It doesn't happen too often around here, but I got "kissed by Lady Luck", and found a very clean condition Rauland-Borg PA amp at the dump today.
The first thing I saw inside was a pair of Mullard 7189s, plus an RCA 6CA4, a 12AX7A and 6AV6. Nice and clean, no corrosion, mouse whizz or dead critters inside,
just a little light dust, which is to be expected - otherwise, it looks almost brand new. There is a date stamped on the underside of the OPT, June 1967.
Made in USA, Chicago, Illinois. The PT and OPT are horizontal-mount, end bell on the top, and open on the underside.
Took off the cover and bottom plate, which look as if they had never been removed before, and what I can see inside is a fairly standard fixed-bias, PP circuit,
I'll have to trace out the circuitry, and draw up a schematic. There is also an octal socket for an optional relay which would be used to allow the microphone PTT to
over-ride the music / phono input(s). It also has a very simple single pot mixer / fader control for the two "aux" RCA inputs. And a single "tone" control and a
"master" volume pot.
As it appears, it looks like the perfect basis for a conversion to an "18 watt" guitar amplifier. If I were to decide to add a tone stack and reverb, that would require
another tube socket or two, no problem, there is still plenty of room on, and under, the chassis. Not at all crowded.
Before all that ever happens, I'll have to check it out, then carefully power it up on a variac, and make sure that no "magic smoke" escapes, the quad section can cap doesn't go BOOM! and that no strange aromas emanate from within...
The other two (lesser) finds were a recent model Ford AM/FM/CD Radio, which was most likely removed for a "Crutchfield Update", and a small 24VDC,
400 mA power supply.
I did a quick web search for "Rauland-Borg", the company still exists, it has relocated to Florida, and seems to specialize in security and audio distribution systems,
probably none of which involve the use of vacuum tubes...
WARNING!!!! Full Tubal Nudity!
Everything seems to work, a tiny bit of hum, but then, that's with the bottom and top covers off.
7189s run with B+ approx 390VDC, G2s approx 370VDC, G1 bias -16VDC.
I have the circuit partially traced out, only "unusual" thing is the extra NFB secondary winding, and that feeds into the PI grid as well as the
12AX7 cathode in the preamp stage, through a three-section resistive divider.
/ed B