Just started my first amp build!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Cobra, Jan 15, 2006.

  1. Cobra

    Cobra Supporting Member

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    I'm just getting started on my first amp build. I've got almost all of the parts & components, & I'm finalizing the configuration & layout. It's gonna have 2 separate preamps:

    1) the brite side of a 5E3 Tweed Deluxe, with 2 inputs, 1 12AX7, 1 volume & 1 tone.
    2) AC-15 preamp with 2 inputs, 1 EF86, 1 volume, & global cut control.

    I'm still deciding on what type of phase inverter circuit. It's either going to be a cathodyne, or a long tailed pair.

    Power amp will be like a 5E3x2, or a Victoria Double Deluxe, utilizing four 6V6 tubes, cathode biased, with either a GZ34, or a 5V4/GZ32 recto tube.

    Transformers will be a complete Matchless DC-30 three trannie set: Power, 30 watt output, & choke. 4, 8, & 16 ohm speaker outs. Big iron!

    The chassis is .090 all aluminum, 16"x8"x3" with mounting flanges, & very nicely welded on all four corners.

    The whole shootin' match will be housed in a 2x10 combo cab, controls on the front, like Dr.Z combos, with two 8ohm Scumback 10" speakers run in series, at 16ohms total, to take advantage of all the windings of the Matchless output transformer. Of course, I can also wire the speakers in parallel at 4ohms for a different configuration.

    I'm far from an expert on this, so any amp gurus that want to chime in on anything at all, feel free to do so. This project is, & will continue to be, a work in progress. I started buying the parts for it almost 2 years ago, & I've been doing copius amounts of research so I have some idea what I'm doing. The goal is to build a homebrew amp that has the features I like & want, will hopefully do clean to scream from the guitar volume, & provide me with a cool learning experience on the finer points of the inner workings of guitar amps. I'd appreciate anybody's thoughts, opinions, ideas, or suggestions.
     
  2. donnyjaguar

    donnyjaguar Member

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    I'm building my first (guitar) amp right now myself too. I'm wondering if you put the cart before horse though as I designed mine around the major components, which I already had, but didn't go looking for the balance until I'd decided on the finished circuit. I found this board to be a good resource for ideas!

    I opted for one instrument input and a selector switch for hi and lo input level. I configured the first voltage ampliifer with the typical Fender K-bypass circuit of 22/25uF or with a flick of the switch a u68 bypass cap a la some British amp's. No pentodes in the low-signal area because I wanted to stickwith the common 12A?7 style of tubes throughout. Pentodes are a great way to get lots of gain in a hurry though and I don't think they are any less musical than triodes. I'm using the typical differential-amp phase invertor becuase it has a little bit of gain that I can take advantage of if I lose too much signal elsewhere. Every hi-fi ampliifer I've built I've used a split-load style, but not sure how these would sound on a guitar amp so I just stayed away. By rights they should work better than the long-tailed pair, but this ain't a hi-fi amp. I stay away from 6V6 tubes myself. Its hard to find good NOS ones and to get the kind of power I desired would call for 4 of them instead of the 2x EL34 I'm using. Like you, i went also with the GZ34. I like the gentle turn-on characteristics. I did however wire it in such a manner that I can sub in a 5U4GB. I have a surplus Hammond audio transformer. This thing is old, but not as old as the power tranny & choke which came out of a 1957 laboratory power supply. I'm going with the 1x12" combo 'cuz I've already got a big amp and a little amp. I need something in the middle. The other unique features I'm incorporating into the amplifier front panel are a switchable pentode/triode output section, AB1/AB2 mode switch, a NFB defeat switch and another boost switch that should provide some level of overdrive before the master volume. I went with a star grounding system and DC filaments on the small-signal tubes. The whole thing is coming together fine, and the output section works like a champ, but there's a lot more work to do yet.

    The hardest part for me will be making a nice front-panel and putting together the wooden cabinet.

    DJ
     
  3. hasserl

    hasserl Member

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    Sounds like an ambitious project for a first build. I'd recommend you build something simple, like a Champ, or a AX84 P1 first time out, then build something a little more challenging, like 5E3 Deluxe, then keep going from there.

    There are a lot of details to work out, mixing it up with channel switching and then designing your own circuits just adds to the perplexity. I wish you well and great success, but I hope you are patient and willing to work thru the details to get it right without loosing your mind.
     
  4. Matt H

    Matt H Guest

    if you've done two years of research- it sounds like a very feasible first project...

    honestly, a cathode biased tweed deluxe output section is not very complicated... with no NFB, you do'nt even need to worry about which plate tap goes where... no more difficult than a single tube running single-ended- just gotta remember to parallel in the second tube- then do the same thing for the other half of the push/pull.


    the part i AM a little concerned about is that you're planning on an ef86 in a combo amp- they're known for microphonics... you might have to try a lot of ef86s to find one that doesn't squeal when you're plying the amp through it's combo cab. some people shock-mount the tube socket.
     
  5. Chiba

    Chiba Gold Supporting Member

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    I would shock-mount the EF86 whether it was a combo or not, just to be sure :)

    Quite an ambitious project indeed - but more power to ya. I've been gathering parts for my first build as well. Mine's going to be slightly different, based on the Normal side of an AC30, with a few changes I'm working out. I have the chassis & cab (2x12 combo, top-mounted w/controls in front), Vintage 30s on the way, and what sounds like the same transformer set you're planning on using.

    Have fun - take pictures - and keep us posted!!

    --chiba
     
  6. Matt H

    Matt H Guest

    oh- suggestion for first time builders...

    too many people just "Build the whole amp" only to fire it up and find out it doesn't work... then, because they're inexperienced, get totally confused on how to troubleshoot.

    solution?

    don't build the amp all at once!!!

    built it in stages and test as you go.

    first do the chassis- (namely switches, transformers, tube sockets).

    then build the power supply. At this point- you can fire the thing up and test voltages. they'll all be hot, and there will be very little voltage drop throughout the various high voltage nodes- but that makes sense- no tubes, no current!

    then build the power amp. fire it up, test the voltages. then inject a signal into the grid of one of hte stages- do you get sound? (a cd player's headphone connection turned up can provide enough signal to get some sound, it won't be loud, but it's a quick way to check. run it through a coupling cap to make sure it's DC isolated. you might consider making a cable up for this purpose with a cap soldered directly to it... then you can use it like an injection probe).

    then build your PI (if a push/pull w/ a PI)... now- test it! use that cd player injection probe you made...

    then build (one) of the preamps, test.

    then build the other preamp, test.

    doing it this way, while it "takes a little longer" reduces troubleshooting time... if somethign doesn't work? you've seriously limited what the possible problems are.
     
  7. Wakarusa

    Wakarusa Member

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    I'd argue that the most difficult part of the proposed design is getting the two channels to mix correctly without any oscillation or funny hi-frequency artifacts. The other common pitfall is actually ending up with too much gain resulting in anything from an amp where the volume pot might as well be a switch (labeled "silent" and "kill") to something overly compressed and muddy that tends to blocking distortion.
     
  8. Matt H

    Matt H Guest

    it's "difficult", but rigging up a 500k linear pot as a "blend" control for tweaking is a really quick and dirty way to balance the two channels, measure the resistance, and replace with appropriate resistor values.

    of course- i think wiring the heaters is actually harder than that... :)
     
  9. Wakarusa

    Wakarusa Member

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    A small pot is certainly one solution, but there are side effects when there's a large gain mismatch. Typically the bulk of the resistance will go to the higher gain channel and can result in high-freq rolloff. Typical solution is to bypass the resistance with a small cap to restore the highs, but it's never really perfect (you get a small notch in the overall freq response). I prefer to tweak the gains in the preceding stages to allow for equal (and smaller) mixing resistors when the channels meet up. Even better if you have an extra dual triode to use as a mixer (just tie the plates together).

    Of course, we're talking subtle differences here. How far you wanna go with it is constrained by how much you think it matters, how much money and design time you're willing to spend, and whether there's room in the chassis for the required bits and pieces.
     
  10. Barron Wesley

    Barron Wesley Member

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    good luck to you. Sounds interesting.
     
  11. Cobra

    Cobra Supporting Member

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    I wanted to clarify a few things: This build will not be a channel switcher, just 2 separate preamps with 2 inputs for each, so you'll plug into either the 5E3 preamp, or the AC15 preamp. I have a Z28 combo, & so far have not encountered any unusual microphony. I like the way the EF86 accepts pedals big time, & I also like the sound of the AC15, as well as 6V6's. Also, I have my amp tech available to me with some advice & help, as this project progresses. He's pretty much a vintage Vox AC15, & AC30 guru, & he's built several of his own amps. My plan is to accomplish as much as possible on my own, & consult with him, you fine fellows here on TGP, as well as others on the Weber & Ampage boards if I run into roadblocks. I'm in no rush, so I'm gonna take my time, & try to do it up right. I do like your suggestions for first time builders, Matt H, & I'll definitely put them into practice. I was fortunate to purchase alot of varieties of matched NOS 6V6's, including 3 sets of matched quads, before the prices got so high, which had alot to do with this project getting off the ground. Right now, I'm still undecided on the PI. I love the sound of the 5E3 with the cathodyne inverter, & this voicing will undoubtably change if I go with the long tailed pair, ala the AC15. I need to do a little more research on this, but I'm leaning towards the going with the cathodyne PI for starters, & see how everything gels, & then maybe try the long tailed pair PI later on if I feel the need to change it, or if the cathodyne doesen't work out in conjunction with the AC15 preamp. I'm not even gonna consider trying to utilize both both types with a switch, or anything like that! LOL!! Next up is finalizing the layout on the chassis, & start drilling, & cutting the holes for the trannies.

    Thanks for all the input so far, & I'd appreciate any further thoughts, opinions, ideas, or suggestions.
     
  12. donnyjaguar

    donnyjaguar Member

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    Well, I got my chassis all built and fired up the amplifier on the weekend. I'll have to find someone to host the pictures so I can show it off. This thing works like magic. I haven't measured the power of the amplifier but it drives a 2x10" cabinet to ear-splitting volumes. There's plenty of headroom left in the amplifier if I need to hot-rod it later but I really designed this as a strong practice amp. It has an interesting Fender-meets-Marshall sound with very tight bottom end - especially when I invoke Triode mode. I was very happy that there wasn't much fine-tuning required to the amplifier, probably because I was very careful with the design and routing of grounds. The reverb circuit works very well too. I'm glad I went with the master volume addition which someone at a party suggested last month.

    Now to design the cabinetry (yikes, you mean I have to use a saw??).

    DJ
     
  13. donnyjaguar

    donnyjaguar Member

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    Pictures of my amp project so far. Working chassis. Hopefully this LINK works.
    I've tidied up the wiring since I took these pix.

    DJ
     
  14. donnyjaguar

    donnyjaguar Member

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    Just an update to let you know I've updated the Donny Jaguar Guitar Amp Project site. I just wanted to keep anyone up-to-date. I noticed there were quite a lot of people viewing the files. Any feedback welcome. I've got a couple of minor bugs to work out with the amplifier - but they both could very well be a bum tube.

    DJ
     

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