Velleman K8020 rebuild

[awkwardbydesign]

I just bought this over on pfm as a little project. I already have a K4000 clone (95W EL34 power amp), and thought this would go with it.
Obviously I can't leave it unmolested, so I found this - http://www.hardwareanalysis.com/content/article/1826/ which sets out a circuit mod. The only question is, which resistors to use? At a price I am prepared to pay, the contenders appear to be Kiwame 2W or Takman metal film, 0.5 and 1W. I have used both in the past, so unless someone gives me a compelling reason not to, I think I will go with the Kiwames (cheaper, higher wattage, physically larger).
As I had some RCA 8514 valves I stuck them in, in place of the EH 12AU7s, and there is an immediate improvement, especially the bass. Hopefully the circuit changes, with better resistors and output caps will lift it further.
Listening to it now, it isn't yet as open as my Promitheus TVC, which in turn isn't as good as my Stereo Coffee LDR (battery powered). I'm in the process of adding a remote control to that, but it seems there is a fault with the remote board; input 4 relay stays closed when 5 is selected. Bugger, more circuitry to check.
Getting better output caps into this will be tricky, as there isn't much space, but I'll have to see what I have that will shoehorn in.
http://i1159.photobucket.com/albums/p625/graham-ar/Audio%20and%20Electronics/Velleman%20K8020/DSC_0199.jpg
http://i1159.photobucket.com/albums/p625/graham-ar/Audio%20and%20Electronics/Velleman%20K8020/DSC_0192.jpg

[Firebottle]

I have good results with the Takman carbon film.

The circuit mods suggested are excellent, certainly go with those.

[awkwardbydesign]

That's interesting. I looked for DIY reviews of the resistors, and it seems that some find the the carbons too warm. I used Kiwames throughout my rebuild of my Ben Duncan active crossover, (http://www.saturn-sound.com/history/hi-fi,%20diy%20projects.htm) and they sounded good. Mind you, I changed other things too, so it's hard to separate individual improvements.

[awkwardbydesign]

And now I have a question, for Alan or anyone else. (I have just bought Morgan Jones' "Valve Amplifiers"; the first chapter is giving me a headache already!)
The 8020 has a 50k pot at the input. And it sits at 10 o'clock for loud listening. My question is, would a 100k pot travel further round for the same volume? And would that cause any problems in this circuit?

[Firebottle]

Simple answer, no and no.

You could fit almost any value pot you like and it won't make any difference :)

[awkwardbydesign]

Ah, so no advantage then? Except possibly a higher input impedance, with a commensurate (minor?) increase in thermal noise? I have been reading Morgan Jones! And a little knowledge is a dangerous thing. :hmm:

[awkwardbydesign]

I've just finished the mods, and when I tested it, it didn't work! Grr! Eventually after much chasing the signal with a sig gen and AC millivolt meter, I found the link I had forgotten to install :doh:
So it's working now, but too soon to tell if it's better. But at least I didn't break it. :yay:

[Juha]

I have one of these.
It will be interesting to see, if the mods made a difference.

My K8020 is unmodified running old DDR valves.
The DDR valves have a warm sound, but something is lacking.
The sound is too dark and there is a hint of "grandma's old radio" which I hate.

[Firebottle]

I'm sure the mods will make a huge difference, as outlined in the links referred to.

The valve will be running in a far better position on its operating curve :thumbsup:

[Ali Tait]

I have good results with the Takman carbon film.

The circuit mods suggested are excellent, certainly go with those.

+1, I like the Takman Carbons too.

[awkwardbydesign]

I went with the Kiwame 2W. Partly because they are 2W, and so will be running understressed, and I liked what they did in my active crossover. However they are longer than the originals so needed the legs bending.
Adding 100k to the top leg of the pot has moved the usable range upwards, which makes me happier. I removed the +6dB relay, as I will never need it; I don't mind different volume settings for different inputs, and less is more in my book.
It's hard to tell if there is an improvement, as I have been modifying my LDR pre at the same time, so have no fixed reference, and I know that my speakers are not right. But there was no point trying to finish them until the amplification was finalised. Now I can get on with them.

[awkwardbydesign]

Oh, and here's a question for anyone who might know (or have an opinion!); is it worth the effort replacing the ceramic valve sockets, (and their associated extra mounting pins) with some Bakelite CMC turned pin sockets? At least for reliability, and possibly for sonic benefits? There appears to be some support for Bakelite over Teflon, not least because apparently the softer Teflon allows the pins to loosen.
http://www.ebay.co.uk/itm/331354325513?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I use CMC binding posts, and like them. And assuming these sockets are genuine!