PRF members' tech journal

71
scott wrote: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.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.Cool. Doesn't Mesa hold a patent on that simulclass thing?Ben

PRF members' tech journal

74
My drum machines [Roland DR-202] AC jack seemed screwed up. When normally inserted, it wouldn't turn on - but if I applied pressure to the jack holding it 'just right' - it'd work fine. I had the cable taped in place to make it work - but this was/is annoying. At first I suspected a cold solder joint - so I rewet all the connections from the jack to PCB - but that didn't do shit. So I ordered a new jack - thinking the problem must lay internally on the jack side. (something like 16$ from Roland with shipping - retarded, must be about 16 cents of material).Yesterday tried to install it - but it was a huge pain in the ass getting the old one out (5 connection points to the PCB). After multiple solder suckings and wedging - I finally got it out - but look to have fucked up 2 of the traces having had to heat/re-heat each pin way more times than I'd planned - and they're now sitting slightly off the board. I installed the new jack and now it won't power up through AC (before it at least did jury rigged). Fortunately the machine works off batteries still - but this is annoying.Any suggestions? This should have been a pretty straightforward repair. No schematics for it - and some of the traces seem to go to nowhere? (Obviously they must do SOMETHING, but I haven't figured it out yet). If I could find good points down the path I'd at least do a continuity test, but haven't been able to do that yet.
post honeymoon | bang! bang! | new black

PRF members' tech journal

75
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

PRF members' tech journal

76
madraso\_ wrote:tolexed an amp I built for the bands guitar player. It came out well, I used 3M spray adhesive, and found it to be very unforgiving, but manageable. Will post some pix sometime.Anyone have experience with using other adhesives to do this?wordI've had friends do this with a very liberal amount of elmer's white glue and a lot of clamps.ben

PRF members' tech journal

80
I just finished building the BYOC 2 Knob Bender pedal. It sounds great but I want to put a knob in the circuit that allows me to blend clean signal in to the fuzz signal. The kit also came with a LPB1 knockoff that I can use to buffer the signal. I don't really know where I need to place this in the circuit and would love it if one of you more techy types could help me out. I have a Audio taper (I think 500K) pot but if that isn't ideal, I could get another value. Schematic can be found here: http://buildyourownclone.com/esvmk2instructions.pdfThanks.

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