Sound City Dallas Arbiter 120
In a moment of weakness, I agreed to take this amplifier in lieu of payment for a cassette machine repair. She is a well travelled middle aged lady who has lost her virtue many times back stage I reckon.
Always had a soft spot for Sound City equipment. No silly bells and whistles, good circuit design, good transformers etc. 6 x EL34s and the usual brace of a ECC83s and ECC81, I hoped that I wouldn’t have to replace too many valves.
The owner said that it hadn’t been used for a while, so powered it up on the current limiter with standby off. With all looking okay, all heaters on, wound the mains down and put standby in ON position. Left it for a while and eventually gave her full beans. Dummy load connected and an input fed into the normal channel, the output looked awful. Low and very distorted, showing that only one side of the push-pull stage was working. My hopes of a quick repair vanished out the window.
With the chassis removed, things looked reasonably standard. Normally you get all sorts of bodges and make-dos. The 6 x EL34s all tested acceptable on the AVO valve tester, so that was good. As always, I check the primary windings of the output transformer, which was good too. Investigating around the phase splitter and driver stages, DC conditions were way off and only one half of the phase splitter was working. Numerous high value resistors were found to be either open circuit of far out of tolerance. So, all were replaced. As you would normally find, the yellow Mullard capacitors (or capacitators, as my customers advise me), were all great. Even the electrolytics capacitors proved to be in good order. Given their age and having had a hard life, I would normally change these, but they will stay for the moment. The drive to the bank of EL34s was still not perfect and a replacement ECC81 cured that. Then noticed that there had been cathode resistors to ground fitted in each EL34. The diagram shows the cathodes are grounded. A low value resistor is a good idea and it gives some indication of bias etc.. But, these 6 resistors varied between 20 Ohms and 68 Ohms! Why?? Those were replaced with suitable replacements and the grid stop feed resistors also replaced as these had changed 20% or so. Now had a good output with no distortion. The main volume control was sulking and didn’t respond to cleaning fluid, so to teach it a lesson, it was replaced. I have to admit to wasting a bit of time trying to find where a strange hum came from with the MIDDLE tone control was advance. Sitting the open chassis on a metal plate (similar to the screen inside the cabinet!), got rid of that.
I know a lot of energy goes into giving you 120 Watts, using 6 x EL34s and all the supporting bits. But, its like a steam train. Stand alongside it and feel the heat and get the smell. As you open the taps, you know something great is happening.