[Hallicrafters] SR-400A cathode resistors

[james.liles]

Hi Glen:
Good conversation and thanks for the response.  The cathode on the 6KD6 has two .02 caps in parallel and two 13 ohm 1w resistors in parallel for a total of 6.5 ohms and .04 ufd combined.  .04ufd at 3.5 Mc = xl = ~ 1.1 ohm excluding any inductance.  The resistors are carrying ~ 17% of the rf load.  At 30 Mc, they carry ~ 2% of the rf load but the skin effect is more profound.  Capacitive bypass in this case gets you close to ground but no carrot.  If these composition resistors were subjected to pure dc at these levels, they would never fail.  The film resistors do not have the problem.  Using carbon composition resistors in this configuration is simply not appropriate but was common at the time.  Kindest regards Jim K9AXN

Original message:

Actually, the cathode resistors are not carrying r.f.  The main reason that they are in the circuit on most equipment is to provide a shunt resistor for measuring the cathode current (which the "plate" meter in most "boat anchor" equipment reads instead of true plate current).  The cathode resistors are bypassed to ground with capacitors so that they do not have any r.f. across them.

Glen, K9STH

Website:  http://k9sth.com


--- On Sat, 8/15/09, james.liles <james.liles at comcast.net> wrote:

The problem with these resistors is that they are carbon composition carrying RF. They have the same skin effect properties as wire --- the flow is forced to the surface causing the resistor to overheat even though the rated heat as calculated for DC is not exceeded. The 10 meter segment of the SR-150 tank is nearly always melted --- same reason. A good choice for a replacement is carbon film or metal film resistor. Or if using a carbon composition, watts times four at HF.