Thoughts on Audio Research D70

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.

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?

yes and yes.
6922 plate remains 75V as prescribed.
I did not change the 1 1/2 6922s for each channel.
Just REMOVED the first 6FQ7 and reconfigured the 2nd as common cathode.
Note that in the orig circuit the 6922 with plate = 75V is directly coupled to the grid of the 2nd (driver 6FQ7 anyway)
6H30 plates are now 280V (B+ still 455V)
I can send you a spice file.