Amplifier identification needed.

[Heatercathodeshort]

A couple of months ago a mate of mine pulled this off a skip. He tells me there was a lot of similar equipment half buried under muck and rubble.

At first I thought it was an old Philips tape recorder but on lifting the lid I discovered this amplifier. The valves are 2 X KT88 1 X ECC81 1X EF86 and a GZ34. There appears to be an octal socket for a tone control pre amp?

It appears to be well built but I do not recognize it's maker. Any ideas? John.

[lesmw0sec]

Looks a bit home made to me.

[RojDW48]

There's some serious 'transforming' going on there! Maybe some version of the Williamson with the PS incorporated in the single chassis? Should sound very sweet.

[Edward Huggins]

This has a classic valve line up for a c.30 watt Monoblock. I would guess it has an input sensitivity of around 250mV. It certainly looks like a homebrew. Do you have any plans for this? ideal as a good mono unit, maybe with just passive controls - but replicating this for stereo could be very expensive...

[Heatercathodeshort]

Thanks for your interest.
I have removed the amp from the box and it looks a lot better. The transformers appear to be of Gardner manufacture. The amp is very heavy.
The date on the main smoothing blocks is November 1958.

Not sure about home construction but possible. The valves have transfers on the chassis giving the output pair as 6550's. The valves fitted are Osram KT88's.

It looks incredibly easy to work on. I'll put it aside until I have some time and give it a full check over. The transformer windings are continuous. All the components appear to be mounted on a single tag board.
I have no interest in Stereo so this should make a good project. Thanks again, John

[Refugee]

That is a nice amp. It looks like a cinema monoblock.
I have a stereo based on a pair of Paramount ones. They go very loud.

[thermionic]

Hi John.

The layout of the tag board is slightly reminiscent of a Leak layout. I'm guessing this could be a home brew with more than a 'nod' to Leak. I bet it's very similar to the TL50, although Leak didn't use Gardner transformers.

It makes me wonder what else was in that skip......any chance your mate could have another look before it's carted away?

I'm sure you will soon have this up & running!


SimonT.

[dseymo1]

I think there's little doubt that it's home-made, given the unused mains and input connector positions, and the hand-written legend for the fuse. Some nice bits to it, though!

[thermionic]

Here's an underside view of a Leak TL50 amplifier I've just grabbed from the net. It has obviously had components replaced, but you can see the similarities.......

SimonT.

[yestertech]

It's a Leak amp for sure ! Heavily customised by the look of it.
They made 100V line versions of the TL50, usually with black terminal blocks at the rear for input and output and three miniature pots at the front, marked T B and V. The remains of these and the engraving can be seen.
Also, the component board is identical to the TL50.

I'd wager the power supplies are more robust than the original, probably needing to be left switched on 24/7 !


Andy

EDIT : I think looking at the pics again, someone has crudely cut away the rear half of the original TL/50 and fitted a new plate with the transformers and caps on - the cutting lines look a bit rough

[Heatercathodeshort]

Many thanks for your very useful information.

I think I have solved the mystery. While breaking up the rubbish Philips tape recorder cabinet it was housed in I discovered this circuit tucked under the lid.

It appears to be a slightly modified BBC 'Loudspeaker Amplifier LSM/10.

The balanced input transformer has been removed leaving the input connections across the 'gain' control R1 250K.

The mains input connector has been modified to accept a standard Bulgin socket.

The mounting holes for the input transformer can be seen in the pictures together with the space for the original mains connector.

The top plate has certainly been replaced but it appears to be a professional job as care has been taken to fit it exactly to the chassis with countersunk 6BA nuts and bolts, finished with grey crackle paint.

The original valves in the circuit are KT66's but the pair fitted are KT88's. This may have something to do with it. Thanks once again, John.

[Heatercathodeshort]

Another couple of pictures showing the 'sub plate' more clearly. It is sprayed in exactly the same colour as the main chassis.

Unfortunately he just casually brought it from the boot of his car having recovered it some months before! Oh well, lucky to have just this I suppose. J.

[barretter]

Originally Posted by Heatercathodeshort

Many thanks for your very useful information.

I think I have solved the mystery. While breaking up the rubbish Philips tape recorder cabinet it was housed in I discovered this circuit tucked under the lid.

It appears to be a slightly modified BBC 'Loudspeaker Amplifier LSM/10.

The amp may be based on the BBC LSM/10 (or the similar Leak circuit) but it is not an actual first version LSM/10 as manufactured by Sound Sales for the Beeb or the later version (ultra-linear withot choke).

[Heatercathodeshort]

Thanks, The mystery deepens.

Simon's under chassis view appears to be almost identical even down to the EGEN gain pot. John.

[barretter]

I see that somebody has written "EF86" against the input valve on the LSM/10 circuit diagram but, in fact, the CV2135 is a Brimar valve, the 6BR7, which whilst electronically very similar to the EF86 has a completely different pin-out (apart from the heaters) so don't try to rewire your amp according to that diagram. I expect it's correctly wired at the moment.

[Heatercathodeshort]

Thanks for that. The base connections certainly appear correct on the diagram for a 6BR7.
It's a project for the winter nights but as others have said, I bet it sounds good! Regards, John.

[Heatercathodeshort]

Just an update. Thanks again for your interest and help with this heavy amp!

I have replaced the leaky oil filled caps and generally tidied up the circuit board.
It now operates very well with what appears to be a limitless output.

I have two problems, first a very high hum level developed in the first stage EF86. I have tried all the usual causes but surprisingly it is difficult to trace. The scope should sort this out.

Second is more serious. The anodes of the KT88's are glowing a very dull red after approximately 15 mins use. The grid voltages are zero [new coupling caps] and the cathode bias capacitors have been replaced. The heavy duty bias resistors are of their stated values.[560 ohms 10 w] This needs more time and a check of the actual values employed and the cathode currents of the two output valves. I will keep you informed. John.

[GrimJosef]

Could it be that the bias resistors were correct for the original KT66s but are wrong for KT88s ? If you know the HT voltage a quick check of the data sheets would resolve that.

Cheers,

GJ

[Heatercathodeshort]

Thanks for backing up my suspicion here GJ. I will check up this weekend. John.