Unusual Leak TL12 +

[yestertech]

Ive had a number of TL12 + amps over the years.
The very first pair I bought turned out to be 100v line variants, (which I hadn't realised ) but luckily I managed to wire a speaker across the low impedance feedback winding and they worked happily like this for several years before being replaced with a proper set.

I know Leak did some other versions but my latest acquisition is a mystery.
It's definitely a 100v line version chassis wise, in gold and the O/P transformer has no impedance selector.
However, there seems to be no high impedance secondary for the 100v line connection. All secondaries measure less than 1.5 ohms or thereabouts.The O/P transformer has been replaced at some point ( rough soldering ) and the number '3744' is stamped on the top of the tape covering the laminations, while the tape type does not match LEAK's usual matt black
Attached to the base cover is a cutting from an advert (pictured ) which describes a 15 ohm version which presumably was surplus stock for sale in the 60's from one of the electronics magazines, albeit it states that it had a 'crackle finish' chassis which this does not.

No idea about its origins, other than the advert

Any thoughts ??

Andy

[GrimJosef]

The list of the six numbered Leak output transformers in Stephen Spicer's Firsts In High Fidelity has the numbering increasing in chronological order, from 1360 for the TL/10 through to 8778 for the final versions of the TL/12+ and Stereo 20. The earlier commercial TL/12+'s used type 3921.

Obviously 3744 is a lower number than this. I wonder if that transformer might have been some sort of prototype, albeit with enough being made for there to have been adverts for them ?

Cheers,

GJ

[Valvepower]

Hello,

This is long shot, but the Leak TL12+’s was used as part of the modulation amplifier in Marconi transmitters.

The MWT BD228A 50 kW MF transmitter installed at BBC Brookman’s Park in 1961 used the TL12+ as a preamplifier...!! I came upon this when I was helping Mo Foster revise his book 17 Watts (the maximum output of the EL84) and it came to light that the TL12+ was used in these transmitters for Radio 1. This means the output for Radio 1 from Brookman’s park was fed through a pair of EL84’s.

Here is a link to Dave Porter’s cracking article on these transmitters.
https://www.bbceng.info/Technical%20...l_Issue_18.pdf

Regards
Terry

[yestertech]

Thanks Terry
That was one aspect of these amps I had never heard about.
Looking at the circuit on that link, the OPT configuration looks a lot like this one, but of course the impedances are wrong.
I wonder if anyone recognises the 'ad' attached to the base plate ?
I certainly recall seeing a TL12+ in crackle grey finish some years ago, doubtless a 100v line version.

I was prepared to believe that a replacement OPT had been retro-fitted using the original end cheeks and tags given that the black tape is of a different material. However, the white printed '3744' tends to imply it was made by one of Leak's original suppliers. Could be coincidence of course....

Andy

[See_Mos]

Long ago my mate worked for the Beeb and I got a couple of TL12 'modulators' when they dismantled some of the older stations.

I seem to remember that there was also a couple of the bigger ones, possibly TL32?

[Valvepower]

Hello,

I remember a friend who worked at Leak in the 1960's telling me the TL50 was also used a modulator amplifier in a transmitter.

At tangent he also chatted about the TL12.1's used in the PA system at the Firestone Tyre Factory, West London.

Terry

[See_Mos]

Thanks for the reminder Terry, yes they were TL50's