Trio W-50 Super Delux Stereo Amplifier

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About 40 years ago I was given a big valve (tube) Tuner Amplifer that was made about 1962 when FM radio stations were just a thought DownUnder and Stereo Broadcasts were achieved using two AM stations. The two ABC National stations in each capital city broadcast two hours of classical music on Sunday nights, one channel carried by one station, the other by the other station. I wonder if this was also done in other parts of the world ?.

Trio and Pioneer were at least two manufacturers who produced Tuner Amplifers with two AM tuners and the W-50 was one of these. One of the tuners tuned the broacast band and the international FM band, 88 to 108 MHz, the other tuned the broadcast band and short wave from 3.7 Mhz to 11.0 MHz. At the time the FM section was of little use and by the time I was given this monster, the AM stereo broadcasts had been discontinued. I had however built a twin AM tuner for my Brother some years before.

When I aquired the W-50 I had to replace a number of coupling capacitors in the audio stages as they were ceramic types and were going leaky, generating noise. I used this amplifier for about twenty years as my lounge room sound system and then when I bought a big Akai system with CD-4 and Matrix Quad I pensioned off the W-50 and put it away under my work bench. I could not bring my self to throw it out.

After visiting some of the valve amplifier fraternity in the HRSA I remembered I had this fine old valve amp and rescued it from a big pile of rubbish under the bench. It was covered in solder splatters from twenty years of flicking solder into the bin next to it and full of dust. I checked it over and plugged it in via an isolation transformer and a fuse. It had only one fuse in it as it was made and this was in the mains wiring and was in a dangerous spot on top of the chassis.

To my surprise it burst into life and both tuners worked like Trio had intended. It was soon evident it was going to need another rebuild as the remaining ceramic capacitors were generating frying noises and the hum level indicated the electrolytic capacitors were no longer very well. The first thing I looked for was a manual and was successful this time round because we have that wonderful resource, the World Wide Web. I found a fellow offering a manual in PDF format posted from South America. The circuit diagram is almost correct but the pin and valve numbering is wrong.

I had most of the parts I needed but I did source a BIG chassis mount twin 50uF capacitor and set about rebuilding for the second time in its life. I also decided as it had an FM Multiplex output so an external Stereo Decoder could be used, to look on the Web for one of these. I soon found complete stand aloan decoders were expensive and of course, the only place I found anything like it was in the USA. They were not much use to us DownUnder in the 1960's. I was lucky enough to find a FleetWood add-on decoder that was produced as an option for a radio gram and no one wanted that so it was cheap, even when I had to take into account postage.

I had just finished replacing components when disaster struck. I was looking round with the volt meter for a suitable point to take off the HT supply to the decoder when I noticed the HT voltage was dropping slowly. What could the problem be ?. It soon became evident when I heard the gurgling noises comming from the power transformer. It had suddenly developed a shorted turn. Now where do I find a BIG transformer with four 6.3 Volt heater winding, a 25 Volt bias winding and a hefty 300 mA HT supply ?.

It was at this point a friend came to my rescue. He builds fancy valve amplifiers and has his transformers wound by a fellow who is semi retired and makes them in his back shed. To make it easier for this fellow to fit my transformer into an available core I decided to use a solid state bridge rectifier for the HT in place of the centre tapped winding and the two 6CA4 rectifiers. This would also eliminate one of the heater winding. The transformer turned out well, just fitting into the case with only a few mm to spare.

It took me about six months to pluck up the courage to get back to the project but the Christmas/New Year break took away any excuses I had so I set to it and soon had it singing again, complete with the FM stereo decoder. It is now in my radio room as a monitor amplifier for my record to CD conversions. As there was only one dodgy fuse I added fuses to all the heater supplies, the bias and HT supplies and of course a safer mains fuse. I also put in a power socket so I do not have a power chord getting in the way when working on it.

The specifications are quite impressive for the day it was made. It has push-pull 7189's in the output stage and at 440 Volt HT they produced about 30 Watt RMS. I dropped the HT to 400 Volt to be a bit conservative. In addition to the AM and FM tuner positions, the input selector switch included positions for Tape Head, Microphone, Phono (with input connectors and resistor networks for Magnetic and Xtal) and an Aux.

I have rewired the selector switching so the old Mic input is now a line in. The Aux input was intended to accept the stereo output from the decoder so I cannot use that as a line in. I have also rewired the switching so the decoder HT only comes on in the Aux position. Interestingly, the original wiring put power back on the FM tuner section in the Aux position so a decoder could be used in this way.

In the original configuration the heaters of the two preamp twin triodes were supplied with DC from a selenium rectifier but as these are a health hazard if they fail, I replaced this with a modern bridge rectifier. I also supplied the heaters of the two tone control stages with DC. Trio had gone to great lengths to avoid hum from the heater circuits, using balancing pots on two of the heater windings and biasing the heaters on the pre amp tubes to plus 120 Volt. As I was getting the heaters sorted out I found another problem that would have been evident sooner or later. Only one of the heater sections in one of the 12AX7 tubes was lighting up. Half the first preamp tube was dead. I also replaced the selenium rectifier in the bias supply and stabilised it to 15 Volt with a Zener diode.

With all that HT available as soon as the power is turned on because of the use of a solid state rectifier, I decided to put in some delay circuit to hold the HT off until the heaters are up to temperature. I looked at all sorts of fancy ways of doing this and just when I was going to use a 555 timer and a relay I found some interesting delay tubes on Ebay and got them for almost nothing. I was actually after the hearing aid tubes that were with them but when the parcel arrived I soon realised I had the solution to another problem. In the box of tubes were also twelve XT120A and twelve XT30A devices from Miniatron, a division of Thompson-CSF. Web searches for data on them did not turn up anything but it did turn up data an just about all the other tubes made in France by Miniatron. I soon worked out what they were and a bit of trial and error soon confirmed what I could do with them. If anyone has the actual data on these I would appreciate a copy some time. Other part numbers written on them are F9035A and F9030A.

Thanks to John from Pacific TV for details of the Fleetwood decoder. Check out Johns web site for what he has to sell and his good collection of free service data.

John confirmed the extra lead from the decoder was in fact for a neon stereo indicator so I pulled the front dial assembly apart and drilled a hole for a little neon to shine through next to the FM section.

Download the W50 manual
Download the W50 circuit

If you are interested in the history of FM broadcasting and of Community Radio in Australia here is a very good paper from Radio Adelaide (101.5 FM) and the University of Adelaide. Just in case you are wondering, NESB stands for Non English Speaking Backgound.

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Written by and Copyright, Phil. Storr © Last updated 21st December 2012