Yikes! There's some gold in there.

Marcc. yep the HTs and CT and cap were disconnected when measuring and the cap polarity is correct. I have another 123, it's windings were hanging out and I glued them back with varnish and a wee brush, it's never had a problem. This one looks good and the insulating strips look good but it has definitely found ground somewhere. I tried to separate the two halves of the core out of interest but they won't budge.

Ian, I knew you had an angle, using a bridge on the full secondary is brilliant. That's going in the memory banks. I'm interested in mixing diodes with the existing rectifier valve, is there issues with that seeing one starts conducting immediately and the other has a heating delay. I assume the diodes go from each HT to GND with the diodes pointing to the HT, I'll have to nut it out. The addition of a thermal fuse is a good idea, I'll check that out too, Thanks.

Hi Tippy, Ian is correct, for example I used diodes hidden in a dud valve base for the HT supply, in my Reinartz receiever.
Have a look in special projects to see how I did it. Doing that let me wind a tranny without the 5v heater winding and I was glad of the space as the winding was very tight.
Fred.

Re the mix of solid state and thermionic in a bridge, no DC volts get out until the cathode heats up.

The "soft" nature of the valve still applies, a silicon diode is in series with the valve on each half cycle. The behaviour of the circuit is that of a full-wave centre tapped rectifier with 2X the AC voltage. In other words, 230V AC into the bridge gives the same result as a 230 - 0 - 230 CT transformer winding without the diodes.

Your connection assumption is correct.

Re the thermal fuse, the fibreglass sleeving is for insurance. If you can locate the (metal bodied) fuse safely against the transformer winding without the sleeving you don't strictly need it.

Marcc is correct, you should heat sink the fuse's lead wires with a pair of long-nose pliers when soldering. When you see these used in appliances the wires are normally crimped on.

I normally use a 77 degree C device, if an old tranny gets this hot under the chassis there is definitely something wrong!

https://www.jaycar.com.au/77-c-thermal-fuse/p/ST3800

There is a significant difference between a heater and a filament rectifier and a filament rectifier with a cathode sleeve.

Filament rectifiers produce "B" voltage before the heater tubes have heaters hot enough to cause conduction so do diodes & metal type rectifiers.. Heater & cathode sleeved rectifiers are slow developers of voltage.

What that translates into is a massive voltage surge, close to twice the running voltage with fast devices like diodes. This will require all caps on the B rail to be upgraded to 500V, or more.

Yep, thanks Ian, got it. I mud maped it and can see how it functions but I suspect with the Philips 123 in question I should connect the diodes to where the centre tap used to connect to achieve the required negative bias, that way it goes through the bias resistors and cap to ground, is that right?

Sorry to pick your brains but it's so good to get all this information from people who know.

Hi Marcc, thanks for your input too. In the case in question using the rectifier valve in conjunction with the diodes as Ian says it will not start conducting until the valve starts conducting. Even if I use a full bridge it should be ok, all the caps are rated at 600V.

The only reason I was toying with retaining the valve was so the heater would glow but I guess I could still run the heater in the valve without using it in the circuit???

I'm off to Jaycar tomorrow so will pick up some thermal fuses, thanks for the tip.

Fred, going to check your solid state rectifier shortly, thanks.

Cheers

I would tend to wait until you get a transformer & see if you can get one with a CT. Where there is back bias the negative goes where the CT goes if you use a bridge. Be aware that the higher voltage cap still applies & there may be a need to put more resistance into B+ to cut back the increased voltage, with the more efficient rectifier.

Reducing the capacity (even eliminating) the first filter cap will also lower voltage.

Thanks Marcc, as I thought and as you say, I need to check what difference and any adjustments that are required with the non standard transformer and rectifier fitted, Thanks!

I hope to get a replacement tranny in the next few days but this information is invaluable to me and my steep learning curve. If the new tranny doesn't work or doesn't fit I have something to fall back on plus it makes me think and understand what's happening.

Many thanks to you and all who all that have helped me. Tippy

Yes Marcc is correct, the junction of the anodes or the two diodes connects to wherever the CT was before.

There is no issue here with excessive voltage due to the more efficient rectifier (as there would be with an all-silicon bridge). All the silicon diodes are doing is making a centre tap. The thermionic diodes continue to work as before, complete with their soft start, soft knee and voltage drop. As Marcc implies there is a difference between indirectly and directly heated rectifier cathodes in terms of warmup time but that would be unchanged from the original design.

If you model the two circuits using, say, LTSpice you will see what I mean.

I will download a copy of LTSpice, thanks for the tip.

I worked out the diode/rectifier valve combination with a pencil and paper and can see it will act as a centre tap and the 6V5 valve would ensure the warm up period was normal. I have a replacement 'almost original' transformer in the set now, not tested yet but should be ok.

The info you guys have proved will be a great help in the future though. Thank you for your time and knowledge and prolonging the restoration of vintage radios.

There is always a lot less stress, if you can get the right bits to get it to run, without having to inflict modifications on it. Often modifications translate into, or become ramifications when it all goes awry.

Had an STC chassis 59 that had been hacked. Neither I, or HRSA could find a circuit for it, & bits were missing & the circuit had been changed to put the volume in the Grid of the OP tube. I kept the mess of ceramic resistors used in a futile attempt to get the bias right: Never happened. It was supposed to be on cathodes, up front. That is ramifications.

Fortunately I found out the value of the missing pot. & in the process of putting it back to era, actually found two factory faults that ended up causing modifications one lead dress & the other controlled by altering bias. Again ramifications as that caused extra work fixing up the problems inherent, & caused by mods.

This was also missing the Local / DX switch more issues.

There is always a lot less stress, if you can get the right bits to get it to run, without having to inflict modifications on it. Often modifications translate into, or become ramifications when it all goes awry.

Had an STC chassis 59 that had been hacked. Neither I, or HRSA could find a circuit for it, & bits were missing & the circuit had been changed to put the volume in the Grid of the OP tube. I kept the mess of ceramic resistors used in a futile attempt to get the bias right: Never happened. It was supposed to be on cathodes, up front. That is ramifications.

Fortunately I found out the value of the missing pot. & in the process of putting it back to era, actually found two factory faults that ended up causing modifications one lead dress & the other controlled by altering bias. Again ramifications as that caused extra work fixing up the problems inherent, & caused by mods.

This was also missing the Local / DX switch more issues.