SIR JOSEPH:



Sorry for the time lag for further reply . . . . other pressing “bidness” demands.



Indeed, that volume control on your set is different than the usual, in its respect of toying with the frontal RF tubes gain instead of going back to the audio stages and inserting it between the freshly detected AF and the audio amp string.

It may end up not giving you the smooth transition that the other circuitry provides, BUT, you describe an abnormal effect, in the respect of the need of bringing up vol control adjustment, to a threshold level where punch thru finally onsets, albeit, with somewhat distorted or choppy sound, I would expect, and that along with your mentioning of the deficiency of perceived max volume not being attained.

By their inspection, any chance that the speaker or output transformer could be other than the original ?



Initially, lets touch on the operation of the volume control circuitry of your set, referring to the add on schematic mark up at the bottom.



Take note of the leftward feed of the Sub B+ over thru the [ ORANGE BUSS ] and its branching up to feed the I.F. transformers and then hitting ITEM # 23 to reduce the voltage level down a bit to feed the ‘D7 and ‘A7 screen grids along with their receiving a RF decoupling of that supply via their ITEM # 12 being .1 ufd capacitor.

Now, our real point of interest is the cathodes of the pair, [ MAGENTA BUSS ] where one would normally see them getting to ground quite readily, but in this case, they are encountering your 3K “volume control” as a variable cathode resistor.


If your controls wiper is at min volume position, with it being positioned such that the whole 3K is into play, you will have totally knocked the gain on those stages. . . .WITH. . .the consideration that if you additionally look at ITEM # 21, it is additionally placing a lowered bit of B+ level on that [ MAGENTA BUSS ]as a counter poise biasing.


(‘ole ITEM # 10. . . .yells out. . .HEY, don’t forget me, I’m over here smoothing down and filtering this mess, that’s on the buss, so that it doesn’t have the gain fluctuating slowly up and down with a woooooompa-wooooooompa-wooooompa.)


Nooooow, as one slowly turns the volume control CW, its resistance incrementally decreases, and eventually cancels the effect of the positive voltage being put on that buss and then the tubes start kicking in more gain as the controls resistance diminishes below the 1 KΩ barrier and then finally. . . . . .HOUSTON. . . . . . .we now have a cathode resistor !


The progressed RF gain ends up as increased audio production down at the grid detection stage of the ‘76.

A final referencing back on that "vol control" / emitter resistor, note that they don't want you dropping its minimum set value below 250 ohms, seems like they have internally added resistance in the space where the wiper travel stops until the point where the connective interfacing to the
end of the element is made, thus stop action on the minimum resistance it can be set to. Now by crunching the numbers on that Buss, that voltage presented to the wiper to the low resistance setting seems to not present a problem for a conventional 2 w control used in that position IF you subbed in a conventional 3 k pot.


NOW, if you used a common 1/2 carbon watt unit, that just might burn the resistance coating of that control when used for the subbing ,when its run down to that very last low resistance end area.('moke. . .'moke. . .I smell 'moke !)

Normal sub replacement procedure would be to use a fixed 430/450/470 Ω fixed resistor where I have shown that magenta square bump out on the schema. That accomplishes your resistive stop action and permits a conventional controls use.

Now, in looking at the audio on past that ‘ 76, I’m assuming credit for your already having confirmed that the 1st grid circuit item 22 resistor is to tolerance, as well as that tubes 250k plate resistor and the following. . .#41 tubes. . .500K grid resistor. . .along with the correct installation polarity of 41 cathode resistor bypassing electrolytic item # 9. . . .along with all of those parts values being OK ?


The specs for the #41 tube is having it using an ~680Ω cathode resistor, in order to attain the 1st grid biasing values, when using lower values of plate supply B+ to it, but with your encroaching up into the mid 200V, range the spec is for a 480 / 500 Ω cathode resistor, which your circuitry is deploying.

Additionally a full driven #41 would be using up in the order of ~38 ma and the screen using ~6 ma.

You cathode current is suggesting of ~28 ma with your volume being down, plus there is your additional thoughts of how much it seems that your B+ is being pulled down.


A 250 supply should give a hefty sound level, within the limitations of the ~3.5 watt power spec of the tube.

( Along with its magnificent 10% distortion specs at that level.)



How about being SURE that item 14 (.015 ufd grid coupling cap) . . . .even if new. . .presents no upsetting of the 1st grid bias of the
#41.


Should that cap let micro levels of B+ creep thru it, it would upset the biasing on the 1st grid and not be fully acquiring the level being established by the cathode resistors presence / utilization.


A positive upward shift of the 1st grid voltage level would be progressively upping the current draw by the tube and thus not be fully responding to incoming audio levels, since it is already at a level higher than the positive peaks of weaker incoming AF info.

That is, unless you progressively run up the volume, on up to that point where the positive and negative nodes of the AF are at such a level that the tube will start responding solely to the PEAK NEGATIVE NODES and PARTIAL POSITIVE NODES and then start a partial coming out of saturation and the creation of distorted and choppy audio during those partial transitions.



To confirm that #14 caps “leakage health”, disconnect the caps lead at its #41 side and have the volume down, power up the set and have DC metering. . . .DVM preferred. . .or VTVM. .. . and use negative meter lead as ground referencing and positive lead to that caps loose lead.
Then you take a grounded jumper lead to swing over and temporarily short that that testing juncture to ground, then swing away to release that shorting action, and THEN see if any DC voltage above ~100 mv comes back as leakage. If so. . . . .find a GOOD capacitor for its replacement.



Two final thoughts would be a 41 with grid emission or a gassy condition, which would require a subbing in of another 41 to confirm that as being the cause of low audio / excess current consumption / distortion.