Thoughts on Audio Research D70

@Hifigeek1: I was trying to do some final checks on the regulated voltages to the first stage B+ and regulated screen grids and I found an ultrasonic oscillation of about 20mV peak to peak. My mistake for not turning the horizontal dial of my scope all the way to look at ultrasonics. Thought the 20mV was just hash or the probe.

This was traced all the way back to the plate of the 12AT7, and then back to pin 6 of the TL071 (U1) which was sending 100mV of sawtooth.

I'm leaving this for another day now but am curious if you think I should replace the 3 0.22 caps surrounding the input pins of the TL071 opamp.

Much appreciate any guidance.

@Hifigeek1: Never mind, the relcap 2.0uF bypassing the cathode (regulated B+) output of the regulator 6550C has deteriorated enough that when I paralleled in another 1uF all is good. Time to replace them bypass Relcaps too I guess.

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

Point well taken, esp with coupling caps where I often choose not to parallel a tiny 0.01 for the sake of coherence. Would rather invest more and get a better single cap.

For B+ bypass my experience is YMMV. I noticed that blackgate lytics will sound hashy and smear if bypassed with a small film whereas others may benefit.

the D-115 B+s are all lytic + Rel2.0uF + Wonder 0.01 with regulated B+ to the screen rel2.0uF + wonder 0.01.
I also notice that the B+ to the 6922 gain stages have nothing at regulator (TIP41) output at all (!).

I think I am ready to move on to triode mode. Just realized the screen grid supply is actually the 6550 pass element's output. This will be useless if I convert to triode, kind of a waste.

Wonder why they use the solid state buffered regulated output for the small signal B+ and not directly from tube reg.

My guess is it's not quiet enough