first build 5e3 - question on tubes and parts

Discussion in 'Shock Brother's DIY Amps' started by agent_zed, Aug 22, 2019.

  1. Nickfl

    Nickfl Friend of Leo's

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    While we're nit picking, attaching the 68K grid stoppers directly to the tube pins would be even better than shielded wire.

    I would also point out that there are a couple of things in that picture that are more about keeping the build pretty than they are about best practice, like the heater wires looped around the sockets when it is better to run them straight across and some of the other socket wiring looping around right next to each other when the should be kept apart and cross at right angles if needed. I doubt these things actually cause any problems in a 5e3, but they are not best practices.

    Also, elevating the heater center tap on a cathode biased amp is a no brainer mod IMO.
     
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  2. agent_zed

    agent_zed TDPRI Member

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    sorry for the dumb question but what are 68k grid stoppers and what do they do? Don't have the schematic to hand to know if these are in additional or just moved to the tube pins.

    Don't know if it's of interest to anybody but Ive been busy today building my chassis. Still got a few more holes to drill and/or enlarge but i need a step drill now.

    This is the flat shape cut out of what i assume is an old night storage heater front.
    [​IMG]

    I drilled as many holes as i could before bending as it is more accurate that way.
    Starting the bends....
    [​IMG]

    All done and spot welded in the corners.
    [​IMG]

    Will strip the paint back and give a flat black for neatness. I've got some 2or3 mm ally plate at work off an old piece of IT equipment which i am going to make a faceplate out of. It's already anodised black so should make a nice looking front.

    This is for a head unit which is why it's so wide as the valves will hang down from the bottom of the chassis. I used measurements off a production version plus a 5e3 standard layout so hopefully it's rigMy steel is about 0.7mm but the production versions are 1.5mm! can't believe they need to be anywhere near that thick. I mean it's nice but must be way over engineered.

    Got to build the box to house it shortly, while i wait for electronic parts.
     
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  3. agent_zed

    agent_zed TDPRI Member

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    Ok so making some progress now and hopefully will get this up and running in the next weeek but a few more questions...

    The plans i'm using show 2 resistors off the lamp which i believe is used if you don't have a center tap PT. My PT is a center tap so i assume i don't need these and instead run the CT cables to earth/ground. My PT has 2 CT's one on the 320v winding and 1 on the 3.15v winding. Do i just run both of these to earth/ground?

    This is the wiring for the PT https://www.inmadout.com/download/pdf/set5E3.pdf which if i understand correctly the CT wires are the gray and black wires.

    And again on the OT the whte cable is the CT so do i also ground/earth this cable?


    I have a lot of spare cables including the 230v input and the 4 and 16 ohm cables. Do i just cut them short and cover them? or is there a proper way of capping them off?

    This is where i am at the moment. The circuit board is fully soldered as is the input jacks and potentiometers.

    Hopefully there is nothing competely wrong sticking out but I guess its better now than missing something electrically dangerous.

    many thanks

    [​IMG]

    [​IMG]
     
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  4. Paul-T

    Paul-T Tele-Meister

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    cathode resistor.jpg


    Congratulations on the chassis - it looks great!

    I would follow Rob's advice. 6.3V Centre tap goes to the cathode resistor. This elevates the supply and reduces hum. You're correct, no resistors.

    Then the HT centre tap goes to the first filter cap.

    Q: is the cathode resistor on your board physically separated from the capacitor? It needs to be - it gets hot.

    On my 5E3, heater center tap is black and HT centre tap is grey, like yours I think.

    Spare wires, do whatever you think is neat. I actually added a SPDT switch on my OT to switch between 8ohm and 4ohm taps, which is useful, so I suggest keeping the other OT winding cables reasonably long.

    Rob Crop .jpg
     
    Last edited: Aug 23, 2020
  5. Paul-T

    Paul-T Tele-Meister

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    In MadOUt look interesting, good to have another transformer source? How much was the set? Did you buy direct, or from eBay?
     
  6. andrewRneumann

    andrewRneumann Tele-Holic

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    White (OT primary CT) would go to the + side of the first filter cap. That is where all the DC power for your power tubes is coming from... it goes through the OT on its way to the power tube plates.

    As @Paul-T notes above, the HT CT should connect to the - side of that same cap for lowest hum operation. This keeps the lion’s share of the return current out of the chassis.

    Good luck, hope this helps.
     
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  7. agent_zed

    agent_zed TDPRI Member

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    thanks both that is really useful info and thanks for the kind words about my chassis. I'm glad there is nothing crazy jumping out on my pic that you are noticing.

    "Q: is the cathode resistor on your board physically separated from the capacitor? It needs to be - it gets hot." ....

    Just checked and it's pretty close but there is a 1mm gap. Is this going to be enough? There is quite a lot of space in my chassis as it's wide as it's a head unit and the PT and OT don't come up into it, instead hanging below.

    I've been mostly sticking to the stewmac plans but my transformers are different hence the deviation so i'll pick up on the robs layout and wire the power like that with the CTs. I have already added in the safety diodes on the rectifier which is so far my only deviation from stock.

    I was going to ask about the 4/8/16 ohm outputs - so i can just add a switch and change it then? What advantage does this give me in terms of sound or power?

    yeah got the transformer set off ebay. I wondered about some of the other ones mentioned but they were either out of stock, expensive or i'd have to get the OT and PT from different places. i figured this was a set specifically for the 5e3 so it should go together ok. The values quoted also seemed to be within the desired range unlike the greek one off ebay i looked at previously. Think they were about £100 delivered. Hoping they are good as it's half the cost of the build! Will find out soon i guess :)

    I've only put 1 output at the moment A) is there a need for a second output? B) If i understand correctly i should put the tip switch so the output goes to ground if there is no speaker plugged in?

    thanks your help is much appreciated
     
  8. Meteorman

    Meteorman Tele-Holic

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    you'll want to match the OT output with your speaker impedance.
    The advantage of that is that you will in fact HAVE sound, as in, mismatches in the wrong direction (speaker impedance > OT impedance) will increase the chance of letting the smoke out of your amp.
    If you are dedicating a single speaker of known impedance (e.g. 8ohm) to this project - you can hook up that matching 8ohm OT wire, and just shrink wrap off the other leads and cable tie them safely out of the way.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I would try to get about 5mm.
     
  10. agent_zed

    agent_zed TDPRI Member

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    sorry I guess i asked a bit of a vague question there. I get that you need to match output and impedance but what i don't really understand is whether a 4ohm output and speaker gives any difference in sound/power to a 8ohm output and speaker? or is that essentially equal.

    Cool thanks, I've managed to resolder one of the legs and move it a bit to get more space although prob only just over 3mm but hopefully that will be enough. I've also raised the cap off the board to allow air movement.

    Next question...

    So i wanted a slightly more modern look so bought a 6.3v LED for the indicator light. Will this work? I've looked at the layout and note the heater cables go to either side of the lamp but a filament obv doesn't care which way the voltage is flowing.

    thanks all
     
  11. Paul-T

    Paul-T Tele-Meister

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    No, it's essentially the same result as long as the OT tap and speaker are matched.

    You might have seen this link already, but it's helpful for LED. I think you could reduce the value of the resistor used as you have a 6.3V LED .
     
  12. agent_zed

    agent_zed TDPRI Member

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    awesome thanks so much for the link, it's exactly what i needed. I missed that one on his site, although I'm not surprised he has a page on it though as he has pretty much everything else :)
     
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  13. andrewRneumann

    andrewRneumann Tele-Holic

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    Some higher voltage LED assemblies have the resistor built in. If you have the data sheet it should tell you.
     
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  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Well... A speaker impedance does not stay constant so an exact sound replica even if you could guarantee a same/same primary/secondary ratio would not be possible. Maybe my ear is trained for what I expect to hear but I generally (not always) prefer 8ohm speakers. Now maybe that is because of speaker design, IDK. OMMV.

    There is certainly more 8ohm speakers produced. More to choose from. Special orders can be obtained of course.
     
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  15. agent_zed

    agent_zed TDPRI Member

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    Ok so getting pretty close now to having everything soldered Last piece of the puzzle is the LED. I had a look last night and tested it with a dc powersource (one of those plug in multivoltage adaptors) at 6v. Light came on first try and I though i'd better check and switched wires around and it came on again. So it must have a an inbuilt diodes to direct the flow.

    Thinking back I think i bought it specifically as it said ac or dc and checking the ebay link https://www.ebay.co.uk/itm/LED-Pilo...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649 that appears to be correct.

    So I am assuming i can remove the additional diode from the led setup? but should i still include a resistor to prevent overvoltage?

    so close now.... (then the dangerous bit!)

    thanks
     
  16. andrewRneumann

    andrewRneumann Tele-Holic

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    IDK.

    I would guess that if you didn’t burn up the LED on your test then you are fine without the resistor.

    If I wanted to be certain, I would do that test again but have my DMM in series with the one of the leads of the LED and the power supply. If current was anywhere under 30mA I would be happy not using the resistor. But that’s me, and I could be wrong so it’s your choice!

    Or try it on the bench with the resistor and see if it works. It would be dimmer if it did, or may not work at all.

    Good luck! Hope this helps.
     
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  17. agent_zed

    agent_zed TDPRI Member

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    cool thanks. I'll hook it up to my dmm and see what it draws and go from there.
     
  18. agent_zed

    agent_zed TDPRI Member

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    ok so that measured ~18ma so i guess i'll give it a go without additional resistors etc.

    just finished soldering all of the connections now minus the filament ones to the valves/tubes and the OT ones.

    Just want to check though my OT has BLUE/WHITE/BROWN cables where as on the plans there is a RED cable instead of the white. The WHITE cable is CT on the specs https://www.inmadout.com/download/pdf/set5E3.pdf am I correct in thinking they have just used a different colour cable and this is one and the same. I mean i'm 99.99% sure it is having looked at specs for other OT's but I'd rather ask a dumb question than blow it all up.

    thanks
     
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  19. jtcnj

    jtcnj Tele-Holic

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    You are correct; your white should be to the "B+1", shown in red in the layout in post #84 above.
     
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  20. agent_zed

    agent_zed TDPRI Member

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    Ok everything soldered. Just going to make a snuffer stick with 2x 56k 2w resistors wired in series (hopefully that is correct - just copying what i've read online).

    Something else i need to check before turning it on is the stewmac plans i've been following listed a .047μF 600V Orange Drop (page 3 https://www.stewmac.com/freeinfo/i-10730/10731.doc.0718.pdf) and i've just found it in my parts box and panicked that i'd missed something, but i've looked over the plans and looked at rob's layout and I cannot find it anywhere. A search of the pdf only shows that one instance of that cap being mentioned. Is it safe to assume this is a mistake as I cannot see it anywhere on the plans and i've been careful to double check as i've gone along.

    Just going to read and re-read the safety instructions again...

    thanks
     
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