[AudioCats]

Starting a thread to document mods being done to my Stax 007t amp. Hopefully someone else will find the info useful/entertaining as well.

This amp is the older version with Champaign colored face plate (not the current silver face plate 007tA). Black PCB 08450-00131 PB-102.

Tubes inside are marked "6FQ7 6CG7 USA", no other marking, not sure who made these tubes.

Listening impression (driving sr007 and sgl-jr), before the mods:
-- sound is somewhat "tiny"
-- rather narrow stage
-- bass is adequate, but doesn't go very deep
-- somehow has a bit of "mid-range burn" feel, irritating especially with the sr007. Made me what to keep turning the volume down.

Not very impressed. I will try a different set of 6FQ7 and see if the sound improves.

***** follow up: looks like the amp needs a bit of warm up. The sound opened up a lot one hour later, stage much wider, no longer tiny, and irritations are gone. Still a bit dull sounding. Bass still not very deep, a bit loose when driving the sr007.

 

[AudioCats]

voltage measurements:
across D3: 1.77v
across R11: 3.2v
across R12: 10v
across R9/R10: 1.2v
across R5/R6 (160r): 0.27v
across D4: 1.77v
across R8 (330r) : 1.14 / 1.17v
across Z1/Z2: 102v
across R13/R14 (51k) : 89V
across Z4: 16v
across R18/R19: 0.17/ 0.18v (therefore Q7/Q8 are not conducting?)
across R30 (20k): 15.4v
across R20 (1.5k): 22v

anode resistor (47k) 352v across, 125C (with amp's top cover removed).
cathode sits at 23v above the HV- rail => Vgk = 15.4-23= -7.5v

LV+: 27v pre-reg, 15v post-reg
HV+: 360v at C11, 355v at C12

 

[AudioCats]

List of potential mods:
a) Remove input-2 (RCA&XLR), since I don't use XLR at all. I don't see the value of XLR in a headphone amp set up.... in Pro audio yes, in a speaker setup maybe, but not in a headphones system.
then upgrade the wiring between RCA and volume control
b) change volume control to shunt style. Maybe use the front panel input switches to select between two sets of input/series resistors.
c) regulate the HV- to maybe -275v. HV- is more important (than the HV+), signal in stage-2,3 (emitter-follower for tube driver) , and stage-4 (tube) are referenced off HV-.
d) remove Z1/Z2 (only after HV- is regulated)
e) signal-diode mod (D1/D2)
f) change tube to 6S4a or 12B4. For my low volume listening style the 12B4 might be a better choice, it has lower Rp. But 6S4a is much more straight forward to use.
f) CCS modules for tube anode, set to maybe 15mA. Will try the DN2540/IXY10m90 style first.
g) external supply for tube filament power.
h) LV+/- supplied by LM317/337, separate set for L/R channel (at that point the signal is still referenced to the GND).
i) regulated HV+. It will likely be supplied from an external source.

 

[lupoal]

and?