QUAD 33 restoration (Pt 2)

Following Pt I, next, the Amplifier boards are removed for modifications:

  • Reduce input gain by 50-60%, to suit higher output from  modern digital sources (Roe MK2 and Takman REY metal film resistors, change values to increase feedback, output stage).
  • Replace electrolytic capacitors (Elna Silmic II).
  • Renew transistors (3 per channel, BC550).


  • Existing Hunts green metallised film coupling capacitors (3 per channel) are not replaced – film and foil would be better sounding, but space is too tight and we are on a budget here!
  • Only one input coupling cap (C400, 0.68uF) has been replaced with a slightly larger value 1uF Wima MKS4 cap from my parts stock – which should also slightly increase LF bandwidth.

Removing the plug-in Amplifier Boards, L & R channels.


Amplifier boards – before


Amplifier boards – after

Disc (phono) preamplifier board:

  • PSU dropper resistors increased to suit higher 16vdc supply.
  • Replace transistors (4 x BC550).
  • Electrolytic caps replaced (Elna Silmic II).
  • PSU decoupling cap C313 increased to 470uF – should improve LF bandwidth.

Phono preamp board – before


Phono preamp board – after (C313 needs to be hot-glued sideways onto board).


Phono preamp board – re-installation


Phono preamp board installed – it’s a tight fit (on the right)

Disc Adaptor board:

  • Increase gain of M1 position (optional) by changing feedback resistors R105/106 and R107/108 to 470Ω and 220Ω.

Tape Adaptor Board:

  • Replace transistors (2 x BC550).
  • Replace carbon film resistors R203/204 and R215/216 with metal film (Roe, Takman REY or similar).
  • Replace output coupling caps with larger value (increased from 0.33uF to 1.5uF, Panasonic metallised polyester) to increase LF bandwidth.
  • Replace input coupling caps with larger value (increased from 0.33uF to 0.68uF Panasonic metallised polyester). These were the largest physical cap sizes that would fit in through the rear hatch.
  • Adjust the 3-position, input and output sensitivity screws to suit your system levels.

Tape Adaptor Board – completed (note ‘sensitivity’ setting screws for Record and Replay).


Tape and Disc Adaptor Boards – re-installation.

Volume control:

After almost 50 years the volume pot on this Quad 33 is giving trouble – the shaft bushing is very worn and loose.  Signal ‘drops out’ occasionally, at low volume settings especially.


Original dual volume pot – note the very tidy Quad wiring and early ‘ribbon cable’.

The case & front panel need to be removed….


I found a NOS Noble (Japanese) dual-gang A50kΩ pot (audio/log taper) with switching.


Quad British pot (top), Noble pot (below).

The first problem….
The new Volume Pot is Japanese, so its ‘bushing’ is metric 10mm, slightly bigger diameter than the ’60s British 3/8 inch (9.525mm).

The chassis hole is enlarged using a Dremel tool with grinding stone.


Secondly, the shaft is cut down to length to match the old one – using the Dremel with mini-cutting wheel…..


The ‘flat’ on the new shaft is not in the right place for the knob markings – grind the new shaft, with some trial and error.


Finally complete – the new pot with re-soldered wire connections.  As the new pot is a bit smaller, the wires are a bit stretched.  Not quite as tidy as Quad’s original work!


Interconnect cables:

DIN to RCA cables, similar to this one, are needed for connection to non-Quad amplifiers….



5-pin DIN plug .

Although fiddly to solder and unsuitable for large wires, in my opinion, DIN connectors are superior to RCA connectors for signal fidelity – minimal metal-mass, silver-plated pin connections, ultra short signal path and no mechanical joints or threaded surfaces to distort the signal.


3.5mm TRS plug for iPod, etc.


Quad 33 – amended block diagram.

6 thoughts on “QUAD 33 restoration (Pt 2)

    • Hi Tomas – thanks for visiting.
      The overall tone is warm, with no edginess – as you would expect from Quad.
      After replacing the volume pot, there is more sharpness, treble presence & some welcome brightness & detailing.
      The Silmic caps have lovely tone colour, no harshness – I think they contribute.
      For a more ‘hifi’ sound, you would need to replace the carbon film Rs – but this is nice for general & family listening.
      Cheers, Owen

  1. Pingback: Quad 33 restoration (Pt I) | D a r k L a n t e r n

  2. Hi Owen. I have a 33 upgrade by myself and I wondering whether the replacement of the 2u2 caps on each amp board would meake some imrpovement, since they be kept on yours.

    Cheers. P. Nunes. SP-Brazil

  3. Nice work! I had mint ‘33 I would have let you work your magic on if it hadn’t gotten away from me some years back. It was part of the complete setup including the Woodsleave for both the tuner and Pre along with a 405 amp. Cheers from TX 🙂

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