Bogen amps are high powered power amplifiers suitable for driving 70
volt line speaker systems, such as in amusement parks, drive-in
theatres, etc. They use 2 807 tubes with about 900 volts on the plates
of the 807's (this is pushing the tubes pretty hard!)
DANGER: These amps use deadly voltage levels (as much as 2000 volts!) We used great caution in working with them!
These particular amps had new output transformers installed for 4, 8,
and 16 ohm speakers. The original transformers only provided for a 70
volt or 110 volt output. Also they had been converted to have
rack-mount cabinets. Some repair work had been done on them but there
were a lot of problems remaining.
Here is the newer of the two amps before repairs. The large transformer
supplies the low voltage DC supply and the filaments. A 5Y3 (or 5R4) is
used as the LV rectifier. The filter choke is in the middle, and the
left transformer supplies the HV, which is rectified by a 5R4. The HV
is 900-1000 volts.
The newer Bogen amp before repairs (bottom view.)
Here is the other amp before repairs. Most of the old capacitors had
been replaced, but the old HV filter capacitors were still installed.
Here are the new capacitors in the "old" Bogen. To replace the old 2
mfd. 1000 v. HV cap, we used 2 Solen 4.7 mfd 630 volt mylar capacitors
in series, with 1 megohm equalizing resistors across them. Without the
resistors, one of the caps was seeing more voltage across it than the
other. We also replaced a paper cap still remaining in the audio
circuit, and added a .47 mfd cap in the audio input to block DC coming
in from a preamp.
As you can see, burned wiring at the power transformer was repaired by re-insulating with heat shrink tubing.
On the "new" amp we replaced the HV cap the same way. Upon testing, we
found about 1200 volts on the plates of the 807's. We then found that
the ballast resistor (seen at the left side of the picture) had been
incorrectly hooked up by a previous repairman, causing it to not
connect to ground. When the resistor was hooked up properly according
to the schematic, the voltage dropped to around 1000.
Also, we adjusted the bias voltage on the 807's by adjusting the tapped
resistor in the bias circuit. A 6SN7 is used as a bias regulator and
was overheating due to improper settings on the resistor. We set the
tapped resistor for the proper voltages at the 6SN7 and adjusted the
top of panel bias control for proper negative bias at the 807 grids
(around -38 volts).
Here is the "old" amp being tested. 2 voltmeters were used, to monitor
807 negative grid bias and 6SN7 regulator grid bias simultaneously.
Testing the "new" amp while monitoring bias voltages.
Another problem we found was that someone had wired a switch directly
in the 1000 volt HV line to act as a "standby" switch. Although the
circuit seemed to function O.K., toggle switches are not rated for 1000
volts and eventually the high voltage could pose a shock hazard if it
shorted out to the switch handle. We removed the switches from the HV
Testing the "new" amp (view 2).
Testing both amps. There was so much power available from them that we
could only turn the preamp volume up halfway before we thought it might
damage the test speakers!
Bogen amps operating, dark view.
Upon testing the amps, the "old" amp showed arcing in the rectifier
tube and the fuse blew. We then found that someone had used a terminal
strip to connect the 807 plate leads to the transformer which was
arcing over due to the HV. We spliced and soldered the plate leads
directly to the transformer leads and insulated the connection with
three layers of heat shrink tubing, which solved the problem. This was
done on both amps.
We found that on the "new" amp, a power surge was occuring during start
up after the 5R4 tubes warmed up. The surge was only for about a
second, but was causing arcing in the rectifier and the fuse to blow.
We could not determine what was causing the surge, but were able to
solve the problem with a ballast lamp. A 60 watt 120 volt lamp was
connected in series with the HV transformer primary. The lamp limits
the current into the transformer, thus preventing the current surge.
After about 20 seconds, a time delay relay shorts out the lamp,
allowing full power to flow into the transformer.
As you can see, the lamp mounts completely inside the chassis. As it is
only on during start-up, heat production is not a problem.
Here is another view of the ballast lamp.
The time delay relay tube is seen between the 807 and 5R4. It uses a
filament which heats up a bimetallic strip which closes the circuit
after a certain amount of time. The tube we had, had a filament voltage
of 2 volts. A diode and one-ohm resistor were used to drop the voltage
from the 6-volt filament line for the tube.
Also we found that the fuses were completely mis-wired in this amp, not
giving proper protection. We rewired them according to the schematic.