Q418C Thermionic's thermionic effect amplifier

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thermionicthermionic Frets: 9498
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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Comments

  • paulnb57paulnb57 Frets: 3044
    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: 9498
    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: 27343
    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?
    Having trouble posting images here?  This might help.
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  • thermionicthermionic Frets: 9498
    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: 27343

    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!
    ;)
    Having trouble posting images here?  This might help.
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  • thermionicthermionic Frets: 9498
    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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  • TTonyTTony Frets: 27343
    :+1:
    Having trouble posting images here?  This might help.
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  • JayGeeJayGee Frets: 1254


    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: 9498
    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: 9498
    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: 9498
    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: 9498
    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: 683
    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: 2272
    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: 1553
    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: 9498
    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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  • CorvusCorvus Frets: 2924
    tFB Trader
    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: 9498
    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: 9498
    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: 9498
    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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