We replace all the original resistors with high quality 1%
metal film types. These offer several advantages. The
most important being tolerance and stability. The closer tolerances allow
the ideal and original operating points to be more easily obtained by
careful choice of valves. Also the noise floor will be reduced slightly.
The full set of Visconol -X grid coupling/decoupling capacitors are
replaced with 1000V and 600V polypropylene capacitors. Rob prefers
polypropylene to oil filled due to their superior temperature v leakage
resistance and lower distortion figure despite being below audible
Unfortunately in the majority of S20's and Varislopes
we service, the Vx caps are far from ideal. Surprisingly, the value
and breakdown voltage is usually still within tolerance but the leakage
resistance versus temperature performance is far from satisfactory. The
increasing leakage with temperature, has the effect of adding a small positive dc bias to the following stage. The
leakage over rides the
automatic grid bias and changes the operating point. The effect of this is
increased distortion and possible over heating of the output stage.
Our policy is to replace all the coupling and decoupling caps with polypropylene types. We also choose
large high voltage types to keep the physical size similar to the original
The finished amplifier is tested to ensure it conforms
exceeds the original specification. Including frequency response, output
power, and distortion.
Finally an electrical safety check is performed (PAT).
Note: There is a lot of nonsense on the
Internet regarding the audible difference between the original Vx
, modern polypropylene (PP) and oil filled (OF) capacitors. This is
made worse by some companies 'cashing in' on the inflated price of
paper-in-oil audiophile caps.
blog on this here. We normally fit high quality polypropylene
replacements. However if you prefer paper-in-oil types or any other
audiophile type we will happily fit them at cost but please don't expect to hear
any difference.** The subtle differences in the inherent distortion between
good quality polypropylene and oil filled coupling capacitors is lower than
the noise floor of a stereo 20. In other words, it is too small to be
measured when fitted in a Stereo 20 and way below aural perception.
The tiny distortion inherent in all capacitors is often greater with some
makes of OF than PP or PE capacitors and vice versa. The important point to bear
in mind is the relative amount of distortion added by a capacitor is
miniscule, compared to the sum of
distortions from the music source, the loudspeakers, your own aural system,
and rest of the amplifier itself. The capacitor make and whether
it is oil filled or polypropylene is the least thing to blame if someone is
unhappy with the sound of their system.
Compared to today's electrical safety standards, the Stereo 20 in its original form is
inherently unsafe. To improve the electrical safety we fit a new IEC mains
block with combined fuse and on-off switch. We also fit an internal or
external HT fuse. In the
event of an output valve going anode /cathode short the fuse will blow.
The stereo 20 is often used in its open chassis form and placed on a shelf
or table. This is very unsafe practice. The chassis must be screwed firmly
to a metal bottom panel with small rubber feet.
We can fit a bottom cover and feet when we service your
Stereo 20 for £15.00 extra.
*We now fit an internal HT fuse
and holder rather that the rear chassis fuse. The fuse is added to effect a safer alternative to
the original green wire wound resistor over-heating, and de-soldering itself.
Under short or near short circuit HT fault
conditions, e.g. an EL84 internal electrode s/c, the fuse blows.
** Some paper in oil capacitors on sale exhibit a
substantial leakage resistance that becomes very significant as the amplifier
temperature increases. This will have a marked effect on the DC operating point
of the pre and output valves. This results in distortion and can lead to overheating
of the output valves.