Thoughts on Audio Research D70

I work on them and retube them. Use ARC tubes only. Excellent amp. After 2000 hrs it's time for an output retube. I also set the bias down to 62-63mV @ 120Vac. Doesn't hurt the sound and runs a bit cooler. Very good amp. The D-115 is basically the same circuit with 2 more 6550's per channel.

No doubt it's a good amp. The D115mkll has a bit more drive, dynamic range, and slam in the bass.

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.

Bypass with the same manufacturers caps as the coupling caps..

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

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.

That's the one. Also make sure you measure 10 ohms at ground to common tap at output transformer.

My guess is it's not quiet enough

No flaming. My attitude about spending gobs of money on NOS tubes is kinda like buying expensive flowers. Sooner or later there going to die...LOL I'm not a big fan of NOS output tubes. The D115mkII was designed around the Philips/Sylvania 6550. Look for a set of those. BTW just because the voltages are ok on the amp doesn't mean the voltage regs are quiet. That's important too. Noisy voltage regs. have a negative effect on that amp. Also, there is a B+ reg and a buffer reg. Make sure the voltage differential is the same or close to the circuit diagram. If the power supply caps are more then 20 years old, replace them. All new caps will make the amp sound better especially in the bass provided there's nothing else wrong with it. There are a few axial caps scattered throughout the amp. Replace those as well. On top and underneath. There is also a 27K ohm resistor near the main filter caps. Make sure it measures 27K. Those do open with tube arcs. There should also be red cap plugs on the capacitors. The support bars always get bent because people try to lift the amp with them and their aluminum. You can remove them and straighten them out a bit to get some of the kinks out. Use a rubber mallet, however the rack handles will come off too. Do one at a time. I strongly suggest you make sure that amp is in good working order before you attempt to modify it.

You realize you will only get about 35-40Wrms across 8 ohms in triode configuration...

KT-120's are not recommended by the factory for use in a D-115mkII. I would however use caps furnished by ARC as well as the red cap plugs that go on top of the cap .

Yes. I was thinking it was a D-70. D-115 will get you 60Wrms..

I have some 1mfd@600V teflon caps if you are interested...

These caps are huge and made by the company that makes them for ARC.

Nice huh...lol

The D-70 is truly an old friend....

6H30's draw more heater current...

I think you mean cross couplers? So insted of using 2)6FQ7 per channel you are using one 6922 and a 6H30 as the driver. The input tube remains a 6922. What are you setting the plate deferential voltages to? I believe with the 6FQ7, now a 6922, it was 75V matching from plate to plate. What percent THD are you getting?