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Post by thetragichero on Dec 16, 2020 1:31:00 GMT -5
facebook group I'm a part of does a couple pedal swaps per year. talking to the guy i'm paired with, he doesn't currently have an amp, records into an interface with one of those starved plate preamps. hopes to one day save up to get a real marshall plexi so instead of sending a pupu platter of pedals i don't use, i ran the idea of a plexi-type preamp with a line out. will have a single-ended el84 output to a switching jack. no load plugged in? bank of three 7.5 ohm 10w resistors in parallel providing close enough to the 4 ohm load (less than 4 is actually ideal since I'm using a 4 ohm: 9.5k output transformer. 2 ohm would be ideal but who makes two ohm cabinets?) schematic (crc filter before b+ replaced with clc filter and using 32/32 and 16/16 cap cans to simplify my power supply wiring, haven't updated schematic)  test fit on chassis to figure out where everything is gonna fit  repurposed hammond vibrato board with the components i have (waiting on the rest)  next up is fitting the iec socket, figuring out the rest of the back (will throw a small pot to give some output control for the line out), figure out tube sockets, and paint and label the chassis. should all be done by Monday when the rest of my stuff gets here |
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Post by b4nj0 on Dec 16, 2020 4:21:24 GMT -5
Nice. Unless it reduces the power handling too much, how about 2x7.5 ohm in series all in parallel with the third 7.5 ohm? That would give around 5 ohms which is a bit more to kick into than 2.5 ohms for all three in parallel?* (Going up is OK but going too low is a no-no?)
e&oe ...
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Post by thetragichero on Dec 16, 2020 13:40:12 GMT -5
we don't really care what the load is nearly as much as we care what the reflected impedance the power tube(s) see from the output transformer
el84 datasheet has a few listings for 250v played/250v screens operations: 4k, 5k2, 7k impedance. ot i have is 9k4:4 ohm, so solving backwards a 2.5 ohm load translates to a 5k875 primary load on the power tube. close enough to 5k2 for me
another reason why three identical resistors in parallel? they'll share the load equally so each 10w resistor would dissipate less than 2w at full blast. maybe they'll get warm but shouldn't be able to cook an egg on em
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Post by thetragichero on Dec 18, 2020 4:24:42 GMT -5
another late night in the books this afternoon i painted the chassis and added labels (man acrylic paint is nice; super quick dry time)  still waiting on a few components along with the retainer for the el84 but i wired up almost everything i could. silly me made an error on the board, having the mixer resistors right off the coupling caps of the first two gain stages (kinda hard to have working volume controls that way) but there was already a hole in the chassis by the volume controls so i have these on a vertical terminal strip, which also have me a great tie point for the preamp grounds (all preamp grounds including from the filter cap connect to the chassis in one place: the 'both' jack sleeve. power grounds all connect to the chassis at the ht winding center tap, right next to the power filter cap. should be nice and quiet)  |
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Post by thetragichero on Dec 18, 2020 17:26:56 GMT -5
banged out the output wiring. would've done before bed but I've learned to stop working when i feel tired. usually end up having to redo   assuming 4w output, the line out is adjustable from 590mV to 1.4mV which is about perfect. could even be used in front of a clean amp like a plexi distortion still unsure if i will have negative feedback. saving that decision until this is all together |
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Post by thetragichero on Dec 19, 2020 19:20:03 GMT -5
banged together the headshell today using offcuts (which is why one side is pine). el84 retainers didn't show up today and tracking for the rest of the components i need hasn't updated since the 15th. 'tis the season  |
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Post by thetragichero on Dec 20, 2020 20:11:59 GMT -5
box is together and routed, painted with cheap (96 cent) flat black spray paint. still have to fashion a piece for the top but then it's ready for tolex (this one is getting purple. should pop with the stainless chassis paint)  |
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Post by thetragichero on Dec 22, 2020 4:30:12 GMT -5
shipment of components came today as expected, so i was able to wire up the power supply, place the last few components in the board, and install it  el84 retainers are still *somewhere* between maryland and here so until they appear i won't be installing the el84 socket and wiring up the heaters cut tolex for the headshell. that'll go on in the afternoon |
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Post by b4nj0 on Dec 22, 2020 8:31:58 GMT -5
Coming together real fine Trag. I hope it's appreciated?
What method do you use / intend to use to bond on the Tolex? I have an extremely long overdue project to finish that needs new Tolex. I have a large aerosol of "Super77" which I think is like a contact adhesive, but it's donkey's use old now ...
e&oe ...
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Post by thetragichero on Dec 22, 2020 13:18:45 GMT -5
I'm better at taking compliments online than in person. I'm digging building with boards as opposed to point-to-point. looks way cleaner
first amp i (re)tolexed i thought that kind of stuff would work. bubbled up over time and will need to be fixed
regular contact cement (ubiquitous brown glass bottle with brush inside) will work, but there's zero work time: stick it and it's stuck
for months now I've been using dap waterbased contact cement (green can, says nonflammable). i guess it's neoprene-based. a lot more work time and cleans up with water. just super not fun when you get a bunch in your leg hair
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Post by thetragichero on Dec 27, 2020 3:35:25 GMT -5
and she's done (tone testing notwithstanding)!  150 ohm power tube cathode resistor puts me right at 72% plate dissipation at idle which is rather cold but I'm hoping tames some of the el84 harshness. ended up just using two 7.5 ohm 10w resistors for when no speaker is attached. they should get warm but not hot (after sound testing i plan to put it on the scope anyway. if they don't get hot with a 1v square wave input they should be peachy with a guitar) |
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Post by thetragichero on Dec 27, 2020 15:32:01 GMT -5
72% is much too low. 64 ohm cathode resistor puts me almost at 97% and the amp came alive. plugged into my bass power amp and 1x15 and lots of low end there
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Post by thetragichero on Dec 29, 2020 19:46:48 GMT -5
added 390pf plate bypass capacitor to the normal channel, 1.8nf plate bypass and 1uf cathode bypass capacitors to the gain stage after the channel mixer resistors and before the cathode follower/tone stack. slightly increased attenuation on the line out (was BLOWING UP the input on my interface). I've updated the schematic in my notebook, may photo and upload later. fun little amp through a 4x12. direct input would sound best with some eq/speaker impulse responses
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Post by reTrEaD on Dec 30, 2020 9:58:18 GMT -5
72% is much too low. 64 ohm cathode resistor puts me almost at 97% and the amp came alive. plugged into my bass power amp and 1x15 and lots of low end there That seems right. Single-ended class A outputs run a consistent power dissipation in the tube throughout most of the curve (although power drops to zero at cutoff where the voltage is highest and the current is zero). So best results occur when you're near 100% power dissipation at idle. |
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Post by thetragichero on Jan 1, 2021 14:29:07 GMT -5
never posted the finished pictures! finished board  aaaaaand glamour shots    |
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Post by frets on Jan 1, 2021 15:16:05 GMT -5
Trag,
That is stunning. In my favorite color too. You are so talented.
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Post by thetragichero on Jan 1, 2021 21:22:20 GMT -5
as-built schematic  |
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