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Rebuilding a vintage Croft Mega Micro pre-amp

Discussion in 'd.i.y.' started by Dowser, Apr 13, 2021.

  1. Dowser

    Dowser Learning to bodge again..

    Picked this up cheaply from Greece late 2020 after keeping my eyes open for years (since 2006 I've run a mildly modified Croft Micro II in preference to any other pre...and I've tried loads). My original post in Classics is here.

    Glenn always varied his design's frequently, but based mainly on 3 different levels; Micro, Super Micro, Mega Micro. From the little info available on the internet, the Mega was his attempt at state of the art, and retailled at around £1500 in mid 80s I think.

    I bought it with a known issue - one of the line stage transformers had failed, and so B+ was being shared by one transformer. Previous owner had also swapped the original phono stage valves from ECC807s to ECC83s (as Glenn did at some point too).

    Pre-amp
    [​IMG]

    PSU
    [​IMG]

    Since September last year I think I have been happily living with it, it sounds much better than my lowly Micro II.

    After having to pop it open recently to diagnose & fix distortion on one channel via phono stage (broken wire from transformer to stripboard, due to transformer isolation rubbers failing), I decided to dig a bit deeper with a view to bringing it back to full health.

    For the PSU my Micro II used a PSU made up of ECL85, a neon tube reference and a 85A2 regulator - after research I think this was actually the PSU design of the Super version of Glenn's. Certainly when I first bought it (faulty) I sent to Glenn - he initially diagnosed a failing neon tube and suggested swapping to solid stage rectification as cheapest solution, but admitted original rebuilt would sound better, so I went with that.

    The Mega has 3 tubes in the PSU too - but what looks like a 9 pin dual triode instead of the neon tube, and an adjustable pot to set the B+ rail (I assume, haven't tried yet :)).

    No markings on 2 of the 3 tubes, but after a little research I think they are EL86, a 12AX7 (ECC83) and the 85A2 regulator. This is based on this site, which suggests this mod to improve the supply of an ECL85 based Super Micro PSU (http://www.cma4ch.org/chemo/attivita/croftsmic.html - see towards bottom of page). Can anyone confirm? I know practically zilch about valves :)

    For the replacement for the failed transformer, I think I need a secondary of 330v AC for B+ and another secondary of 6.8v AC for the EL86 heater. Again, can anyone confirm? This is based on AC readings of the existing PSU with the pre connected. Interestingly, the transformer that is feeding 2 B+ PSUs only has 325v AC on the secondary....I assume due to additional load or different caps used (not checked the detail of them yet).

    The pre-amp heaters are today provided via a single transformer with a 17.6v AC secondary, rectified to 22.5v DC, and then dropped down by a (I assume 78x series, not checked yet) reglulator for each channel in the pre. I wonder the sound quality benefit of providing a regulated heater per valve in the pre? Seems quite easy to achieve.

    So, any recommendations on a good place to get replacement transformers? I'd buy at least 2, to keep the channels matched, and potentially all 4 if transformer tech has moved on so much in recent years?

    I'd appreciate any other comments on above from those in the know too please - especially if any of my assumptions are wrong! Thanks.

    One last question - assuming it is a 12AX7/ECC83 in the PSU, does the quality of it matter as much as when being used for signal duties?!

    Thanks again, Richard
     
    Darren L, chiily and Old Valve Git like this.
  2. Dowser

    Dowser Learning to bodge again..

    I bought some Ultron ECC82s and fitted earlier this week for line stage duties - not that I was unhappy with the Philips in there before, but it was clear the seller I bought from fitted cheaper valves before selling (phono stage 83s were mismatched). I'd forgotten about tube rolling differences! Much deeper imaging with the Ultrons. No idea who actually made them, and they were cheap (€30 with shipping from Germany). So what is a good ECC82 in the Croft line stage?

    Today I also swapped the output coupling caps - there was a 1uF Wondercap in there before, bypassed with a .034uF German film of some sort. I fitted 1uF mustards for my sins :) Did the same in my Micro II a few years back (shit, just checked, 5 years ago!) and was impressed. They take a while to burn in, but already sounding lovely. Again, has anyone tried other 1uF output caps? Recommendations?

    What was in there before;
    [​IMG]

    Now;
    [​IMG]

    Still looking for a replacement transformer - need to hone my Google skills this weekend :)

    Richard
     
  3. Vinny

    Vinny pfm Member

    Very limited help here - I have a Super Micro IV (if memory serves), and the power supply on that is an 85A2 and ECL85. No neon. 2 x 12BH7 output/line.

    That replaced a Micro A (still here), which has an ECC82 cathode follower and I have tried endless variations on the ECC82 and the only real difference was when using a modern Tun-Sol (I'd have to check to be 100% certain), and that lost all bass but mid's and uppers were sublime.

    Not a believer in "tube rolling" folklore here, not in the two Croft pre's here anyway. That said, there are LOTS of "sworn as true" statements online that PSU valves make a difference - again - not convinced and with 85A2 at £3-5 each, and ECL85 not much more, that is eay to try.
     
  4. martin clark

    martin clark pinko bodger

    34nF/630v Polystyrene is a large value of a very-high-quality capacitor - you 'll have difficulty finding anything-like such these days; do keep them in the bits-box.
     
  5. Dowser

    Dowser Learning to bodge again..

    ah, shit Martin - I unfortunately turned one into a 17 ohm resistor by burning it with the soldering iron :( I originally intended to leave them as bypasses to the Mustards, but shorting one meant I could not... Why are they special, because of the high voltage capability?
     
  6. Dowser

    Dowser Learning to bodge again..

    yeah - the a mega has a 3 tube PSU, still need to investigate it further - all rails use different makes of tubes currently except the EL86 which all look the same.

    first priority is finding a new dual secondary der with the right secondaries.

    But to be clear - it sounds stunning as is :)
     
  7. martin clark

    martin clark pinko bodger

    @Dowser - Polystyrene is right up there with Teflon as lowest DA material for capacitors. As a rolled film/foil style - ESR/ESl is limited only by package size.

    Since they had to be a rolled film/foil combo, these are long out of fashion; availability in such a high voltage rating was/is rare, and because its a rolled film/foil cap, the values available were always small. Up to 10nF, usu; I have a small handful of 100nF caps at c 100v , made by LCR in Wales who might be/have been the last makers of sim. I've a very small stash of 1nF radial 1% 100v PS caps made by Siemens, which are (imo) the DBs for things like I/V conversion rolloff and similar top-qual items in analogue filters.

    Polystyrene caps went away not only because expensive to make - and sources of good-enough-quality PS film was always rare, & now I understand non-existant - but also simply because the melting point (as you found!) is only c 80-83degC and that is utterly incompatible with surface mount production techniques; as is, their large physical size/ through-hole format - being rolled-film parts. GOTO 10.

    All that said - Polypropylene film/foil really is just about as good/ beyond contention. Wima FKp series (there are several) caps are outstanding, widely available, utterly reliable and good value - so don't fret :)
     
    Last edited: Apr 24, 2021
    Dowser likes this.
  8. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Oh dear.... 1% tolerance as well... About the best caps money can buy and now rare... VERY rare in that value which is big for a polystyrene and that voltage rating.
     
    Dowser likes this.
  9. Vinny

    Vinny pfm Member

    Polystyrene has a very similar melting point to the majority of common everyday polymers - up in the 200's C.
    I have never worked with extruded PS, I have worked with PS-coated polyester, but possibly PS isn't easily heat-set, so may distort at lower temp's than things like PP, PE, PETP etc. etc.? Maybe? It certainly doesn't melt below the boiling point of water - an awful lot of food and drink containers are made from it (un-expanded).
     
  10. martin clark

    martin clark pinko bodger

    Over c 80degC sustained kills PS caps though. Not my opinion, well known data; the limit of service temps.

    (ETA - and I'm an interested amateur, not an industry pro - the film used was godawfulthin, sub10 micron and as such - also has substantial electrostatic stresses & voltage gradients applied. Perhaps part of the 'why..?' )
     
  11. Dowser

    Dowser Learning to bodge again..

    This was just a split second touch - because one of the resistor legs jumped off the tube base and I was too lazy to remove everything again to refit it, I tried to do it with the cap still in place stupidly :(

    [​IMG]

    I didn't imagine I'd killed it with the above damage, but put it all back together and one channel down - the cap was shorted :(
     
  12. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    /\ Yep. They can often even look a bit melted and distorted in shape and sometimes appear to have cuts/splits in the outer layer of the styrene. Indeed to the best of my knowledge LCR in Wales are the last Western manufacturer and still make them when I last checked maybe 6 months ago.
     
  13. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Probably £20 ish for that cap... if you could find one... and that's at Farnell/Mouser/RS prices not from a "boutique" vendor:(
     
  14. Vinny

    Vinny pfm Member

    Packaging PP is not a great deal thicker.
    Unless the film is thin you get two linked problems - both amount to huge cap's per uF.
    Polystyrene film has always been rare - it is crap as a film - I have handled it - no tear strength for starters. At least one Japanese company used to make it as BOHIPS - biaxially orientated high impact PS - a VERY cheap (but yellowish) plastic, paper replacement.

    The hassle making any polymer film for cap's is how to make profit - when I worked for ICI, they were not interested. You make a ton and that will last a cap' manufacturer half a year at least, sort of thing. The other thing is imperfections, which kill cap's (and things like video tape - remember that?). Mostly, in ICI films, the zits were smaller than minute bits of catalyst from polymer manufacture - lumped under all zits as "fly specks".
     
  15. Dowser

    Dowser Learning to bodge again..

    yeah, right, thanks for making me feel better Jez :)

    I hate myself for taking such shortcuts and failing - all it meant was that in the end I pulled everything off valve base to rebuild it properly anyhow, and I killed a perfectly good cap.

    on the good news front, I really like what the 1uF mustard is doing :)
     
    martin clark likes this.
  16. russel

    russel ./_dazed_and_confused


    Good to know, I have a bag of about 70 22nF 630V polystyrene caps, only 10% tolerance though.
     
  17. Dowser

    Dowser Learning to bodge again..

    After fitting the mustards I've been happy, overall a much more organic sound...but missing some "ting" somehow (really don't know how to describe it) compared to before. I'd originally intended to fit the .034uF bypass caps to the mustards too...but given I destroyed one that makes it hard :)

    Went through my spares box and dug out some old Audio Note .022uf "****y" caps - rewired things to ensure I don't need to keep stripping valve base just to change caps and tried them with the mustards. So far, so good. I never messed with output bypass caps before...I wonder why?

    Using spare tag strip to make future changes easy - pure silver .6mm used :)
    [​IMG]

    One done;
    [​IMG]

    FInished
    [​IMG]

    Need to sit back and listen a while now, I've made far too many changes this weekend!

    Richard
     
  18. Dowser

    Dowser Learning to bodge again..

    Small update time;

    - Managed to find a NOS supplier of those Siemens poly caps in Greece - 6 ordered at around £4 each with shipping :)
    - Been hunting more tubes - bought a batch of 20 mixed ecc81/82/83 recently. Tried 2 matched TAD (tube audio doctor) ECC82 "selected" (whatever that means - I think all TAD tubes are selected in Germany but made in China?). Too early to say if better than Ultrons, need to listen some more
    - Replaced the unbranded ECC83s in the PSU with some JJ ones from the batch of 20 above. One was faulty - measuring HT after (was 260v DC before), one rail dived up to 470v before I switched it off...swapping to another JJ from same batch fixed it. So far, sounding better, but need to listen more again, and probably swap back...see my questions below.

    Inevitable questions;
    - see first photo below, looking at the plates I'd say the unbranded ones longer plates must be better...but initial impressions are they are not. Anyone aware of online resources where design and manufacture of valve types and components are discussed, I'd like to learn more. Initial impression of JJs compared to unbranded ones are much better imaging.
    - What has failed in the faulty JJ ecc83 that caused HT to hit such high levels? It wasn't connected to pre at time, so no issue, but I do not really know how valves work.... :) PSU cct in 2nd photo below. Top valve is an EL86.

    Thanks! Richard

    [​IMG]
    [​IMG]
     
  19. Vinny

    Vinny pfm Member

    In my Croft different valves sound almost exactly the same. That said, I like RFT/RWN/Erfurt (E German state-run electronics conglomerate) and the plates in those are tiny.

    There is more than stacks online about identifying valves, but so much is total garbage, you'll end up reading for hours and then suddenly realise that at least 50% of what you've read can only be nonsense. The clasic goof is people talking about Pinnacle, Bentley, Zaerix, TAD, Marshall and loads besides, being good, bad, great or whatever, when they did nothing but sell badged valves from whatever manufacturer offered the best deal.

    Your long plate looks to be Philips, reading the silver ink data (usually described as etched by those with no lateral thought process - it is proud of the glass), through the bottom of the glass - flip it round and the code should tell all. If it works, it will potentially be worth a mint, if Philips Blackburn (Mullard).
     
    Dowser likes this.
  20. Vinny

    Vinny pfm Member

    I haven't forgotten BTW, no box as yet :)
     

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