Here is one, finally, for all the DJs of the world. The Isolator circuit has been used for years to create the classic effect of dropping out a specific frequency range, although it can also be utilized as a very effective general EQ as well. This particular circuit is a hybrid affair, the state variable filter is solid state and it is married to a tube output stage stage. This offers a number of benefits: truly wide ranged filters are possible while at the same time offering the compression and harmonics of a tube stage. This means overloading no longer generates nasty distortion artefacts but a wonderfully smooth response.
Even better, as the equalizer is so flexible, it is great on virtually any source and is recommended in the studio just as much as at a live gig. This particular unit is fully transformer balanced on the input and output and has a bypass switch that removes the filter from the signal path but keeps the tube circuit and master volume connected for even more variation of tone.
The original case was rough and ready with plenty of scratches but that adds to the appeal sometimes especially when the front panel can be produced in a matching red color. Further variations of this idea could be reduced in size even more with subminiature tubes and unbalanced connections for portability.