E.I.S./Leutz Model C-7

     Advertisements for the Leutz Model C-7 superheterodyne started to appear in the radio magazines in September 1924. The Model C-7 was an advancement over the prior Model C that was introduced in 1923. You’ll find Model C superhets with four different types of IF transformers (UV-1716, ACME, General Radio, and E.I.S.), but you’ll only find the Model C-7 superhet with E.I.S. transformers. Here is a small list of some of the other differences of the Model C-7 from the Model C:

1. The C-7 had a regenerative first detector that improved the efficiency so much that one IF stage could be eliminated. The regenerative amplification is controlled by a small variable capacitor that is located on the front panel between and above the two large tuning dials.
2. The C-7 had vernier tuning controls for each large tuning dial (the verniers are located on the front panel below each large dial).
3. The C-7 only had two filament rheostats and the customary 400-ohm potentiometer was eliminated.
4. The C-7 did not operate with a loop antenna. Either a short wire antenna or a long wire antenna could be used.

     Here is a small list of some of the particulars of this set:

1. Most of the parts, including the cabinet, were probably supplied as a complete kit by E.I.S.
2. The E.I.S. factory engraved front panel is 40” long by 8” high by 1/4” thick.
3. E.I.S. antenna inductance (coupler), oscillator coupler, IF transformers, and output coupler (filter transformer).
4. E.I.S. 4-inch main tuning knobs.
5. General Radio tuning capacitors, audio transformers (one replaced), rheostats, and tube sockets.
6. Jewel ammeter and voltmeter (voltmeter having a 5-position selector switch).
7. Seven tubes.

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Leutz Model C/C-7<     HOME     >Leutz Navy Model C-10

Charles R. Leutz

     On our way home from Nebraska we stayed the night in Cheyenne, Wyoming. Well, Cheyenne just happens to be the home town of Richard T. Ammon, one of the greatest 1920s superhet experts on the planet. I thought that it would be a great opportunity to meet him. I gave Rick a call on the phone and told him that I was in  town and had a C-7 that I had just purchased, and asked him if he would like to come over to the hotel and see it. Rick was over in about 20 minutes, and brought a lot of paperwork and information on the C-7 that he gave me! It was great to meet and talk with such a knowledgeable person on these old superhets!

     I purchased my C-7 at a radio auction in Aurora, Nebraska, on July 2, 2011. The above photo shows how the set looked along with a few other relics the day before the auction. The set a couple of rows behind the C-7 (above the right side meter in the photo, and with a loop antenna sitting on top of it) was a battery operated 8-tube Silver-Marshal superhet (it had four S-M bakelite IF transformers).
     My wife Carrie and I drove nearly 850 miles from Eagle Mountain, Utah, to attend the auction. We made a mini vacation out of the event and really enjoyed ourselves. The people of Nebraska are great!

     Norden, Hauck & Co. advertisement for the C-7 superhet from the January, 1925, Popular Radio magazine.

     Since my set uses a C battery to bias the first two IF transformers at 4.5 volts negative I made the measurements again with the C battery in place. Surprisingly, the two batteries in this set still hold a charge of 4.3 volts. As would be expected, the higher negative bias produced a narrower bandwidth. The relative gain actually went down though. I suspect that the lower relative gain is due to a high impedance of the battery to high frequency currents. The battery should probably be bypassed with a capacitor to reduce the impedance to high frequency currents. The table below shows the bandpass measurements of the first two IF transformers with the 4.3 volt C battery as a secondary reference.

     A bandpass measurement of the various IF transformers provided me with valuable information on how this set would actually perform. Period documentation indicates that the Model “C” and Model “C-7” transformers peak at 47 KC; I have found that to be incorrect. These types of transformers have large iron cores and their secondary windings have massive amounts of wire, both of which dictate a low peak frequency. The peak frequency of the first two IF transformers in this set is close to 30 KC (very similar to the RCA UV-1716, the ACME 30 KC, and the General Radio 271). Moreover, these E.I.S. transformers readily amplify frequencies down into the upper audio spectrum. This makes for a noisy radio.
     The Model C-7 output coupler, or filter transformer, is a totally different animal. This transformer is an air-core transformer with much smaller windings and the primary coil is tuned with a .01 MFD capacitor (indicating a primary inductance of 1.15 mH for a peak at 47 KC). The C-7 output coupler in my set actually peaks at around 52.4 KC and it has a very narrow bandwidth of about 4 KC. The peak frequency of 52.4 KC may be due to an out of tolerance primary capacitor (perhaps .008 MFD instead of .01 MFD). Since the first two IF transformers are fairly wide banded, and have an upper frequency limit that includes the bandpass of the output coupler, the entire chain of the IF amplifier is controlled by the output coupler at 52.4 KC (in my set).
     The table below shows the bandpass measurements and resistance measurements of the IF transformers. The first two transformers were measured with the secondary winding referenced to A-. The output coupler was measured with the secondary referenced to A+ and the grid leak resistor and capacitor in place (grid leak resistor measured 2.7 megohms).

     Jewel voltmeter, with 5-position switch.

     Jewel ammeter, indicating total A battery current.

     Another close up of the front panel. The business card on the right is affixed to the inside lid of the cabinet. This radio may have very well been built by “Radiotrician” A. S. Lundy, of Omaha, Nebraska. The other label indicates identical numbers on the voltmeter switch; D for oscillator filament voltage, H for the A battery, M for 2nd Radio Freq (3 to 5 volts), R for the main buss (5 volts), and B for the total B battery (90 to 100 volts).

     Close up view of the front panel and the 4-inch E.I.S. tuning dials. The filled-in holes for the antenna terminals are visible, along with an engraved L for long antenna, an engraved S for short antenna, and an engraved G for ground. The terminals were repositioned to a terminal strip in the rear of the set. The control for regenerative amplification in the first detector is labeled “volume control.” The front panel, knobs, and other instruments are somewhat dirty and dusty. I am not going to clean this radio at all. It is in good enough shape to leave it alone.

     E.I.S. Model C-7 oscillator coupler.

     E.I.S. Model C-7 output coupler (filter transformer).

     Two different views of the E.I.S. IF transformers. The model number is not known because these transformers do not have E.I.S. labels. Most E.I.S. transformers of this style have a label that indicate either Model “C” or Model “C-7” (there was probably no difference between these transformers electrically). The transformers in my set are merely engraved with primary and secondary letters and terminal numbers. The type of mounting for these transformers is also totally different than the customary large aluminum base. The transformers are definitely E.I.S. though. They could be very early transformers, before E.I.S. had labels to put on their transformers, or they could be very late transformers, when E.I.S. didn’t want to get into trouble from RCA. Either way, they appear to be a rare style of E.I.S. transformer. I personally suspect that the transformers are early. These transformers are what someone familiar with leutz sets would probably immediately notice while peering inside the set.

     E.I.S. antenna inductance (antenna coupler).

     General Radio tuning capacitors.

     Schematic diagram of the Leutz Model C-7 (courtesy Richard T. Ammon). My set has slight differences from the standard C-7. The standard C-7 uses two 4.5-volt C batteries in series to provide nine volts bias for the first detector. My set has the two C batteries in the correct physical location, but it only uses one C battery to provide 4.5 volts negative bias for the first detector. The other C battery is used to provide 4.5 volts negative bias to the two IF transformers and the two audio transformers. The schematic above shows an “X” in the return lead of the secondary of the IF transformers; this is where the C battery is connected.

The audio amplifier

The IF amplifier

The RF front end

     Top inside view of my set. The C-7 was designed to have antenna terminals on the left side of the front panel and battery terminals on the right side of the front panel. The original builder of this set filled in the terminal holes on the front panel and installed the terminals in the rear of the set. Another modification to this set is the addition of a small knife switch on the lower right-side front panel. A person familiar with Leutz sets would probably notice something else that is different about this set right off the bat (more details to follow).