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 detection. We often find Leak Stereo 20s with open circuit EL84 cathode resistors and blown bypass capacitors. This is usually caused by an internal electrode short on an EL84 but it can also be caused by leaking coupling caps. To help reduce the unpleasant aftermath of such a failure, we can fit a fuse and ‘failure indicator’ circuit in each EL84 cathode that will blow and indicate a failed or compromised valve. We drill four neat holes adjacent to each EL84 with an led protruding slightly above the top of the chassis. The led will only illuminate when there is a blown fuse, o/c cathode resistor and/or failed EL84. Ask for this if you want it. The finished amplifier is tested to ensure it conforms to or 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. See the 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. SAFETY: 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. For an additional cost, We can fit a bottom cover and feet when we service your Stereo 20.   *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.