Sir Bill. . . . . .

Final ADDENDA upgrade:

The resistor is a shunt across the last tubes heater in the string. Without it the heater may flash from a cold start.

A ponderance . . . ? ? ? ?

(How comes it do be 'dat alls 'dem 12AV6- 12SQ7's , etc . . . equivalencies in all of them 'lebenteen kazillion AC/DC tube radios in all 'de worlds . . . dont's do be also havin' 'dem 'dose 'dere sames identical resistors done beings installed acrossed thems filaments also? . . . (The devil made me do it !)

After evaluating that sets design circuitry, my sole thought on that filamental shunting resistor, would be an encountered situation of emissive hum related with that stage.

E.G. . . .if you will look at all AC/DC series operated AM (AM-FM)receivers you will see the highest audio voltage gain stage in the audio is its first stage . . . therefore being the most susceptible to hum pickup. Tubes typically used are 12AV6 / 12SQ7 or equivalencies .

Examine additionally and you will find that they will have that particular tube wired in its positioning in the filament string . . . right at the very end . . . at the closest proximity to chassis (or floating) ground.

That precludes or minumizes the potentiality of hum pick up from the filament string being its source.

Now look at your set and you will see that they have deemed that 12AL5 as having MORE importance in that respect than the 12AV6, since it is preceding the 12AV6's proximity to ground.

Still ??? . . . they must have encountered a problem, since they additionally shunted the filament to cut back on its emissivity to ~3/4's of its expected value by the use of that particular shunt res value .

As per the Eff- Emm alignment of your sets detector circuitry . . . and its particular designs cutting of discrete component count . . . they are giving you two temporary in circuit add on alignment interfaces using the initial 250k resistor off from detector plate and metering the on /frequency strength of the FM IF signal all the way to even including the secondary the detector transformer.

The common procedure of the monitoring of the final limiter negatve grid voltage does not include the detector stage following it.

Once that optimal IF frequency tuning of the stages is accomplished then that test aid attachmant is pulled and the set of matched resistors are added on to do the zero setting of the detected S curve.

WHAT ! you're fresh out of 1% 100K tolerance resistors ?

Then why not just use this precision work around, requiring those two 100 k resistors plus one 10k pot OR a mini trimmer pot (a la modern solid state tecnology).

Then the 100k's go to the outer legs of the pot with the center termimnal (rotor) free.

You then temporarily tie the 100k resistor ends together and connect them to another isolative/current limiting 100k resistor.

Now you will additionally need a DVM set to its DC measurement mode and a DC voltage supply of ~10 VDC . . or an El Cheapo brand of 9 V transistor battery will serve you admirably.

That will then receive the following hook up, with the battery negative going to the rotor of the pot and the outer sole 100k going to the positive of the battery.You then place monitor metering from the rotor of the pot to the COJOINED ends of the 100k units that are coming off the two ends of the pot.

Move the pots rotor to guesstimated mechanical mid positioning. Set meter ranging for the highest observable voltage and take note of that voltage and see if that initially read voltage can be increased by slight pot positioning.

When that voltage peaks you have set up the trio for a balanced condition of having the "pair" of 100K ohms plus balanced in values , being potentially set much better than that specified 1% tolerance.

To wit . . the 10K pot value option was just giving the consideration of the 100 k parts used being at the extremes of tolerance swings.

If they actually were initially falling into closer matched conditions a downscaling to a 5k or 1k pot selection would permit even more exacting vernier adjustments of balancing . . . limited by resistor temp characteristics . . ambient operating temps and pot stability.

Ehhhh whats that . . . you say . . . .01% matching being achieved?

When balanced . . . hopefully might that pot have been one with a locking nut ? . . . or just use a drop of cement on the shaft . . .then you just use the ends of the 100k's across the FM detector circuitry and the center rotor of the pot receives the metering connection.

Then you just box up and store your "contriviance " away . . . just awaiting for the next aligment job of that nature . . . as I have often seen 47K, 150k 220K 470k choices of resistor values being used for different equipment, but with their tolerance of being balanced values being the critical factor.

Orrrrrr . . . . now finally after alignment you COULD use my oftimes touted FM evaluation technique of using a tuned in FM station and using the same alignment initial procedure of using the 250 k resistor off from one 'AL5 diode plate to be able to get precise metering of the incoming received station and capability of precise tuning "RIGHT-ON" to that station and then, just temporarily, don't touch station tuning again.

Disconnect 250K resistor and its metering and then set up with the "matched" 100 k resistors, see how close you are being to ZERO volts alignment to the S curve from your previous final tuning /zeroing.

Now you can touch the tuning again to slowly tune in THE direction that gives an increase in voltage.

HOLD . . . needless to say that the use of a DVM will be giving far too slow af a settling response, with least significant digit bobbling . . so use a VTVM or a 20k---> upwards VOM or a TVM / FET VM for doing this monitoring.

This voltage readout will be in coincidence with that of a FM response " S " curve display, where you see a single sine wavelet that shows a single positive and a negative node , with the center portion being the ZERO voltage crossover point (where you were fine trimming in that final detector tuning for 0 volts.)

Keep slowly tuning on the selected station until that prior monitored voltage peaks and then just as the curve shows, it would be past the very top of the peak and then start slowly declining in value.

BUT go back to the peak voltage and log its voltage down.

Then come back down the tuning curve and hit the ZERO and switch metering polarity . . or switch leads polarities . . . and watch the negative voltage ascension . . confirm its peak and tune just past it to confirm its decline then come back to the peak voltage and log it down and compare the prior other reading logged down.

If they are not the same you can then further fine tune by micro adjustments of the detector secondary ZERO adjustment to finally end up with a like set of peak positive node and peak negative node voltage outputs.

Additionally this procedure is extremely usfull for using as an evaluative analysis as to the " health " of the alignment of the FM detector stage of an FM receiver.

One just ties in metering to that stage and monitors the peak positive and negative node voltage and compares them for balance.

Taking a hypothertical peak value of +10.5 VDC and -10.4 VDC, you are getting quite close in balanced tuning.

However should you find a +12.5 and a -8.5 (or conversely on polarity) that wouldn't be sounding so good or possibly if even finding a +4.25 and -16.75 . . .-4.25 and + 16.75 for the reading . . . that would be sounding quite atrocious and be quite squirrelly in ease of even tuning into the station.

(And possibly a prior techie or even the less aspiring Casper P. Clodd or Delmer Dumbnuttz with his diddling stick in hand and thinking that he can just start turning slugs and make a "listening" evaluation.)

Lastly . . al fin . . . as per that shunting filament resistor, just lift one end of it it out and see what it does on final evaluation pending the situation that there is no final induced HUMMMMMMMM . . . . . Chummmmmmm.

Depiction of An FM " S " Curve's Tuning /to/ Outputted Voltage Relationships

73's de Edd