Q418C Thermionic's thermionic effect amplifier

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  • thermionicthermionic Frets: 9499
    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: 9499
    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: 9499
    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: 9499
    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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  • Modulus_AmpsModulus_Amps Frets: 2569
    tFB Trader
    Well done, good solution with the front panel text too!
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  • thermionicthermionic Frets: 9499
    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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