[OZ4]

If I were working on this radio I would determine which path the hum is taking. Again assuming that both the tube and the socket are good there are only two likely paths . Either thru #19 cap or #20 resistor into the 27 tube.

The first thing I would do is to lift the #19 cap as that won't affect the bias of the 27 tube and will break the path from the IF detector. If the hum stops then we know it's from there. Hum still there then it's the bias line thru the #20 resistor.

If the hum is from the IF detector then as Steve mentioned the #14 cap and the #18 resistor should be checked.

[Tubesforme]

If I were working on this radio I would determine which path the hum is taking. Again assuming that both the tube and the socket are good there are only two likely paths . Either thru #19 cap or #20 resistor into the 27 tube.

The first thing I would do is to lift the #19 cap as that won't affect the bias of the 27 tube and will break the path from the IF detector. If the hum stops then we know it's from there. Hum still there then it's the bias line thru the #20 resistor.

If the hum is from the IF detector then as Steve mentioned the #14 cap and the #18 resistor should be checked.

I have tried various 27 tubes and each causes different amounts of hum. I have changed out part 19 twice, with different brands, NOS with no change. Part # 20 measures right on 470K. I have cleaned the tube socket, by hand and tightened the contacts, no change in the hum. I had connected only part # 20 to the grid of the 27, leaving part #19 disconnected. The hum was still present. Naturally when the grid is left open the hum is extra loud. I have tried bypassing part #20 with a small electrolytic, connected to the supply side of the bias. Still had the hum. The only way to eliminate it is to ground the grid of the 27. Part #14 and 18 have been changed out twice, and both test good. I do suspect feedback from part #21 even though it is NOS. Also I have a Philco speaker with the correct output transformer coming because I am using a universal output transformer and a different speaker with a higher ohm field coil. The hum might also be some feedback from that transformer. Once that speaker arrives I will retest the radio and if the hum remains then replace the interstage transformer. This job really needs a scope, which I don't have anymore.

[Tubesforme]

In an earlier post mikethedruid asked if you have checked the ground on condenser 14. Have you done that? Also, check the value of that condenser and resistor 18.

Steve

Yes, yes and yes. First things that I did before even posting this request. I also changed both parts out with known NOS good parts. All grounds have been tried with jumper clips and none changed the hum.

[Ron Ramirez]

I also changed both parts out with known NOS good parts.

Perhaps this is your trouble (or a contributing factor)? New-old-stock capacitors are...still bad. Brand new capacitors are usually (though not always) good. Ditto for resistors.

[OZ4]

What happens when you apply a something like a 100uf cap to the bias line B to ground. This should swamp any ac hum riding on this line. There should be -50vdc here.

[Tubesforme]

What happens when you apply a something like a 100uf cap to the bias line B to ground. This should swamp any ac hum riding on this line. There should be -50vdc here.

I don't have anything that large. I tried a 10uf and that did nothing.  The bias is correct or I would not have the -56V on the grids of the 71's, measured from one side of the filament. The grid of the 27 reads -7V measured from the cathode.

[Tubesforme]

I also changed both parts out with known NOS good parts.

Perhaps this is your trouble (or a contributing factor)? New-old-stock capacitors are...still bad. Brand new capacitors are usually (though not always) good. Ditto for resistors.

Ron, none of the capacitors or resistors are NOS. All are new and I have tried more than one of each, from different vendors. The resistors that I uses are carbon comp type and are new, but I consider that style NOS. All capacitors and resistors test good and are right on spec, when tested with a digital meter.

[Tubesforme]

It looks the problem is after the 24 but before the 27. It could be lead dress. Try moving the 27 grid lead to see if that makes a difference.

Steve

I agree that is where the problem is. I had already done everything that everyone has suggested before I posted this request and I have redone everything again with no results. There are now only two items I question. One is the 250uuf capacitor #16 off the detector plate and the replacement interstage transformer that I put in new. (It might have leakage feeding back into the grid circuit of the 27, but it's not affecting the 71's and they are biased off the same circuit.)

[Ron Ramirez]

Tubesforme, you initially said NOS. I could only go by what you said.

Carry on, folks.

[Tubesforme]

Have you checked the ground on condenser 14, the 0.25 mfd between resistors 17 and 18, and that of condenser 16 ? a bad ground on either might be causing your problem.

I originally did rebuilt this condenser block. It has two Xicon .22Uf @ 630V caps inside the housing with the commons soldered directly to the can, as in original Philco cap was and then bolted to the chassis with the original self taping screws. Resistor #17 is a new 470K 1W and resistor #18 a new 100K, that I replaced a second time because the first one read low. Both are new 5% carbon comp type. Capacitor #19, the .01 was replaced a second time, this time with a new Illinois Capacitor .01uf @ 630V. All grounds have been redone as have been all soldered connections to the capacitor blocks. All excess rosin was brushed off every connection. Grounds were also jumped to the chassis with jumper clips when I was testing it. The only part between the 24 detector and the 27 audio tube that has not been replaced is #16, the 250 uuf mica. Tubes have been substituted three times with NOS tubes. All tube sockets were cleaned by hand with a Q tip and Deoxit D5 and tightened. When the 27 grid is grounded or the tube pulled the hum leaves. Pulling the detector does nothing to help the hum. Disconnecting part #19, and leaving part #20, in circuit the hum remains. All of this was done long before I posted on this site for assistance. I do not suspect anything between the detector and audio tube as the problem. Part #21 the interstage transformer was replaced by one from AES, supposedly equivalent to a Stancor A53-C. I wondered if by chance there might be leakage in it. The hum sounds like 60Hz. The voltages on the grids of the 71's is correct. I now have the original Philco speaker assembly and that brought the voltage at point J to the correct value. Bias on the 71's was measured from one side of the filament. Bias on the 27 was measured from its cathode. Voltage on the 27 cathode was measured from the PT, CT. B+ voltages were measured from the PT CT. This response isn't  met to be flippant, I just wanted everyone who has given advice to know what has been done and what didn't work. Thanks to everyone for your ideas.

[radiohenry]

Hi Tubesforme,
    I would like to suggest reviewing the rebuild of the filter block. I suspect there in lies the problem. Certainly it can be rebuilt using electrolytics  however the original did not have electrolytics or if I recall correctly .22uf.
  The filter block used two 1.0 uf , one 1.5 uf and one .13uf and if I read this correctly pin #1 was the common ground.
  On the last 20 that I rebuilt the filter block I removed the old components and using non electrolytics wired all the components inside the old can to the terminals indicated on the filter block diagram . When I finished replacing all the other caps and resistors it played like a new radio.
   Just my thoughts after I was fortunate to have an excellent experience in the rebuild of a model 20.
                                                    Good luck,
                                                             Henry

[Tubesforme]

Hi Tubesforme,
    I would like to suggest reviewing the rebuild of the filter block. I suspect there in lies the problem. Certainly it can be rebuilt using electrolytics  however the original did not have electrolytics or if I recall correctly .22uf.
  The filter block used two 1.0 uf , one 1.5 uf and one .13uf and if I read this correctly pin #1 was the common ground.
  On the last 20 that I rebuilt the filter block I removed the old components and using non electrolytics wired all the components inside the old can to the terminals indicated on the filter block diagram . When I finished replacing all the other caps and resistors it played like a new radio.
   Just my thoughts after I was fortunate to have an excellent experience in the rebuild of a model 20.
                                                    Good luck,
                                                             Henry

Henry,  I did not use electrolytic's. I used the original values and they were all new Metalized Polyester rated at 630V. The filter uses two 1uf, one 1.5uf and one .13uf (1-.1 paralled with 1-.033) The 1.5uf is the AF supply filter and yes it has been changed twice. I would never use electrolytic"s to rebuild a capacitor that originally used paper type.Tubesforme

[Tubesforme]

If I were working on this radio I would determine which path the hum is taking. Again assuming that both the tube and the socket are good there are only two likely paths . Either thru #19 cap or #20 resistor into the 27 tube.

The first thing I would do is to lift the #19 cap as that won't affect the bias of the 27 tube and will break the path from the IF detector. If the hum stops then we know it's from there. Hum still there then it's the bias line thru the #20 resistor.

If the hum is from the IF detector then as Steve mentioned the #14 cap and the #18 resistor should be checked.

You are very right on isolating where the hum is coming from. I know it is from something in the bias supply and the only part I am questioning is the interstage transformer (even though it is a new transformer) as all other paths have been eliminated. The bias resistor and the power divider resistor all are good.

[LASJayhawk]

The interstage transformer center tap is connected to point B and not the chassis ground, correct?

[Tubesforme]

The interstage transformer center tap is connected to point B and not the chassis ground, correct?

You are correct. It is connected to terminal #1 of the main filter pack, which connects directly to the center tap of the PT and pin 2 of the power divider resistor. This is the exact way the original transformer was connected. If it wasn't connected correctly, there wouldn't be -56V on the grids of the 71's.