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Q418C Thermionic's thermionic effect amplifier

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thermionicthermionic Frets: 5548
edited October 2018 in Making & Modding
Barry from Ampmaker's imminent sabbatical prompted me to buy a P1800 kit sooner rather than later.

https://i.imgur.com/m6rdxlM.jpg

I was hoping I would have sold my house by now and moved to a nicer place, where I didn't have to build amps on the kitchen table, but there we go.

https://i.imgur.com/n3SaaOn.jpg
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  • paulnb57paulnb57 Frets: 1340
    Nice!
    Stranger from another planet welcome to our hole - Just strap on your guitar and we'll play some rock 'n' roll

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  • thermionicthermionic Frets: 5548
    There's a lot of tiny pieces of hardware in this kit:

    https://i.imgur.com/e4szEU1.jpg

    So it pays to group the items together by size:

    https://i.imgur.com/Kj78f9J.jpg

    Oh noes! I was missing one essential nut. I checked my bike spare parts drawers and found what I needed. This is why you should never throw anything away!

    https://i.imgur.com/neo5gkE.jpg

    Most of the parts fitted to the chassis:

    https://i.imgur.com/cFKw9g8.jpg

    The main part of the power supply done:

    https://i.imgur.com/3gTkpRL.jpg
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  • TTonyTTony Frets: 15409
    So, as someone who's *never* built an amp, but always paid heed to the "you'll die if you get it wrong" warnings ... what's the trick to building something like this?
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  • thermionicthermionic Frets: 5548
    edited October 2018
    Start with something simple, like a 5W single-ended thing. It’s not really much easier than a bigger amp, just less work and less potential for making mistakes.

    Make sure you understand power supply capacitors, bleed resistors etc. Know how to use a multimeter.

    Learn a decent soldering technique. For some reason, that’s been a problem with this build, despite investing in a nice station as an upgrade to my trusty 25W Antex. Maybe I should get some lead solder again.

    No use now as Barry is taking a sabbatical, but Ampmaker’s build guides are top-notch, full of practical advice that helps you avoid getting stuck in fiddly situations.
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  • TTonyTTony Frets: 15409

    Make sure you understand power supply capacitors, bleed resistors etc. Know how to use a multimeter.

    Ahhhh.

    That answers the question of why I've never built one.

    I keep thinking that I should learn something about electrickery and its various components, but then remember that I've thought that many times over my life, and not got around to it yet!
    ;)
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  • thermionicthermionic Frets: 5548
    edited October 2018
    You just need to know that capacitors hold charge, and large ones can supply 100s of Volts. Make sure sure they are discharged before going near the inside of the amp, either through bleed resistors in the circuit, or know how to safely discharge them yourself. I’ll have a little video about this in a future update...
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  • JayGeeJayGee Frets: 553


    No use now as Barry is taking a sabbatical, but Ampmaker’s build guides are top-notch, full of practical advice that helps you avoid getting stuck in fiddly situations.
    The build guides etc are (for the moment at least) still up, and very well worth a look even if you can’t order the kits anymore because there’s a wealth of practical, helpful information in them and they give a very good clear idea of what’s involved in an amp build.

    I’m a two left hands kind of guy when it comes to practice and know just enough to be really wrong about stuff on the theoretical side when it comes to electronics yet still managed to get an SE5A up, running, and sounding great at the very first attempt, and I think that says a lot about both the quality of those guides and the practicality of an average bloke tackling at least a basic amp build.
    Don't ask me, I just play the damned thing...
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  • thermionicthermionic Frets: 5548
    Here's a little demonstration of how the ~ 400V DC drops down to safe levels in about a minute, as the large power supply capacitors discharge through the bleed resistors that are connected across them. 


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  • thermionicthermionic Frets: 5548
    edited October 2018
    And here's how to make a neatly-twisted heater wire pair:



    Remember to take a step forward as the wires tighten up and shorten!
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  • thermionicthermionic Frets: 5548
    edited October 2018
    Heater wires done, and power supply tested:

    https://i.imgur.com/VO7S7zz.jpg

    Got a selection of valves from @karltone to try. Went for the Tung-Sol EL84s:

    https://i.imgur.com/5HREHZh.jpg

    One good thing about Maplin closing is that I had to look online to buy solder.  Hello, my old friend!

    https://i.imgur.com/jK7mXLt.jpg
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  • thermionicthermionic Frets: 5548
    Finished the turret board and the valve socket wiring:

    https://i.imgur.com/4au97DZ.jpg

    The 12-way rotary switch was... challenging.

    https://i.imgur.com/AAmmBok.jpg

    I was somewhat surprised to find out it's a 12-way switch? A quick google reveals that a little washer with a tab restricts the movement to any number of positions between 2 and 12. Surprising that this isn't mentioned in the otherwise excellent build guide.

    But I run into a more serious problem:

    https://i.imgur.com/DdoLSPv.jpg

    The locating tab on the switch doesn't line up with the hole. I think the build guide mentions bending the locating tabs back on the pots, and the tabs on the toggle switches line up ok, so this is surprising. I can either file off the tab, or undo everything off the front panel and enlarge the hole with a dremel...
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  • FatPeteFatPete Frets: 573
    The little tab will just snap off with pliers. It shouldn't be necessary as long as you tighten the nut.
    Trading feedback: Trading feedback
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  • KeefyKeefy Frets: 458
    And here's how to make a neatly-twisted heater wire pair:



    Remember to take a step forward as the wires tighten up and shorten!
    I tried this once, didn't work for me, the wire just went all coily-uppy :(
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  • GagarynGagaryn Frets: 1546
    edited October 2018
    Keefy said:

    I tried this once, didn't work for me, the wire just went all coily-uppy
    You need to maintain tension while twisting and know when to stop.
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  • thermionicthermionic Frets: 5548
    FatPete said:
    The little tab will just snap off with pliers. It shouldn't be necessary as long as you tighten the nut.
    Yes, I know I can remove it, but I’m thinking that if there’s a hole there and no mention of it in the build guide, then maybe it’s supposed to be located by the tab/hole. It’s a rainy day here, so I’m going to take a good look at it soon.
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  • BillKatBillKat Frets: 1410
    I seem to recall chopping the spigot off mine, there was a hole nearby but oversize and not close enough, as if for something else or maybe a different make of switch.
    I had to ring Barry cos I took the washer off that sets where the four clicks go without realising its position mattered.. rookie error


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  • thermionicthermionic Frets: 5548
    edited October 2018
    Well, I took a chance and snipped it off. It all mounted ok with the knob marker lining up well with the legend of the faceplate.

    However, when I was wiring up the input sockets I had 1M and 100k resistors left. I thought, that makes sense, because one is a higher gain input. But no, they should both be 1M! I'd wired in a 1M on the switch by mistake. I thought I'd have to stop, get a replacement switch and resistors and start again. Luckily I had a spare 1M, so snipped the wrong one off the switch and carefully wired the 100k onto the tags. It's very fiddly at the best of times, but this made it trickier. I somehow succeeded to do a reasonably tidy job, so carried on.

    It's a great feeling when you realise you're about to solder your last connection. Having a little break now before I do the final tests.
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  • thermionicthermionic Frets: 5548
    edited October 2018
    Aargh... problems. All the voltages tested out ok but I had a feeling something was wrong with the input jack wiring. True enough, I got a hum through the speaker but no signal when I plugged in. I snapped the leg of a resistor as I was desoldering, so time to stop, buy some replacements on ebay and start the input jacks from scratch. I think somebody posted here a few months ago with a similar problem? The way the diagrams are laid out is not 100% clear and a photo of the wiring (as there is for the speaker jacks) would be a big help.

    Still, you’d expect some minor problems assembling anything from a kit with 200+ parts!
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  • thermionicthermionic Frets: 5548
    Well, I got the input jacks wired correctly eventually. I was too impatient, I should have taken some time to work out what was going on, and why. Anyway, the jacks are wired up correctly now, but I'm not quite there yet.

    There seems to be a problem with the switch. Mute, mutes. SE (single ended) is also silent - this appears to ground the grid of one of the EL84s when testing continuity, but the other doesn't seem to be connected when the amp is powered up. The two push-pull positions seem quite low in volume, and there's a tremolo quality to the note decay. I think maybe the signal from the phase inverter is not getting to one of the EL84s, and this is happening in all three of the non-mute positions. Needs more investigation...
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  • thermionicthermionic Frets: 5548
    Final testing:

    https://i.imgur.com/j17wXo0.jpg

    All voltage measurements within the acceptable range, and a plate dissipation of 11.7W - spot on!

    https://i.imgur.com/kyYg5Lh.jpg

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  • thermionicthermionic Frets: 5548
    edited October 2018
    Meanwhile, I'd been mulling over my problem. I suspected the switch, because it's extremely fiddly and there's the potential for errors, shorts or even heat damage. But it grounded the grids of the EL84s ok, and it was clearly switching in the low-pass filter in the PP+ mode. I concluded the fault must be in the phase inverter - complete lack of single-ended output, low power and ugly distortion in the push-pull modes... clearly only one of the output valves was receiving a signal. I quickly found a wiring error in the phase inverter valve socket, and bingo!

    https://i.imgur.com/jHkoLFn.jpg

    Plugged in and it's as loud as it should be with a really natural and smooth valve distortion. Success!
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  • thermionicthermionic Frets: 5548
    OK, now I hit another problem. Not a showstopper, but a bit annoying. The cutout for the top panel is not quite wide enough for the panel, and it obscures the legends above the knobs.

    https://i.imgur.com/FA8Gqrq.jpg

    https://i.imgur.com/YQEgXjU.jpg

    The cab was built for the N5X I built a few years ago - part of the reason I wanted to build an 18-watter was because the cab turned out to be a fantastic 1974X style cab, possibly better quality than a real Marshall (it's nicer quality than my Marshall-built AC15 for sure). The legends on the N5X panel are below the knobs, so that was never a problem.

    I could a) live with it, or b) get a custom faceplate made. But looking at the cab, the back panel is not flush with the side panels but is recessed by a few mm:

    https://i.imgur.com/gsZsQ8a.jpg

    So, I could shim it with battens, it might be enough to pull the legends from under the top panel...
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  • thermionicthermionic Frets: 5548
    I don't have the tools, space, or skills to do any serious woodworking, but I found that B&Q sold strips of wood that were near enough to what I required:

    https://i.imgur.com/g3GqubD.jpg

    Ah, much better!

    https://i.imgur.com/dnTSUQj.jpg

    The back panel is about a mm proud of the sides, but I can live with that.

    https://i.imgur.com/t3F8U4O.jpg

    And here it is, finally working and fully assembled. The cab was built for me by Mark at AF Custom Cabs when I built an Ampmaker N5X a few years ago. I put a Celestion Greenback in it, and ever since I thought it deserved a proper Marshall 18W circuit in it.

    https://i.imgur.com/qexjjzI.jpg

    These things don't do clean at all do they? Even on 2, it's crunchy - but a really nice, smooth overdrive sound that's better than any pedal I've tried. It was a bit microphonic, so I've swapped the JJ ECC803S that I had in V1 for an Electro Harmonix "normal" ECC83. 

    This is the second Ampmaker kit I've built, and somewhat surprisingly, I ran into a few more pitfalls the second time around:

    - Wiring mistake on the phase inverter socket
    - Wiring mistake on the input jacks
    - After correcting the input jack wiring, it buzzed loudly. I knew what it was straight away - I'd forgotten to solder the ground wire back to the chassis tag.
    - Intermittent no signal/loud buzzing, which I traced to a short between two pins on the PI socket. They're extremely close together and loose in the sockets so they move around and are not rigid. If one of the solder connections sticks out a tiny bit, there's a danger it could move and short to the adjacent pin. Next time I will sleeve all the socket connections with heatshrink.
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  • Well done, good solution with the front panel text too!
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  • thermionicthermionic Frets: 5548
    If my £1.43 B&Q solution didn’t work out I was all set to get you to make me a new faceplate with the legends below the knobs @Modulus_Amps :)
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