Low voltage in Philharmonic power supply

[Troy Taylor]

Hello-

I am in the midst of restoring a pointer dial Philharmonic that I bought this past summer from a forum member.  It is in great condition and had had very little previous work done to it, so there were no unpleasant surprises or shoddy workmanship to deal with.

All the caps have been replaced now, including the 'hidden' ones, and the set is now playing (with the occasional hiccup I still need to find and remedy) and sounds quite good!  But there are a couple of things that I think are not right.

Most noticeably, the 10H choke gets warmer than both the 30H choke and the power transformer.  The 10H is not ever too hot to touch, but it gets hot enough that you cannot leave your thumb on it indefinitely, which you can easily do with the other choke and power transformer.  This heating up is gradual and takes more than an hour of continuous operation to get that hot.

I went through the voltage checks for the power supply listed on pages 14-48 and 14-49 in RIDERS and found that all of the measured voltages are quite low.  Please note that test instructions include 110VAC supply with the HI-LOW switch DOWN, and I used a variac to ensure that the input voltage was as stipulated.  These are the results:

TEST 1: Correct voltage = 430VDC.  Measured = 330VDC

TEST 2: Correct voltage = 410VDC, Measured = 286VDC

TEST 3: Correct voltage = 375VDC, Measured = 300VDC

TEST 4: somehow I neglected to measure this one

TEST 5: Correct voltage = 125VDC, Measured = 101VDC

TEST 6: Correct voltage = -28VDC, Measured = -23.7VDC

TEST 7: Correct voltage = -3VDC, Measured = -4.0VDC

TEST 8: Correct voltage = 300VDC, Measured = 244VDC

The filament voltages measure correct. 

The 390-0-390 AC center tap transformer measures 331-0-331

So my main questions are:

Could there be something wrong with the power supply circuit that is dragging all these voltages down?  Or might I have a substitute transformer with a lower nominal voltage?  It certainly looks original and operates cool to the touch, so if it is actually not the right transformer it was otherwise well adapted to fit and handle the current requirements.

Would these low voltages somehow contribute to the 10H choke getting hotter than the other components?  Or could a faulty choke cause these voltage problems?

Any suggestions or advice would be much appreciated!

Thank you and best regards-

Troy Taylor
Edmonds, WA

[Robert Feenstra]

Troy, on the negative voltage divider, I've had to play with the resistances to get the voltages close to spec.  If the -27 volts is high ( as you indicated: -23.7), it will affect the bias on the output tubes causing them to conduct a bit more ( and the bias for the AVC system).  Perhaps your voltage divider needs a bit of work.  Once you get that right, you might find the rest of the voltages will fall into place.

The bias on my AM/FM outputs are -23 volts Grids to cathodes with 290 volts on the plates.  This would indicate it is in the Class AB region of the push pull output - that is if I am reading the tube specs properly.  someone please correct me if I am wrong.

Reducing the resistance of the cathode resistor would reduce the negative grid voltage, turning the tube on more.  Increasing the cathode resistor would increase the negative voltage on the grid, reducing the plate current.

[David C. Poland]

I am thinking spot on voltage readings are not too critical, because the radio is designed to operate well over a range of of 100 to 130 volts AC, in conjunction with the hi-low switch, allowing a  15 volt range on the power transformer primary which translates  into an allowable  50 volt or so variation of the 400 volt B+ ahead of either B+ choke. 

I did have -C divider voltage problems on  my BOL amp I built it from Scott Philly amp parts. My wiring of the interstage transformer on the amp was the problem. Has a center tap primary for 3 wires of the set cable. And two separate secondaries feeding the signal to each pair of 6L6 grids. I had crossed connected the secondary wiring which created oscillation issues evidently consuming power and dropping all -C voltages - and the supply -C from the amp of -27 to about -20 volts. That one leg of each secondary connects to the earphone jack was what lead to my error.  A simple fix which fixed the -C bias bring it in to proper ranges, and my -C candohm was actually good after all.

But if the amp's interstage transformer has issues, I can see it might affect the -C divider.