Velleman K8010 KT88PP monoblock

 

Please note this here is my opinion and may not reflect the truth, it is followed by the story of a customer of mine and it is his opinion which i also agree with.

I certainly dont want to deter anyone from buying a Velleman tube amp, but he can allways go and listen to one if in doubt.

Velleman and product (historic) overview according to me :

Velleman is a Big Belgium electronics company who recently was taken over by a group of dutch investors i believe. Originally they started a long time ago with kits and selling gadgets and today it is a reasonably parts supplier with still its core business the kits and next to that they import and distribute a lot of goods from Asia from gadgets to speakers to alarmsystems like their German counterpart Conrad. The main problem is the low quality of these goods, but thats another story. Velleman entered the tube business because they saw a market in it, i knew the company fairly well and the engineers have zilch tube experience. So when the marketing manager asked for a tube amp, he said it has to be cheap, foolproof, good looking, failsafe and 100 Watts at least.Thats how the first kit the K4000 came alive, it was a copy of a tube amp published by a dutch magazine called "The last of the Mohicans!". The original amp was running on EL84 and Velleman substituted that for EL34 and changed powersupplies and output transformer as well. The first models ran on Irish toroids made also for Amplimo, nowadays all toroids come from China or somewhere in Asia. When eventually sales dropped and demands for better quality arrived, the same filosophy stayed but the EL34 was just substituted for KT88 and the big quality increase was monoblock configuration.

The main problem you see is that people who are not audiophiles themselves cannot understand the demands that audiofiles require, they are also not tube specialists so they dont understand the circuitry either, so basically this is an amateur amp build by a novice who has little cash to spend on quality parts. This is offcourse a shame for such a big company as there are plenty of consultants who specialise in tube audio design.

From a commercial point of view :

The cost of the pcb and the chassis and the transformer are the biggest cost.To include for example top quality resistors and capacitors would add 5% to the cost. But you see companies like these ( 99% are like these ) want that 5% in their pocket. The general attitude is that they are aiming for 2 types of clients.

The audiophile on a budget who buys it build or someone builds it for him.

The audiophile DIY that is too lazy to build his own, and who can improve things if he has the skills and interest.

 

Comparative schematics : here are the last of the mohicans and the original velleman K4000 schematics

( i really hope you appreciate as i spend hours digging this out of my archives )

As you can see the direct coupling was omitted and the cathode resistors are 4 1/4 watt in parallell, that burn out quite often when powering up as they cannot handle the high voltage, the first tube is incorrectly biased and changing the values to the above version will certainly improve sound a lot, also the input is overfiltered and overprotected.

Schematic of K4040 , the K8010 is practically identical :

As gain was just on the limit an extra stage was added, but instead of switching the ecc83 for an ecc81 ( µ=60 ) they added an ecc82 for extra gain, the ecc83 is not really suitable for driving a parallell push pull of EL34 or KT88.

First Customer for this

I often call my company the Prototype company because i do a lot of one offs, and here the same happend. Danny walks in the shop and asks me about a mod for this a simple one, so i give him the standard K4040 mod as the circuit is the same. A week later he asks what more can we do as he is very happy with the progress, so this is his story and the complete mods are on this page. Every time he completed one and it had to be a significant improvment to justify to continue the quest.

Here is his story :

This is the story of the ugly duck who became a beautiful swan.

When I was very young someone told me that story of Hans Christian Andersen.
And now that my amps are completely changed it is in fact the same story:

The story of the mediocre sounding Velleman K8010 who became a top performer.

One year ago I started looking for a new amp, in the audio newsgroups many people were
raving about tube amps and how good they sounded compared to solid state.
To keep the price down I went for a kit, I knew how to solder and the difference between a resistor and a cap.

So I bought the Velleman K8010 kits.
When they were finished I was disappointed, I expected to hear a huge difference compared to my NAD solid state amp,
instead they sounded a little different than my NAD but not that much better.

So I started searching and reading on the internet, then I met Benny from http://www.diyparadiso.com
I explained him my disillusion with the Vellemans. He designed a new circuit for it with better components (dale resistors, audyn caps)
The new circuit looked much better on paper:
the signal didn't have to pass any unnecessary components like in the original circuit, no input filter, direct coupling, reduced feedback,
more current for the input tube and the concertina splitter (in the original circuit they only get 1ma, the ECC82 works best at 8ma, go figure).
circuit:
12au7 gain stage --> coupling cap --> 12au7 gain stage --> 12au7 concertina splitter --> coupling caps --> KT88 PP --> OPT LL1623

After these mods the amps sounded better, I was going in the good direction.

Next mod advised by Benny: changing the Output Transformer to a Lundahl LL1623,
these made a big difference, after that only one channel had the LL1623, I could clearly hear the difference between the 2 amps.
The LL1623 gave lots more detail, performers came more alive, the soundstage was there. These Lundahls are fantastic.
I started to listen much more to music than before.

After a few months being happy with the modified amps, I asked Benny if there was another mod which could get my amps to a higher level,
answer: raising the B+ from 380vdc to 440vdc and changing the concertina to a Lundahl LL1660/18ma interstage transformer.
Raising the B+ also changes the operating points of the tubes if you don't change the resistors, but implementing an IT LL1660/18ma is also completely changing your circuit.
So I did both mods together.
First I designed everything in B2 Spice which I found very helpfull for getting the DC voltages right.

When I first made the Vellemans (without any mods) and I checked the voltage, I had 7vac on the heaters instead of the 6.3vac (big flaw),
so I had to lower the voltage by selecting a different tap on the primary, I got 6.5vac but it also lowered my B+ from 410vdc to 380vdc.
By putting a small transfo in series with the secondaries of the power supply transfo I raised the B+ to 440vdc.

I changed the concertina splitter to a IT LL1660/18ma interstage splitter,
at the same time I made a better power supply for the input and driver tube (seperate chokes) and I rearranged the signal path and the grounding.
I didn't like how the signal went around on the original pcb and how the grounding was done. Now, almost everything is point to point wiring.
From the cinch connector to the grid of the input tube I've put a shielded cinch cable. The grounding is changed to a distributed star.
The input stage stage uses only one triode of the ECC82, on one amp I use the first triode and on the other amp I use the second triode,
so when I swap the tubes the other triodes are used. The heater supply is also changed for 150ma instead of 300ma.

New circuit:
12au7 gain stage --> coupling cap --> 396a driver(2 parallel) --> LL1660 IT splitter --> KT88 PP --> OPT LL1623

If I compare the IT splitter based amp with the concertina splitter amp,
the one with the IT splitter sounds much better, especially the highs are more detailed. The sound is more open.
The seperation between the instruments is also much clearer, there's more air, space around the voices, soundstage has greatly expanded.
So the Lundahl IT LL1660/18ma is definitely a winner for me in my amps.
And the amp is now dead silent, no noise or hum, even with my ear close to the tweeter.

The LL1660 splitter makes an as big difference as the LL1623 OPT.

compared to a concertina splitter I see the following benefits for an IT splitter :
1. perfect balanced splitter
2. works as splitter and interstage coupler, no coupling caps needed to the output tubes
3. works also as a choke for the driver tube.

I also installed some swithes so I can switch negative feedback on/off and the KT88 between triode and ultra linear.
I like most no NFB with the KT88 in triode.

I've configured the Lundahl irons as following:
-IT LL1660/18ma:
primaries: all in serie (alt-S)
secondaries: for PP (alt-U)
see http://www.lundahl.se/pdfs/datash/1660.pdf for the schemas.

-OPT LL1623:
primaries: all in serie (PP connection) see http://www.lundahl.se/pdfs/datash/1620_3_7.pdf
secondaries: 4 parallel groups of two in serie, not like in the lundahl sheet,
but alt-2 from http://www.diyparadiso.com/lundahl1.htm

You can also configure the LL1623 for UL.
On the primary there are 4 windings (all in serie), see the Lundahl pdf,
connection for UL:
4 : tube anode
2 + 5: UL
3 + 7: B+
1 + 6: UL
4 : tube anode
I've put a switch so I can choose between UL or triode,
only switch between triode or UL when the amp is off.


I started with the Velleman kit because I wanted a tube amplifier for a good price and I didn't know anything about tubeamps and how they work.
Lucky for me I met Benny who could help me and turn the K8010 into something very nice.
I think I will rename my amps because there's not much left from the original K8010 kit: only the case and the soft power startup with the relays,
everything from the cinch connector to the speaker terminals has been changed and replaced with better components.
It was journey that took almost a year but I learned a lot about tube amps.

I didn't thought I could get my amps to sound as good as they do now.
Many thanks to Benny of http://www.diyparadiso.com for his designs and great advice,
also thanks to the helpfull people of the TubeDIY asylum at http://www.audioasylum.com

What's next ?
- a CCS (C4S) for the input tube. ( the std CCS is the best one, he tried many ccs )
- try different coupling caps between the input and the driver tubes.

On this i can tell you Danny trieed many capacitors and finally settled for Mundorf silver foils. ( he also tried auricaps and teflon caps and the jensen copperfoils and all the others )

New project:
- kusunogi styled non oversampling DAC http://www.diyparadiso.com/dac1.htm

Pics After the MOD:

New Schematic

the tubes used are Western electric 396A ( not ECC81 or 12au7) and ecc99.

And the one with the CCS that sounded worse then a simple CCS or then even no CCS

Now Danny finds its a good amp, but it probably costed far more then my boully kt88pp amp in kit!

Copyright webdesign 2005 Aquablue Benny Glass