Philco 38-2 Low Volume - Printable Version
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Philco 38-2 Low Volume - dconant - 11-09-2024
Hello
The volume on my 38-2 seems low. I need to turn it up about half way to get normal volume. The data sheet for the 6F6 tube using PP Class AB (Pentode) Fixed Bias shows max 250 volts for the screen, 375 max for the plate and -16 min for the grid. My plates are 220 volts, screens 230 volts and grids -13.5 volts. Is the grid voltage too high? Looking at the schematic I can see why the screens are higher but should they be? Schematic https://philcoradio.com/library/download/service%20info/wiring%20diagrams/vol2/split/Part34-Philco%20Wiring%20Diagrams%20Vol.%202.pdf
Thank you,
Dan
RE: Philco 38-2 Low Volume - morzh - 11-09-2024
Check your 94 Candohm wirewound resistor. Those are known to go bad.
Though I would not say right off that the voltages are bad.
RE: Philco 38-2 Low Volume - MrFixr55 - 11-09-2024
Hi DConat,
Generally, the lower the plate voltage the less negative the grid voltage has to be. Since the plates and screens are fed by the same voltage, the plate voltage will not be higher than the 250V that feeds the output transformer and the screens. This set looks like it is fixed bias but when you further analyze the schematic, it is cathode biased. the Candohm is between chassis ground and the return to the B+. Some folks will consider this to be a way to get C- but the voltage will vary with load, just like a cathode biased circuit will.
The grid voltage is probably a little negative than usual. The RCA Receiving Tube Manual (RC14, ca 1940 does not list the correct resistor value for cathode bias for 250V for push pull. Grid voltages are only listed for fixed bias circuits. The total Candohm resistance from B return to chassis ground is 126 Ohm. The grid return resistors are tied to the B supply center tap and ground is at the other end of the resistor. with this low a cathode resistor value, the grid voltage sounds OK to me.
There are many things that can cause low volume in a radio. The issue could be in the "amp" section(the 1st AF, 2nd AF, phase invertor, output tubes, output transformer, speaker, etc.), or in the "radio" section (RF, convertor, IF amp, detector, etc.
Due to the fact that this has a rather nice amp circuit (I don't know why there are 2 stages of AF amp before the phase invertor, but it should be pretty sensitive) with a rated 7W output in what is likely class A or AB1, I would consider adding an Aux In jack just to play mp3s through the amp. This will also tell you if the issue is in the amp or in the radio. Bhis is not difficult to do. Break the connection between cap 61and R43, R44 and C39B. Connect C61 to the common of a SPDT switch. Connect one pole to the point that C61 was disconnected from and the other pole to the aux in jack. This will allow "divide and conquer" troubleshooting, and playing big band music directly through the radio's amp will make it ROCK!
Have you checked the tubes? Do so or sub if you can. Verify that one end of the secondary of the audio transformer is grounded. Unplug the speaker and measure between ground and Pin 8 of the 2nd AF amp. It should be a very low reading, under 8 Ohm and more likely close to 2 Ohm or less (remember, impedance and resistance are two different things.
Did you recap the radio? If not, do so. A leaky cap could drive a grid too positive to amplify correctly. Check all the resistors having to do with the 1st AF ap, the 2nd AF amp and the phase invertor. A resistor in the plate or cathode circuit of the 1st, 2nd AF amps or phase invertor that is out of tolerance high will lower plate voltage, increase cathode bias, etc., all causing lower gain.
Issues in the "radio" section include tube strength, alignment issues, an open antenna coil, similar cap and resistor issues as the rest of the radio. try connecting the antenna directly to the grid cap of the RF amp and to the convertor. If much louder when connecting to the grid of the RF amp, then troubleshoot the antenna circuit. If much louder when couching the antenna lead to the grid of the convertor, then troubleshoot the RF circuit.
Hope all this helps. Let us know the outcome.
RE: Philco 38-2 Low Volume - RodB - 11-10-2024
If you check all of that and still have a problem (my first inkling would be one of the 6F6s) then look at the 6J5 phase inverter plate and cathode resistors. The 51k plate load should be pretty close to the cathode 50k. Then check the 4 resistors on the grids of the 6F6s. The top pair should should measure the same as the bottom. If any of these are not matching then the amp is doing more pushing than pulling. In addition, the 6F6 grid bias should be closer to -16 volts which could be a problem with the candohm resistor.
RE: Philco 38-2 Low Volume - dconant - 11-10-2024
Thanks guys for the help. I started looking at these points but I have to quit for the day. I will be looking into this stuff more Monday.
RE: Philco 38-2 Low Volume - dconant - 11-10-2024
I managed to get some more time on this.
Hi Mr. FixIT,
The radio has been recapped. Many of the resistors were replaced. The tubes all checked good except the 6F6 they were marginal. I subbed in a good set and found no difference. As for the other tubes I don't have any others to try replacements. The secondary has one point grounded. Get .5 ohms from pin 8 to ground which is same as secondary of transformer. My radio is code 121 so we need to use the second schematic with all those X parts. The plate of the 2nd audio is 90 volts vs 135 per schematic. But the main schematic shows the plate resistor is 40K. The X parts schematic shows 99K which reads 101K. I did not replace that resistor. Maybe use 40K? That is one of the only two resistors in that third of the chassis I didn't change The plate of the phase inverter is 170 volts vs 180 volts per schematic. That is probably ok. I did not do anything with the aux jack you mentioned. Not 100 percent sure where to cut.
Mr. RodB,
The the three section candohm reads total of 139 ohms (96, 17, 25) vs schematic 88, 15, 23.Is that 96 ohm reading a problem? The 4 resistors on the 6F6s are pretty close to what they should be, not exact though (1K off). I don't understand with this: The 51k plate load should be pretty close to the cathode 50k. The schematic shows 51k plate and 5K screen. Please explain as I'm not sure what resistors. Is it the total of 85 and 87 resistors. If so they are close 51K cathode and 49K plate.
Thanks Guys,
Dan
RE: Philco 38-2 Low Volume - morzh - 11-10-2024
Both sch, the one you showed in the beginning and the one I see on Nostalgia, show 40K for the 2nd audio plate.
RE: Philco 38-2 Low Volume - MrFixr55 - 11-10-2024
unless otherwise stated most parts have a 20 % tolerance. If both versions have the same tube count, I would consider changing the resistor to 40K IF the resistor was previously changed.
Did it ever work while you had it?
I will try to mark up the schematic as a picture is worth 1,000 words or 10,000 of my words.
Remember that low volume could be caused by alignment issues. If you do not have a signal generator, you can attempt to "peak" by ear but turn only a little bit and mark the original positions before "tweaking". As of now, there is no proof that that the issue is in the amp stages as opposed to the radio stages.
RE: Philco 38-2 Low Volume - RodB - 11-10-2024
I guess the candohm is OK. The cathode of the phase inverter has 2 resistors, 85 and 87. The two in series are 50k. So that looks good. My schematic, which I think I got from the library, also shows 40k for the second audio plate load. If possible lift one end of the 100k and tack in a 39k.
RE: Philco 38-2 Low Volume - morzh - 11-10-2024
Or just tack anopther 100K in parallel.
RE: Philco 38-2 Low Volume - dconant - 11-11-2024
Mr FixIT, when I get ready to restore a radio I do just enough to verify it works or get it working. So if it didn't have good volume I would not know that. I then set about doing the restore. The set did not work when I got it. I have done a partial alignment up to the point of doing the adjustment for the 2XC compensator. I can't go further as I don't have a dial yet. Waiting on Radio Daze. I have a generator. Please see last page of the link provided below for 2XC compensator and alignment instructions. I have the IF cans with the three compensators on each on the sides. Both versions of the schematic use the same tubes. The set is pretty sensitive picking up a lot of stations in my area.
RodB, if you look at the last page of this schematic https://philcoradio.com/library/download/service%20info/wiring%20diagrams/vol2/split/Part34-Philco%20Wiring%20Diagrams%20Vol.%202.pdf you will see the 99K plate resistor. I tacked in another 100K in parallel and it made no difference, the voltage on the plate of 2nd audio is still 90 volts. Makes me think the tube is not drawing current.
The 1st audio plate is low too at 80 volts vs 100. There is a 70K (37X) resistor feeding both 1st and 2nd audio plates. If I short out that resistor 1st audio goes up to 120 and 2nd audio goes up to 125. There was a slight increase in volume.
I touched the antenna to the grid of the RF amp and the volume went down noticeably. I don't know what tube is the converter.
RE: Philco 38-2 Low Volume - RodB - 11-11-2024
So, if the 2nd audio is not conducting current then you get no response from the speaker. Let's back up and start at the B+ and follow it to the audio circuits. Find resistor 14X, there should be about 260 volts on one side and 245-250 volts on the other. Next up the line is resistor 37X with the 245 on one side and say 160 on the other. Next is 2 resistors, 31X with the 160 and 100 volts, and 32X with 160 and 80 volts. See how that rings out.
RE: Philco 38-2 Low Volume - dconant - 11-11-2024
14X measures 235 - 225, 37X measures 235 - 128, 31X measures 128 - 90, 32x measures 128 - 90. Now that you had me do this I see a problem in that 14X was not connected to 37X at the output of 14X it was at the input of 14X. I made the correction now I measure
14X 235 - 220, 37X 220 - 124, 31X 124 - 86, 32X 124 - 88. With 32X shunted with 100K I get 120 - 94. No volume change.
RE: Philco 38-2 Low Volume - dconant - 11-11-2024
I went back and looked at my picture of that area and it shows I had the new resistors connected correctly. So the factory did it wrong. Probably not much difference. I measured both side of field coil 290 - 230. Coil measures 752 ohms vs 660 on schematic.
RE: Philco 38-2 Low Volume - RodB - 11-11-2024
OK, so I'm seeing the B+ is about 25v low causing the overall performance to be reduced. How much I can't say. I looked at the screen voltage of the 6K6 output tubes and the schematic calls for 260v, which is the B+ right off the power supply. That B+ feeds resistor 14X. See where I'm going? Generally, a ten percent reduction is within tolerance but in this case it bears investigation because it's ten percent for each stage.