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I returned to DIY electronics in 2009 after a 20 year pause by building a few kits to get in shape. This blog is for me to keep track of my progress.
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Velleman K4040 mods

Posted 8th May 2011 at 12:32 AM by alexcp
Updated 4th May 2014 at 12:59 AM by alexcp

I posted some time ago a note about my Velleman K4040 amplifier. The sound of the amp as built from the kit was unremarkable and definitely not in line with the price. Lacking experience with tubes, I was looking for some time for well documented mods to the kit.

I found two 2006 posts here and here and implemented them rather directly. The only difference is that I used IXCP10M45S instead of a pair of J508 current sources in each channel and replaced the 1N5408 rectifiers with UF5408 instead of adding noise suppressing capacitors in parallel to them.

The mods did not leave much of the original Velleman design in place; the result, however, is a remarkable improvement in sound!

However, two problems remain: (1) I got hum that wasn't present before the mods; and (2) a two-year old running around doesn't get along with eight hot, attractively glowing tubes. The project is shelved until a better time.

For future reference: The toroidal output transformers of K4040 have approx. 1.8k Raa primary winding (15:1 turns ratio, anode-anode to 8ohm) with 33% grid (ultralinear) taps, and a 8ohm secondary with 4ohm tap (which is also used for feedback). The DC resistance of the primary is 41.4ohm anode to anode (20-21ohm B+ to each anode) and about 7ohm from B+ to each grid.
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Posted in Tube power amps
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Total Comments 3


  1. Old Comment

    I have just bought a bog standard k4040 from Ebay and this is my first valve amp or valve anything for that matter! Actualy I don't think the sound is too bad but their is still room for improvement and by the sound of the mods that you have made i think this amp has real potential! Do you know of anyone in the UK that could do these mods for me?

    Many thanks

    Posted 14th January 2012 at 02:46 PM by monkeyman123blue monkeyman123blue is offline
  2. Old Comment
    alexcp's Avatar
    Hey, sorry I did not see your message earlier. I have no idea about who could modify your K4040 in th UK. You may want to check these people, but I have no experience with them. I suggest that you collect information over time and do it yourself. It is not that difficult, you'll learn in the process, and will have fun!
    Posted 23rd January 2012 at 04:03 PM by alexcp alexcp is offline
    Updated 17th February 2012 at 01:09 PM by alexcp
  3. Old Comment
    I've now got a PCB Layout for Alexcp Mods to the K4040 should anyone wish to use it?. I don't know what the bias current is on the KT77's though.

    Mark Snape.
    United Kingdom.
    Posted 8th February 2015 at 12:06 PM by SNAPEFU SNAPEFU is offline

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