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Post by thetragichero on Jul 22, 2020 0:49:29 GMT -5
so this thing was a mess before my terrible attempts at mods on so many years ago. decided to put the chassis and headshell to good use by turning it into a Marshall jcm800 bass amp. using the power transformer and choke from the hp test oscillator i salvaged for parts along with a 125w mercury magnetics output transformer i got for dirt cheap. power transformer is 435-0-435 so my plan is to do a dual supply (something like 600v on the plates and 300v on the screens) running kt77s. reusing the power switches and the power board with the iec plug and fuses I've already applied waterslide decals and will show pictures of that later. for now here is the guts as i build, so far the screens are wired and some of the heaters (multiple heater taps on this transformer and I'm trying to spread out those hungry kt77s)  |
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Post by thetragichero on Jul 23, 2020 1:45:28 GMT -5
installed the bias circuit along with the bias sense resistors. if i plugged the figures in correctly should give me about -45 to -65v for bias  |
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Post by thetragichero on Jul 24, 2020 2:54:14 GMT -5
heaters done, started on the preamp and figuring it some power supply wiring. gonna need another terminal strip but much too late to drill  |
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Post by thetragichero on Jul 24, 2020 22:47:14 GMT -5
bright preamp basically done. started normal preamp. tone controls mostly done brain is getting fuzzy so time to tap out for the night  |
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Post by thetragichero on Jul 26, 2020 3:45:44 GMT -5
first two preamp tubes done, mixed into the first triode of v3 and out to the tone stack. started the "mid slope" control which appears to be a third order high pass filter with a 1M pot between it and ground. interesting  |
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Post by thetragichero on Jul 27, 2020 1:29:31 GMT -5
preamp is done up until the phase inverter plates. so we're in the home stretch!  |
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Post by thetragichero on Jul 28, 2020 2:18:19 GMT -5
all that's left is attaching the ot to the b+ rail and firing 'er up  |
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Post by thetragichero on Jul 31, 2020 17:34:17 GMT -5
haven't neglected this project just posting about it. it's basically there. it's a working amp although not nearly as loud as it should be. i suspect the power transformer is the limiting factor which i will get to shortly. I'll measure plate and screen voltage as i hit an E chord to verify
so why do i suspect the pt? sounded good with guitar (but still not 100w loud) so i plugged in the bass vi i recently got that i have believed to post about and slammed some low chord stuff. sagged/sputtered (shouldn't expect that with 235uf first node and a choke before the screens/preamp) and then i see a flash from inside and the amp loses power. after checking over everything it was the mains fuse, some dingbat (aka me ten years ago) put a fast blow fuse instead of slow blow. installed proper fuse and it works again
i did what i usually do with the baxandall James tone stack and wired the treble pots backwards so i fixed that. mid control wasn't working and i realized it was because in the interest of an intelligent grounding scheme i left that connection for last. once soldered to ground it works GREAT. I'll be stealing this for future builds
little dumb mistake using shorting jacks for the low inputs. will swap traditional ts jacks to resolve that
stay tuned
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Post by thetragichero on Aug 14, 2020 20:27:58 GMT -5
I've got a power transformer for this just have to put it in
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Post by thetragichero on Aug 16, 2020 15:13:18 GMT -5
finally set this up with a meter on the plates. plate voltage dropped almost 200v (from 540 at one point i saw 349) so when i find the time I'll put in the power transformer i ordered from tubedepot
also realized i was measuring bias and setting as if the bias sense resistors were only for one time instead of two, so it was set super duper cold
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Post by thetragichero on Aug 19, 2020 2:54:03 GMT -5
 got the new power transformer in, completely rewired the power (going from full wave rectified to a big ole bridge rectifier bolted to the chassis), bias (since this was intended for the jcm900 there was a nice 45vct winding meant for all those op amps it made for a nice bias supply with pretty much perfect adjustment range... -33v to -59v), and heaters (other pt had two heater windings with unknown current capabilities i split em up) 510v unloaded b+ will fire it up with tubes tomorrow! |
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Post by thetragichero on Jan 6, 2021 20:22:56 GMT -5
so this worked but I'm guessing it had a bum ot as one of the tube sockets arced and before then didn't sound as loud as it should. so i replaced the socket and charred parts, out in a 100w twin ot i had along with changing the bright cap and adding cathode bypass cap to the second stage of the bright input. guy is interested in buying in, plays a lot of hendrix stuff so i recorded a little demo with the channels jumped, lace sensor silver neck pickup, and a klon clone and about 5 minutes watching a youtube tutorial www.dropbox.com/s/dkt9lou6450jgk4/hendrix.mp3?dl=0the normal channel has a lot less gain than the bright channel, which is what you'd expect for a bass amp. i like how it sounds |
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