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Leak TL/12 Plus / Point One Plus

Discussion in 'classic' started by Tony L, Dec 15, 2017.

  1. Woodface

    Woodface pfm Member

    I have no real interest in owning vintage valve gear but I love watching thee kind of threads. Be interesting to hear how it compares to the stereo 20.

    Excellent work.
  2. Tony L

    Tony L Administrator


    I decided to keep going and just not fit the big K40Y-9s up by the EL84 bases until I have the new smaller resistors fitted. There are still a couple of gaps as I’m waiting on stuff, and I haven’t fitted the 32/32uF can caps yet (I’ll do that last). I made a process error and totally forgot about the 3M3 resistors so had to desolder that area to fit them on the ends. Rather pleased with my resistor bending there, it looks far neater than I managed with the S20. My secret weapon, found after watching hours of far more skilled computer and audio restoration folk at work, is a pair of surgical forceps, the things that look like a cross between a pair of tweezers and a pair of scissors. They are exactly the tool I was missing! Perfect for holding stuff, perfect for bending stuff.

    PS I’d really appreciate it if any of the folks ‘who know’ here could debug the above. If there is anything obviously wrong please let me know before I blow anything up! As I’ve likely got a bit of a wait for the other bits to land I’m actually very tempted to build the other one to this point. It makes sense as to be honest if either amp doesn’t work or blows up I’d not know if it was from my work as I can’t read resistor codes! I’m very confident that what is in the amp is what is on my plan though, and I’ve just copied the cap polarity so that should be ok.
  3. snowman_al

    snowman_al pfm Member

    That is very tidy Tony, you should be pleased,
    As far as you can tell it is correct. The electrolytic caps are the right way round (critical), but it is difficult to see the resistor colours due to the orange cast.
    Tony L likes this.
  4. Tony L

    Tony L Administrator

    Thank you. Assuming my lists in #119 is correct I’m pretty confident I’ve stuck the resistors all in the right place! Thankfully the bags were all labelled with the values, so I tested them with the meter a bag at a time and wrote the Leak position on the bag. I never had more than one bag open, so no way they could get mixed up! That done it’s just like building Lego really and hopefully colourblind-proof.

    The areas I’m a little concerned about is the Voltage/Wattage as I couldn’t find any bill of materials that specifies that. I suspect I’m well over in most cases. The 47uF caps are 100V, the 8 & 16uF 450V, the 32/32uF 500V and the K40-Y9s 400V and a crazy 630V for the .22uF (all I could find when building my S20).

    The resistors are all 1W aside from the ones on the valve bases which are 0.5W and the Mills which are 5W.

    I don’t understand the implications here, e.g. by using over-spec components am I more likely to take the transformers out if there is a fault condition at any time?

    PS I don’t understand my colour sense! Stuff looks fine to me, I can tell the difference between all colours in daylight, but for some reason I just can not read resistor colour codes. In that specific situation I can’t tell the difference between red, orange, brown etc. I don’t get it, though I do have a multimeter so I guess I don’t need to.
  5. Tony L

    Tony L Administrator


    I’ve now got amp #2 to the same place. The Philips 1uF caps turned up, I bought ten of them as they were real cheap and picked the two closest to 1uF on the meter. They all measured between 1.1uF and 1.2uF on the Fluke 87V so hopefully they are ok, though I’ve no way of checking leakage, ESR or anything. I still don’t understand how to read a schematic properly, but they look to be around the EF86 (input) area of the circuit, so I assume less than perfect ESR will result in hum rather than explosion/fire?

    PS Now waiting on the tiny K40Y-9 (C1) to land from Ukraine and the smaller sized resistors I screwed up ordering for the EL84 bases and then it is time to see if they explode or not!
  6. S-Man

    S-Man Kinkless Tetrode Admirer

    I think the old carbon resistors tend to drift due to long term high voltage use.
    If the 3M3s are for grid stoppers, you should be OK in this respect.
    Tony L likes this.
  7. Tony L

    Tony L Administrator

    I don’t know what they do, but they do measure 3M3 in circuit with the valves removed so I can keep an eye on them easy enough.

    PS I thought the ‘grid stoppers’ were the 470k (R16 &17), but I’m at the Lego kit building level here. I do not understand how the circuit works beyond real broad-brush terms (mains rectification/PSU then conceptually input, phase-splitter, output valves, output transformer etc). I do want to understand more though, e.g. what exactly ‘ultra linear’ is, how negative feedback is used within this amp etc. I’m basically at the level of an assembly worker; I can follow instructions and processes easily and neatly enough, but I lack the education, training or knowledge beyond that.
  8. Barrymagrec

    Barrymagrec pfm Member

    They are grid leaks for the phase splitter so not under much strain.

    R16 and 17 470k are the gridleaks for the output valves and the gridstoppers are R18 and 21 10k.
    Tony L likes this.
  9. Tony L

    Tony L Administrator

    Can someone explain to me what C15 (the flat one under R5) is actually for? I notice my bronze Stereo 20 doesn’t have it, though my stock/unused later grey has. My bronze Stereo 20 had been extensively worked on by others before I got it so I have no idea whether it was there originally, or whether I should add them. The amp works absolutely fine without it, but I’d like to understand what is going on here.
  10. Barrymagrec

    Barrymagrec pfm Member

    It rolls off the stage gain at high frequencies - it affects stability so may differently arranged for different OP transformers.
    Tony L likes this.
  11. Darmok

    Darmok "Didactic Prophylaxes"

    You could start with reading and understanding, "Conventional Current.""Conventional+Current."&rlz=1C1VDKB_en-GBGB938GB938&oq="Conventional+Current."&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

    Read, "K Ibrahim" student text book containing the C&Gs Electronic Servicing 2240 course Pt1 & 2, it helped me too no end during my "Training for Work" during 1993-1998 college years, not to difficult to follow for me that is...


    One on ebay 1st edition for a fiver + free post...

    Hope this is of assistance?
    torstoi likes this.
  12. Tony L

    Tony L Administrator

    Thanks for that, bought, got to be worth a fiver!
    Darmok likes this.
  13. Darmok

    Darmok "Didactic Prophylaxes"

    Me too, my other copy is in storage along with all my vintage gear and vinyl collection. :(

    A little bit more on the Leak TL series of vintage valve amps.

    TL stands for "Triple Loop" feedback.
    Tony L likes this.
  14. Tony L

    Tony L Administrator

    I need to go and study a few more early-60s Stereo 20s! My bronze one is 1961 and has the same (later spec) output transformers as the 1965 grey one. I strongly suspect it never had the capacitors as whilst it had been (quite strangely) recapped when I bought it the resistors appeared stock throughout (and were way, way out of spec!). Pic of how it looked here, and how it is now here. It had the ‘new’ resistor values fitted (e.g. 470k rather than 1M) even though its manual/schematic has the old values.

    The thing I find puzzling is the timeline as my bronze Stereo 20 is obviously a fair bit later than the TL12 Plus, both in colour and in output transformers, yet lacks that cap. Then the far later grey Stereo 20 has it again (pic here)! It also has two resistors chained in that position as well as the cap, I assume to make up the value (82K I guess).

    This sort of archeology is fascinating but also frustrating in how little actual information seems to survive. As an example I can’t find precise dates for the chassis colour changes. As I understand it the TL12 Plus came out in about 1956, the Stereo 20 in 1958 along with the stereo record itself. There were some ‘champagne’ coloured Stereo 20s with black print like my TL12 Plus, but they are rare as hens teeth. Virtually never seen, so my guess is the colour change had occurred by ‘59 or 60 as the absolute latest. I think most Stereo 20s are bronze, with a fair few grey ones too. I suspect that colour change occurred about ‘64, but again I don’t know for sure.

    PS I really respect what Jim (Audiomisc) here is trying to do in preserving UK hi-fi history (link), but there is so much that already seems lost to time. Frustrating.
  15. Darmok

    Darmok "Didactic Prophylaxes"

    I have a champagne Stereo 20 with the black ink Leak logo in storage along with a bronze and a charcoal edition.

    They are buried deep with a whole load of my other life possessions locked away in a shipping container.

    I bought the champagne one just to have one of each to compare, I don't recall any useful info on circuit details or components.

    Leak TL25+ underside...


    Have a peek at the underside circuit component tag board of a Leak TL25+.

    See the Green wire wound bias resisters are mounted to stand away from the paxolin material, the WW resistors get very hot underload and scorch the paxolin till it turns to black, it may be useful to follow the technique to preserve your TL12+.
  16. Tony L

    Tony L Administrator

    I think the equivalent in the TL12 Plus are R19 & 20, which are the 270R and in my case 5W Mills mounted a good cm at least above the board. If you look at the pic in #142 you can see how far beneath them the shadow is! The TL25+ looks interesting, I’ve not done much research there. I’ve never heard a pair, or any of the more powerful Leaks. I heard TL12.1s in an unknown system at the WigWam show ages ago and liked them, other than that my Leak experience so far is just the S20.
  17. Woodface

    Woodface pfm Member

    Would these have been sent back to factory for updates or repairs?
  18. Barrymagrec

    Barrymagrec pfm Member

    In the TL12+ each cathode resistor dissipates less than 0.5 watts whereas in the TL25+ they each dissipate over 2 watts each.
  19. Tony L

    Tony L Administrator

    Many certainly were as the fix to the circuit (especially of the Stereo 20) is well known. As I understand it there were two ‘design errors’:

    a) initially the amps ran the then recently introduced EL84 valves right at the edge of their envelope, and only Mullards could take it, other brands just blew up despite being in ‘spec’, this was fixed by changing some resistor values and eventually the output transformer was redesigned.

    b) the Stereo 20 uses pretty much the same mains transformer as the mono TL12 Plus and once it was partnered with an also twice the circuit in the new stereo preamps, exported to Australia or other hot climates and stuck in a cabinet they overheated really badly spewing wax everywhere. The transformer was later changed.

    My understanding is a lot of both amps ended up back at the factory or national distributor as warranty repairs, but after that it is anyone’s guess. These amps are the very definition of ‘Right To Repair’ being easy to work on and coming with a full schematic and layout map in the manual, which is one reason so many are still serviceable 60+ years on!

    One problem is the schematic that shipped with them often doesn’t reflect the actual amp, e.g. my Stereo 20 looks to have been shipped with the later more gentle resistor values though the manual is of the original amp, i.e. almost certainly was ‘wrong’ from new. My suspicion is both my TL12 Plus hadn’t ever been back to Leak as they were still running the more aggressive (1M) resistor values though the each have had some minor work done (a capacitor on one, cathode bias resistors on the other). I’d obviously love to know more of their story, but that is lost to time. I don’t even have birth dates for these, but judging by the supplied valves, some of which may be original, my guess is they are both from 1958 or thereabouts. My bronze Stereo 20 still has it’s sign-off tag so I know for sure that is 1961, and I suspect the grey 20 is 1965 as it had a lovely test as NOS full Blackburn Mullard valve set dated 65. Matching date codes on both the quad of EL84s and the trio of ECC83s.
    lordsummit and Woodface like this.
  20. Woodface

    Woodface pfm Member

    Hopefully they will sound great once done.

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