Unknown GEC

[Malcolm G6ANZ]

I've decided to have a change from CCTV and go back to an old radio that I've had for a long time. I did think it was a GEC universal all wave 6. Looking at it now it probably isn't. It has four controls on the front, no mains transformer and a barretter (sp) for the heaters.
It will need a lot of work as the smoothing cap has white growth on it, the mains lead is twisted lighting flex and its thick with dust.
Does anyone recognise the set from the photos? The back panel looks like a hardboard replacement and the chassis label is missing.

Malcolm

[Richardgr]

Looking at the text on the dial ... "Quality control, Brilliant to Mellow" ... is that some sort of tone control?

[HamishBoxer]

I would say so and probably switches different caps in circuit.

[Paul_RK]

If it is a switch - I think the most similar GEC set I've owned had an indicator like this one coupled to the tone control pot. Looks as though there's a matching waveband pointer on the other side of the dial, another feature I remember.

[Malcolm G6ANZ]

The tone control is not switched, it's a normal rotary control. The waveband indicator has the same pointer style.

Malcolm

[Mr 1936]

Hi

Looks very similar to the GEC BC3890. Late 1930's styling.

[McMurdo]

It looks like a BC3865 from 1937, (Universal Super-6) but this has the tone control as a concentric knob combined with the volume on the left, with the tuning on the right with combined wavechange.

I can't find a 4-knob version. Is it original? The two outer knobs look rather odd, mounted on the joints of the cabinet front like that.

[Malcolm G6ANZ]

I don't think its a BC3890. Mine has four knobs on the front and no mains transformer/choke. Where the 3890 has the transformer mine has the barretter. Although the styling is the same.

[Malcolm G6ANZ]

The outer two knobs must be original as they are for the tone and waveband. As such they have drive cords going to the dial indicators.

[Malcolm G6ANZ]

I took the chassis out of the case. If it has been modified from concentric controls then was done very well. I cant see how the controls would have been concentric.
Also there was a big hole roughly hacked in the base under the tone control drive. This was probably done to adjust the string driving the indicator. There have been repairs to it at various times as there are several 'new' capacitors and the output transformer looks to be a replacement.
I'll go with a BC3865 model as it looks the closest and assume it was modified by person or persons unknown.
I find out more when I extract the electrolytics and look et their date stamp.

[Malcolm G6ANZ]

The plot thickens! The chassis now out of the case and it has had a hard life and many repairs. A new output transformer and several replacement capacitors, I won't call them new as they are at least 50 years old. That the length of time I've had the set.
More concerning is the frequency changer. The original was a 7 pin base, this has replaced with an octal base and a different valve. It looks to have been done professionally except for the bank of 330ohm resistors in parallel. The problem is that the markings on the new valve are non-existent. Once I can trace out the connections I may be able to find the valve.
This set is now a real challenge.

Malcolm

[ms660]

X76, the 330 Ohm resistors being heater shunts?

Lawrence.

[McMurdo]

Wonder if it was subject to wartime shortages and some creative work-arounds, to coin a modern phrase?

[Malcolm G6ANZ]

I'll keep adding to this thread rather than start a new one for each small step of progress.
With a bit of detective work I have put a number to the replacement unmarked valve. I knew that it was a triode hexode frequency changer and had an octal base, with the grid on the top cap. I tried the internet for such a valve but found searching quite impossible. So I resorted to my ancient Babani valve data books. Part of the data set are frequency changers. I narrowed down the search to octal base, heaters on pins 2 and 7 a grid to the top cap and a triode hexode. This gave me 12 possible results. Some of these I discounted as the valve shape was wrong. I then went to the valve museum site and searched for a picture of any of the possible valves. After a couple of searches I struck gold. The valve was most similar to an X61M made by Marconi-Osram.
I could now look at the heater chain in more detail.
In the heater chain the two replacement valves were bypassed with resistors. The rectifier was a U30 and replaced with a UY31, and the frequency changer X31 replaced with anX61M. the UY31 bypassed with a 200ohm and the X61 with 4x330 ohm in parallel. I presume that this is to maintain either the current or the volt drop in the heater chain.
The other curious modification that is beyond me is that there was a length of line cord dropper in parallel with the dial lamps. One of these was open. The line cord is about 6 inches long and is open circuit. It seems strange that this would be fitted instead of a new lamp.
The volt drop across the heater chain with the original valve would be 90vand with the replacement valves it adds up to 94v. The rectifier having a lower volt drop and the frequency changer having a higher volt drop. Can someone explain why the shunt resistors are needed as the volt drop along the chain is close to the specified voltage? And why the line dropper across the dial lamps?

Malcolm

[ms660]

The X61 was the first one I looked at but the shunt for its 6.3 volt heater didn't tally up and the valve base didn't have a metal skirt in the example I looked at, the X76 did have a metal skirt and the heater shunt tallied up for its 12.6 volt heater for a 300mA heater chain.

Lawrence.

[Malcolm G6ANZ]

So I got it wrong��. It should be an X76. Not the X61. Oh well I tried

[McMurdo]

the service sheet for the 3865 mentions alternative FC valves:

[Malcolm G6ANZ]

I'm starting to look at the heater chain on the modified GEC set which is probably a 'Universal All Wave Super 6'. In the past two of the valves have been replaced and the heater chain modified with resistors put in parallel with the new valve heaters. I am assuming that the set was working at some point after this modification.
With all the original valves and dial lamps the heater chain volt drop is 90v, the barretter (type 304) drop for 300 mA is 150v so the total volt drop at 300mA is 240. This I understand.
The new frequency changer has a 6.3v 300mA heater (the old one was 13v) and this is bypassed with an 82 ohm. The rectifier is 50v at 100mA (the old one was 26v at 300mA) and this is bypassed with 200ohm. Is this to give the 300mA?
The volt drop with the new valves without the bypass resistors is 107v this leaves 133 volts across the barretter, which is within its range. So why have the parallel resistors? and how were these values calculated?
Just to make life really interesting ,there was a length of line dropper resistance wire across the dial lamps. This is open circuit and now removed . It was asbestos covered so waiting for disposal. Could this be to do with the new valves or a spurious addition. One of the dial lamps was open but I would have thought that replacing the lamp was easier than bodging in a resistance cord.

Malcolm

[MurphyNut]

Someone is selling one almost the same and they call it a BC.3960.

[ms660]

The heater shunt resistors are there to enable 300mA (or there about) to flow through the series heater chain, eg: if say a valve with 13V 300mA heaters was replaced with a valve with 13V 160mA heaters then you need an extra 140mA to flow through its heater shunt resistance....R = V/I which is 13/0.14 which equals approx. 92 Ohms.....the resistance of four 330 Ohm resistors in parallel is 330/4 which equals 82 Ohms.....which isn't 92 Ohms but it's near enough to work ok.

Lawrence.