Thoughts on Audio Research D70

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.

Hi Johnsonwu, I may be misunderstanding your intent. If you are not planning on changing the circuit that is inherent in the amp, then there are no real risks that I can see.
BTW, I agree with you that the circuit is superior to the Classic 120's and the VT series..... this has been borne out in the listening tests that I and my a'phile group have conducted vs. the VT series and the Classic 60 that we also AB'ed against. ( presumably the Classic 120 is the same basic circuit).
I have yet to hear a better ARC amp in the mids than the ARC D70mk2, even in the ARC VS series of today.

The Classic series had triode configured output tubes. This tended to make the output tubes last longer and gave it a sweeter presentation. I believe the front end was fet transistor up to the driver tubes. Very different topology from the D-115mkII

@Hifigeek1: Yes indeed triode vs NON-Ultra-linear fixed bias tetrode mode. But same B+ @ output stage.
420V B+ is a bit low for my liking but with 4 tubes per side power should be more than enough.
I have every intention to convert that to triode at a later stage, after I am done with the caps and diodes.

I had the CL120 and CL150s before. CL120 at least has a tube driver stage (different plate voltage from D115 though). CL150 is just raw muscle.