75
by scott_Archive
Bandmate's YBA-3 was sounding fucked up. Took a look and two of the output tubes (on the same OT primary leg) were red-plating, so we pulled the amp aside and threw another in its place.Tested all the output tubes, and one of the two offenders showed unreasonably low transconductance, and the other had a shorted element. Took the two remaining output tubes and moved them so one is on each side of OT primary. Tried out every 6L6GC I have in the two empty spots, but none idled anywhere close to the two existing tubes. Existing are modern JJ, all my spares are old US or UK manufacture.Tried out EL34s in the vacant spots, and was able to find a pair of them that idle almost identically to the existing 6L6's. Adjusted bias so that the EL34s run on the hotter side and 6L6's run on the cooler side, but all are within reason.This amp had previously been gone over, and we wired up the ground switch on the rear as a "suppressor grid selector switch", so one position grounds pin 1 of the output tubes and the other sends the raw bias voltage to pin 1. Because the JJ 6L6GC has no pin at all for pin 1, we were able to mess around with running the EL34 pair in either mode without affecting the 6L6's. Saw a difference of about 1.5ma in idle current depending on switch position, and both my bandmate and I agreed the amp sounded better with the bias voltage being sent to the suppressor.Noticed that there were still two electrolytics (preamp cathode bypass caps) that we hadn't replaced in previous work on this amp. Lifted one leg of each and measured capacitance. where both should have read 250uF, one was closer to 300 and the other closer to 475. Replaced both of these guys with 220uF and saw a dramatic effect on amp performance. Where before the amp was distorting at about 3 on the volume knob, now it stayed cleaner up until about 5. Played the amp cranked up pretty good (maybe 5 or so) with his baritone plugged directly in. Sounded damn good. Rocked out for an hour or so, checked the idle characteristics, and it was stable and sounding good, so we closed it up.Today it will see its first real band practice. Before it was using 4x6L6, and the YBA-3 that was filling in for it during its time of illness is Sacred Hat, which uses EL34s. Tonight will be our first practice ever with a hybrid EL34+6L6 output stage. Looking forward to it.
"The bastards have landed"
www.myspace.com/thechromerobes - now has a couple songs from the new album