[tludka]

I have a 38-7X Console radio which is missing the speaker. I have a speaker for a 38-38X however it has a different output transformer attached, and the speaker is stamped as a HR-20 and is driven by a push pull output tube 1J6G. The schematic / parts listing for the 38-7X calls for a H 31 speaker driven by a 6F6G output tube. Is there a way to modify the HR 20 or use a totally different speaker?

[Radioroslyn]

You'll need an opt to match the 6F6/42 to VC vs the 1J6. And something to replace or act as the field coil. The 1J6's output is somewhat less than the 6F6 so wouldn't be a good idea to try the the opt on the spk. The current rating is going to be too low and the load resistance too high.

Replacing the spkr w/a modern one is the same. Find an opt 5k to vc and something to replace the fc.

https://philcoradio.com/library/download...%20280.pdf

[tludka]

I noticed in the Philco Library / Speakers that an H20 is listed for models - "Central Contact System." With a "1000 Impedance online" listed for the output tube. Can anyone clear up my confusion on the Model and output tube references?

[MrFixr55]

If the speaker was meant for a portable radio with a 1J6 Dual Triode output tube in Class B operation, the output transformer for the speaker has a 10K Ohm Plate to plate impedance. The output of the 1J6 is 2.1 W but runs at 135V as opposed to the 42 that puts out 2 W into a 7K Ohm impedance with 250V on the plate. You could use the speaker with the 1J6 transformer, but it would be a mismatch with reduced output. You also have to deal with the fact that there is no voice coil to act as a choke for the power supply. Speakers for most transformer powered AC sets have a field coil impedance of about 1,150 Ohms, except for older Philco sets like the Model 20. If a suitable electrodynamic speaker or a choke can't be found, substitute a resistor for the field coil. Start with a 1,150 Ohm resistor. However, consider upping capacitor 11A to about 30uF to compensate for the lack of inductive filtering. If this causes too high a B+ voltage, increase the resistance of the resistor that was substituted for the speaker field coil.

IMHO, leave the speaker for the battery operated radio, as these radios are historically significant, and get a modern replacement, and find a correct transformer. A transformer for a single ended 47, 2A5, 41, 42, 6F6 or 6K6 should work.

[tludka]

Thank you for your response. The H20 is a 10 1/2inch speaker so I doubt if it was for a portable battery-operated radio. I do have some 5W 1200-ohm resistors and I can substitute in the circuit for the 1140-ohm field coil. I can change the 8uf to a 30uf that won't be a problem. The audio transformer on the H20 speaker is 32-7198. I will see what I can find.

[MrFixr55]

There are several speakers, including one labeled H-20 for "Central Contact" Systems. The transformers on these speakers are not designed to connect between a tube plate and B+. I am not sure, but these central contact systems may be public address systems, such as may be installed in a school, office or factory, where each classroom or room has a speaker. In this type of system, the output of the amplifier has a transformer that matches the tube or tubes to a "70V Line". These speakers may then all be connected to the 70V line in parallel. Even though the output stage of the amp has a transformer, each speaker will also have a transformer with taps to adjust the wattage that the speaker will draw. The total selected wattage should be less than the rated output of the output stage. Even in sold state PA systems, this 70V line distribution scheme is popular. In some systems like for a school, the system may have an intercom or "talkback" function, where the person operating the central system in the main office can select one speaker, address that room and then place the system into "listen" mode so the occupant of the room, such as the teacher in a classroom can address the office. Kind of like the squawk box at your local fast food drive through. These systems were common in schools. I went to Catholic school from grades 1 through 11, and it was not uncommon that when the class cut up (usually with a new teacher or a substitute teacher), the principal (usually a nun or brother) would come running with the "board of education" (a paddle). I don't know if Philco made or sold these systems, but I assume so. The 70V Line sound distribution is popular to this day, even though "powered speakers" (self contained speaker systems) fed by "line level" audio signals are gaining popularity in some installations.

[MrFixr55]

Yeah, don't use the transformer currently on the H-20 (Dopf, I thought "H20, is that new technology, a speaker using water technology?" ), the 1KOhm impedance is too low for almost any output tube, especially a 42 or 6F6.

An 8", 10" or 12" Speaker is very common for PA or distributed sound systems.  For stage sound reinforcement systems, smaller speakers ganged into a "Line Array" are becoming very common.

Try to salvage an output transformer (and hopefully a speaker) from any AC set with a power transformer that uses a single 2A5, 6F6, 6K6, 41, 42 or 47 output tube.  Most single end 6F6, 42, 47 or 2A5 output stages have a transformer with a 7,000 Ohm impedance.  41 and 6K6 have a 7,500 Ohm impedance.  The other consideration, however, is the voice coil impedance of this H-20 speaker.  At 1.25 Ohm, it is very low.  By the time the 1940s rolled around, most non-Philco speakers had voice coil impedances between 3.2 Ohm and 8 Ohm.  By using the same chart that you are looking, use the Transformer Part #, Output Tube and Voice Coil Impedance columns to find the correct Philco P/N for an output transformer compatible with the speaker and tube used.

I cannot find the 32-7198 output transformer in the speakers section or the output transformer section. of the Library parts pages. 32-7198 was likely not the standard transformer for the H-20 speaker.

Hope this helps.

[tludka]

FYI the Impedance of the HR20 (the one I have) secondary of the audio transformer is 5 ohms. The 38-7XX speaker is supposed to be a H31 with that secondary of the audio transformer being 1.25 ohms. So essentially what I want to do is to see if I can find an audio transformer with a primary impedance of 7.5K ohms and a secondary of 5 ohms that can handle the power output of the 6F6.

[MrFixr55]

7K should be fine. While not a perfect match (This transformer is meant for 6V6 tubes), Antique Electronic Supply (AES) has a nice (meaning CHEAP!) transformer, meant for small single ended Fender amps using a 6V6 output tube. AES P/N P-TGO-001, price $20.00. Input taps for 5K Ohm and 8K Ohm. (For 6V6, the primary should be 5K Ohm with 250V B Supply but 8.5K Ohm with 315V on the plate.) Output for 4 and 8 Ohm speakers. Power handling is 5W. This transformer is used with amps that may have a plate voltage as high as 350 VDC, so it should definitely handle the voltage and current of a single 42 or 6F6. Use the 8 KOhm Primary. Use whichever secondary (4 or 8 ohm) that gives you the best sound. Mismatch is relatively minimal. Another transformer that would work is AES P/N P-T291, rated at 8W with 4K and 9K Primaries and 4, 8, 16 Ohm secondaries. This one is $30 but has specs that justify its use for 41, 6K6, 42, 6F6 and (interesting that they group this with the others) 6V6, using the 9K primary. $10 more but more flexible.

Hope this all helps.

[tludka]

I have let this one sit because of other duties. Now I am back, and I have a couple of questions. I hooked up a Hammond 125BSE audio transformer to a 4-ohm speaker. I receive a crackle and the putt- putt like a bad filter cap. Since the original speaker was not with the radio, I am using a 1200-ohm resistor for the field coil. The voltage checks from the Philco spec are close to the specs except for one of them. The filter caps have been replaced. If I remove the cap lead from the IF 6K7G tube the reading on the plate seems ok and the putt putt stops. When I attach the cap the reading Justs fluctuates and will not stabilize, and putt putt is back. The cap reads a negative 3vdc disconnected from the tube. I have checked all the resistors and replaced the caps. Checked the wiring several times. The 1st I.F. coils ohm out per the schematic but not sure about the mica caps in it. Could this be the problem?

[morzh]

Just to make sure, you chose either 4 ohm into 5K or into 10K? (blk-org or blk-grn)

[tludka]

4-ohm speaker. Black, Green leads.

[morzh]

The put-put is not like the speaker problem.

[tludka]

I know that when I first started working on this radio, I did not even have a speaker. Once I finally found one it was not the original for this model, but I found one to work with it. However now it seems like there is a problem in the radio that I was not aware of because I didn't have a speaker to try it out before.

[morzh]

Also, check if any wires going to Grid Caps of the tubes that are supposed to be INSIDE of their respective shields (if such exist, I do not know 38-7) are indeed inside (of course with the shields installed), and not outside.