+ Reply to Thread
Results 1 to 6 of 6

Thread: An interesting day with a Croft VC amplifier

  1. #1
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

    Default An interesting day with a Croft VC amplifier

    Some time back I offered to look at a Croft VC power amplifier for Rob (Muggleton).

    Well today Rob made the trip up from rather further south in France. I had not seen or heard a Croft VC before so it was interesting for me to find out what made it tick.

    For those that don't know it is EL84 based, with parallel '84s in ultra linear fixed bias guise, giving about 30W per channel.
    Here is a photo, basically there is nothing to see from the front:



    Being fixed bias it was prone to excessive current flow when the valves aged or decided not to play the game regarding published specifications.

    So after hooking up the necessary test gear it was discovered that the quiescent current was jumping about all over the the place, with nothing to control excessive spikes, this was what was causing the HT fuse to blow.

    After a break for lunch (we are in France after all), Rob was then given a demo of the Firebottle amplifier set-up through the ESL57's, and was impressed by the air and clarity.

    After returning to the workshop it was decided that some auto bias needed to be added to the fixed bias of the VC in order that balance could be achieved between the parallel output valves.
    Resistors of 16.4 ohms were fitted to each valve cathode and satisfactory balance was achieved, with only a very small loss of output power.

    On testing in the lounge driving the ESL57,s the VC definitely didn't like the reactive load and promptly blew a fuse and one of the cathode resistors. After fitting a Zobel network to each channel output, peace was reinstated and a very smooth and open sound was the result.


    So a good day in so far as I could bring a problematic Croft VC back to proper operation, plus meeting up with a fellow music lover.

    Cheers,
    Alan
    I love Hendrix for so many reasons. He was so much more than just a blues guitarist - he played damn well any kind of guitar he wanted. In fact I'm not sure if he even played the guitar - he played music. - Stevie Ray Vaughan

  2. #2
    Join Date: Oct 2012

    Location: NE England

    Posts: 4,173
    I'm Jez.

    Default

    Nice job Alan

    I fixed a Croft power amp many years ago which IIRC had 4 x EL84 per channel and was push pull ultralinear but it was more conventional looking for a valve power amp in as much as valves, transformers etc on top of a chassis. Repair completed I give it a try in my system but was unimpressed I'm afraid... rather mushy and indistinct from memory (it was about 20 years ago at least) and I well recall that my Leak Stereo 20 thrashed it on everything but max power.. I can't recall ever seeing anything similar again so it may have been a prototype or a model that was quickly dropped maybe...
    Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)

    Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
    ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
    Digital: Yamaha CDR-HD1500 (Digital Swiss army knife-CD recorder, player, hard drive, DAC and ADC in one), PC files via 24/96 sound card and SPDIF, modded Philips CD850, modded Philips CD104, modded DPA Little Bit DAC. Sennheiser HD580 cans with Arkless Headphone amp.
    Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco

  3. #3
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 99,005
    I'm Grant.

    Default

    Well done Alan... nice walkthrough too...
    Regards,
    Grant .... ؠ ......Don't be such a big girl's blouse

    I've said it before and I'll say it again: democracy simply-doesn't-work
    .... ..... ...... ...... ................... ..... ..... ..... ..... .....
    FIIO K7 BT, M11 PLUS, BTR7, KA5 - OPPO BDP-103D - PANASONIC UB450 - PANASONIC 4K ULTRA HD TV - PIXEL 6 - AVANTREE LR BLUETOOTH - 2* X600 SOUNDCORE - HEADPHONES INCLUDE, FIIO, NURAPHONES', FOCAL, OPPO, BOSE, CAMBRIDGE, BOWER & WILKINS, DEVIALET, MARSHALL, SONY, MITCHELL & JOHNSTON - 2*ZBOOK'S- MERCURY BD ROM, ROON, QOBUZ, TIDAL, PLEX, CYBERLINK, JRIVER - MULTI HDD'S -

    Oh my god! There's nothing wrong with the bidet is there?

    “Nothing discloses real character like the use of power. It is easy for the weak to be gentle. Most people can bear adversity. But if you wish to know what a man really is, give him power. This is the supreme test. It is the glory of Lincoln that, having almost absolute power, he never abused it, except on the side of mercy".

    “You see these dictators on their pedestals, surrounded by the bayonets of their soldiers and the truncheons of their police ... yet in their hearts there is unspoken fear. They are afraid of words and thoughts: words spoken abroad, thoughts stirring at home -- all the more powerful because forbidden -- terrify them. A little mouse of thought appears in the room, and even the mightiest potentates are thrown into panic.”

    "You don't have free will. You have the appearance of free will.”

    “There's a war out there, old friend. A world war. And it's not about who's got the most bullets. It's about who controls the information. What we see and hear, how we work, what we think... it's all about the information!”


    ***SMILE, BE HAPPY***

  4. #4
    Join Date: Jan 2013

    Location: Birmingham

    Posts: 6,772
    I'm James.

    Default

    Thanks for enlightening us with your interesting days work on a poorly Croft. Glad it sounded good after you had sorted it.
    Main system : VPI Scout 1.1 / JMW 9T / 2M Black / Croft 25R+ / Croft 7 / Heco Celan GT 702

    Second System : Goldring Lenco GL75 / AT95EX / Pioneer SX590 / Spendor SP2

  5. #5
    Join Date: Mar 2016

    Location: Mascayroles, France

    Posts: 22
    I'm Rob.

    Default

    Indeed a most interesting day!
    The chance to hear Alan's system, with his lovely sounding Firebottle monoblocks, was a real treat and the sense of stabilising the rather skittish circuit of my Series VC was also deeply satisfying.
    In the best sense of the phrase, it was all a bit of a "hi fi adventure" for me and has given me back a well-loved power amp.
    We've had a bit of subsequent correspondence, and the final improvement is probably to replace the 5963 driver valves (250V versions of ECC82's) with full spec '82s, since that might also help obviate any potential issues.
    But anyone needing a really open, transparent and fast amp set-up should try to audition the Firebottle pre/monoblocks - they really are something special.
    Cheers
    Rob

  6. #6
    Join Date: May 2012

    Location: Toulouse, France

    Posts: 6,562
    I'm Kevin.

    Default

    The pre and power amp do a good job too.
    I do need to have a listen to the Monoblocks at some point though.
    Kevin

    Too busy enjoying the music....

    European loan coordinator for Graham Slee HiFi system components..

+ Reply to Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •