Service manual for an Australian transformer set

[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.

[Ed_Dinning]

Hi Sing, if you mean the HT rectifier this should be the easiest part of the circuit.
If you want a valve rectifier then it normally (but not always) needs its own heater winding due to the high common mode voltage on the rectifier heater (6X4 does not have this restriction).
Most valve rectifiers have a limit to the size of reservoir cap that can be used (but not the smoothing cap). Rectifier circuit tables also specify the minimum value of anode series resistor to be used. This is often covered by the DC resistance of the transformer HT sec winding.

Cheers, Ed

[crackle]

Radio Museum shows the following Australian radios with that valve line up.
1947 Radiola 512M
1948 Radiola 519M
1949/50 Radiola 524M
1950 Radiola 720C
1947/48 Hotpoint Band-Master H55ME
1948 Hotpoint-Bandmaster H55MEX
1949 Hotpoint-Bandmaster H55MEY

Mike

[regenfreak]

Brilliant Mike. Thanks

Hi Sing, if you mean the HT rectifier this should be the easiest part of the circuit.
If you want a valve rectifier then it normally (but not always) needs its own heater winding due to the high common mode voltage on the rectifier heater (6X4 does not have this restriction).
Most valve rectifiers have a limit to the size of reservoir cap that can be used (but not the smoothing cap). Rectifier circuit tables also specify the minimum value of anode series resistor to be used. This is often covered by the DC resistance of the transformer HT sec winding.

Ed, Yes its the HT rectifier. This is the part that I know least. I haven't had the experience of building a valve rectifier circuit yet. 6X5 seems to have a bad reputation. I don't know where to look up the rectifier circuit tables. I have looked at the Radiola 524M and 519M, it is not clear what is the secondary voltage spec of the transformer with center tap...I am guessing maybe 300V, I don't know.

I have overcome the challenges of tracking, winding own antenna and oscillator coils etc , the next challenges is to avoid oscillation and instability with good shielding, decoupling and bypass caps, wirings and well-thought components layout within the chassis homebrew set.

[joebog1]

If its Australian, the only place I know of that will have the circuiot, and perhaps the whole service manual is Kevin Chant.
https://www.kevinchant.com/r-series.html

Hope thats of some help

Joe

[Nuvistor]

This is a good source of information on valves.
http://www.r-type.org/

Eg. http://www.r-type.org/exhib/abh0072.htm

Eg. http://www.r-type.org/pdfs/6x5.pdf

[regenfreak]

Thanks Frank, I have bookmarked your first link.

The 6x5 seems to earn a bad name for shorts/transformer killers but 6X5GT is ok as long as the series resistors and reservoir cap are within manufacturer limits. It is just a question of adapting my HT/filament transformer (240V-250V, 6.3V) to this valve, figuring out those values is the first uncertainty. The DC resistance of the secondary of my HT transformer is 110 ohms.

The valve manufacturer manual is not always clear...

Probably initial guesses of anode resistor of 100ohms and reservoir cap of 8 microfarads based on Radiola 519m service manual

[regenfreak]

Originally Posted by joebog1

If its Australian, the only place I know of that will have the circuiot, and perhaps the whole service manual is Kevin Chant.
https://www.kevinchant.com/r-series.html

Hope thats of some help

Joe

Thanks

[joebog1]

Its very similar to lots of radios from the late 40,s and 50,s. There was a radio "factory" on almost every corner. I would be looking up Kriesler!!. BUT dont be surprised if the circuit doesnt exactly match the radio. As new valves became available it was common to blank out an octal base and fit a noval. see my pic.
This chassis has an almost identical circuit to your article.

Joe

[regenfreak]

Yes, its like different generations of AA5, they are almost all the same with mostly variations in components layout within the chassis, decoupling, AVC, shielding and wirings. I have downloaded about 30 schematics and service manuals of all kinds of AA5 from 1930s-50s...i have been studying them.

[Argus25]

Originally Posted by regenfreak

While I am looking for information on the suppression techniques for oscillation, instability and motorboating

The trouble with this is that the term motorboating is not specific often people call it that when they hear low frequency repetitive bop bop sounds coming out the speaker.

Classical motorboating is caused by a common impedance or resistance in the power supply and/or poor capacitor bypassing of the audio output and driver amplifier, so audio frequency signals from the output stage get back to the input amplifier in a positive feedback manner. One classical cause for this is transistor radios is a high resistance in the ON-OFF switch. In valve radios its usually poor electrolytics.

Although you can also end up with motorboating sounds in audio amplifiers with negative feedback around them, and where the coupling capacitors are too small and the phase shift at the low frequency end causes the negative feedback to become positive. So there are many potential causes of these sounds.

Likewise oscillation can refer to all manner of instability due to unwanted feedback pathways in a single valve stage or around more than one stage. And it can present to the listener like motorboating. And it can represent radio frequency oscillation within a single stage or , due to feedback pathways in a single stage or around multiple stages of RF and IF amplification.

For example one notorious example is in the IF amplifier. This is because it has identical tuned circuits in its anode and grid (typically 465KHz or thereabouts) so there is is risk they can exchange energy with each other. Oscillation is supposed to be prevented by the screen grid action of the Pentode, isolating feedback from the anode to the control grid. So, if there is anything wrong with the screen bypass capacitor in this stage or its grounding the stage will oscillate. Often it pays to connect this directly to the valve's cathode.

However, feedback signals can get in from the power supply rail, needs RF bypassing (0.1uF capacitors typically), or the heater connections too. The agc control line is another place, if not properly bypassed where feedback can cause oscillations in the IF amplifier. And that is not all, poor shielding of coils and proximity of components can also induce feedback.

It can take quite a while to learn the construction & design techniques to help avoid all of the above, so if you start with a proven electrical & mechanical design it helps.

When it comes to suppressing RF parasitic oscillations , its best not to have them in the first place, but one trick that can help is to add 100R stopper resistors in series with the valve's control grid, this creates an LPF and blocks very high frequencies.

As for the 6X5, there is nothing wrong with these at all, but like any valve you have to respect the manufacturer's rating sheet, and specifically for valve rectifiers do not load them directly into a higher value filter electrolytic than they were designed for unless you add some resistive or inductive peak current limiting.

There were a number of home project radios published for building in Australia. The classic was "The Little General" superhet, initially with octal valves, later others, these are a very appealing starter radio build with a proven stable design. So search for that one.

http://members.ozemail.com.au/~radio/general.html

[joebog1]

As far as the rectifier goes, most aussie radios began with a type 80, and then seemed to go to a 6X5. in the mid 50,s it was a 6X4. Its obvious advantage is there is no need for a 5 volt heater wiring for the rectifier. As I have mentioned in other threads here, I am 67 years old, have been playing with radios all my life, and have yet to see an AC/DC radio made in Australia. They did exist however. Every cheap radio made here ( and there were hundreds of different models) had a mains transformer.
For what its worth, I have the whole of Radio and Hobbies, which later became Radio Television and Hobbies magazine. and can drag out at least 20 different radios that are a proven stable design if you want them.

Cheers
Joe

[regenfreak]

Originally Posted by Argus25

Classical motorboating is caused by a common impedance or resistance in the power supply and/or poor capacitor bypassing of the audio output and driver amplifier, so audio frequency signals from the output stage get back to the input amplifier in a positive feedback manner. One classical cause for this is transistor radios is a high resistance in the ON-OFF switch. In valve radios its usually poor electrolytics.

Although you can also end up with motorboating sounds in audio amplifiers with negative feedback around them, and where the coupling capacitors are too small and the phase shift at the low frequency end causes the negative feedback to become positive. So there are many potential causes of these sounds.

Likewise oscillation can refer to all manner of instability due to unwanted feedback pathways in a single valve stage or around more than one stage. And it can present to the listener like motorboating. And it can represent radio frequency oscillation within a single stage or , due to feedback pathways in a single stage or around multiple stages of RF and IF amplification.

For example one notorious example is in the IF amplifier. This is because it has identical tuned circuits in its anode and grid (typically 465KHz or thereabouts) so there is is risk they can exchange energy with each other. Oscillation is supposed to be prevented by the screen grid action of the Pentode, isolating feedback from the anode to the control grid. So, if there is anything wrong with the screen bypass capacitor in this stage or its grounding the stage will oscillate. Often it pays to connect this directly to the valve's cathode.

However, feedback signals can get in from the power supply rail, needs RF bypassing (0.1uF capacitors typically), or the heater connections too. The agc control line is another place, if not properly bypassed where feedback can cause oscillations in the IF amplifier. And that is not all, poor shielding of coils and proximity of components can also induce feedback.

When it comes to suppressing RF parasitic oscillations , its best not to have them in the first place, but one trick that can help is to add 100R stopper resistors in series with the valve's control grid, this creates an LPF and blocks very high frequencies.

Many thanks for the insight. From my little study/observation of the AA5 schematics and service manual, here the suppression techniques of motorboating, birding and instability:

1. Add 100nF RF bypass to earth cap to the convertor grid, the primary of the first IF. Add 100nF bypass to the primary of the second IF at the grid of the IF amp valve. Add 100nF bypass to the HT of the detector anode.

2. Add 100pF-100pF RF bypass cap to the secondary of the 2nd IF detector diode anode.

3. Avoid capacitive/inductive coupling. Isolation of RF and audio by careful layout of valves and IF, and wirings. Move plate wiring away from the grid. Keep their wirings short. If an RF line is crossing an audio line, they should at 90 degrees to each other.

4. Use earthed coaxial cables for the detector grid to volume pot

5. Solid contact between IF base and the chassis; connection of the air variable gang using tinned copper braids to earth point if they are bolted on rubber rings to the chassis. Some AA5 maker soldered the IF cans to the chassis!

6. Use of earthed tin shields for the detector and audio valves. Also GOAT forming fitting shields are used for old sets.


I have found this Canadian book on fixing motorboating and squealing:

https://canadianvintageradio.com/wp-...Motorboats.pdf

It can take quite a while to learn the construction & design techniques to help avoid all of the above, so if you start with a proven electrical & mechanical design it helps.

I actually bought a working Stewart Warner C51T2 AA5 two weeks on ebay. I have been studying the wirings of the real thing before homebrewing my own

There were a number of home project radios published for building in Australia. The classic was "The Little General" superhet, initially with octal valves, later others, these are a very appealing starter radio build with a proven stable design. So search for that one.

http://members.ozemail.com.au/~radio/general.html

I have successfully built a number of two tubes to four tubes superhets with one IF stage only. However, it always goes into oscillation if I add IF amp stage building on modern breadboard (two much parasitic capacitance indeed!). My first battery farm AA5 built on PCB went birding and motorboating like mad. Therefore everything must be built with proper metal chassis ground plate with good shieldings and wirings.

My latest Frankenstein lash-up is a three valves, single IF superhet line-up: 6SB7Y, 6SQ7GT and 1SQ5GT with a 6 inches Super Ferrite Sleeve Antenna (made of 40 pieces of 15cm x 1cm ferrite sticks) that is directly connected the RF tank. It is so sensitive that it probably as good as the commercial superhet with IF amp. Clearly the Q so high it overloads the circuit and bursts my ear drums across all stations!

[regenfreak]

Originally Posted by joebog1

As I have mentioned in other threads here, I am 67 years old, have been playing with radios all my life, and have yet to see an AC/DC radio made in Australia. They did exist however. Every cheap radio made here ( and there were hundreds of different models) had a mains transformer.

Is there Australian electrical standard requirement for transformer?

[joebog1]

Yes there is and has been for many years. I don't know exactly when this became "law" so to speak. I mentioned my dad, he was a master craftsman of transformers (and winding in general) and his teaching was superior to commercial designs. But after pulling apart and rewinding transformers built way back (approx 1920) they were always very safe. Although they mostly didn't have side cheeks of the former, there was always ample insulation space left at the end of each layer. Especially the primary (mains) winding.

Between primary and secondary there was a variety of insulation techniques used, ALL dependent on quality required, or dollars available. The best I have seen is linen tape soaked in some form of wax, and consisted of 5 individual layers. Each layer was separate and began and finished on opposite sides of the bobbin. Some military radios had oil filled (PCB, Poly-chlorinated Biphenyl) transformers. I'ts deadly for health but excellent for insulation/isolation in transformers.

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

Joe

[Argus25]

Originally Posted by regenfreak

Is there Australian electrical standard requirement for transformer?

That is an interesting question, I'm not sure about the regulations here.

Practically every radio & TV made in Australasia has a power transformer. When I first saw imported "transformerless" sets I threw my hands up in horror wondering "what were they thinking ?".

In fact many technicians here have ended up getting nasty shocks repairing AC/DC sets especially when the P & N got reversed and its a total PITA when trying to connect these sets to lab test gear when the chassis is connected to "hopefully" neutral.

All of my AC/DC vintage sets here I run with isolating transformers and I earth their chassis.

I would never build any mains apparatus without a power transformer and a properly earthed chassis. I would advise the same for anyone building home made valve radios. And read up on how earth lugs should be secured and appropriate fusing and how to secure mains power cords to a chassis (don't tie knots in them as was once the fashion). One thing that helps is to use a chassis mount fused IEC connector, then you won't always have a power cord and plug dangling out of your radio.

[regenfreak]

All of my AC/DC vintage sets here I run with isolating transformers and I earth their chassis.

I would never build any mains apparatus without a power transformer and a properly earthed chassis. I would advise the same for anyone building home made valve radios.

Yeah I would never run anything homebrew or vintage radio without an isolation transformer. I am not familiar with AC/DC set wiring.

The benefit of connecting the chassis to AC main earth is confusing for me if an isolation transformer is used. I think I remember reading somewhere that connecting the main earth to chassis may cause hum due to a ground loop formation.

Earlier models of All American Five are called widowmakers and later models have 0.1 microfarads isolation capacitor across the RF earth and the chassis earth. My Stewart Warner C51T2 has the isolated RF earth connected directly to the neutral of the main. It has a plastic earth plug pin using a capacitive voltage dropper for 240AC. Therefore it has a RF earth return ground wire running along the chassis. I believe the position of this wire is important; put it the wrong place in a homebrew can cause instability and oscillation.

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

It must be somewhere written in the handbook among these jungle of rules:

https://infostore.saiglobal.com/en-g..._AS_AS_208028/

Clearly it would be difficult/expensive in many cases for importers to add transformers to existing American/European sets. Or maybe the US/European makers mass produced sets for the Australian market..I do not know. Culturally the Aussies seem not to be particularly risk averse compared with the American, so it must be some regulatory requirements for this.

Some military radios had oil filled (PCB, Poly-chlorinated Biphenyl) transformers. I'ts deadly for health but excellent for insulation/isolation in transformers.

Some old EHT transformers like power line or x-ray transformers has this deadly PCB stuff..causing pancreatic cancers

[regenfreak]

Speaking of the recklessness of radio makers using live chassis, the above-mentioned homebrew with 6SB7Y, 6SQ7 and 1QS5GT homebrew with the fully tracked super ferrite sleeve antenna is based on the schematic of this metal body Silverstone 8003:

https://www.youtube.com/watch?v=eib6GSYS4CQ

[Ed_Dinning]

Hi Sing, I have plenty of transformers if you want one to experiment with. This would have a centre tapped HT winding as well as 5v (or4v) and 6.3v for the heaters.
I dislike these 6X4 style rectifiers due to the potential for causing damage, but in practice I have rarely seen it happen.
The decoupling of HT rails with additional non electrolytic caps is always a good idea. You also need to be careful with the layout of your earthing points. An earth busbar, connected to chassis at one point only is a good idea, as are "star "earth points.

You cracked the theory relating to RF so power supplies should present you with little difficulty (apart from the possible HV hazards) !


Ed

[regenfreak]

Many thanks Ed. You are always so kind I will be in touch..

I have ordered the 6x5GT. It seems like these Australia transformer sets are running at much harder plate voltage for the converter and IF valve at about 240V plate because they use 6V6. They also have a delayed AVC. I reckon they are more sensitive than typical AA5 counterparts typically running at HT of 100-110V. I prefer lower HT like 110V for homebrew set for safety reason.

I learnt the hard way it does matter where I placed the HT decoupling caps on the HT rail, and where I put the volume pot to avoid oscillation...Also the outside foils of the axial capacitor should be connected to RF earth side of the circuit to improve shielding...

When I first read about the importance of the location of the Earth point on the chassis or star points, I thought what the heck ..then it makes senses thinking about it. I have actually spent quite a bit of time studying the locations of the Earth points of different model chassis.

Actually I am a bit scare of experimenting with valve transformers and rectifier valves while I had no trouble with experimenting 10KV neon sign transformers and 20KV flyback transformers