Post by thetragichero on Dec 1, 2020 15:00:34 GMT -5
the guts are pretty much done pending testing. this is 10lbs of junk in a 5lb bag and not entirely pretty but managed to keep the preamp ground on the complete opposite side of the chassis from the other grounds so I'm hoping it'll be relatively quiet besides the fact that the pentode in the ecl86 is very similar to an el84, the dna is very fender brownface (tone stack used from one of the bigger models). i used the triode sections of the ecl86 for a long tailed pair phase inverter so i had to add an additional 12ax7 (I've already seen this done with other conversions so I'm not concerned about the extra heater current draw, especially since the transformer must have a rather stout heater winding since the 6ca4 rectifier is a hog). looked to sluckey's peewee hammond conversion for help, especially the ecl86 pinouts
I'll take a photo of the schematic later. will be going into a combo with a 12" organ pull cleveland speaker. already have the plywood just gotta work up the nerve to work outside in this sub-60 degree weather (the perils of being used to 80+, i know our members in the PNW must be cackling)
Post by thetragichero on Dec 1, 2020 16:20:35 GMT -5
need to remove the first stage cathode bypass cap because it's a bit too gainy and bassy. need to add a "fizz" cap between the phase inverter plates to remove high frequency yuck (also may mess with negative feedback, lower resistor or bypass with a small cap). getting some motorboating which i think I've tracked down to the one reused capacitor (the big old yellow Hammond-pull). when i parallel it with another 100nf cap the motorboat speed doubles need to tweak to get more clean out of it, and need to pickup a 1A fuse (have a 2A in there right now for testing purposes but much too large for when it's all packaged up). but i get sound and it sounds like guitar, so the rest is just gravy, baby
just gotta work up the nerve to work outside in this sub-60 degree weather (the perils of being used to 80+, i know our members in the PNW must be cackling)
Pacific Northwest, whatev, I'm looking at about 5 inches of the white fluffy stuff outside and it's still coming down. The wife's got the 4WD pickup at work (it's hers anyway), so I'm going to have to make my Honda plow its way out of the driveway in the next hour or so.
I love the snow when there's enough to ski on, otherwise it's just a hassle . . .
Post by thetragichero on Dec 1, 2020 23:49:18 GMT -5
just relaxing playing guitar tonight but the yucky "whoosh" I'm thinking may be solved by adjusting the negative feedback circuit. i went with a 5k6 ("close enough" to the 6k8 i didn't have) tail resistor and 4k7 nfb resistor with 47k coming from the output transformer secondary (think i took this from a bassman schematic). why i didn't take a peek at the 6g9 tremolux which team el84 is beyond me. that used 6k8 tail/1k5 nfb/27k from ot. I'm thinking that whole thing is my problem (i remember similar "whoosh" from my excelsior before adding nfb and from the 5e3 build i did)
removing the cathode bypass cap on the first stage tamed the too bassy. just enough with the neck humbucker on the les paul to have some authority. we'll get there
Post by thetragichero on Dec 2, 2020 13:57:56 GMT -5
ecl86/6gw8 is an el84-like pentode (9w plate dissipation) and 12ax7-like triode on the same bottle. got rid of the "whoosh" and just have distortion on note decay, which I'm guessing it's likely either relying on the chassis for the output jack ground (worthy of a nun ruler slap for bad) form or similar crossover distortion (which should be fun to alter as the individual 270 ohm 5w cathode resistors are at the bottom of everything). there's an easy path to run a ground wire from the center of the chassis where the output secondary lies (in the event that i needed to swap leads to turn squealy positive feedback into negative feedback) to the pi filter cap ground point with such a cramped layout i was surprised i got any sound at all. have been enjoying my recent eyelet board builds for troubleshooting purposes
edit: quick measurement before getting on to other stuff (have to clear out a couple organ husks before picking up another on Friday) has me at 124% dissipation at idle so those is likely my issue
Post by thetragichero on Dec 4, 2020 1:37:06 GMT -5
hmmm so one tube was biased too hot and the other too cold, even after changing to individual cathode resistors so i ran a wire connecting the two cathodes and it's now biased to 80% plate dissipation, which i think is good for el84-esque output section with a tendency to get a bit brittle/crunchy. saving tone test for the daytime hours but I'm guessing my crossover distortion issue will be gone. measured just over 18w at full tilt into a resistive load after last little tweaks all that's left is to build the box!
Post by thetragichero on Dec 5, 2020 21:12:31 GMT -5
finally got the effing thing working. had wired the volume/tone control wrong (and copied it down in my notebook a different wrong way). turns out this is the "moonlight" tone control from an ax84 project and i dig it. can get bright or dark but at least for the les paul i tested it with they're all musical settings. and now the volume control has a lot more range somehow i didn't have a resistor between the phase inverter and preamp power supply nodes so they were effectively the same node. added a 1k resistor and rerouted wires a bit probably didn't need to replace both pi and preamp filter caps (i think it was just the preamp cap that was causing motorboating) but i did and went bigger than i probably needed, 47uf and 22uf respectively change the nfb resistor from 47k to 22k just like the tremolux oh and i removed the second gain stage cathode bypass cap because i felt like i was just hitting those power tubes too hard. sometimes these el84-type make me miss octal power tubes
kept getting harshness on the organ pull yamaha 12" speaker i keep in the shop so i tried on the kustom 2x15 and it sounds fantastic! nice clean and damn near crunch sound cranked. responds to guitar volume control and picking dynamics which was my hope
Post by thetragichero on Dec 7, 2020 19:40:12 GMT -5
started on the cabinet. basically the same dimensions as a brownface princeton although angled back slightly and I'll be sticking a 12" speaker in there. should be just enough room (and thankfully the chassis is small and the tubes will be horizontal
Post by thetragichero on Dec 14, 2020 23:25:24 GMT -5
starting to look like a real amp
have some tidying up to do with the extra tolex on the inside. will put grill cloth on the baffle and mount it. then it's just mounting the speaker, the chassis, corners, and handle. by far doing the tolex is the most stressful part of any build for me
Post by thetragichero on Dec 15, 2020 21:24:09 GMT -5
still have to rotate the speaker, make the speaker cable, and make a name badge for the front but I'm mostly satisfied. both my drill press and pneumatic staple gun went out on me so there was a lot of nervous hand drilling and grill cloth wasn't as quick as usual
Hand-drillled or not, still a beautiful job, Trage!. The old classic design, just a V and T control, good stuff. And a salvaged organ speaker as well, I guess "everything old is new again" should be your motto.