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Post by thetragichero on May 27, 2021 13:39:01 GMT -5
didn't see a thread for this, I've been picking it up and putting it down for probably a year trying to get it to work right... anyway onto the back story my first build was a 5e3 kit into a cannibalized raven solid state combo cab and chassis. being a first build it had issues so when i went to start modifying my pawnshop excelsior i started tearing parts from it (components, transformers). so i had the 5e3 turret board and a Thomas organ chassis that was super narrow along with power and output transformers from a fender blues deluxe. started building basically a 6g3 brownface deluxe, fixed bias 2x6v6 with a long tail pair phase inverter and the tone stack from the brownface pro amp (although i have volume before tone stack instead of after). had issues with motorboating cranked, changed some power supply caps (one particular style of 16uf 475v caps i got from antique electronic supply have always seemed to give me trouble) and eventually added a big coupling cap between the tone stack and the grid stopper for the second preamp stage. that did it! since the orientation of the chassis is like a marshall head, i decided on a "piggyback" style combo with a sealed 1x15 (dimensions are basically bassman 1x15, 24ish x 24ish x 12"). pine plywood construction, everything but the rear baffle braces and the braces for the front panel above the 'head' (shot some nails in to hold until the glue dries. neither of these need to be particularly strong) held together with oak dowels and titebond. need to make the front and rear baffles, finish sand, and then get tolex/grill cloth on it. will have a handle on each side and casters on the bottom
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Post by thetragichero on May 29, 2021 0:09:09 GMT -5
no picture but all pieces are cut, sanded, painted black, T-nuts for speakers in speaker baffle which has also had grill cloth attached (not my best work but it'll be okay). tolex has been cut into sections for each piece, just need the fine cuts and to attach it. thinking i'll remove the cathode bypass capacitor (2.2uf) from the first stage because there's a bit too much gain and treble (not really clean above 3 on the volume control and maxed it sounds pretty metal... plus a little too bright. brightness may change when i test it with the organ pull square magnet eminence that'll go in here so i'll wait to do anything until it's all wrapped up)
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Post by thetragichero on Jun 1, 2021 12:24:41 GMT -5
yesterday i got the cabinet and back baffle tolexed, just have to do the head part and cut out where that will be connected to the cabinet. tapped on the front baffle to make sure it'll fit. not my best tolex job but it'll do
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Post by thetragichero on Jun 1, 2021 16:52:32 GMT -5
test fit, still need to add all the hardware and speaker. not terrible
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Post by newey on Jun 1, 2021 20:37:14 GMT -5
I'd say a good deal better than "not terrible", looks to be coming along nicely.
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Post by thetragichero on Jun 1, 2021 22:46:06 GMT -5
I'm a perfectionist. i often find myself standing at a urinal looking up and getting angry at the horrible paint lines between the wall and ceiling. it's absolutely miserable lol speaker, corners, handles, wheels are in. going to hunt for washers because the speaker screws are slightly too long (i did not account for 3/4" ply being 19/32" thick. lumber measurements aggravate me)
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Post by unreg on Jun 2, 2021 16:52:53 GMT -5
Regardless, your project here looks exceptional to my untrained eyes! ๐ 3/4 is 0.75 19/32 is 0.59375 Maybe your lumber source is lacking in quality? Anyways, great job so far thetragichero! ๐๐
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Post by thetragichero on Jun 2, 2021 20:43:25 GMT -5
23/32". my bad would've gotten this completed today but one of the lock nuts to attach the head was apparently placed in the wrong bin at ace so i have to go back tomorrow. removed the 2.2uf cathode bypass cap on the first gain stage, might put a small one back in its place, might leave it. this has a lot of low end! cleans really can only be had on the low input gutshot of too tiny chassis. board is on top of the tube sockets. not ideal but was able to track down oscillations and/or add plate bypass caps to get it to behave
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Post by b4nj0 on Jun 3, 2021 3:36:02 GMT -5
All looks great but above all is interesting. All your builds hold attention from start to finish. IOW- thumbs-up.
3/4 is a nominal size as in rough sawn, once it is PAR (planed all 'round) it obviously loses a little, but since they want you to pay for the wasted timber you get charged for 3/4 in addition to the planed upcharge. Win-win for someone? I could be off target but I think I'm correct.
e&oe ...
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Post by thetragichero on Jun 3, 2021 21:48:29 GMT -5
well here we go! had to replace the second power supply filter cap as motorboating came back when cranked with bass cranked and treble turned down. replaced 22uf with 47uf (only three boxes on the power supply so issues don't surprise me). added 220nf cathode bypass cap to first stage and that works fine for me. there is some sort of mechanical buzz coming from the 'head' second that's annoying and I'm still trying to track down, but so far I'm happy with this. while there's not a lot of clean travel on the volume knob on the high input, what's is neat is that around 7-8 it's cranked fender and it changes to more of a marshall sound at 9-10 (more bite). completely unintentional but neat
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Post by reTrEaD on Jun 11, 2021 10:28:59 GMT -5
removed the 2.2uf cathode bypass cap on the first gain stage, might put a small one back in its place, might leave it. this has a lot of low end! Cathode bypass caps can be one of the more interesting parts of amp design. Because of the negative feedback that occurs through cathode degeneration, you leave a lot of gain on the table without them. But it can be somewhat difficult to go through the calculations to find the appropriate size. It's been a good long while since I've done this but if I recall correctly, you use the parallel combination of the cathode resistor and the internal cathode resistance and pick a capacitor with an equal amount of capacitive reactance to achieve a -3dB rolloff at the desired frequency. iirc, the reciprocal of the tube's transconductance is involved in determining internal cathode resistance. But it's been a while and I'm not certain I remember that correctly. You can move the rolloff point higher to deal with a system of amp and speaker that seems to 'bass-heavy'. Or move it much higher to just emphasize the treble. Also worth noting ... electrolytic caps tend to have a small amount of inductance in series with them because of the way they're constructed. In some cases, this can affect the bypass at treble frequencies. To counteract this, you might choose to parallel some smaller film/foil caps with your electrolytic bypass caps.
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Post by thetragichero on Jun 11, 2021 17:19:12 GMT -5
oh definitely. i'm of the opinion to learn what's going on, and then forget the equations and use handy online calculators like this one: www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/ (they also have nice plate bypass capacitor and long tail pair calculators) i'm okay with this not totally accentuating the treble as in my experience this allows overdrive pedals to sound better (at some point passing too many high harmonics sounds terrible) i'm pretty sure the speaker cable is what rattles/buzzes. i have some big rubber grommets i can try to secure them with but have been working the "honey do" list this week (considering it's last year's honey do list, i have zero room to gripe) excited to record a demo of this amp whenever i can find a few hours to lock myself in the studio
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Post by reTrEaD on Jun 11, 2021 20:36:46 GMT -5
Nice calculators. i'm pretty sure the speaker cable is what rattles/buzzes. Should be easy enough to locate. Oscillator and your thumb.
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