philco: 60B

Hello all,

I'm replacing the speaker in this 60 philco with a permanent magnet speaker. I've replaced the field coil with a 10watt 1k ohm resister. the radio works ok but the resister gets very hot. Is this normal or am I doing something wrong?

Kind regards,

Bill

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Hi Bill,

a possible overheat of the resistor could derive from a fault condition as an improper bias of the power amplifier. Anyway remember that the nominal power ratings of a wirewound resistor could be specified assuming a component temperature as high as 250°C. In this radio we have a current in the order of 70 mA and a power dissipation in the order of some 6 W. The original field coil stayed cooler, because of its mass and dissipating surface and even because the surrounding metal frame. The temperature of small resistor could easily rise to a value not excessively harmful for the resistor itself, but enough to melt the solder joints.

A neat solution is to use a 25 W aluminum housed ww resistor, screwed to the metal frame of the radio. You can refer to the Mouser types 72-RCH25S10000JS06 or 756-WH25-1K0JI, both 1 Kohm, 25 W.

Emilio

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Thanks for your help Emilio, Checking the voltage drop across the resistor With a 1000 ohm 10watt resistor installed It tests about 150vlts. Pluging this into ohms law, you're right, I'f my procedure is correct I should use about a 25watt resistor. 

Bill

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Dear Bill,

if you read 150 volt across the resistor, you have also to investigate for the reason of a quite heavy overload. From the Ohm's law we can assume a current flow in the resistor of about 150 mA, about twice the expected value. The most likely cause should be a leaky coupling capacitor between the plate of the audio preamplifier, pin 2 of the 75, and the control grid of the power amplifier, pin 4 of the 42. The capacitor is identified as 33 in the diagram. Try to check the voltage from pin 4 of the 42 to the chassis. In normal condition you should read about - 19 volts. If the absolute value is lower, replace the capacitor with a polyester dielectric one.

The overload can also explain the failure of the original field coil.

About the power of the wirewound resistor, as I told before, its nominal value usually specified at very high core temperature, around 250°C. I prefer to use larger resistors, to operate at considerably lower temperature. It is safer, lasts for the lifetime and costs about the same.

Emilio

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Hello Emilio,

When I test pin 2 of the 75 I get about 170v DC. Pin 4 on the 42 shows about 4.5v DC. Thats not close to the -19 DC required. When I install a new #33 condenser it doesn't seem to make much of a defferance. Is there anything else I can try?

Bill

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Hello Bill,

what kind of capacitor are using as replacement? Anyway disconnect for a moment the capacitor lead from pin 4 of the 42 and check again the negative bias voltage on the grid. It should be around -16 or -17 volts. Also check the 0.5 meg resistor 35 for open.

Let me know

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Hello Emilio,

With the capacitor disconected the negative bias  tests at -2volts on pin4 of the 42 tube. I'm using a .02 630v metal film polyester capacitor.The radio does work with the old or new cap installed but has very little volume.

Thanks

Bill

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Hi Bill,

first measure the resistance from the center tap of the HV winding to chassis ground. It should be about 260 ohms. Maybe that the metal can of the aluminum filter capacitor touches the chassis. Then measure the DC voltage across the bias resistors, 235 + 32 ohms, identified as 47. If here the voltage value is right, -17 volts or even more negative, and the grid resistor 35 is good, we must assume that the power tube 42 is gassy and must be replaced.

Emilio

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Hello Emilio,

The center tap of the HV winding to chassis tests at 269 ohms. the DC voltage across the bias resistors test at -20 v DC. The grid resistor tests a little out of range at .66m ohms. I've tested the tube and also replaced it with one I have on hand with similar results. I did however notice that  the reading on pin 4 of tube 42 is not consistent. it seams, when the set is cold, the reading is close to correct and as the radio heats up the reading changes and goes close to 0 volts. Could it be that as the  10 watt resistor, I'm using for a field coil, heats up it changes my reading?

Bill

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Hi Bill,

the bias voltage source, based upon the divider 47, seems to operate quite well. Since its negative value increases in overload conditions, a reading of -20 volts can be acceptable, even it is not consistent with the drop of 150 volts across the 1 k filter resistor. In fact the current flowing in both the resistors, the 1 K and the voltage divider 47, is the same. If the drop across the 1 K resistor is about 150 volts, we must assume a current of about 150 mA. The same current, flowing in the 269 ohms resistive divider, should result in a voltage drop of about -40 volts. Anyway we can assume that the huge ripple usually present in overload conditions can affect the accuracy of measurements. Even the poor emission in a tube well beyond the end of its life could explain different readings taken at a later time.

What cannot be explained is the voltage drop across the grid resistor 35. The only possible sources of current in the grid resistors are either a leakage current in the coupling capacitor or a grid current in the tube. With negative bias, the grid current in a good tube must be zero. Since we read a drop in the grid resistor even with the coupling capacitor disconnected, we must assume the presence of a positive grid current. This is generated by positive ions when the vacuum fails, probably due to overheat of the tube electrodes or even to small leaks in the glass. Ion current increases with heating and this also explains why the grid bias falls near to 0 after a short warm up.

You must replace the 42 with a good one. Then you must check the drop across the grid resistor 35 for a near to zero value.

Emilio

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Hello Emilio,

I've replaced the 10 watt resistor with an aluminum 25 watt 1.2ohm replacement. After doing this all readings have become much more consistant. The voltage drop across the new resistor is now 105 volts. Pin 4 of the #42 tube is now -3.4 volts with or without the new capacitor. with the capacitor disconnected it measures -.7volts.  I then tested the voltages on both sides of the #35 resistor to chassis ground. One side tests at -14.78 and the other side tests at -3.5. the resistance measures .772m. The HV windings measure 33ohms and 267ohms. I tested the voltages of both windings to chassis ground. The 33ohm ohm winding tests at-1.78 and the 267ohm winding tests at -11.5. I get nice clear audio but no sensitivity (very little volume). Is there anything else i can check?

Thanks,

Bill

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Hi Bill,

the bias is still wrong and I even see some inconsistencies in the values reported above. You read a drop of 105 V across the 1.2 K resistor and hence the current can be evaluated in about 90 mA. The same current flows across the voltage divider identified as 47 on the diagram. Since the total resistance of the divider is about 270 ohms, we should read about - 24 volts here. You read -14.78 volts the first time, then you find -1.78 across the 44 ohms resistor and - 11.5 volts across the 268 ohms one: but now the sum is  -13.28 volts. Anyway, since there is no dedicated filter capacitor here, the biasing voltage is certainly affected by even feeble loads, as the meter input impedance or the grid current. Probably with a scope you could see a huge ripple across the divider 47.

The low volume is mainly due to the wrong grid biasing of both the 75 pre and of the 42 power amplifier.

Anyway no further investigation can lead to reliable results until the proper bias conditions are restored, even because of the feedback loop in the bias voltage generator. You must read near to zero drop across the grid bias resistor 35. The drop you are measuring today across the resistor is 14.8 - 3.5 or 11.3 volts absolute. This can only be due to the summing action of two possible currents, the grid positive current of a gassy tube and a leakage current through the coupling capacitor 33. Both currents must be negligible in a proper working circuit and the normal drop on the grid biasing resistor must never exceed about 20 or 30 mV.

This is the reason why my advice is to replace the old 42 with a good one, leaving the coupling capacitor 33 disconnected, and see if the drop falls to zero. Then connect the capacitor 33 and check again the drop across the grid resistor 35. If greater than few tens of millivolts, replace the coupling capacitor with another type. Once this problem is fixed, we can move to other items.

By the way, you replaced the field coil with a resistor, but the smoothing action of such a filter must be too poor. I see on the diagram a 50 nF capacitor, 43, and a 5 k resistor, 44, that probably were part with the coil itself of a notch filter intended to attenuate the hum. Maybe that the value of smoothing capacitors must be somewhat increased to compensate the absence of the said filter. I also fear that the rectifier could be at the end of its useful life, if the overload conditions went on for a while.

Anyway, as I said, be patient and first restore the proper bias conditions.

Good luck,

Emilio

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Sorry, I forgot a third possible cause of the drop across the grid resistor 35, which is a leakage to ground in the tube socket or in the base, maybe due to moisture or to carbonized phenolic layers. You can check with a megohmmeter after disconnecting the resistor.

Emilio

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Here are some pictures of the radio with tags. I'm thinking maybe I'm taking the readings in the wrong place. 

• IMG_5258 (167 KB)
• IMG_5259 (159 KB)
• IMG_5261 (153 KB)

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Hi Bill,

I am sure that you are taking the readings in the right places. Unfortunately there can many causes of drift. The voltage depends upon the current flowing through the tubes, about 80% through the power amplifier. But the current might be limited by a poor cathode emission in the power amplifier itself or in the rectifier. Consider that certainly you have an oveload condition even in the power transformer, hence a resistance increase of the windings. In these conditions you could observe a drop in the heater supply and a consequent further decrease of the emission. Another probable reason of drift can be the progressive ionization of the gas inside the 42 as consequence of temperature increase. It is hard to follow the behavior of the whole circuit in these overload conditions.

Just as an experiment, waiting for a good 42, you can connect a smoothing capacitor, say 0.1 microfarad, 50 volts or more, across the voltage divider 47. You should observe a relevant increase of the negative bias, at least across the divider itself.

Emilio

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Hello emilio,

It sounds like you're very much convinced it's the #42 output tube. I've tried 3 different #42's with the same result. I installed a .1 cap also  cap across the voltage devider and it didn't seam to have any affect on the voltages. I'll keep digging, Maybe something will jump out at me. Thanks for helping

Kind regards,

Bill

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Dear Bill,

I have no preconceived opinion, I am just analyzing the facts on the basis of the diagram downloaded from Radiomuseum.

In normal operating conditions the current in the control grid of the 42 must be zero. Always in normal conditions the total current from the rectifier may be evaluated in some 70 milliamps, instead of the about 90 millamps that you are reading now. The excess current is almost certainly flowing through the power amplifier tube. The grid resistor 35 is connected from the top of the voltage divider 47 to the junction of the control grid and of the coupling capacitor 33. You read a relevant drop across the grid resistor 35, rising to approximately 12 volts after a warm up. Hence the grid bias voltage of the 42 is someway around -3 volts, well below the typical values given for this tube, ranging from -16.5 to -20 volts. You say that a drop is still present when removing the grid coupling capacitor. You also checked the resistor for open, with readings of 660 K and 770 K. Unless we want to believe in the supernatural, the drop across R 35 must derive from a current, about 15 microamps, flowing in it. There are very few components involved indeed, here is a simplified diagram.

When the capacitor 33 is removed we have just two options left.

1. A current of opposite polarity, flowing from the control grid of the 42 to the bias source
2. A current flow to ground through a leaky wiring or insulanting wafers of the tube base.

The third option to explain the different voltage at the two ends of the grid resistor 35 is an open R 35 itself. If the circuit was not modified, I can't see other possible causes. And until this wrong bias is not fixed and the overload removed, other investigations can only lead to wrong conclusions.

Best wishes,

Emilio

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Hello Emilio.

Thanks for the diagram, that's helpful and puts it in perspective. I've taken some more readings with the radio cold and a NOS #42 in place. With the #33 capacitor disconnected I test at .319 volts at the #4 pin. On on the pin #4 side of the #35 resistor I test at -16 and on the high voltage winding side I test at -22. I'f I understand correctly ,and assuming no supernatural entities,  should there be no drop across the #35 resistor and the #4 pin of the 42 tube, with the #33 capacitor disconnected, should read 0 volts?

Thanks,

Bill

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Dear Bill,

you should find the same voltage, about -16 to -20 volts, from the top of the divider 47, the lug also connected to the center tap of the HV winding, to the grid pin of the power amplifier. Maybe you have a cold joint in the wiring from the resistor 35 to the base. Anyway there are very few components to investigate and the bias problem looks close to the solution.

Best wishes,

Emilio

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Hello Emilio,

After digging a bit deeper into schematics for the model 60, I found tube socket data. In this data it says that the control grid volts on the #42 tube (Pin #4) should be .18 volts. I know you have sugested it be -19 volts. am I looking at something incorrectly?

Bill

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Hi Bill,

the values I gave were just typical ones. The exact value of the grid bias voltage was set by the designer, depending upon the wanted operating conditions. In the Frank's pages, also accessible from the Link section of Radiomuseum, you can find several datasheets of the 42. The GE datasheet gives values ranging from -16.5 to -20 volts, mainly depending upon anode aperating voltage.

Since you found the exact value specified by the radio manufacturer, you must use that one. Remember that the accuracy of the reading is affected in this case by the impedance of the meter. To have the same reading, you should use a meter similar to that usually specified in the service notes. A tolerance from 5 to 10% is usually accepted.

Emilio

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Hello Emileo,

I've uploaded the data for the model 60 and had a question in regards to the readings. on the tube socket data it tells me to read form cg to k to get my reading. Am I to understand that I take the reading form the cathode to the control grid to read the voltage for pin 4?

Bill

• philco 60 (272 KB)

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Dear Bill,

K is anyway connected to ground, so you are reading the voltage on the control grid. There is something strange in the ervice sheet you loaded. I read a value of .18 volts, no sign, for the grid of the 42. If this means 0.18 volts absolute, this value is certainly wrong. I am not familar with the symbols of these notes. Anyway the plate and the screen voltage are in the order of 250 volts and, according to the datasheets for the 42, at such a supply voltage the negative grid bias must range from 16.5 to 20 volts. I guess that the right nominal value from the control grid to the cathode (or the chassis ground) is -18 volts. Maybe that the values given in the service notes refer to an arbitrary scale of the Philco 048 tester. Use a high impedance voltmeter, 10 Mohm would be fine.

Anyway an alternate way is to measure the voltage on the bias source 47 and check for zero drop across the grid resistor.

Emilio

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