PASSIVE MODULES

The V72 based mixing desks of the tube era mostly consisted of passive modules such as faders, equalizers and other controls, with the associated amplifiers housed in racks close by.
Next to faders such as e.g. W66/67/68, there were two modules worth noting:

W75k - high pass (passive) with several switchable frequencies, inserted between mic and any amp (except V76)
W77 - external gain switch for V77

However, passive modules most interesting for today's use are filters and equalizers made by Eckmiller:

W86 - high & low shelving EQ
W86a - dto, but more versatile controls
HV53 - ("Hörspiel-Verzerrer" = radio play EQ), bandpass with variable Hi-/Lo-cut
HV55 - dto, more versatile controls
HTV34 - ("Hoch-Tief-Verzerrer" = high/low EQ), quite versatile stereo unit
TS10 - ("Tiefensperre" = low cut), with variable frequencies, steeper roll-off than HV5x
HS10 - ("Höhensperre = high cut), dto.
(for impedance reasons, TS10 & HS10 were intended to be used as a pair only, and in the correct sequence!)
NF11 - ("Niveaufilter" = tilt equalizer), a simple yet effective unit with just one control knob: Centered around a mid frequency, it offers either LF boost and HF cut or LF cut and HF boost
PF5-8h - ("Präsenzfilter" = Presence EQ), with 5 center frequencies; boost only - in 8 steps of 0.75dB each (sic!)
(There also was a PF5-8m, which offered presence cut only - obviosuly one of the most obscure and rare items of the lot. Never came across one yet)

All of these models were intended for use with a V72 style amplifier as follow-up; they had an insertion loss of 34dB, exactly matching the gain of the make-up amp (hence the term "multi purpose" for the V72 - it could be used as a micpre or as a make-up amp for faders, filters, equalizers, panorama controls…). They use a quite rare 8-pin Tuchel connector; if you can't find one, modern flat connectors (e.g. like these here) may be used to attach wires to the individual pins.

Something to look out for is that these EQ's need to see a balanced source and load; with their transformerless, fully symmetrical circuits, they will not perform properly when connected unbalanced. Also, the source impedance should be around 50 Ω, the load impedance no more than 4 KΩ (these being typical o/p & i/p impedances of V72/a/b). Otherwise, chances are that there will be too big a drift from the specified frequency responses.

There are some interesting passive EQ designs by other companies that also fit the physical dimensions:
Maihak W49 - bandpass with variable Hi-/Lo-cut, early version of HV53 (see above)
Telefunken ELA 150a - stereo, fixed Hi/Lo Shelf, with controls for both channels together
Teldec MPE-30 - stereo, fixed frequency Hi/Lo Shelf, individual controls for each channel
Teldec MPE-300 - mono, three band design (Hi/Lo Shelf and mid Bell) with selectable frequencies each, unbalanced I/O