Thoughts on Audio Research D70

Thanks for the opinions. Now to find an old beat up unit for an affordable price so I can do a complete rebuild.

Found a D115 mkii. Can't wait to hear how it sounds.
Will definitely verify each test points voltage and go through the usual hexfred/schottky diode mods and coupling cap upgrades... then triode of course.
The circuit begs for a better driver tube than 6FQ7.
Will see if filament supply can support 6H30 (driver and power tubes have AC filament).

Johnsonwu, I will be very interested in your outcome after you have modded your D115Mk2. WZ Johnson got the circuit very right, IMHO. Changing what he designed could be a risky business, OTOH I applaud your experimentation.

Just the throw my 2 cents in, I'm not a big fan of Hexfreds. They can be very problematic down the road. One the other hand, some good soft recovery diodes in the filament supplies would work nicely, (non-Hexfred types). Also because 6H30's pull quite a bit more heater current I would avoid them in that amp least you over heat and damage the heater winding of the power transformer. Although the transformer might still be available from ARC, it is quite expensive. Instead, replace the stock coupling caps as well as the filter caps if they're old, and bypass them with good film caps. Make sure all the tube sockets are clean and tight, the 6FQ7's triode halves are well matched and the D.C. voltage around the 6FQ7's are properly adjusted. Check that all the voltage regulators are functioning properly and quiet.

@DaveyF: Totally understand the risks. I think the circuit is in many ways superior to the Classic 120s and VTXXXs I used to own, and provides more opportunities for asjustments, thats why my quest for this amp.

@HifiGeek1: Thx for the cautions. Guess I have been lucky. The only Hexfreds that have exploded on me was the IXYS bridges. Been lucky with fairchild 1200V stealth and ST 100V Schottkys. Knock on wood. The filament supply is good for 4 x 625mA (6FQ7), AC filament in parallel, and I am only changing 2 of those guys to 900mA. The 6922s use DC filaments (15VAC winding) and thats where I plan to use the 100V Schottkys.