Have been unable to locate the correct schematic for this. All the schematics I find show a 7J7 for the detector/oscillator, mine has a 6K8. The chassis is stamped 40-155, code 121 and the schematic is similar except for condenser 6 and 22 which are a different value listed that what is in this radio. Performance seems about the same compared to the Philco 39-25 that I restored earlier. The 40-155 uses a 1232 (7G7) for a r.f. amplifier and an additional 41 for push pull output so I really expected better sound and better reception; I even replicated the built in antenna on the 40-155. Both of the radio's came without speakers so I installed the same type of pm speaker in both, would the original speaker make a difference in sound quality?
First off, Welcome to the Philco Phorum! Secondly, apologies in advance for the length of this post.
I am sure that Gary will answer you the next time he is on line. On the bottom right of the page, you will see the Avatars(?) of those members who are currently on line.
The following link is from the Philco Library on this Phorum and shows the version with the 7J7 Pentagrid Convertor. Triode-Heptode First Detector. (Edited 01/14/2025 for Correction to description of the 7J7) I must apologize that in this post, I described the 7J7 as a pentagrid convertor based on my interpretation of the schematic in the Philco library. After consulting the RCA Receiving Tube Manual, I note that the 7J7 is a triode-heptode (a heptode being a tube with a cathode, plate and 5 grids) tube, similar in function to the 6J8 other than the base design and pinouts. The 6K8, a triode-hexode (a hexode being a tube with a cathode, plate and 4 grids) while different in design does the same function. The difference between the triode-hexode or triode-heptode and the Pentagrid or autodyne detectors is that the triode-heptode or triode-hexode provides a "separate electron path" (sharing only the cathode) for the oscillator and mixer with the advantages of having both "tubes" in the same envelope. The Pentagrid Convertor (Heptode) uses a "grid - plate" as the oscillator plate, but the "electron paths" of the oscillator and mixer are the same, passing through all grids. Part48-Philco 1940 RMS Year Book.pdf
Now this is where it gets interesting!
The same document states that there are certain runs with a 6J8 Octal tube replacing the 7J7. Per the Radiomuseum website, The 6J8 is supposedly a Russian version of a 6K7 remote cutoff pentode, hardly the tube one would want to use as a convertor. However, Radiomuseum also lists a 6J8 Triode Heptode, where the triode is used as an oscillator and the heptode section is used as a mixer. This triode oscillator / heptode mixer configuration usually works better at high frequencies than the typical pentagrid convertor. Remember, that in the late 1930s to 1960s or later, shortwave was to many people what the internet is today. Shortwave was the window to news (true or propaganda) of the developing conflicts that culminated in WWII, and for many immigrants, news of the land that they left. (Edited 01/14/2025- The fact that the 7J7 and 6J8 are both triode-heptodes with similar construction and function other than basing, and the fact that Philco does not give tube pinouts on the schematic may be the reason why a separate schematic for sets using the 6J8 was not published.
The next issue is that your radio has a 6K8, which is a triode hexode, not a triode heptode. These are not the same tube. Is the tube location sticker still in your radio? Does it state 7J7, 6J8 or 6K8?
To further complicate things, there is no insert in the Philco Service Document for your radio showing the 6J8 circuit.
The following may help:
Attached below are 2 pages from Radiomuseum describing the 6J8 and the 6K8 and showing typical oscillator mixer circuits with tube "pinouts":
Attached is an "Emergency Tube Substitution Guide published by RCA during WWII. (New radio and replacement part production stopped in 1942 to build the electronics that were required by the US Military.) The manual does not show that a 6K8 can replace a 6J8. Both tubes are listed.
Now, for the poor sound issue:
Are you dealing with a distortion issue? The fact that you have a 6K8 instead of a 6J8 may be an issue.
I am a believer in "divide and conquer troubleshooting. In addition, I like to add "aux inputs" to my radios so that I can pipe music from an MP3, computer, etc., a great way to get "old time" radio to play from your "old time radio" (pardon the pun). It also helps in the diagnosis of where the distortion comes from. The amplifier section of this radio should sound rather clean. Not Fisher, Scott or Marantz clean, but clean. The best place to "inject" an outside signal is at the junction of the detector and the Volume control. If you are interested, I will do a diagram. Any modern speaker is likely better than the original. However, there are 2 issues that must be dealt with:
Correct voice col impedance for the transformer in use
Correct resistor or choke in place of the field coil (and likely the necessity tto increase the capacity of the filter caps).
Do recap the radio and check all resistors. For this and other radios, Philco has a rather novel method of getting the inverted signal for the second Pentode in the Push Pull Circuit. Note that there is no Phase Invertor Stage. Note the 3900 Ohm resistor between B+ and the screen of one of the 41s. Note the capacitor between the screen of that 41 and the grid of the other 41. That is how the inverse phase needed for the second pentode in the push pull output is generated. It is critical that all caps and resistors in this section of the circuit be exact (within stated tolerances) and in good shape.
Again, sorry for the length of this. Hope it helps!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
(This post was last modified: 9 hours ago by MrFixr55.)
Thank you for the thorough answer and explaination!
The radio sticker on the inside of the cabinet states 7J7, seems like it's likely that someone put a different chassis in the cabinet. I was wrong saying that the radio had a 6K8 installed, it has a 6K7 installed. Would performance be better if I installed a 7j7, or would the difference be slight?
I did thoroughly go through the amplifier section replacing the resistors and capacitors since the values of the resistors had drifted more than 20%. To compensate for a pm speaker I installed a 1k ohm resister and bumped the second capacitor to 47K'. Also added bluetooth but I injected the signal to the volume control and ground. Would be interested in how you would connect.
So Philco didn't release a schematic showing the 6k7 installed instead of the 7j7?
It sounds like you installed the Bluetooth in the correct place. Short answer to how I would install the "aux input" (Bluetooth, phono, etc.) is to break the connection between capacitor 38 and volume control 41. Use a SPDT switch to select input (radio or Bluetooth). This is similar to what Philco does with radio-phono combinations. They may shut off B+ to the radio section or short the detector output to ground to prevent bleed-through of the radio signal when listening to the aux input but this is not always necessary. Does the sound with Bluetooth fail to meet your expectations or is the issue only with the radio?
I also must apologize that in my previous post, I described the 7J7 as a pentagrid convertor based on my interpretation of the schematic in the Philco library. After consulting the RCA Receiving Tube Manual, I note that the 7J7 is a triode-heptode (a heptode being a tube with a cathode, plate and 5 grids) tube, similar in function to the 6J8 other than the base design and pinouts. The 6K8, a triode-hexode (a hexode being a tube with a cathode, plate and 4 grids) while different in design does the same function. The difference between the triode-hexode or triode-heptode and the Pentagrid or autodyne detectors is that the triode-heptode or triode-hexode provides a "separate electron path" (sharing only the cathode) for the oscillator and mixer with the advantages of having both "tubes" in the same envelope. The Pentagrid Convertor (Heptode) uses a "grid - plate" as the oscillator plate, but the "electron paths" of the oscillator and mixer are the same, passing through all grids.
Does the radio even work with the 6K7 as the convertor (the stage that converts whatever the chosen frequency to the IF frequency)? The 6K7 is a "remote cutoff" pentode, used for RF and IF amplifiers, and is rarely used as a convertor (sometimes referred to as the 1st detector or oscillator mixer). The 6K7 may be used as a mixer when there is a separate oscillator tube, as used in the first generation of superheterodyne radios. The "sharp cutoff" 6J7 was used by Philco on 37-84 "entry level" superheterodyne as an "autodyne" oscillator-mixer 1st detector. This "Autodyne" detector is generally unstable and is not used at high shortwave frequencies. Ever since the introduction of the 2A7, the first "pentagrid" (heptode) convertor tube designed specifically as a combined oscillator-mixer. The heptode (pentagrid convertor) was the most popular form of convertor (Oscillator - Mixer or 1st Detector) used in superheterodyne radios. Other popular Pentagrid Convertors include the 6A7, 6A8, 6SA7, 6BE6 and their 12V equivalents.
One of the Experts or maybe Ron Ramirez coined the term "Franken-Philco" (or is it "Phranken-Philco"), changes made by Philco but not documented. The service notes for the 40-150 and 40-155 from the 1940 Philco Yearbook discuss the substitution of the 7A7 Pentagrid Convertor with the 6J7 but did not publish the circuit. No documentation on this chassis or any Philco chassis of this era having a push-pull output and diode 2nd detector that I have found either in the Philco Library or the Nostalgia Air Website uses a 6K7 either as an oscillator-mixer or as a mixer using a separate oscillator tube.
Note also, that this radio was built between 1939 and 1940. From 1942 to 1946, manufacturing of electronic devices and parts were reserved for the military and parts or tubes for civilian use were scarce, often requiring substitution of a failed tube with either one not ordinarily used bty the manufacturer or a less than ideal substitution which negatively affects the performance of the set.
In the previous post, I posted the circuit diagram of the radio, and the tube pinouts for the 6J8 and 6K8 tubes. Below, I provide the pinouts and description of the 6K7, 6K8, 6A8 and 7J7 tubes from the RCA Receiving Tube Manual version RC14, ca 1940.
The best advice that I can give about sorting this issue out is to:
Please list all the tubes actually installed in this radio. This will help sort out whether this chassis is correct for the cabinet, a substitution. by a similar or a total oddball user-built Phranken-Philco
To the best of your ability by wire tracing, please confirm that the socket occupied by the 6K7 is actually the convertor socket.
To the best of your ability, determine if the socket was replaced or rewired. If all sockets except the convertor socket are riveted to the chassis but the convertor socket is attached by screws, it was likely replaced. If the soldering and wiring "job" does not match the rest of the radio in technique and quality, then the socket was likely replaced or rewired.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Thank you for patiently writing a thorough answer.
The radio indeed did come with a 6J7 instead of a 7J7, the tube socket is riveted in. Would performance increase rewiring the radio with a 7J7? Listed tubes are 1232 although it has a 7G7, 6K7, 7B7, 7A6, 7C6, 2-41's and a 84.
I replaced the 3900 ohm resistor with a 4.7K, sounds like I should get the value back to 3900 ohms.
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