Thoughts on Audio Research D70

The CL-150 was a totally different animal and frankly pretty average by today's standards. It started life as an M-300 and used Chinese coke bottle 6550's which at that time, were just awful. Their were no Russian 6550's at that time, and the Sylvania 6550 was history. So the only choice was to use the Chinese 6550. The Chinese tubes would fail after a few months, but the heaters would still work. So basically the light was on but no one was home...ARC decided to change the amp to the Classic series and changed the output configuration to triode making easier on the Chinese 6550 output tubes. So it became a CL-150. It's got great slam and sounds good and is miserable to work on. The cooling fans blowing on the fets and tubes while the amp is on is not an advantage. Cooling tubes yes, cooling fets no. Funny thing is a modified M-300 to triode have bigger transformers then a stock CL-150.

Just to chip in for a minute, i have a pair of CL120's since 1990. Those have been modded by Steve Huntley and the inputs were also balanced.
I agree with Hifigeek about those Chinese bottles. The day i got them home from the dealer and plug them in, 2 tubes failed within 5 minutes. I have retubed those amps with 6550 SED Winged C many times since then and have not had a single issue with any of them. I have had the opportunity to "test" a VT100MKIII and VS110 and a VS115 in my system and i'm still holding on to the 120's. Having separate power supplies in monoblocks configuration have advantages which the D series don't have.

I would like to listen to a REF150 next. Last time i talked to Steve before he closed GNSC he recommended i listen to a 150 but only in my system in my room as at a dealer sound room with different ancillaries would not be a fair comparison.
Sly

So the verdict for my unit is:
Faceplate is dinged up and dented. Supporting bar bent. I cant get a replacement from Audio Research as they are out of spares.
Seller graciously offered a partial refund which is very nice of them.
The 1A fuse popped several times and made me worried the screen regulator circuit might have hidden issues. 3 x 1A fuses and a trip to the local store with a tube tester I am fairly certain it was a bad 12AT7 that tests weak on the tester. Replaced it and I have powered it up 10 times since with no issues.
I am hoping that the fuse pops were just a problem with the tube.
Since the unit is of late vintage and confirmed to be a mkII I think the lytics are still OK.

I meticulously adjusted the diff amp balances and bias and now it sounds better than initially, and so far the voltages (esp screen regulator output I am cautious of) look stable.

Over sound is that it's really foggy and a bit slow. Crank it up and I notice glare and roughness.
Space is real tight for any cap mods. Auricaps will fit but how much better are they than the Rel + Wonder paralleled.

A friend has a pair of Reference 300 that's in the store room for at least 4 years now since the last power up. Maybe this little project, if successful, can get him to power them up and play again.

@HifiGeek1: What do you think of the recent frenzy regarding those whatever 50 years treasure funny black bottles and the whatever black sable 6550s? PM me if you want to avoid flaming :)

Johnsonwu, sorry to hear of your travails. I would say that there must be something still wrong with your D115M2 if it sounds foggy and a bit slow.
Did you replace all of the tubes and check the caps?

All tubes are known good tubes from my collection. All voltages were checked and verified AOK.

I have no way of checking how good the lytics are cos I dont have a big cap meter and I dont want to pull out the lytics.

Foggy is expected of the Rel-cap/Wondercap combo, as I have over the years owned quite a number of Audio Research amps. No surprises there. With this amp, everything is like 95% there but not quite excellent. Nothing annoying, but nothing that leads me to say, wow this track plays good.

CL120
VT150
CA50
VT200
CL150
VT130
and I might have missed a few...
Most of them I ended up redoing the coupling caps with Infinicap-S, Dynamicap, Auricap, or Jensen (wish I had tried V-caps and worked on the all-tube models more before I sold each of them one after the other).

Problem right now is that space is at a premium and its very difficult to put in premium caps into this amp due to their size.