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HEATHKIT MA12s

Discussion in 'd.i.y.' started by djdstone, Oct 31, 2017.

  1. djdstone

    djdstone pfm Member

    I’ve found a pair of these for reasonable money and I was wondering if they are really worth spending any money on? They look to be in original condition so might need a few parts replacing. Most people seem to replace rage original smoothing caps with F&T types. They also need the mains cables replacing and possibly some of the smaller cap values inside. Should I take the plunge ? I’d love to have a go at building my own from scratch but the kit prices seem quite high. Would be grateful for any advise
     
  2. Barrymagrec

    Barrymagrec pfm Member

    They are basically a Mullard 5-10 so a pretty reasonable amp.

    Assuming the transformers are O.K. It wiil always be cheaper and simpler to replace the electrolytic caps and check the resistors than build new amps.
     
  3. snowman_al

    snowman_al pfm Member

    Yes well worth spending money on.
    Good quality output transformers, nice easy to work on chassis, hard wired, plenty of room what more do you need? Treat them as a kit even.

    Loads of info on the web, but basically a Leak TL12+ 'copy'. As long as you have original transformers everything else can be replaced as or when needed. (Nicer still if you have a matching pair ie; 2 x Hinchley or 2 x Radford output transformers... )

    I have never had 2 together, but mono SQ on a par at least to any TL12+ I've had.
     
  4. djdstone

    djdstone pfm Member

    So I understand - what’s the best and safest way to remove the smoothing caps? Even though I’m assuming that it hasn’t been used for some time.
    The circuit is pretty simple apart from that. One the amps also has a separate mains lead running from the transformer to the original Pre amp. Any idea where I can get hold of 500v rated smoothing caps - hifi collective only stock ones rated to 300v
     
  5. djdstone

    djdstone pfm Member

    Hi Snowman that are Hinchley Transformers. They will need updating to accept phone cables and one of the GZ34 Tubes needs replacing. They look original apart from that. Basically I thought that they would be a simple amp to work on
     
  6. snowman_al

    snowman_al pfm Member

    Good to have a 'pair'.

    Not sure you will get brand new 60uf + 250uF 350 volt caps any more. The ones you do see are mostly old stock and probably no better than what you already have. Better to use a 50 + 50uf (350 - 500v) and add another suitable 200uF under the chassis. That will give the GZ34s a slightly easier time too.
    But unless the originals are obviously swollen or leaking goo you might find they are still serviceable. Try measuring their capacitance first. If it is close you might be able to 're-form' them.

    Before you spend any money though (or power them up) do continuity checks on all the transformers. Just in case...

    I have sent you a PM by the way.

    Alan
     
  7. djdstone

    djdstone pfm Member

    Thanks Alan - have PM’d you back
     
  8. djdstone

    djdstone pfm Member

    [​IMG]
    Does any one know where I can source these?
     
  9. djdstone

    djdstone pfm Member

  10. snowman_al

    snowman_al pfm Member

    Well they are pretty original and look very tidy! You must be pleased.

    Do not be tempted to plug them in and try them just yet though...
    A couple of things jump out from the pictures.
    First is change the red Hunts capacitors (0.1uf 400volts) C4, C5 and C9. They are notorious for leakage (electrical), if not now then pretty soon.
    Check the grey Plessey(?) 0.5uf coupling caps C7 and C8 very carefully for electrical leakage. If in any doubt replace them.
    The 'mustard' Mullard/ Phillips work well as replacements for all the above, but any decent axial MKT or MKP will be fine. Right values of course.

    Next the cathode bypass caps (C10 and C11) for the EL84s look like Mullard's 25uf jobs. They will be dead by now. Fit 47uf or even 100uf 35volt axial types.

    You are right about the 60uf + 250uf caps too. They are leaking goo. I would still test them before replacing them though. 20% chance I would think.

    The speaker sockets are 'Deltron' 4mm panel mounting types. They are the best. There are cheaper ones available, but not nearly the same quality.

    As for the sad Mullard GZ34, I would buy a couple of new JJs or Golden Dragons or what ever, and fit a 1N4007 diode in series with each 47/50 ohm limiting resistors. Plenty of room on the valve base. Keep the good one till you find a mate.

    You have the makings of a very nice pair of amps.
    Alan
     
  11. djdstone

    djdstone pfm Member

    Thanks Alan - that’s a very comprehensive list to go through - really appreciate all your help. Where do you get your parts from?
     
  12. snowman_al

    snowman_al pfm Member

    No problem.
    I do not have a single source and I'm lucky to have built up a fair stock over the years.
    Part of the 'fun' is finding the parts you need too. Try:-

    CPC.farnell is probably the first place for the Deltron sockets. I use them for standard value resistors and caps too.
    Farnell have a bigger range that suits valve stuff, axial caps and better values.
    Cricklewood have a fair range too.
    Hifi Collective for caps and bijou parts.

    I search ebay too, sometimes you find the best price there. There are a number of sites that cater for valve guitar amps (like Jellyfish etc.) and they often do good prices on caps, valves and hardware.

    I try to use / find replacements that match the original. I do not like to use a radial cap to replace an axial for instance, you do have to sometimes though. Also modern parts are often much smaller than the original. So you can use a 2 watt resistor where it was 1/2 watt, you can use a 35v or 40v cap when the original was 16 or 25 volts, etc. You will find that many 630 volt caps are cheaper than 400 volt types I guess due to the quantities made?

    I'm sure others will be along with other suppliers too.
     
  13. djdstone

    djdstone pfm Member

    Hi Alan - the IN4007 diode goes between the GZ34 and 4 & 6 on the schematic? [​IMG]
     
  14. snowman_al

    snowman_al pfm Member

    Yes basically.
    Lift the 50 ohm resistor from pin 4 and move it to pin 3. Lift the other 50 ohm resistor from pin 6 and move to pin 5.
    Those are the HT AC feeds from the mains transformer.
    Now shape 2 diodes to fit from pin 3 back to pin 4 and the other from pin 5 back to pin 6. Just make sure you get the polarity right.
    The bands should go to the old pins 4 and 6.

    Like this
    [​IMG]

    Different amp, but a GZ34 base.
    The new diodes are in series with those in the GZ34. Just a precaution should you get a 'duff GZ34' then you wont get AC on the reservoir capacitors etc.
     
  15. djdstone

    djdstone pfm Member

    Thanks Alan - that makes more sense now. What caps would you recommend for the 0.1 and 0.5uf. Is there any need to buy the fancier brands here?
     
  16. snowman_al

    snowman_al pfm Member

    I wouldn't choose exotic caps.
    Just concentrate on getting the amps in good working order first. Once you have, you can experiment as you want. (As I mentioned above the Mullard 'mustard' caps would be a good choice if you can find some in the future. You will not get 0.5uf any more. The modern value is 0.47uf = 470nf.)

    I have used this make in a couple of repairs and my recent Audio Innovations rebuild and found them excellent value, quality and sound fine, and far better than the original Hunts.
    https://www.ebay.co.uk/itm/10x-Capa...ylene-Axial-Valve-Metal-Film-UK-/190816561218
    https://www.ebay.co.uk/itm/2x-Capac...ylene-Axial-Metal-Film-Audio-UK-/200978940509

    If you want a better known brand go for something like these Vishays
    http://uk.farnell.com/vishay/mkt1813447405g/cap-film-pet-470nf-400v-axial/dp/1166858
     
  17. djdstone

    djdstone pfm Member

    I will order some of those Alan. What’s special about the Mullard caps?
     
  18. Tony L

    Tony L Administrator

    If replacing vintage paper in oil caps the Russian military K40Y9 is the one to beat. Affordable, reliable, and won’t alter the voicing of the amp from the designer’s original intent. I did a lot of research when rebuilding my Stereo 20 and would happily have paid out for fancy boutique paper in oils or whatever, but these are allegedly better and more authentic. Have a reputation for superb long-term reliability too. I couldn’t be happier with the S20 rebuild, it sounds stunningly good!
     
  19. Arkless Repairs

    Arkless Repairs Trade: Arkless Repairs

    They don't go wrong.. ever. Probably the most reliable electronic component ever made.
     

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