The AM Forum

Hi all,

I'm working on a circa 1938 HRO  (serial no. Y161).  I'm in the middle of a recap/re-resistor job on this receiver and I have a question about R9 (V6 cathode resistor for the 2nd IF).  The question I have is simply, what value should this resistor be? This HRO appears to have been equipped with 2.5V tubes originally but upgraded to the 6V tubes at some point. The 2nd IF tube is now a 6D6. The early HRO manual calls for a 300ohm 1/2 watt resistor. The 1939 HRO manual specifies a "1000 to 5000 ohm 1/2 watt resistor".  Why the range of resistance given in the later manual? 

This receiver had been heavily (and poorly) worked over in the past by a prior owner. R9 currently in the receiver is not the original so I can't trust that it is the right value.  Hopefully the receiver experts here can set me straight.

Thanks!
Brian - W8KHZ

Brian

From the HRO 1935 schematic available on the BAMA web site... it appears that all of the 6D6 amplifier stages (1st and 2nd RF, and 1st and 2nd IF) are biased the same way (with a 300 ohm resistor from the cathode to a common bus). It is unlikely that this higher level stage of 6D6 RF/IF amplification would be biased to produce a lower plate current than the lower level 6D6 RF/IF amplification stages. In the list of components, R9 (2nd IF bias resistor) is listed as 300 ohms 0.5W... as are all of the other 6D6 bias resistors.

Therefore... it would appear that 300 ohms is the correct value for R9

Stu

A number of later receivers have a "gain set pot" to set the IF strip gain to a desired value during alignment.  I imagine the range of values was specified because during the original factory alignment this resistor was chosen to set the total set gain at some desired level.

A lower value of cathode resistor provides less bias and higher gain so if the gain seems excessive (IF overload or instability) you can increase the value.  I would treat 300 ohms as the minimum value and go up from there if/as needed.