This Stromberg-Carlson unit shows model AV-39 on the product tab, and was historically upgraded with transformers and a meter to make it similar to an AV-38. Based on the signature of a tech in March of 1959, the tech added the 3 transformers to the mic inputs. Brand of transformer is not known (see pics). Another tech wrote in pencil on the schematic that he added the VU meter in October of 1962. At some point, it appears someone added an unbalanced 1/4" jack on the side for inserting an instrument, so I labeled this input, and it is connected to the phonograph knob. My opinion is that the addition of the "instrument" input is novel, but not an ideal result. I would recommend using a standard passive DI box into one of the mic inputs to use it this way. In addition to the back panel line outputs on barrier strip and 1/4" TRS jack, a 3rd XLR jack was added which I labeled output. They are all wired together. 600 ohm output.
A new power cord was added, and unit was serviced with some internal resistors or other components (see pics). I labeled the instrument input, XLR output, added the missing feet on the bottom, replaced missing or worn out screws as needed.
Functionally, the unit passes signal and sounds good, definitely tube colored. The EQ options seem to work. The VU meter does work. Cosmetically, one of the original knobs (phonograph knob) was cracked when I originally got it and it eventually came apart. At some point, an epoxy was used to fuse it back together. You can see a slight line on the knob, but it seems very solid and has never had any further issue. Also, the handle has a missing piece which also this way when I got it. The handle is solid otherwise and there are no signs of it breaking further in the past 20 years. 1 of the knurled knobs that connects the top does not match the other 3. This is also the way it came to me. I left it that way. Otherwise, it has scrapes and bruises from age, but seems free of any major rust or grime.
This unit could be a fun 3-channel mono mixer, or just used with a single mic channel in studio, and also makes for a cool show piece.
Paraphrased Original Sales Description from 1950
The Model AV-38/AV-39 Pre-Amplifier is a multi-purpose line amplifier designed for sound system installations. These pre-amplifiers are ideal for remote program pick-up. Their gain is adequate to amplify a microphone signal to as much as 18 V.U. across a telephone line.
Model AV-38 Pre-Amplifier is equipped with an output level meter and three low-impedance microphone input transformers. Model AV-39 Pre-Amplifier is identical except the meter and input transformers are omitted. A meter kit is available that can be installed in the Model AV-39 without the necessity of drilling any holes. TM-2 Input Transformers can also be added when a combination of low and high-impedance microphone inputs are desired.
Many uses will be found for these pre-amplifiers. They will be used to drive power amplifiers in large systems and also used for remote program pick-up amplifiers in school auditoriums, hotels, banquet halls, stadiums, sports arenas, and churches. They will be connected to the main system by a telephone line.
Size with Cover: 13 1/2" wide, 11" deep, 10 1/2" high. Includes allowance for knobs.
Weight: Net 18 lbs, packed for shipment 22 lbs.
Tubes Supplied: 6 x 6SC7, 1 x 6N7, 1 x 6X5GT G.
Power Supply: 105-125V, 50-60 cycles, 40W at 117V.
Inputs: Three microphone, One Phonograph with low-gain (equalized for crystal pickup) high-gain (.25 Mohm not equalized) connections.
Controls: Three microphone, One phonograph, one bass boost for phonograph only, one microphone bass attenuator, one treble attenuator, on-off power switch, meter range switch
Output Level Meter: Model AV-38 only, calibrated -20 to +3 db. 0db equals 1.945 Vrms. Meter range switch reduces meter sensitivity 10db.
Power Gain: Microphones: AV-38, 86db at 400 cycles, 38 ohm source impedance. AV-39, 89db at 400 cycles based on 50kohm input source impedance. Phonograph: high-gain connection 44db at 400 cycles based on 600 ohm input source impedance or 62db based on 50kohm source impedance. Low-gain connection 42db based on 50komh source impedance.
Power Output: +18dbm
Output impedance: 600 ohms, center-tapped at 150 ohms.
Frequency Response: 60 to 12,000 Hz within +0db -3db
Tone Controls: Treble control provides up to 34 db attenuation at 10K on phonograph and up to 17 db attenuation on Microphone. Microphone bass control provides up to 17 db attenuation at 50Hz. Phonograph bass control provides up to 12 db boost at 90 Hz.
Hum Level: 50 db below rated output
Output Regulation: Approximately 6db from full output load to no load
Finish: Glacier gray over copper plate