Thoughts on Audio Research D70

In the mid 80s, I used to put the D-70 on Quad 63s (with custom stands) for my clients.

Sometimes used Entec subs, sometimes not.

To this day, some of the most lifelike and stunningly involving music reproduction I was ever privileged to hear (within its dynamic envelope, of course).

I suppose the D70 was colored to some extent, but, IMO, it remains one of the most listenable amps ARC ever made.

And I go back with ARC to be early 70s, and worked there from '76-'77.

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

Ampex53: I dont have a thread on modding the D115ii.
The schematics are floating around the Internet.
The docs Kalvin sent me are the schematics (same) and parts list.
A 1987 build would be a D115ii.
Hope the fuse holder is repaired by now.
Biasing is a nightmare but since bias V is regulated you can put in 1 row of KT88s and bias them then add the other row. You MUST use DVM probes that have smaller exposed tips otherwise you risk shorting out things.
More importantly is the diffamp balances.
The gain stage 75V plate voltage and the 250V driver plate voltages must match (the 250V is hard to match you need to have 6FQ7s that are very symmetrical within 1 envelop)

Mods that I have done:
* All coupling caps to 0.33uF Russian K40y9 PIO
* Triode mode (pull the screen grid resistors and connect plate to screen grid with 100Ohm resistors @ each KT88(need change out a few resistors @ the bias regulator circuit to raise regulated ference bias voltage to close to 50V otherwise you smoke the KT88s)
* a very involved mod which involves REMOVING 1 pair of the 6FQ7 (V4V5) and substituting the V6V7 driver 6FQ7 with 6H30. THis involves tying the 2 6kohm (schematic says 5k) R27R28 together to form a common cathode R of 3kOhm and reducing the Plate R and pot to 13k plate resistance. (this is NOT for the faint of heart, I came up with those values running many rounds of spice simulation)

I am extremely happy with the results.

6H30's draw more heater current...

No worries hifigeek1 I removed the 2 6FQ7s (V4V5) which are the capacitatively coupled bottom tube of the 4 triode criss cross diffamp driver subcircuit so the net filament draw of this new config is 1 pair of 6H30 vs 2 pairs of 6FQ7 ie 30% lower than before.
The 4 6FQ7s previously shared the same 6.3V AC filament winding.
My amp now runs with 2 less tubes and 4 less coupling caps than stock,
6922 phase-split->cap->6922 gain->6H30->cap->KT88.