In addition to boatanchor receivers and transmitters, I occasionally enjoy working on tube audio amps. I was impressed by the solidity of this Dukane audio amplifier found at a yard sale. It has a 100 watt power rating (75 watts RMS) and is of heavy duty construction. Design and construction date is approximately1960-61. It looks as if it was intended for continuous commercial use. It can be rack mounted. An angled shiny aluminum heat deflector carries heat toward the front of the rack by convection. The nameplate indicates power consumption of 400 watts.
It sports two 5U4GB rectifiers. Its output tubes are four 6CD6GA in push-pull parallel. It also has a pair of 12AU7 for preamp and phase inverter and a 0D3 voltage regulator for the screen grid supply. It uses fixed bias.
The 70 volt tap on the output transformer is also used for the inverse feedback circuit. I cleaned the set, checked the fuse, and replaced the power cord with a three wire grounded cable. I also did the usual power line leakage tests and resistance checks in the high voltage section.
I reformed the electrolytics slowly, using a Heath PS-4 variable high voltage supply with a 25 mA lamp in series to limit the current and act as a visual fuse while reforming both electrolytics at the same time. The caps are rated at 500 volts and reformed very quickly. With the tubes removed, I checked the power transformer voltages and also measured the output of the selenium rectifer bias supply. The bias supply was less than the required -39 volts. I replaced it with a 1N4007 diode but left the disconnected selenium rectifier in place. Dukane also changed the bias rectifier to a silicon diode in the "C" version.
The schematic for the nearly identical 1A475C amp called for connecting a 40 watt resistor to the 25 volt balanced output for testing purposes. I connected the resistor load and powered up the amp with a variac while monitoring B+ voltage. The 25 volt output matches an output of 8 ohms. After testing with the resistor load, I connected an 8 ohm speaker load and provided an audio source. After cleaning a noisy input pot, the amp worked very well.
Click here for the schematic to the very similar Dukane model 1A475C.
In checking the large 0.25 MFD coupling caps, I did not find any needing replacement, a tribute to the quality of the original components.
The Dukane amp has moved to the West coast in order to join a matching Dukane for super-power stereo.
1-06 . , update 10-09
A Heathkit HG-10 VFO was the previous item on the bench.