[Patrick Turner]

On Oct 21, 3:36*am, "Edward Morris" wrote:
Hello,
* * I'm interested in Velleman tube amp kits. *Does anyone know if Velleman
has quality products?

Thanks,
Eddie

And I had a look at the Velleman schematics which can be found in the
online kit manuals.
There is an amp with 4 x EL34 in each channel, Kit K404.
It has separate fixed bias adjust pots for all 8 output tubes - good.
The setting of the bias involves adjusting pots until a red LED just
begins to turn on while fiddling with little tiny switches.
There is an LM3914 chip which works the LED but better would be to
have 2 x 4 way opamps working differentially to work a yellow and red
LED so that bias is too low when te yellow comes on and too high when
red turns on, and when both are not on the bias current is correct at
+/- 2mA from a wanted 40mA. Alternatively a red-green bi-colour LED
could work the same way, and LEDS placed adjacent to each tube so the
bias condition can be checked at a glance any time the amp is on.
Its a small additional step to get such a circuit board to also
include active protection and turn off the amp if Iadc in one or more
EL34 goes to say 60mA for longer than 4 seconds.

The K404 has a single 12AX7 set up with 1/2 for a concertina phase
inverter and the other 1/2 as an SET input triode which has to make
slightly more grid drive voltage than is applied to either side of the
EL34 PP grid circuit, ie, about 20Vrms max.
There is considerablr global NFB used but I don't know why 12AX7 was
used as the input & driver tube when much better would have been a
6CG7 LTP plus another 1/2 6CG7 SE input tube. The remaining 1/2 6CG7
could have been for a stand alone preamp if wanted..
There is a preamp with 12AU7 and no GNFB and there is a switchable
choice of whether you include an input pot or not.
methinks without the gain pot the power amp plus preamp would be much
too sensitive.
For CD players, it might be best to omit the 12AU7 altogether and just
have the CD player feeding the gain pot which then feeds the power amp
input.

I cannot view any specs for the OPT or PT but they are all toroidal. I
have seen many really horrible toroidal OPTs which have been designed
and wound by morons who have not used sufficiently thick insulation
betwen P and S windings which results in far too much transformer
shunt capacitance. Judging by the advertised LOW prices methinks its
probable the whole range of amp kits or the main parts are all made in
China. Unfortunately, the Chinese have some way to go before I can
endorse most of their audio frequency transformer integrity.

I'm sure knowledgeable ppl might find more to criticise but with minor
modding one could probably greatly improve the product.

One would have to say the K404 might give you far better performance
than a Dynaco ST70 kit, and in real dollar terms allowing for the
passage of 40+years the Velleman are cheaper, probably we need again
send thanks to the Chinese slave labourers which continue to be
ruthlessly exploited by anyone buying anything made in China.

Patrick Turner.