help with audio resesrch d76

Larry, hopefully it doesn't have the white relay in it either. That can become problematic.

I like the D-76 a lot. I'm working on a D-90B right now. Both are great but the D-76 with new caps actually sounds better then when it was new as all the poor twist lock lytic caps get replaced with current technology filter caps. The D-90B uses different output tubes as well. The B version utilized 5881/6L6's vs. the D-76's 6550's as well as a servo circuit which is somewhat audible. Also for reliability reasons, the white relay in the D-90B should be removed as the audio can hum for a few seconds after the relay closes. Some people find that objectionable.

The original D-90 had 6550 outputs.

...also the D90 did not have a servo in it. The D90B does.

Larry, In the D90, V19 is the 6550 regulator. In the D90B the regulator consists of a 12BH7 V15, and a 12AT7 V16. The B version does not use a 6550 for voltage regulation. It is possible the unit started out as a D90 and got updated to D90B. I have seen updates like this before. You unit needs to be serviced by someone who knows ARC products. I would be happy to take care of it for you if you do not wish to ship it back to ARC. The servo was used to illuminate coupling caps.

In certain situations a servo ic is used to eliminate coupling caps from one section of a given circuit, utilizing the ic in a feedback loop, usually at the input. Although coupling caps are a necessity in tube products, they do have their own set of coloration that add to the sound of the product. A tone control of sorts. Some designers felt that by removing them at the input of the circuit and replacing them with a servo ic there would be less coloration. What we now know is there's no free lunch. Servo ic's do have their own set of coloration with each manufacturers ic's sounding different. In some cases they can actually sound worse than a high quality coupling cap.

If you mean the output configuration the answer is no. I believe the output circuit topology is partially cathode coupled, not triode configured.

Well they are different transformers. The CL-60 is triode configured. On the D76, the screen voltage sits at 360Vdc, while the plate voltage sits at 575Vdc. In order for this to be triode configured the screen and plate would have to sit at the same voltage, with the screen and plate tied together through a resistor. Only one voltage winding off the power transformer would be necessary.