An example is a Quad 22 preamp in which I did some serious rewiring back in about 2001.

You can see that there are 7 tubes, all twin triodes, which replace the original two pentodes and two twin triodes.
I have fitted a well drilled back plate with gold plated RCA sockets, to make the unit compatible with today's cables, and use in any system, not just one with Quad power amplifiers.

The control knobs have a slightly different function set up from left to right as Volume, Balance, Bass, Treble, HF Filter. Of the original 5 plastic buttons, 3 were broken so I made new aluminium buttons, hand engraved them, and bolted them onto the metal switch levers with some epoxy glue, ( Araldite ).
The face plate of the amp itself was slightly altered since there is no concentric balance lever on the volume control and the Quad badge has been replaced with an led.

All the original ganged press button switches have been retained, but with the simpler and more effective circuit, only 1/3 of the contacts are used. I have found these old switches to be more rugged and reliable than the tiny switches mounted on printed circuit boards found in more modern equipment.
Different value gain and balance controls are fitted. The line stage gain amp and tone control amp is fully deletable.
The complex arrangement for multiple eq for 78 shellac records has been abolished.
There were about 12 different contours for equalization of records produced before 1955, but few people wish to play them now, so the preamp has only RIAA standard eq, with hard wired passive RIAA filters within the amp, rather than having the eq circuit for records in a can that plugged into the back of the original Quad.  The tape eq can has been entirely deleted
because nobody ever now uses the tape source used in the early days of tape replay.

However, for those wanting to replay ancient recordings Quad II preamps are best simply restored to exactly original condition, rather than modified like I have in this example.
Such a restoration ideally involves replacing all the R and C components since the degradation of these parts
over the last 50 years can cause serious distortions. The original Hunts capacitors and carbon composition resistors are notorious for going rotten with moisture absorption and corrosion.

In the amp I modified,  I have retained the LC filters to give a steep 12dB/octave HF cut starting at 5 kHz or 7 kHz, or no cut at all.  Having done that I found out how useless such filters were for making old records sound less noisy. Such filters dull down the music, and the right way to combat noise from vinyl is to clean the record, not try to filter noise away.
I have never used the filter feature.
Cleaning your records properly can only be done with a cleaner that has a special brush and vacuum cleaner and with special record cleaning fluid.  But anyway, I used the original well made Quad 22 filter inductors, but I abolished the variable filter slope function since it seemed to be useless function when I tried to use it with a particularly noisy record in an unaltered amp. The unit was repainted and aluminium knobs polished. Aluminium switch buttons were fitted to replace broken plastic originals, and engraved.
The improvements to the control unit allow compatibility with today's source components or connection to any power amp, and give outstanding performance.

MODIFIED QUAD 22 PREAMP AND POWER SUPPLY, 2001.

Another image shows the highly modified Quad 22 control unit (or preamp) on the left side, and the power supply on the right side which is an old tubed power supply I bought at a ham sale for $2.
I completely rewired the power supply to suit the needs of the preamp, made the perforated steel cover and gave it some gold paint to match the preamp. The power supply would normally be mounted well away from the preamp on a shelf below the control unit.
A new umbilical cable was hard wired to the control unit and octal plug fitted to suit the octal power output
socket on the rear of the supply.

PHONO AMP STAGE, 2001

For each channel, Phono input 6DJ8, V1, both halves in parallel, feeding passive RIAA filter, ( no negative feedback ).
The 318us and 3180us time constant filters are before the second gain stage and the 75us time constant filter is after the second gain stage but before the cathode follower output.
Phono gain stage, 12AX7, V2, with cathode follower buffered output, V3.
The phono amp gives 46dB of gain at 1 kHz, which is plenty for all MM cartridges but for MC a step up transformer will have to be used for low output MC since I found the noise of the 6DJ8 was still too high for MC with outputs below 0.5mV.
And not shown is the earth terminal for turntable grounding leads.
Cartridge loading can be changed from the defacto values shown by using additional components mounted on an RCA plug and plugged into the RCA socket shown beside R1.

The original four Quad input source press button switches are shown near the three IN 1,2,3 sources plus the fourth switch for the phono amp output.
S2 and S3 are the two remaining button switches which can be used as shown for gain and tone amp deletion.
Later made Quad 22 had one less switch section included that was never used to begin withand so in the last revamp of a Quad 22 in 2006 the whole set up was slightly different, and will be dealt with further down this page.
But in this amp when tone and gain isn't used, the input from the pole input from the 4 input switches becomes directly connected to the top of the volume control. If the following power amp is a sensitive enough then no gain is needed, and the signal does not experience unecessary bits and pieces in the signal path.

S4 is a NOS replacement rotary wafer switch because the original had fatigued brass connections which broke very easily.
The tone control amp is a 12AX7 which probably will rarely be used.
For better tine amp micro detail use a 12AT7 with slightly different RL and Rk.
The tone amp is a "unity gain" type with a a standard Baxandal  feedback tone control network which is the *only* way to build a blameless tone control that won't have any sonic signature when in the flat position.
V2 12AU7 line gain amp is a simple SET stage with current FIB with R14 unbypassed.
V3 is another 12AU7 which has a very high resistance input with low capacitance to make sure HF losses after the volume and balance control are minimized.
The output resistance from the 12AU7 cathode is about 1k, so long cables, power amp input resistance and input capacitance will not affect the preamp output signal.

In this latest reformed Quad 22, V1 is an SET gain triode driving a V2 bootstrapped follower output in what is now called a mu-follower amp stage.
V3 is a normal SET stage used to drive the deletable tone control stage and had "unity gain" because the tone network is a Baxandal feedback type with about +/- 9dB maximum boost and cut to extreme frequencies.

V4 is a C output stage with CCS cathode current sink, so that the only signal load is the load that is connected to the output of the amp. This ensures the cathode follower has the lowest distortion possible, and maximum available drive current.

S3 is to provide 3 HF shelved HF cut starting at about 1.5kHz so that there is a selection of 0, -2, -4, -6 dB cuts to HF. This is useful where harsh speakers, or "digital" over processed recordings with excessive HF content above 1 kHz  would ruin the enjoyment.

Note that this particular Quad 22 didn't have enough switches on S1 to have more than 1 switch used for gain deletion, so when the gain stage is switched out, the input source is still connected to the R5 47k input feed to V1.
But this will have no effect because most source impedances these days are from an opamp follower or cathode follower and are low impedance below 1kohm.
A high impedance source is one rated at over 10k ohms, but it won't be affected by V1
input permanently connected.

Fixed bias is used to bias the grid of the  cathode follower, the V2 follower after V1 and the CCS MJE340 bases using divider R25, 26, 27 and 28.

PREAMP POWER SUPPLY FEB 2006

This simple linear power supply is exactly what was used for the Reformed Quad 22 preamp above but the same schematic could be used for any preamp project.
I placed two octal power outlet sockets on the rear panel to allow for a simple phono stage to be connected if
desired.
I wound the power transformer myself with B = 0.85Tesla  and with 32mm stack of 32mm tongue width material which was amongst my stocks of pre-used transformer laminations.

The transistors for the emitter follower type of regulator can be almost any general purpose transistors; I had a couple of T0220 flat pack power transistors normally used in low power amps and so I used the two, since they have an hfe which tends to be higher than one large flat pack bjt.
As it has been drawn, a short circuit of the heater supply will blow the 4 amp fuse, F3, but other refinements like a slower ramp up of the heater voltage could be incorporated.

Reformed Quad22, rear panel, feb 2006, with hand made new power supply.


The umbilical cable is nicely flexible and will curl up easily.

Reformed Quad22 with cover off, feb 2006.


The 4 x 12AU7 can be seen standing in the original McMurdo tube sockets, and the 150uF caps also can be seen mounted above chassis in the line of tubes. The Alps Black volume pot can be seen to the right.

The existing Quad board for components for the tone controls was retained but fitted with new capacitors, yellow, and new Welwyn resistors, blue. Two 4 mm screws hold the cover onto the amp chassis.

Reformed Quad22 under chassis wiring.
  It is very crammed inside this amplifier!

 You can see that 4 rows of switches are not used since there is a second side to the switch bank hidden under what is seen.

 All the original turret connectors used in Quad22 were removed and replaced with fibre strips with brass plated screws to give a better range of connection points.

 All the capacitors are polypropylene and red coloured ones
 are Wima 0.47uF, 630V rated.
 
 The thick solid wire top to bottom above tube sockets
  beneath the 3 dimensional wiring with very short leads
  is the 0V buss rail, separate from the case and chassis.
 

  Wiring for B+ rail voltages and heater wires is with
  well insulated stranded wiring but wires carrying signal
  is 0.6mm solid hook up wire taken from a multi pair
  telephone cable which had about 50 wires of different
  colour coded wire.
  This makes it fairly easy to trace wires in service work
  and such wiring easily fits the small space and it sounds    
  well.

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