Bogen HO-125 PA Power Amps Repair Journal
by Chad Hauris, Retro Electronics and Audio Lab, Midland, TX. 10/6/07
Please click the photos for larger views.

These 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.

Top of Bogen HO 125 amp before repairs.
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.

"New" Bogen HO 125 amp before repairs.
The newer Bogen amp before repairs (bottom view.)

Bogen HO 125 amp before repairs.
Here is the other amp before repairs. Most of the old capacitors had been replaced, but the old HV filter capacitors were still installed.

New capacitors in Bogen amp.
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).

Testing the "old" Bogen HO 125 amp.
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 Bogen amps.
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 circuit.

Testing the Bogen HO125 amps.
Testing the "new" amp (view 2).

Testing both Bogen amps.
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!

Dark view of Bogen tubes.
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.

Testing ballast lamp on Bogen amp.
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.

Lamp installed in Bogen amp.
Here is another view of the ballast lamp.

Time delay relay in Bogen Amp.
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.


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