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Leak tl 25 plus output transformers..

C

Chris Cooper

New Member
Hello. I have a pair of leak Tl25 plus amplifiers in the process of renovation. It would seem one is an early model and one late as they have different chassis colours and different output transformer numbers. They have been rebuilt at some time with holco resistors and both have identical component values which tally with the only circuit diagram I can find online. Is this correct considering the op trafos are different. They were working and sounding good (and the same) for many years until one of the gec kt66 went bad. I have repaired the fault and have some el34 to use until I can afford another gec!! I also want to repaint them as they had been painted black, but matching the chassis colours is proving a challenge (paint names or codes for the champagne and bronze shades?) I might also look at reducing the sensitivity while they are stripped, does anyone have a recommendation for 775mv sensitivity? They had a resistive divider on the input, but I would consider reducing the amp gain if there is a cct mod available. Cheers Chris cooper
 
I'm afraid the output transformers are quite different. different anode to anode impedance and different ultralinear tap percentage... They changed 1961.
6L6 is a decent replacement for KT66 BTW but not EL34.
 
Jez is right. The output transformers, as well as some of the circuitry mostly around the front end, will be different. The TL25 Plus used the EL34 pentode valve and that is what should be used in this amplifier.
My advice would be to see if you could find another TL25 that matches one of the others you have and then get the two rebuilt the same. Preferably not with Holco resistors...
 
Chris Cooper said:
Thanks guys I can only find one cct for TL 25 http://www.44bx.com/leak/Leak/Circuits/tl25Plus.gif which the amps match, but don't know if it's early or late.
Click to expand...

That's the same as I have in the Leak book. It's likely there will be some differences in the feedback and in the step network on the anode of the Ef86. There may be other small differences. If you want a proper stereo pair you are going to have to get another one....

Strange that they show EL34, KT66 and 5881 as suitable valves... 5881 is a special quality version of 6L6, which is more common and readily available so it's odd that they don't mention this... Stranger is that EL34 is there! KT66, 6L6 and 5881 are all very similar and can be substituted for one another but the EL34 is a bit different and not usually suitable for use as a replacement to the others without a few small modifications to circuitry... it will work but not optimally, so there is quite some compromise going on there! Either it's "right" with 6L6/5881/KT66 and you can get away with EL34 to get you by... or the other way around.
 
Chris Cooper said:
Or find another late type op transformer... I've a good early one to swap!
Click to expand...

It's possible that mains transformers are also slightly different.... When they changed the OPT's on the Stereo 20 about the same time they reduced the HT voltage a bit as the changes to the ultralinear tap point would mean they could get the specified power for a little less HT and give the output valves a slightly easier life at the expense of slightly higher distortion...

If so it's not quite so bad as they can be roughly equalised by changing the dropper resistor values.
 
I think i have now found an early circuit diagram, and there are indeed a number of differences, so I am on the lookout for an 8615 output transformer.. does anyone have one?? I have a good early 3925 to trade...
 
Chris Cooper said:
I think i have now found an early circuit diagram, and there are indeed a number of differences, so I am on the lookout for an 8615 output transformer.. does anyone have one?? I have a good early 3925 to trade...
Click to expand...

3925 probably the better of the two....

An alternative, depending on how important originality is to you (probably considerably better than originals) and whether you are willing to spend probably £500 on a pair of transformers is http://www.sowter.co.uk/specs/ua23.htm
 
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