finally wired up the output xfmr, sounds great!
had some problems with the line amp oscillating, but after the feedback windings were swapped, most of it went away,
just occasional motor-boating at certain settings, so i shortened up some grid leads,
changed the bypass cps to .33 instead of .68 to swamp out the low end,
still had a slight problem, so i added the stock zobel, 62k/.003 to the 600 ohm output winding and that fixed everything.
i am wondering if there were stability problems with the line amp/xfmr combo,
there is a 20 pf across the plates of the 12ax7a on the original circuit,
this is a fix for an unstable circuit, or it might just mean wiring capacitance,
i also noticed that when the output was run balanced, (ground lifted) that the zobel was no longer needed.
i am also running the orig zobel on the hs-29, 63k/270pf, and using the 620 ohm resistor across the triad hs-56 sec.
you want that resistor in there so the line amp does not distort, the level goes up quite a bit without it, it seems to make no difference on the high end,
tons of bottom end, very happy with the sound, no hum, no tube noise,
i took out the bandwidth control as it seems like a "bells and whistles" thing,
it only really works at certain settings, and i have enuff controls to hassle with if you include the mic pre, compressor, etc.
i did not need the zobel with the Sowter output to fix any problems,
so Brian might have done a better job at controlling phase shift on the tertiary winding,
he uses a EI core instead of a dual coil L lam setup, flux linkage with the EI can sometimes be better than the UI or L dual coil setup, opposite of what you might think because the dual coil wraps more of the core, leakage causes phase shift which causes feedback,
a 0.003 cap is a pretty big value to strap on the output xfmr, this might be the fix for the phase shift.
this is a unique line amp when you look at it, cathode current for V1 goes thru the output xfmr,
there are 4700 ohm self bias cathode resistors on the 12AU7, which elevates the cathode to 40 volts, which cuts off the tube,
but they elevate the grids to about 30 volts with a voltage divider off the pwr supply, to get the operating point back up, i have about -8 on the 12AU7, slightly lower due to a dropped B= voltage, (240 VDC)
this is probably done to make the amp less noisy,
or it could be to limit current thru the transformer pri, a 12AU7 can pump out about 20 ma, so 40 ma might be too much for the pri wire, too much heat, pri dcr is about 395 ohms, times 40 ma = 15 watts!
with the 4700, we get about 8.8 ma times 2 = 17 ma times 395= 6.75 watts,
ok, bolt the lid on and rack this so we can finish the 670 and fender princeton and v72 pwr trans revision,
i recommend mounting the NOS Triad and Peerless stuff flat, rather than using the original hardware as these are wax filled units, and sometimes, when they solder the header plate on, the wax can get hot and boil up into the seam, so you have a contaminated weld,
you can see this over at gearslutz, people asking how to fix their 2000 dollar output transformer that is hanging by the wires which may pull out,
so yeah, a real bummer to finally score a s 217 d and have the leads ripped out, :-\
if you own an original EQP-1a, then you may want to reinforce the transformer mounts with some type of strap over the top,
so you can fix tape hiss, jack up the treble 8 different ways,and best of all,
boost and cut the bass at the same time for that pultec mojo,
we have a Telefunken 12AX7a and a GE ECC/12AU7a for the tubes, offboard pwr supply,
now for vocals, nuthin beats the MEQ-5.
i would like to use this line amp with a meq 5.
this chassis is what i would call a "white trash" chassis, kind of rough, done in a hurry,
it is a rebuild of an earlier chassis, we try to be neater now days as there seems to be some real sharp projects coming down all the time,
