Could someone do a sanity check for me please. Kit amp.

Discussion in 'Shock Brother's DIY Amps' started by VintageSG, Jul 12, 2019.

  1. VintageSG

    VintageSG Friend of Leo's

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    I recently purchased a kit amp from the Far East. It's a low powered valve amp, for which I ( conveniently ) already have a couple of sets of replacements of higher quality. I 'reasoned' that I couldn't buy the transformers and chassis for less elsewhere, so went ahead. Woo!.

    It's a valve rectified stereo amp with a 6n2p driving a pair of 6p1p outputs to a heady, probably, 1 Watt. More than enough for near field use with my computer. The vendor sent across the layout and schematic, and a few things concern me.

    1) Coming off the rectifiers, the first capacitor is a 220uF. This seems somewhat high!. It strikes me as being too high a value by a factor of 10ish and a 22~33uF would give the rectifier an easier time.

    2) Parallel resistors? Why? Other than cost reasons, wouldn't a 120R10W do the same job? I'm also thinking a resistor ahead of the first smoothing cap might help the rectifiers. I may just use solid state for the rectification anyway, and power the heaters for show :) in which case, I'd need to drop the voltage further, and the large first cap wouldn't matter so much.

    3) In the rectifier section, I'm looking at it and I'm thinking there should probably be a pair of 1N4007 between the two 240V outputs and the HT centre tap.

    4) Other than, including postage, I couldn't even buy the three transformers, I went with this as it looks like it'd be fairly easy to rework it to run with 6p14p valves in the output and possibly a 6n4, or rework the socket and use a 12AX7 as the driver.

    So, have I lost the plot?, I have a nagging feeling I've missed something blindingly obvious here. I know my voltages will be a little on the high side as the transformer is a 230V input, and my wall voltage usually runs at 245V. All clue gratefully received. I have been struck with a case of the dum-dums.
     

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  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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  3. sds1

    sds1 Tele-Holic

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    Nice one @corliss1, I couldn't find that datasheet myself.
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, that took a bit more searching than I was used to. 84 is an old-school rectifier that you'd primarily see in radios, but for a kit from the "far east" it could certainly be a thing.

    But yes, it would seem that schematic isn't necessarily the best operating conditions for that rectifier. If you don't have the tube already you could use another 6.3V rectifier in place and get similar results, although the much more familiar 6x4 tube shows only 10uF on the first stage.
     
  5. VintageSG

    VintageSG Friend of Leo's

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    The 6z4 used in the kit ( and which I have rather a lot, for reasons... ) is a cheap little thing often used in the Chinese Champ-a-likes. It uses a heptal socket. I can't find an English datasheet for the thing anywhere. It often gets confused for the 84/6z4. That first capacitor does look to be silly large though. I do have some 22uF and 33uF caps somewhere, and I'm pretty sure they're 450V.

    I'm starting to think a redrill for a noval and acquire an EZ81 would be a better way forward for long term reliability.

    Attached is a picture showing the 6n2p, the Chinese 6z4 ( a name which causes much confusion ) and a 6p1p. The 6z4 is in the middle, despite my dumb filenaming.
     

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  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    As long as the PT you get will handle the other rectifier, that's what I'd do :D
     
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  7. shortcircuit

    shortcircuit Tele-Meister

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    The Chinese 6Z4 is electrically the same as a 6X4 tube, but with different pinout. The trick is to slap 220 ohm 5 w resistors on the plates of the rectifier, and you can run those 220uF with no worries. You'll only lose about 4 volts from your B+. With that much capacitance, you will have no power supply hum.
     
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  8. cyclopean

    cyclopean Friend of Leo's

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    what did you roll?
     
  9. VintageSG

    VintageSG Friend of Leo's

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    Hmm. The 'kit' arrived today. I was looking forward to building it too, but there's an issue or two. Maybe a problem in translation or use of language. Whatever, it'll just take rather longer to sort out than I was anticipating.

    See if you can spot the issues in this kit amp. This box of bits...

    smeggit.jpg

    Hey ho. Dummus Von Dumb-Schmuck here bought a ready built amplifier. While the construction quality in terms of the layout, soldering etc is pretty good, and the components look to be OK, it sure as eggs aren't bacon isn't a kit of parts. This kind of stymies a few avenues I wanted to explore, such as making a few changes during construction to enable me to run 6p14p if I wanted to. There are a few minor, but possibly not, differences between the schematic, the suggested layout, and what I've found.

    rectifiers.jpg

    One change is the way the rectifiers are wired. On each rectifier, the plates are tied and fed from one leg of the power transformer each. So, each rectifier is handling half the waveform?.

    I'm not going to plug it in to test it though. Not until I've gone over it with a fine toothed comb/meter. This does not inspire confidence in the slightest.

    iec.jpg

    No PE connection!, also, look at the rectifier photograph again, only one leg of the mains in is switched. UGH!

    Jobs to do then. Replace the switch with one that switches live and neutral. Provide a PE. Follow the advice of @shortcircuit and fit a pair of resistors to the plate feed ( I checked my box of random bits and have had to order some. I did find some 10uF 450V electrolytics though, which was nice, and a small stack of 1n4007 diodes. Well, nearly 8" of bandolier of them. Where they came from, I just don't know ) I already have some suitable switches, so not an issue, but a minor inconvenience.

    I'm contemplating going to diode rectification, just leaving the heaters of the rectifier valves connected ( for show, squits, giggles etc ). There's a fuse in the IEC inlet. I was intending drilling the chassis for a fuse holder to provide an HT fuse. I'm not happy drilling when everything is installed. Maybe an inline is in order.

    Good points. Those transformers weigh as much as a small car!. They are -really- heavy. The output transformers appear identical in all but colour to the output transformer used in my E-Wave 5 Watt amp, and that sounds glorious. The chassis is thick steel, the valve bases are ceramic, the input sockets and speaker posts are of good quality, and there's no bloody LEDs illuminating the valves during operation!

    When ordering from sites like Aliexpress, DHGate etc, read the description very carefully. Place the item in your 'basket', go to sleep for a few hours, then sanity check your decision. The ordering error, I'll hold my hand up to.

    If you do decide you want a low powered valve amp, double/triple check it before powering it up!
     
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  10. sds1

    sds1 Tele-Holic

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    Perhaps the most thoughtful electronics assembly I've ever seen in a Chinese product. Pretty cool!

    What purpose are the zip ties around filter caps serving? I can't tell from the pic.
     
  11. VintageSG

    VintageSG Friend of Leo's

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    The zip ties are holding the caps in place. Stuck to the chassis are some self adhesive zip tie mounts, the little rectangular 'tower' type that are so dang useful for impromptu mounting, as in this case, or for providing zip tie points for cable runs.

    The whole assembly is quite well done, aside from the switch and lack of PE. Whoever assembled it did a really top notch job.

    IMG_20190715_143747.jpg

    I apologise for the odd angle, but it was the best way to show the bases.
     
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