Or at least a new G*S*G i s born....
I've had all the parts around, but not the time (and somewhat not the ambition) but this weekend I got my arse in gear and built up the scond G*S*G. No wild and crazy mods or major design changes, in spite of all my earlier ramblings, I stayed with about 90% of Shannon's original design.
I built it into a black powder-coated Hammond 12 X 10 X 2 chassis, which allowed room for the somewhat oversized Magnequest RH-60 OPTs and two chokes under the chassis, located at the front to avoid any possibility of magnetic coupling into and from the OPTs.
The first choke, I used a Hammond 158Q (5H, 150 mA, 105 ohm DCR) and in place of the two paralleled 180 ohms resistors, I used a Hammond 156R (1.5H, 200 mA, 56 ohms DCR). Everything else was per specifications.
For the two 6B4Gs, I used some WWII vintage mil surplus ceramic octal sockets, which mount with the "wavy washers" and thusly don't need the two extra mounting holes. These have full surround contacts and grip the pins like a starved pit-bull. Yellow_Light_Colorz_PDT_04
The MQ RH-60s have 5K primary, 30H inductance and 185 ohms DCR, secondary is 8 & 16 ohms. They do have exposed solder terminals, so there is live voltage present (use shrink tubing, as I explain further down...)
(yes, I did manage to tale a 412 VDC "hit" during the initial power-up test before the tubes were inserted) :o Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_21 Yellow_Light_Colorz_PDT_09 :o
Stopped everything, applied heat shrink sleeving over the hot terminals, problem solved Yellow_Light_Colorz_PDT_08
I did all the metal mangling Saturday, and stuffed the board and final assembly on Sunday, and the "smoke test" and listening started Sunday evening.
I've always liked the sound of the James OPTs, but these RH-60s simply blow them away! With the 5K primary, the output power is a bit lower than with 3.5K, but the distortion is also a lot lower, and these trannies have sound quality that is just one more step "beyond"...
With the 158L and 156R chokes, the B+ at the OPT is 312VDC, (about 10VDC lower than with the original configuration) and the extra inductance virtually eliminates any residual hum or ripple.
I started out with a Westinghouse 6SL7, GE 6AX5A and Sovtek 6B4Gs.
Great sound, deep bass and very clear treble response.
After an hour, I tried out the vintage dual-plate Russian 6C4Cs. They were good but the Sovteks are noticeably better all the way around. Now, the 6B4Gs already have a few hundred hours run time on them, the 6C4Cs were NIB / NOS, and almost 50 years old , so perhaps that explains the difference.
I'll do a similar comparison with GSG # 1 with the James OPTs and slightly higher B+ and see how the 6C4Cs compare in that amp.
/ed B in NH