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Leak Stereo 20

Discussion in 'classic' started by Tony L, Jan 31, 2012.

  1. Patrick Dixon

    Patrick Dixon Imagineer

    Looks like you have made a very nice job.
     
  2. Tony L

    Tony L Administrator

    Looks good. Is the little switch a suitable rating? As mentioned a long way back up-thread mine came with one that looked like that which actually turned out to be the cause of all my issues - it was underrated for the power and had burnt inside causing a bad and noisy connection. I just took it out and turn the amp on and off at the mains. I also found I needed a spark cap otherwise I got quite a click through the speakers on power-off, again mentioned up-thread. Since that I've done nothing to mine bar stick a load of nice Mullards in it, it really is a superb sounding amp IMO. My next move will likely be to land a TVC preamp of some kind, I really think this is a good enough amp to justify a Music First, Townsend or whatever upstream.
     
  3. snowman_al

    snowman_al pfm Member

    Well my Russian caps arrived from Tubestore yesterday... (Excellent service again from them)
    and I had them installed soon after. Put a know good set of valves in, or so I thought, and anticipated the first switch on.

    Click - nothing. no power to the transformer? Strange, I tested everything during the re-build. I had only fractured the neutral wire putting the sleeving on. Soon sorted.

    Click (again), powers up this time, volts all check well, a quick prod of the input sockets and I get a nice hum from the left channel and silence from the right? The input ECC83 was faulty. Found another one, an original that came with it, and stereo returns. Put the Squeezebox on and we had music!

    I ran it for 4 hours last night and it is sounding really good. No hum or hiss, no fizz or pops. It has just played SOTS right through this morning - so pictures later then...
     
  4. snowman_al

    snowman_al pfm Member

    Pictures from yesterday.

    Completed under chassis re-build.

    [​IMG]

    Initial tests with, eventually, all good valves! (The EL84s are Philips USA JAN 6BQ5s.)

    [​IMG]
    [​IMG]

    And a bit of tube rolling now with All Mullard valves. I was lucky as it came to me with 4 Mullard EL84s. One pair is near new Mullard Holland and the older pair are Blackburn and measure very well still, Nice?

    [​IMG]

    Tony, yes I read about your problems with mains switching, mine is a 250 volt ac DPDT rated at 3 amps and I have paralleled both poles so should be good for a while ;-).

    And more here: https://goo.gl/photos/KHU1PRgNQEpAXSm69
     
  5. Dowser

    Dowser Learning to bodge again..

    Nice one - are you happy with the sound?
     
  6. lowerexley

    lowerexley Member

    Impressive work, love the attention to straight lines, you get the bug to make sure all components are in alignment! I did one 2 years ago and used all metal film caps and some low esr coupling caps but polyprops. It sounded great and I tested it against a Naim NAP 140. Was just wondering, if I may which POI? caps you used and did you use carbon resistors, some look like metal film. I had to sell mine as it was cosmetically not too good but I have another Gold one to do and this one I will definitely be keeping! Unfortunally, for originality I don't have the obligatory 100 ohm dropper but all original mullards throughout.
    Nice work anyway. I think this amp is a definite must have! Ran mine from a NAC 42.5 btw. Cheers
    Dave
     
  7. Tony L

    Tony L Administrator

    How did you find the gain, e.g. was it blowing the windows out with the slightest twitch of the volume knob?

    I'm still not sure where I'm going with mine yet preamp-wise. Very tempted to try a TVC if a nice one turns up second hand. My Audio Synthesis stepped attenuator passive sounds great, but trying my Verdier tube pre does show there is a bit more weight and scale to be had which I'd rather like to unlock given I'm running it with little infinite baffle speakers (149s).
     
  8. snowman_al

    snowman_al pfm Member

    Dowser: '... are you happy with the sound?', Richard I'm delighted.
    Straight out of the box it sounds smooth and natural. Plenty of bass control too (Dynaco A25s speakers here).
    It is the first Leak valve amp I have had that is silent. Including previous Stereo 20s. So, better than my previous TL12+s that always had the very slightest fizz with your ear to the tweeters. (EF86s for you I expect?)
    It also does spoken voices really well, Radio 4 is a pleasure.

    lowerexley: Thanks Dave, I'm a bit OCD with getting things to look right as well as sound right. Coupling caps are Russian K40Y-9 types. I used the 400 volt ones. (There are green K42Y2 types and pink ones K??? too, but general consensus is the silver K40Y ones are better in a Leak amp.)
    I have used various poly caps in previous Leak rebuilds (admittedly a while ago now) and always gone back to paper or PIO types. By the way I have other valve amps where I prefer the polys to oil caps...

    Here is a TL12+ from the 1990s, where I used NOS ITT paper caps:

    [​IMG]

    Yes, I used metal films or wire-wounds. I think metals are a must for anode resistors and power applications. And as I no longer have a good range of carbon resistors, used metals as they were to hand. I personally do not find them overly 'bright' in presentation and any difference in sound is subtle at least. And I do think metals are more reliable long term.
    I did leave as many original carbons that were still in spec' as I could.

    I've previously tried Naim pre-amps (32 and 42s) with Leaks with no problem, yes a little restricted in volume control rotation. I have also tried my B4 and even built a couple of cathode followers, but overall I much prefer a simple passive. There's just a cheapo Malaysian 100K switched attenuator in the black box in the pics.

    Currently trying a very old set of 6P14Ps and I think I like them even more than the Mullards...
     
  9. lowerexley

    lowerexley Member

    The gain is a trifle sensitive and I think on the last one I stuck a double wound Alps 100K pot in the phono line to trim it down but not really the ideal solution! I think Ill use those PIO caps this time and some carbons as well. I also ran it with Celestion SL600's and it seemed to cope well with their rather poor compliance, didn't overheat and was a good burn in exercise. I needed the Naim for my MC and it was fairly cheap for the sound quality it provides. Nice amp above btw! Nice job. Dave
     
  10. lowerexley

    lowerexley Member

    Sorry " I think on the last one I stuck a double wound Alps 100K pot in the phono line" was actually meant with reference to using my laptop as the source signal. Just a pot in a box! Cheers
     
  11. russel

    russel ./_dazed_and_confused

    Nice job, how did you do the decals?
     
  12. paulfromcamden

    paulfromcamden Baffled

    Also interested in this!
     
  13. snowman_al

    snowman_al pfm Member

    There are no decals...
    I'm afraid it is all Leak original paint and screen printing.

    I was very careful not to use any abrasives, I've seen quite a few Leak amps that have lost their shine, and lettering, after being polished.

    I used car wash products to get it clean and then gave it 3 coats of 'Quick Shine Wax' with a micro fibre duster to restore the lustre.
    I've just checked and it is still 'slippy' to the touch after a couple of weeks use and handling. And now it is clean it is easy to re-apply a little wax in future.
     
  14. Raoul Duke

    Raoul Duke Active Member

    Beautiful work there snowman al! Mine is an older unit from 1959 with the 3921 output transformers. I updated the circuit to the later version which I think is easier on the valves. My mains transformer is the original 3920. Also interested in the 270R resistor and 300pF cap across R5L and R5R. Hadnt come across this before but looking at my TL12+ amps from a few years later, they all seem to have this "mod" Anybody able to shed any light?

    heres mine :)

    [​IMG]DSC_1360 by Bregtje Cals, on Flickr
     
  15. Tony L

    Tony L Administrator

    Cool! Between us we have all three variants as mine is a '61 intermediate model with the old mains and later output transformers. As far as I can ascertain mine was originally built with the later circuit values and I kept to that with the rebuild. Its manual has the original values in the diagram though which was quite confusing!
     
  16. snowman_al

    snowman_al pfm Member

    Raoul,
    The values across R5 are 4.7k and 300pf. I understand they offer an extra margin of stability at high frequencies. (Above audibility at 30k+ hz if memory serves.)
    Ideally they should not be needed, but most designs from the late 40's on, Williamson, Mullard 5-10 and 20, even Radford have them in various forms and values.
    Quote from the Mullard design book, ''A CR network shunting the anode load (Leak's R5) produces an advance in phase which increases the stability of the amplifier at high frequencies''.

    Yes they are fitted to TL10s, TL12+s etc. as standard (not a mod) maybe because the use a pentode first stage? Not sure why they only appear on Stereo 20s at a late stage, possibly due to the use of the later 8778 output tx with the 8152 320 volt mains transformer or other part quality changes?

    By the way I notice that pictures on your flikr site that you have strange values for some of the coupling caps. C4 and C6 are nominally 0.22uf (not 0.1uf) and C5 is nominally 0.1uf (not 0.47uf).
     
  17. Raoul Duke

    Raoul Duke Active Member

    snowman yes interesting that Leaks other amps had this CR network as standard. My Stereo 20 and no doubt many others do not seem to suffer without it however. Re the strange component values in my pics.. well spotted! I was experimenting with some different component values on advice of a friend who had made some measurements and is of the opinion that the circuit tended towards LF instability. My changes resulted in a very bright sound however which was not what I was after so have since reverted to all standard values as per circuit diagram. Very much enjoying the Stereo 20 which I switch every now and again for TL/10's and TL12+'s
     
  18. Emporiumhifi

    Emporiumhifi Well-Known Member

    love the stereo 20, a superb amp that punches way above it's weight, there are some tweaks that are worthwhile. Some noise can sometimes be reduced if a skirted socket or grounded skirt is used on the first ECC83, due to its high sensitivity, I have heard but not tried that a grounded skirt (or metal based which does the same thing) on the GZ34 reduces mains hum radiation.
     
  19. Mr.Nic

    Mr.Nic pfm Member

    Acquired a nice ST 20 a little while back, and just bagged a passive pre. Is there any consensus / preference for interconnects between the pre and the Leak?

    (and yes, I know I'm potentially opening up a whole can of worms here)
     
  20. G T Audio

    G T Audio Trade: Distributor and Manufacturer

    I used to modify the ST20 many years ago to reduce the sensitivity to about 800mV meaning it could be used in a conventional system without the sensitivity and noise issues. Sound quality and performance was identical to the original.
     

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