I am curious about this. The 461A amplifier I spared for switch addition has this method, a wire of maybe 3 turns wound around a carbon resistor.
What is the point of this and how is this designed? Would a modern design still use it, or does it have something to do with lack of proper inductor materials? Does this have anything to do with the resistor material acting differently in the magnetic field, or would it work equally well if it was in parallel with the resistor?
Not much information about this, other then its a form of de-tuning.
It looks to be a simple parallel RL network that is used to define the amount of negative feedback vs frequency in each amplifier stage .
At low frequency the inductor has very low reactance so the resistor is effectively shorted. This means that the other 180R resistor in series with the RL network provides quite heavy negative feedback. This will tend to reduce the excess gain of the stage at lower frequencies.
At higher frequencies the parallel RL network will look like a resistance in series with some inductive reactance and this will tend to reduce the negative feedback. So this maintains gain at higher frequencies and the aim would be to get equal gain across LF through 150MHz.
My guess is that you have to tweak each inductor to get a flat frequency response in the 5 stage amplifier. Each one of these RL networks will be optimised for a particular part of the LF through 150MHz bandwidth of the amplifier. So you would have to know which one to tweak to flatten a particular part of the frequency range. It should be possible to get a really flat response.
Without this tailored negative feedback the amplifier would have lots more gain at low frequencies compared to the gain up at VHF.
I suspect this 'old school' coil over resistor construction method was used out of convenience and it also saves space. It would be unsuitable for today's fully automated assembly systems using SMD though.