50w Plexi Clone build complete

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Beer, Feb 12, 2006.

  1. Beer

    Beer Member

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    Jan 21, 2002
    Location:
    Westbrook, CT
    I finished up my initial soldering on my Weber plexi clone (6M45P). I'm going to have a friend who messes with amps more than I come over with his bias meter and help me start her up. First I've been pouring over the thing trying to find mistakes and already found one this morning. I've marked up the pictures with the schematic's numbering scheme to make it easier to review. If you have the time or inclination, please pick my amp apart!
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    Here's the work around I came up with for the filter cap board.
    [​IMG]
    Here are the links to the weber schematic and layout.
    https://weberspeakerscom.secure.powweb.com/store/6m45p_schem.jpg
    https://weberspeakerscom.secure.powweb.com/store/6m45p_layout.jpg
     
  2. wilder

    wilder Member

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    Location:
    Denton, TX
    The board looks good as near as I can tell. It's really impossible to decipher the pre-amp and control wiring from a pic as it's all the same color. I'm not familiar with this kit, but the layout looked nice and comprehensive. If you want someone who knows what they're looking at to look it over, you might try posting these pics on the weber kit builders board (you may already have) as there is bound to be someone there that has built this kit. Good luck!

    Chris
     
  3. Beer

    Beer Member

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    Thanks for replying. I posted this on the Weber kit forum, but I also thought I might get some other people who don't look at that forum to take a look. Yeah, I know what you mean about using the same color wire, but the kit came with three colors only and suggested that that yellow be used for "signal", red for "power" and green for filament, so I stuck to that as much as I understood those parts to be. I veered a little bit for my own sanity, but also thought of making "every other" wire a different color. It's hard to decide when you only have three colors. Next time I might try to source a few other colors of push back cloth wire to make it a bit easier to read. My next project, if this one goes well, will be a tweed Fender or blackface clone. This has been a really fun experience, and I'm jazzed to get it working.
     
  4. tonezoneonline

    tonezoneonline Member

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    I know it looks neater that way but connecting all of your wires from the tone stack and tube sockets under the board sure makes it hard for anyone to see what you've done.It also make it hard to troubleshoot if there is a problem or to mod the circuit down the road if you want to tweak the amp.
    Just a thought.If it fires up fine the first time and your perfectly happy with the tone it's not an issue.
     
  5. Normster

    Normster Member

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    Sacramento, CA
    Looks good! I don't know if I'd call it a mistake, but I would have pushed the filament wiring deep into the back corner of the chassis. Less chance of AC hum that way. As long as you have it pushed down against the chassis you should be fine.
     
  6. samwheat

    samwheat Member

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    Scranton, PA
    Don't want to cut you down however

    The heater wiring could be neater

    Personally, I build my boards on the hoffman amps layout and use turrets

    I really like the sozos esp compared to orange drops

    I have a Marstran OT in mine

    Mine is a protube 50 gut and clone ...... with a couple mods

    as like Keith said ..... how does it sound
     
  7. Francis

    Francis Member

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    Location:
    Adelaide, Australia
    OK, my first post in this forum, so please be gentle :)

    So, from a long history of obsessive construction a few comments.

    I really hope that the power earth run goes to a bolt into the chassis somewhere under that power board. There is a general principle about power ground. You give it a total priority in mechanical integrity. That little bit of green wire and its connection to the chassis is all that stands between your wife and widowhood if there is a problem.

    On the subject of the mains wiring. It ain't brilliant. Sure, most of the voltages in there can give you a significant belt, and are quite dangerous, but the power wiring will kill you stone dead every time. I know it doesn't have quite the vintage cool factor, but heat-shrink tubing is your friend. (actually I spy a bit on the end of C4). I always use massive amounts of heat-shrink to ensure that no part of the mains wiring is exposed. You can also buy a plastic shroud that covers an IEC mains socket inside - further protecting you. I tend to prefer to use mains power fittings that use push in connectors- since you can get these with plastic covers - making for an automatically much safer construction. All you need is, say, for the solder joint holding the power input lead to the power switch to fail, to be about to test out the integrity of that little green wire. And if some bozo has used a cheater plug on your power feed, things are even more grim.

    The power board is a worry. I assume by "workaround," you have had to deal with oversize components. It looks a pain. I would have complained about the components. But anyway, there are a few general principles about such construction. One thing we try hard not to do is have components only supported by their leads - never regard solder as a construction adhesive. The larger capacitors should have an independent support. Typically this means either glue (hot melt is good, but not in a tube amp - or a silastic rated for high temperatures,) or better, cable ties. This means drilling some additional holes in the fibre board, but tie the components down solidly, and remove the stress from their leads. Otherwise you risk a joint failure.
     
  8. Beer

    Beer Member

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    Location:
    Westbrook, CT
    I have been inattentive to my post, my appologies. Here is an update and some replies.

    I brought the amp to a tech friend's electronic repair shop where he checked over my work and the schematic and layout to look for problems that jumped out at him. After he gave it the thumbs up, he hooked it up to a variac (actually his was more complicated than the standard ohmite looking one you see, with several other bells and whistles including an ammeter to check for spikes) and a fancy schmancy dummy load, signal generator, and oscilliscope (most of which were spelled correctly there, even if they aren't here). He powered it up without tubes first and then with after it passed the initial test. Had some crazy oscillations and way too much gain. He monkeyed with it a bit but we both had to get home (this was after work and he was doing it pro bono) so I collected the amp, bought some tubes from him (JJ power tubes and EH preamp tubes) as sort of a "thanks".

    I basically just wanted to make sure it didn't blow up upon power up. I took it home and hooked it up to my Hotplate (set to 100% load) and a 4x12. I cracked the volume and it immediately made the oscillation noise, so I reversed the output transformer leads to the plates (as described on the schematic) and that fixed the oscillation. I still had a gain problem where anything over "1" was crazy loud. I was also blowing the .5A fuse with anything over "4" on either channel volume. So I poured over the thing trying to find bad connections and components out of spec. The only thing that I eventually found was that the 1M resistors on the inputs were hooked up in a way (although correct to the layout) that was not consistant with the schematic. Also, I was using .5A fast acting fuses and changing to .5A slo blo (as many insisted were what was intended) has solved the fuse eating situation.

    I also had to swap out several resistors in the bias circuit before finding a combination that gave me the "right" bias and some range to play with. I used Radio Shack resistors and plan on getting some better quality ones at the next trip to the electronics store.

    I found the bright channel is wicked crazy bright and experimented a bit with some changes. I removed the bright cap off the pot and changed the 0.0022uF coupling cab to 0.022uF just to see what effect. It has improved the brightness dramatically.

    One aspect I was not prepared for was the shear volume of this amplifier when not attenuated. I'd owned, briefly, a '73 100watt super lead and it was just too insanely loud and clean and I didn't want to mess it up with mods AND found some other shiny piece of gear at the time that caught my fancy, so I sold it pretty quick for a bit of a profit and figured I'd just find a 50watter at some point. I'd owned several non-MV Fenders before and had, in my mind, an idea of what a 40-60 watt amp volume range should be like. I cannot believe how quickly this 50watt amp gets loud. Even after the 1M resistor connection change, "2" is about as loud as I could ever play in any band, and is quite clean. "4" is insanely loud, enough to thump one's chest and leave your ears ringing, still pretty clean sounding at that volume too. This is through a 4x12 in my "studio" over the garage where I regularly play my Bogner XTC at fairly loud volumes, but my lack of experience with non-MV amps really shows here I guess. I've gotten some great crunch rhythm tones out of the amp thus far using the THD attenuator at about -8dB. Anything more seems to be too much of a tone loss to my ears. I am not satisfied with the tone when playing in the upper register. It is not full enough, too piercing still, and, well, it ain't a Bogner XTC Classic which is what I'm used to. I will continue to mess with the amp, try out different values AND brands of caps and resistors in key locations. I've learned enough to be dangerous at this point, but also enough to understand most of what people are discussing when referring to many ampilifer circuit terminology.

    Oh, and on the suggestion of peeps up above, I changed the filament wiring a bit to shift it over to the back of the amp. I changed some of the grounding (removed the speaker ground wire) but it is quite quiet anyway. I also added screen grid resistors and 1 ohm cathode resistors (for checking current, I checked these against a friend's bias probe and got identical results). I will change the power caps around at some point so they look neater. I'd like to clean up some of the wiring, as some have mentioned. I think that, from what I've seen of chassis pictures, the 100watter has a lot more room to put things in neatly. This sucker is a bit crowded. I have to take some things apart just work on others.

    My plan for this amp was to just mess around with it, changing values and trying out mod's, instead of messing up a vintage or more expensive boutique amp. Also to learn more about amplifiers. So far I have been very satisfied and had a lot of fun. I've taken pictures and plan on making sound clips and maybe putting together a webpage with what I've learned and the results of different trials. I have a dual 500K pot on order and will try out the "Rich Mod" that I've read about (a post phase inverter master volume mod). I am more of a master volume kind of player. I don't like things too loud.
     

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