[Patrick]

All my votage measurements were to chassis. Let me go back and remeasure to "B", as you did (how did you decide to use "A" filament center tap?).

I also fed 1400Hz to the antenna, and got nothing. I then fed it to one side of the volume control, and got a very strong signal. I then attached an 18" wire to the volume control, and got strong station reception, but adjusting the trimmers on the 3 variables was squirly, cutting in and out. I'm going to go back and replace the mylar on all three trimmers. And even though both sides of the antenna coil ohmed out good, it looks like there might be a problem there, as well. I'll pull it out and check it. Someone tried to re-cap this unit in the past, and did a really bad job. Lots of flying connections, cold solder joints, wrong wiring, etc.

Maybe all the voltages are O.K., it's just that we were using the wrong point for reference.

[Radioroslyn]

Tuning Condenser Wiring Trouble – Models 20-21-70-90 – Philco Library (philcoradio.com)

[Radioroslyn]

Does the det tube look like it's lighting up as bright as the other 24A's? You may try checking the voltage at the tube's pins while the set is operating. 2 1/2 volt tubes pull a little better than twice as much as 6v tubes. Any sort of oxidation or poor contact can diminish the heather voltage and the efficiency of the tube. Right now your 24A isn't hauling down enough plate current to drop the voltage across the plate resistors.

Happy Hunting!

[Bill Bacco]

Also don't forget to verify your power supply voltages are correct. The numbers you have for the 71A plate seem kind of low. This could be related to where you were measuring from (ground vs. point B). It would be interesting to see what you have on each side of coil #27. Maybe rfeenstra could do the same measurement.

Bill

[Patrick]

New voltages. Mine or on the left, rfeenstra (sorry, don't know your name) are on the right. All voltages taken from "B", except the 71A cathodes are taken from the filaments of the same tubes.

The only BIG differences are the 71A plate voltages.

Variable caps are fine, antenna coil is good. I can pick up stations, but they are all garbled no matter how I adjust the variable trimmers.

[Image: https://i.imgur.com/qaFiy0q.jpg]

[Patrick]

Left side of choke is 261, right side is 232, measured to chassis.

[Patrick]

Well......that last one was my goof. I've had the speaker plugged and unplugged so many times, and it goes in 2 ways. "J" and "D" were reversed. I now have 220 on the plates of the 71s.

So it looks like all might be good. HOWEVER, my sig gen gives me strong signals at different frequencies, but I can't pick up any stations!

I'm going to set it on the shelf until tomorrow, and wait for replies. Need to find which tube is giving me microphonics on my 610. To be continued...

[OZ4]

My armchair calculations.

As for the DC bias on the #24 detector:

(Many times the cathode is grounded. With the grid being driven - v. )
In this case bias is set up by the fact the cathode is raised up off of chassis ground through the resistor #12 causing is to become +v in relation to grid which is grounded through the IF winding.

So measuring from ground to the grid will not give proper a bias reading. 

The bias is determined by the DC current through the tube cathode resistor which is the same at the plate circuit.

It would be helpful if they had listed the plate current for the detector. 

Calculating the bias voltage via current.
So the plate supply voltage comes to point J via the choke #30.
I guess this voltage is approx 150 volts At J. (Measure this and recalculate as below.)
( This is from chassis ground. The chart voltages don't mention the reference point. Measuring from point B would increase this by
 the negative offset of the 187 ohms at #2 and #3. )


From the chart the voltage at the plate is stated to be 35v.
(I am sure this is lower than would be measured with today's voltmeters.)
that means the rest of the approx 150v source is dropped across the plate resistors #17,#18.
And 150-35=115v drop. And assuming they are good values the current through the loop should be approx .19MA. That's less than 1 MA.


So with bias determined by the DC current through the cathode resistor. Now with .19MA x 50,000 ohms we have 9.5volts. And that should be -9.5v.


Apparently the actual plate voltage is not critical so all this is pretty fuzzy.


This seems like very little current compared to the other tubes. But as this is being used as the detector I don't think it needs much.

[radiohenry]

Patrick,
 You were correct in your earlier post . For the grid voltage you need to measure from the filament to grid . You should not be reading 0 volts. 
                                         GL,
                                               Henry

[Patrick]

Grid to filament on the 71's is 30 v. I was just matching rfeenstra.

[radiohenry]

Well that makes more sense but still kinda low I would think.
Henry

[OZ4]

More thoughts on the detector. Maybe this will help.

Correct me if I am wrong, 
The detector is not operating as class A. It just occurred to me that the detector is bias just at cutoff with no signal coming from the IF. That would mean no plate current. When the IF hits the grid only the positive going half of the signal turns on the tube thus acting as a diode. This is filtered by #16 but only to the point of audio frequencies giving us the recovered audio.


In other words  if you ground the grid there should be very little plate current. The positive bias of the cathode resistor should almost cutoff the tube current. There must be some plate current to cause bias on the cathode resistor in the first place. So very little plate current to start with. That's why the plate voltage is so high. Didn't you say there is no signal?

[rfeenstra]

Chris, that's how I thought the system worked. Plus, it gives amplification. but then, I've been wrong before!! With the tube normally very near cutoff, the plate voltage would be high. I still don't understand where Philco got the 35 volts on the plate. Of course, given the very high load resistance in the plate circuit, it wouldn't take much grid voltage to take the plate voltage down. I wonder how varying grid bias affects the sound of the detector/amplifier.

[OZ4]

Yes ideally it should be just at cutoff but since the bias is created by the cathode current there must be some current to begin with.

[rfeenstra]

So I played with the bias on the detector by dropping the cathode resistor to under 10K ohms. This brought the plate voltage down to about 75 volts, but I started to pick up distortion. I presume the tube reached very close to full saturation. No way I can figure how they got the 35 volts unless their 1000ohms/volt meter drags it down that far... It doesn't! I dug out an old VOM and it dropped the plate voltage only about 6 volts.

Chris, the grid is at ground potential. Coil is very low resistance. About 4 ohms. There is some current as the drop across the two plate resistors is about 20 volts. Not fully at cutoff.

For grins and giggles I hooked up the scope to the detector grid and plate. On the grid, with a strong AM signal, I got about 800mv modulated RF with the probe set to 10/1. Even this reduced the volume quite a bit. On the plate with the probe sill at 10/1, I get about 200 mv of RF, and almost 4 volts of AF. No reduction in audio volume with the probe connected to the plate, as would be expected.

Patrick, make sure your coils are good. I know mine measured ok, but there was a lot of green corrosion due to the dreaded Philco coil disease. I just decided to rewind them to be sure. Only the outside winding needs to be replaced.

Rob