The PHILCO Phorum

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This chassis has been fully restored with all of the original value components. It works quite well, however the second you turn it on you get a hum that builds slightly as the tubes heat up. Once the tubes reach operating temperature, the hum remains and it can't be varied in intensity. In normal room volume it will drive you crazy. All tubes have been changed, several times. All resistors and capacitors shown in the picture have been changed, the ones in the B lines twice. The only capacitor that hasn't been changed is part 16, the 250uuf mica capacitor in the 24 plate circuit. Unless indicated, following voltages taken from PT CT. Meter being used is a new Fluke DMM. Voltages taken with volume full on and no antenna. I can't ground the chassis. The interstage transformer was replaced with one from AES which is indicated to be equivalent to Stancor A53-C.

#24 detector plate reads 117V
#24 detector screen reads 17V
#24 detector cathode 8V

#27 audio grid reads -7V common point cathode.
#27 plate reads 130V
#27 cathode reads 7.5V 

The voltage divider resistor measures correct resistance between all sections. Point F reads +19VDC and Point B reads -7V.


The grid voltage for the PP 71's read -52V

The hum can only be eliminated by grounding the grid of the 27 audio tube. Grounding the detector grid or pulling it out of its socket makes no change to the hum. The primary filter has been rebuilt twice, with original value 630V capacitors.


Any idea? Thanks

Could there be a bad connection where a ground lug is riveted to the chassis? The thing that jumps out at me is the cathode of the 27. If you are using the chassis as the reference, the 27 cathode should read zero volts. If you are using point "B" (the center tap of the HV winding) as the reference, it should read about 8 volts.

I would use a clip lead connected to a good chassis ground and jumper every ground connection in the chassis, starting with the cathode of the 27. 

Steve

What make you think that it's the 27 tube? What's going on with the output tubes? What are their voltages? And remember the voltages in the chart are probably taken with an old school meter not a 10 meg ohm + meter.   I would hang a 5meg resistor across a modern meter to simulate the load of an old meter.

Chris, the only reason I mentioned the 27 tube was because of the he had a 3.1 volt reading on the cathode which is connected to chassis ground, which led me to believe the riveted ground lug had a bad connection.

Steve 

I would try poking around the grids with a .1uf cap to ground to localize the source of the hum. The cap will ground out any signal without messing up the bias. Make sure voltage rating is high enough. An oscilloscope would help here too. And don't forget to check the tube sockets for bad connections.

The voltages on the detector may read high because you are using a high impedance meter. Try narrowing it down by pulling tubes to see if the hum goes away. Start with the 24 then the 27.

Steve

(10-23-2015, 09:34 PM)Steve Davis Wrote: [ -> ]The voltages on the detector may read high because you are using a high impedance meter. Try narrowing it down by pulling tubes to see if the hum goes away. Start with the 24 then the 27.

Steve

Steve it will leave when the audio tube (27) is pulled, but nothing changes when the 24 is pulled.

With all the tubes in place, try grounding the grid of the 27.

Steve

(10-23-2015, 11:01 PM)Steve Davis Wrote: [ -> ]With all the tubes in place, try grounding the grid of the 27.

Steve

When grounding the grid of the 27 audio tube, the hum disappears completely.

I also used a 4.7uf/450V capacitor and connected the + to ground and the - to the grid. This too kills the hum but also the signal.

Byu the way I again checked the detector voltages using the cathode as the common point and the voltages on the tube didn't change.

Any other good ideas?

Does the hum go away if you pull the type 24 tube ?

I believe that some hum was considered normal back then. It gave the owner an illusion of bass response, and let him know the set was running.

(10-24-2015, 06:04 PM)mikethedruid Wrote: [ -> ]Does the hum go away if you pull the type 24 tube ?

No, it remains the same. Pulling any of the other 24 type tubes make no change either.

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.

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

(10-24-2015, 11:01 PM)Steve Davis Wrote: [ -> ]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 have already thought about that. In fact I changed out the lead from the .01 coupling capacitor block and the grid of the 27. I tried moving it around first, nothing really changed. I think the problem is the fact that the grid of the 27 is running at -7VDC, which is far too high for this tube. One person told me to try different values of grid bias resistors to lower it. However why, what caused the voltage to get so high in the first place? Further proof is when I put a small electrolytic with the + end to ground and the - end to the grid, the hum drops and I can hear the station to which the radio is tuned. However the capacitor does drop the volume to a low level at the same time it removes the hum. I was suspecting the voltage divider/bias resistor, but disconnected all of the terminals and checked the resistance and all sections tested correct according to the Philco schematic. Tubesforme