Tach filter needed for ready to run distributor?

standard old tach expects a 100-300 spike from the coil negative for each spark

high power ignition can cause that spike to be 400V+ and an aged old tach doesn't like it....
the tach uses a range of capacitors resistors and transistors to count the rate of sparks, those capacitors have to charge and discharge to switch a transistor on and off to make the little needle move, if they are hit with more voltage than designed for, the simple circuit that puts pulses into the coil that drives the tach pointer is just kinda ON for too much of the time when it should be offonoffonoffon at a rate the illustrates your rpm.

causes the needle to quiver a few hundred rpm over what it should be at idle then at high revs be all over the place or pinned to the 7000 rpm end stop.

fixed mine with one of these

Vs-40hfr40 for sale | eBay

VS-40HFR40 zener diode

you bolt it to your coil mount or a bit of angle bolted to your car, using its nut and washer,
run a wire from the solder tab to a connector which you put onto coil negative. that's all you do with it.

absolutely dictates that the spike that triggers your tacho will never be greater than 400 V
but doesn't do anything else. one of those 40s in its name means 400 volt so a one with 20s would be 200 volt or 35 would be 350 volt etc R is an indicator that the bolt end of it is earth.
i chose this oen cos i had one 350 or 300 v version would also be fine

350 Volt one was standard fitment along with an HEI module on a XJS jaguar.
its a clamp in electronics terms

if you were running a modern tach it counts 12 volt square wave pulses, that is when you need a special feed from the ignition box or ECU or a converter of some type.

there are only 2 types of electronic tachos really direct to coil negative or modern ones that like a proper 12 volt feed from some other electronics.
direct drive don't count they use a speedo drive cable off the distributor

each one of those solutions for a modern tacho is creating the right feed either off the circuitry triggering the spark, or is converting the spikes of 100s of volts at the coil negative into something nice square and 12 volt ish, just by a different route

modern car instrumentation is all 5 and 12 volt stuff our old style 60s/70s stuff is generally connected direct to the thing it is measuring hence old style tacho likes big 100+volt signal and a standard 12 volt feed for anything fancy it may have in addition, some for its function and potentially one for illumination.