Having parallel filaments make it a lot easier to debug the stages as well.

Safety comes first

Most of the first radios I first worked on was AC/DC type, live chassis radios, many missing the cabinet. Of course, with our 120 volt mains, the risk of severe electric shock is minimized.
Working as an Industrial electrician, I was a lot more careful handling the higher voltages used.
Dave, USradcoll1.

[regenfreak]

Originally Posted by usradcoll1

I used the 6SB7Y in the Hallicrafters multi-band receivers, instead of the 6SA7, a marked improvement.
The 6DG6GT is a low HT output valve used in many US TV sets as an audio output and a voltage divider, using 280 volts on the anode and screen and furnishing around 120 volts on the cathode. It also draws 1.2amps @ 6.3 volts heater current. The newer design 6X5GT rectifiers are considerably better.

Interesting! Hallicrafters wow! It should be better for SW or FM. For MW reception the 6SB7Y should shine more in a delayed AVC set like those Amalgamated Wireless Australasia transformer sets. I experimented with miniature pentagrid counterparts of 6SB7Y like 6BA7 and 6CS6 as well in superhet lash-up rigs.

I choose 6DG6GT (filament power of 7.5W) over 6V6 (filament power of 2.8W) because I want to stick with the safer 110V plate like the AA5. More energy is required to boil over the electrons running at low plate voltage; the same is true with those 12V space-charge automotive AA5 tube line-up; they are suckers of filament power. 6DG6GT is electrically the same as 25L6GT, 12L6GT, 50L6GT.

I can't make up my mind about 6X5GT depending the transformer that I can get hold of. If the transformer has 5V heater output, I would use 5Y3GT instead.

[Argus25]

Most of the first radios I first worked on was AC/DC type, live chassis radios, many missing the cabinet. Of course, with our 120 volt mains, the risk of severe electric shock is minimized.
Working as an Industrial electrician, I was a lot more careful handling the higher voltages used.
Dave, USradcoll1. [/QUOTE]

Yes I think the lower 120V mains is safer than 240V, though not safe.

One interesting thing about earthing a chassis, and having a power transformer for valve work is this:

Although this practically eliminates the risk of a shock from contact with the chassis and makes it much easier to attach other earthed apparatus like scopes etc for testing and is the preferred way to have it, without a doubt, there is one interesting disadvantage, that applies to all mains powered high voltage mains power supplies that have outputs that are not floating and it is this:

The very isolation afforded by the power transformer defeats the dwelling's breaker box (RCD/ELCB).

Also, since in the valve radio or amplifier one side of your about 300V DC supply is normally connected to the radio's chassis, then its possible to receive a shock from contact with one finger or hand to the +300V rail in the set. The current passes through your body to ground or via the other arm, if you happen to be holding the chassis or another earthed object. This current of course is not detected by the RCD on the primary line side of the power transformer as there is no unbalanced current there.

So oddly, if the person makes a mistake poking around inside a transformerless hot chassis set, they are sometimes afforded the safety action of their fusebox RCD, for a current passing from their body to ground away from the chassis, but not with a transformer based set that generates high voltages.

However, overall it is still much safer to have the transformer and an earthed chassis for the radio or amplifier, than a hot (potentially live) chassis system.

[regenfreak]

So oddly, if the person makes a mistake poking around inside a transformerless hot chassis set, they are sometimes afforded the safety action of their fusebox RCD, for a current passing from their body to ground away from the chassis, but not with a transformer based set that generates high voltages.

I have lots of respect for electricity. My flat breakers are dated without RCD which is good for high voltage experiments but not safe. Ironically I had to remove the GFI modules from those expensive Italian FART (no joking) iron cored neon sign transformers to make tesla coils working. I always use RCD plugs with an extension cord to the bathroom though.

I have put two dry fingers across the poles of a Daisy chain of 9V cells total of 130V. Nothing happens. But I feel a very uncomfortable sharp bite when I increase the 9V chain to 180V. If I am sweaty or have wet hands or the HT is AC, the voltage pain threshold should be much lower. If I remember correctly, the minimum distributed capacitance of a dry human body is about 100-200pF.

I regularly use the 1920's all aluminium SG Brown type F high impedance headset having 100-130V over my skull. The only protection is the Bakelite ear cups but I have not been zapped yet.

I measured the potential difference between floating HT secondary of an 240V isolation transformer and a solder iron (always earthed). It is about 40V. I guess it is a ghost reading due to the distributed capacitance between primary and secondary windings of the isolation transformer:

http://www.datatronics.com/pdf/distr...ance_paper.pdf

Although this practically eliminates the risk of a shock from contact with the chassis and makes it much easier to attach other earthed apparatus like scopes etc for testing and is the preferred way to have it,

The question of whether to ground the chassis to main earth during scope measurement can be confusing and harzardous issue for newbies, fast forward to 28 to 33 minutes Uncle Doug who advises against the grounded chassis during scope measurement:

https://www.youtube.com/watch?v=5SjVm-rF39A

[Argus25]

Originally Posted by regenfreak

I dislike homebrewing by a simple copy of the All American Five designs with series filaments and hot chassis because the design was fundamentally optimised for cheapness instead of safety. Having parallel filaments make it a lot easier to debug the stages as well.

Safety comes first

There is another reason why parallel filaments with multiple valves are so much better than the series configuration.

The large valves help protect the smaller ones and they all protect each other's heaters at power on. Due to the low resistance of the cold filaments, they transiently at least, effectively short out the power transformer and they all get a soft start and rise to temperature at a slow speed. And if the power transformer isn't gigantically oversized for the task, it simply can't support the massive high turn on currents, so the voltage is collapsed and rises slowly as the heaters warm up.

However, it is a much riskier business powering a heater string from a low Z internal resistance supply in series, from the mains power for example.

The larger valves with more thermal inertia take longer to warm. If the series current limiting resistor is large enough to prevent a turn on current peak blowing up the smaller valve's heaters like a flash bulb, it dissipates a good amount of heat when running.

This is why in TV sets with large heater strings you will see a Brimistor (NTC power resistor) in series with the heater chain to protect from this problem, which is never an issue with a transformer based supply. The Brimistors of course sit there running hot, as does the series resistor, so the whole affair is a poor cousin to a real power transformer, which adds of course, to the cost & weight off the apparatus.

It makes me laugh though when people complain about the weight of quality apparatus, when it was not intended to be portable, because it is shackled to a mains power outlet.

[Terry_VK5TM]

Originally Posted by joebog1

I have made a small search, but cannot seem to find when exactly AS/NZ electrical standards began. My apologies for this.

Joe

If you are really really bored , you can get on to the Standards Australia website and search all the superceded standards.

Some that might apply from back in the dark ages are AS3100 & AS3108 (there's plenty more eg a more recent one AS/NZS60335). The earliest reference I could find was 1970.

Standards Australia (under another name) started in 1922.

As far as the change to AS/NZS standards, I can't remember when it was but possibly as late as the early 2000's (<- relying on old, deteriorating grey cells there).

[usradcoll1]

Originally Posted by Argus25

Most of the first radios I first worked on was AC/DC type, live chassis radios, many missing the cabinet. Of course, with our 120 volt mains, the risk of severe electric shock is minimized.
Working as an Industrial electrician, I was a lot more careful handling the higher voltages used.
Dave, USradcoll1.

Yes I think the lower 120V mains is safer than 240V, though not safe.

One interesting thing about earthing a chassis, and having a power transformer for valve work is this:

Although this practically eliminates the risk of a shock from contact with the chassis and makes it much easier to attach other earthed apparatus like scopes etc for testing and is the preferred way to have it, without a doubt, there is one interesting disadvantage, that applies to all mains powered high voltage mains power supplies that have outputs that are not floating and it is this:

The very isolation afforded by the power transformer defeats the dwelling's breaker box (RCD/ELCB).

Also, since in the valve radio or amplifier one side of your about 300V DC supply is normally connected to the radio's chassis, then its possible to receive a shock from contact with one finger or hand to the +300V rail in the set. The current passes through your body to ground or via the other arm, if you happen to be holding the chassis or another earthed object. This current of course is not detected by the RCD on the primary line side of the power transformer as there is no unbalanced current there.

So oddly, if the person makes a mistake poking around inside a transformerless hot chassis set, they are sometimes afforded the safety action of their fusebox RCD, for a current passing from their body to ground away from the chassis, but not with a transformer based set that generates high voltages.

However, overall it is still much safer to have the transformer and an earthed chassis for the radio or amplifier, than a hot (potentially live) chassis system.[/QUOTE]
I was surprised to see that most of the TV sets and radio receivers sold in the UK are the transformer less, series heater chain, live chassis sets. They have those big old nasty heater dropper resistors, plus half-wave rectifiers.
Larger smoothing and storage capacitors are needed because of the 50HZ and the half-wave rectifier setup. Naturally, I understood the DC was still in widespread use in the UK, as in the large cities of the US.
My preference has always been a transformer powered piece of electronic equipment. It just struck me as being a higher quality unit.
Dave, USradcoll1, as usual very opinionated.

[G6Tanuki]

Originally Posted by Argus25

The larger valves with more thermal inertia take longer to warm. If the series current limiting resistor is large enough to prevent a turn on current peak blowing up the smaller valve's heaters like a flash bulb, it dissipates a good amount of heat when running.

How true: in the days of series-heater-chain valve TVs it was considered 'perfectly normal' for the protruding-out-the-ends-of-the-cathode-tube bits of filament on small low-cathode-thermal-inertia valves like the EF91 or 6AL5/EB91 to light up *REALLY* brightly for a few seconds at switch-on.

Not all TV manufacturers bothered with a Brimistor. Despite such abuse, it never seemed to harm these little valves [I've heard it said that the only way to make sure a 6AL5 is truly dead is to hit it - several times - with a shovel].

[regenfreak]

The large valves help protect the smaller ones and they all protect each other's heaters at power on. Due to the low resistance of the cold filaments, they transiently at least, effectively short out the power transformer and they all get a soft start and rise to temperature at a slow speed. And if the power transformer isn't gigantically oversized for the task, it simply can't support the massive high turn on currents, so the voltage is collapsed and rises slowly as the heaters warm up.

However, it is a much riskier business powering a heater string from a low Z internal resistance supply in series, from the mains power for example.

The larger valves with more thermal inertia take longer to warm. If the series current limiting resistor is large enough to prevent a turn on current peak blowing up the smaller valve's heaters like a flash bulb, it dissipates a good amount of heat when running.

This took my newbie's brain a while to understand why. At the first glance, the series filament would have lower inrush current than the parallel filament configuration due its higher equivalent resistance. However, the larger valves have more powerful heaters. The more powerful the heaters are, the lower the cold resistance of the filaments and the thicker heater elements/thermal mass. During the initial inrush phase, the series filaments can be viewed as a potential divider. The voltage across the larger valves raises much slowly and so a large voltage spike is developed across the poor small valves taking the full force of 240V mains. P = V x I so the small valves are lighting up like a Christmas tree.

For the parallel heaters, the voltage is always the same, the initial inrush is not too bad. It may just cause core saturation of the transformer momentarily. The inrush current for the 6DG6GT is about 2A; 6SBY7 is just under 1A.

The NTC thermistor can get very hot. For high power application, it should be mounted on a beefy ceramic holder away from wirings. Also power off and off is a no-no when the NTC thermistor is still hot. I used the Ametherm calculator to do sizing for my 3KW variac:

https://www.ametherm.com/inrush-current/calculators

There seems to have lots of art and science on how to wire the parallel heaters. For example, there is a certain wiring technique to avoid hum. How little details can make a big difference to hum:

http://www.valvewizard.co.uk/heater.html

Probably some of the advice works for audiophiles but not radiophiles!

[MotorBikeLes]

In #28, Regenfreak wrote - "I regularly use the 1920's all aluminium SG Brown type F high impedance headset having 100-130V over my skull."About 25 years ago, just before I moved here, my pal Dave was very friendly with a chap named Bill. He had been a (or THE chief) designer at the old Baird company from Bradford. Retired, he still played "advanced analogue" stuff, and one day built himself a brainwave feedback machine.
One evening, Dave paid his regular weekly visit, but got no reply. He went in, and found Bill dead with his brainwave machine on his head. Some sort of simple PS failure got the better of him.
Les.

[regenfreak]

Good grief! Touch wood. At least you wont find me jumping into a bath tub wearing the SG Brown F like the Ipod generation.