help with audio resesrch d76

Hi I just picked up a couple arc amps a d76 and a 90b both in mint condition. What one is the better of the 2 and what do they sell for. I'm getting rid of one or the other. Gary
The D 76 is pretty old school, i have a D 75a that I bought new back in the mid-70's. It is not bloated sounding, but it doesn't have the resolution in the midrange that their later amps did. I don't know the D 90 at all. There is a database of all ARC models on line that will give you the basic specs, prices and model upgrades. Any amp of this vintage is going to need some attention unless someone has already re-capped and retubed it and gone over it, so that might be a consideration as well. The newer one might sound better, but they made some bombs, too, so I can't help you on sonics of a comparison. (I still have my D 75a, which I keep largely for sentimental reasons, I have maintained it over the years, all the original packing, warranty card, etc). Tube amps have come quite a long way, i'm not sure I'd put any of the old ARC stuff into the super collectible category of highly sought-after early tube amps. Interested to see what others have to say.
The D-76 and D-90b are terrific amplifiers. I have owned both and liked them equally well. However, it is important they are operating properly and the tubes are sorted and matched for these amplifiers. Audio Research is your best bet for tubes even though they are somewhat expensive. Without proper adjustments and properly selected tubes you will never know the full potential of these amplifiers. I guess if I had to choose between the two I would choose the later amplifier.
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.
I own both....very similar driver circuits. The reason ARC came out with the D-90 is that Mr. Johnson's favorite sounding tube was the 6l6 (personal communication) That is one of the reasons he used the 6L6 in the ARC/Dynaco ST70-C mod. He told me his ARC guys always pushed the higher power 6550. Tube brands have a larger effect on sound than the different circuits. Mr. Johnson liked the GE 6550 in the 76/a and GE 6l6 in the 90B. The JJ 6l6 and the mesa boogie STR440 sound similar to the GE 6l6. The 90B set up correctly has the power to "float" an HP use to say. The 76 is gorgeous sounding especially in the midrange...but the 90b has BALLS!! ;)
The original D-90 had 6550 outputs.
...also the D90 did not have a servo in it. The D90B does.
Thanks HifiGeek!! I have never looked at the schematic for the original D90. Clearly shows matched GE6550s. I was fooled by the website. Has a picture of D90 with GE6l6s in it. must be D90Revised...oh those ARC guys. When I talked to Mr. Johnson he said there was a heated debate between himself and his "engineers" between the 6550 and 6l6, but I did not realize they produced a 6550 version. Mr. Johnson seemed pleased he produced an amp using 6l6s. Also, I have an amp stamped on the back with D90-B and it does not have the servo in it. Leonard at ARC finally told me that it was at a dealer and was sent back for the B update and they took a little short cut... ;) Thanks again for the audio trivia. Larry
Larry, hopefully it doesn't have the white relay in it either. That can become problematic.
Hifigeek1 I need your help. What was the purpose of the "Servo" circuit? in the ARC 90B? Also, is the white/gray relay located down in between the large grey tall power supply caps? where the 6550 power supply tube was located in the 90? I do not have one in mine. The reason I am asking is that all of a sudden my 90B(ish) upon start up, the output tubes glow orange after about 30 seconds and then the fuse blows. Techs look at my 90B and say it does not match the ARC schematic for 90B?? Again ARC said it started out as a 90?? or 90Revised. Very confused. Thanks Larry
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.
HiFigeek1 you have sent myself and my fellow audio buddies on a wild goose chase.....for several days...we just thought we were dumb and didn't know how to ask you...Illuminate coupling caps???? We thought you knew an audio term we did not know....WELL DID YOU MEAN ELIMINATE coupling caps...??? Larry ;)
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.
I was wondering if you could answer a question I have, is the arc d-90b triod?
If you mean the output configuration the answer is no. I believe the output circuit topology is partially cathode coupled, not triode configured.
Thanks Hifigeek,

that is exactly what I wanted to know. You have confirmed my suspicions my tech is a very good solder monkey.

I was talking having my sockets replaced in mine and he said that it was wired in triod. I asked if it was suppose to be he said according to the ARC schematic it was. I asked him to double check and he said he just confirmed it by re looking at schematic in the manual.

This made no sense to me that it could be triod with 80 watts output and that small of output transformers.
They dimensionally look the exact same to me as the Classic 60 outputs.
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.
I meant the transformers from my d-90b look identical, at least external as the cl-60.

Would it not be as simple to run the d-90b in triod as

1. Remove the screen tap leads wires from pin 4 of the output tube sockets
2) Connect a 1 or 1/2 resistor from pin 3 to pin 4 (plate and screen) on each of the four output tubes
3) re bias
thanks and have a good one.