My ultimate
intention would be to make the amps by
myself, but lack of time and poor skills in metalworking caused me to
retire this wish and rely on others. For past 15
years I used to collaborate with the great people of ARG
company from Ukraine and we worked hard to make together the best amp. The history of
this can be found in the link below. Now with the
totally new situation - tube amps being made in China and
costing next to nothing, my advice to the people needing an amp is to
shop in china on ebay and re-work the amp to make it decent. Chinese amps
coming to us via Hong Kong look great and sound HORRIBLE.
I mean Hor-ri-ble. But the price
justifies that we buy them, use the beautiful bodies and
all, and just change a little the schematics. In the end - they sing
like a dream. The link
is to be found below.
MODIFICATIONS OF AMPLIFIERS General note
about the tweaking process
It is hard for
me to describe every amp and every
possible schematics - how we want things to be modified. In general -
western schematics are usually a result of a very
long period of preparations, testing and optimization. They are
finished and optimized. Chinese amps are
strange animals. Good looking, with good parts, but
totally strange schematics. The circuits
look like a result of un-qualified copying plus some
"original creativity" plus unknown factors. All in all, the Chinese
products have too many parts and unnecessary components scattered all
over the place. My short list of
mods is this:
1. In pentode
tube amps ALLWAYS make a triode mode conversion (unless
there is a switch to change the modes). This applies to ultralinear
circuits etc. Triode mode
pentodes sound great, but in pentode mode - they sound
really BAD. Bad bad bad. The mod is easy and costs nothing.
2. In the input
section - remove and short all series resistors - we
don't need any of them. Tubes are happy when coupled to each other
directly.
3. sometimes
there is too many paralell resistors which form - together
with series resistors - some signal attenuators. Get rid of all of them
except the resistor which goes from tube grid to ground. Every small
tube needs grid grounding resistor, value between 200K to 1 M. The first input
tube may use the volume pot as a resistor - so the
paralell R can go out.
4. I like
triodes without the cathode capacitor. Most amps use an
electrolytic cap across the cathode resistor. Remove it and maybe you
can decrease the cathode resistor by half. This sounds much cleaner.
5. Coupling
caps: all tubes connect to the next tube by a series cap.
Use sizes like 0,47 uF or more in the first stage, and over 1 uF in
driver stage. use the RADIAL wound, round type not the box type and not
the oval. Radial is the way to wind a good cap. The best type is
paper in oil (PIO) type, copper foil is the best
electrode material. (jensen or audionote) or any decent MKP.
6. I routinely remove the global feedback loop and never I found it to
be a problem. Without feedback EVERY amp that I know - performed better
- more open, louder, with deeper bass, more magic, more what I like.
Thats basically
all we need to do. The improvement depends on the
circuit but usually the mods open up the sound a great deal. Triode
mode is most important of them all. Theoretically we "loose power" but
subjectively the sound is louder, more spacious and free. Wery well
worth doing.
The most basic and simple test of an amp is to run a generator vawe
through it and see if it shows on the scope.
By using the sine vave we can learn a great deal about the condition of
an amp.
1. Is it alive?
2. Does it distort the sine visibly? Does the both phaces of an amp
work equally (symmetrical sine versus a "W" letter wave. )
3. What is the amplification coefficient of each stage and the whole amp
4. When (in volume) the clipping begins. (finding the real maximum
power, not advertised power )
The biggest problem is that that test has no real value unless we can
run the power near the maximum.
It is NOT ALLOWED to play the amplifier without the load, just silently.
Especially tube amplifiers hate to be run at max volume without
speakers or load connected to the outputs.
A human can not possibly spend any time testing speakers and playing
say a 1 kHz tone at 50 Watts in the same room.
So the BIG ISSUE is to find a load capable of dissipating up to 100
Watts of power at 8 Ohms or close.
Here comes my own Fikus solution: Immerse the load in Water. You have
an unlimited power, silent, cheap and lasting infinitely load.
The resistor (in my case a wirewound 20 Watts / 10 Ohms) can be
immersed in a bowl of tap water and the water takes up all the heat. I
tested 60 WPC for half an hour without any noticeable temperature rise
).
Just look:
the dry resistor measures 10.8 Ohms plus 1 Ohm for the probe wires.
The resistor gets immersed in the room temp tap water
Wet resistor measures slightly more - 11,2 Ohms plus probes. That is
still a perfect test load for the 8 Ohm amplifier output.
Contrary to popular belief, water does not short the electrical circuit
easily. (the lights stay on in a sinking car or on the Titanic).
Especially that we are dealing with only a few volts AC here. Of course
a prolonged application og DC would lead to the water electrolysis
(still harmless for the purpose of testing as a load).
