Public Address Amplifiers
Retro Electronics and Audio Lab, Midland, TX. Updated 10/6/07

Rauland PA amp with turntable. Appears to be early 50's model...brand new in box, never used. It has a vibrator power supply to use on 12 or 6 volt DC and a battery hook-up cable, and can also be operated on 120 v. AC. Uses 2 6L6 output tubes. We will replace old capacitors that have deteriorated through time.

Operadio PA power amp. Early 1940's model: uses 4 6L6's, 2 5Z3's, an 80, and a 6SN7. Operadio later became the DuKane corp. manufacturing similar equipment. This is really a military or heavy industrial quality unit. We replaced all capcitors and installed a new 6L6 to replace a missing one and a 6SN7 in place of the mistakenly installed 6SL7. A new volume control was needed, and the high-low line voltage switch was bad. The power transformer was wired permanently in the "high" (120 volt) position...the "low" (110 volt) position provided too high of a tube filament voltage, and most locations today tend toward high line voltage.
We found that the audio was intermittently cutting out...this was traced to the B+ fuse socket. Someone had installed a 15 amp fuse, so we cleaned the socket contacts and installed a new 1/2 amp fuse.
We teamed up the Operadio with a Heathkit preamp, model WA-P1. We used filament and plate voltages from the Operadio amp to operate it. This preamp was recapped as well. Someone had put a 12AY7 in place of the 12AX7 phono preamp, causing weak sound on phono...replacing with the correct tube brought back good sound.
One safety note: the original circuit of the WA-P1 connects the RCA jack output directly to the plate of the 12AU7 tube, resulting on 110 volts on the RCA jack. We wired a .1 mfd 600 volt capacitor from the 12AU7 plate to the RCA cable for safety.
Photos: Rear view of amp, turned on its side.  Testing new volume control.  Testing unit, rear view.

New capacitors installed.  Front view.  Heathkit preamp with new caps. 

Testing completed setup: View 1  View 2

RCA Sound-Reinforcement amp and mixer: uses 4 807 tubes for output: part of our own collection. We use this amp to supply the whole shop and and outdoor horn with sound. All 3 jukeboxes are hooked up to it, along with a microphone, AM/FM tuner, and Garrard 78 rpm changer. The switches on the sides are for turning specific speakers on and off: We are using a 70-volt distribution setup.

Bogen 10-watt amp CHB-10: uses 1 7868, 1 12AX7. We replaced the cheapie ceramic coupling caps with mylars for better sound and added a power indicator lamp.

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