Sunday, March 24, 2019

Velleman K4040 mods

The sound of Velleman K4040 as built from the kit (see my previous post) was not in line with the price of the kit. Lacking experience with tubes, I was looking for some time for well documented mods to the kit.

I found two 2006 posts on, 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. In 2015, Mark Snape (a.k.a. SNAPEFU made a PCB layout for this mod, although I have never seen it myself.

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

However, I still had two problems to resolve: (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 was shelved until a better time.

For reference, the toroidal output transformers of K4040 have approx. 1.8k Raa primary winding (15:1 turns ratio, anode-anode to 8ohm) with 33% ultralinear taps, and a 8ohm secondary with 4ohm tap (which is also used for feedback in the stock amp). The DC resistance of the primary is 41ohm anode to anode (20-21ohm B+ to each anode) and about 7ohm from B+ to each grid.

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