Hi Trevor, it was generally up-market sets that had the variable selectivity facility although I can't think of any EMI models that had it.
The HMV 800 deluxe radiogram has variable selectivity, Till, again using cams to vary coupling.
The pictures you've taken make it clear how your set works - I assume that there is a spring which keeps the "followers" in contact with the two cams (eccentrics).
Hi Trevor, it was generally up-market sets that had the variable selectivity facility although I can't think of any EMI models that had it.
The HMV 800 deluxe radiogram has variable selectivity, Till, again using cams to vary coupling.
The pictures you've taken make it clear how your set works - I assume that there is a spring which keeps the "followers" in contact with the two cams (eccentrics).
Hi Brian,
The followers are indeed spring loaded. The springs are inside the transformers.
It begs the question. Was the radio chassis a special unit constructed for the BT8161 or was it a modified production radio chassis? The Super six radio is on Trader sheet 282.
The HMV 800 was loaded with all sorts of features. The radio chassis in my 801 is nowhere so specced up.
Till Eulenspiegel.
The HMV 800 selectivity control cams. The postwar Dynatron Ether Conqueror also has a selectivity control, operated by wafer switches.
Keith
One picture says a thousand words there Keith. Presumably each wafer switch has an adjustable cam on the main shaft. I bet it would be fun setting those up.
Eddie
There are no Wafer switches in the 800 radiogram, Eddie. Perhaps this is a terminology confusion. The switches illustrated in Keith's picture are not wafer switches but leaf switches, rather like a relay. They are straight leaves moved by cams towards or away each other to perform the switch functions. Wafer switches rely on a central rotor operated by a shaft. Rotors, which are controlled by the shaft connect contacts, mounted on a wafer, to each other to complete the required circuits.
The variable bandwidth function in the HMV 800 is achieved by switching resistors across IF transformer windings, varying the coupling between IF windings (mechanically) and varying the AF feedback in the audio stages.
In the GEC 8161, the bandwidth control is limited to the last two, i.e. mechanical movement in the IF cans and AF bandwidth control. The latter is very easy to implement, but the former a trifle tricky!!
Sorry Eddie and Brian for any confusion, I must have worded it badly. The wafer switches I was referring to are in the Dynatron Ether Conqueror (see pic).
Keith
Oh I knew they were leaf switches Brian, just like the ones in my jukebox project recently but I am always happy to adapt to other peoples terminology which I assumed was the case here. 
Eddie
Sorry Eddie, grandmothers and eggs etc.!
There is a GEC Super 6 on ebay so I have set a bid for it. At least the magic eye mounting looks the same.
I have also sourced a 12" electrostatic CRT (VCR58) as a stopgap until a 15" comes available - probably never, but at least I can hope.
I will post a wanted list in the appropriate section for the valves required and I will require some bases, almost all B7, B5 and B4 - I have some but not enough. Till is supplying me with info and I have cleaned up the circuit scans and will work on the others. Transformers will be a problem but hopefully, this is where Mike B and Ed D can help. The large mains transformer has 7 heater windings plus HT!
I will also cut and veneer a couple of boards for the control panels - the main cabinet is figured walnut, the top being a bit damaged but nothing that a good scrape will not sort out. It will be nice to see it with the two control "escutcheons" in place.
Hi Brian,
Two pictures of the other GEC BT9121. The timebase module is placed to the right of the CRT rear support. The GT1B thyratrons are inside the screening cans.
Till Eulenspiegel.
A bit of progress in the last couple of days! I have, at last, located the owner of the "other" GEC set. It turns out that it is a BT9122 which is the smaller (12") brother/sister of the BT8161 (15"). He has agreed for me to visit him to see the set and take pictures, measurements and the like - that's a real step forward.
I missed the GEC radio that was on ebay, but in a way, it means that I can see the original before making a commitment to buy something which could turn out to be a blind alley.
The next installment:
I have spoken with the 9122 owner who is very pleased to help the project along! One very important thing - he has a full manual which covers both the 9122 and the 8161. Evidently, they are identical except for the central CRT mounting (12" or 16"). The manual includes full chassis layout drawings and component values etc, which were missing from the data to hand at the moment. Hopefully, when I visit him, I will be able to scan the docs - he is not willing to trust the vagaries of the postal system, having had some bad experiences.
Meanwhile, I decided to give the cabinet a bit of a polish and take some pictures. Here are a few of them:
(Chris - if you think these use too much server space, please delete some of them)
Great news Brian and excellent photos. The cabinet looks to be in very good condition. It reminds me a bit of a Bush SW43 receiver in style. I wonder what the intention was of the person who removed the guts, they must have had a purpose in mind otherwise they would have scrapped the cabinet. Shame they dumped the fascia for the radio and control panel.
Best get yourself a 'swansdown duster' 
.
Keith
Oh ... Err! 
Fab!
The speaker cloth is modern(ish) Tygan so not original - I will see what the original looked like when I visit the BT9122! There is also a hole for a 12" speaker cut in the baffle board (as well as the original 8" hole!) so I guess it was converted into a hi-fi cabinet, fortunately with not too much hacking around!
Looking at the photo gives me the impression that the baffle is not original as the cut outs can be seen from the front and are clearly offset.
I would expect to see a 12 or even a 15 inch speaker as original.
A thing of beauty
I could envisage polishing that cabinet for hours....... Ah, bliss 
I'm inclined to agree with Mr Refugee, and if unless my eyes deceive me, the bigger, more central hole does not appear truly round 
Marion
Perhaps it was not very clear, sorry, but I said in my previous post that there was a 12" cut in the baffle board - I meant cut by person or persons unknown. The hole has been either enlarged or has been cut out for the conversion job.
Looking at the three speaker cloth sections i would expect the original speaker to output some of its sound through all three parts.
This would place the speaker in the center and with a diameter large enough to allow for this.
If the larger circle is 12 inch that would make it 15 inch.
The baffle would be a replacement due to the original cut out being too large for the intended replacement speaker.
I can guarantee that the baffle is not a replacement. It has had an extra hole cut in it. The smaller hole to the bottom left hand corner is also original. The large hole placed more centrally has been crudely cut and not that many years ago. The edges are still very clean (but rough).
We will have to wait and see if the TV chassis stops a central large speaker being fitted.
8 inch sounds a bit small for a set that size.
