Filament buss question

Discussion in 'Shock Brother's DIY Amps' started by moosie, Nov 23, 2017.

  1. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Hmm, yeah, that makes sense. I didn't think of it as a feature. Just some weird thing I didn't need. OK, I won't chop it off. :) But I won't use it yet, either.
     
  2. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Before moving on to the power section, I did a painstaking review of every connection, every component and wire. Good thing, because I had the 6K6 cathode bypass e-cap in backwards. Otherwise, everything looked good.

    The board's done now. Also, the OT, reverb jacks, and footswitch. I made up a few shielded cables, connected the floating ends to V1 and V2 grids, and left the other ends free, to be connected to pot and jack that can't be installed yet. I want these leads to run under the board for a neater look. The heaters follow a similar route from V5 to the pilot.

    I drilled a 5/16" hole in the fiberboard, near the power section, just to feed the PT secondary leads, and heater center tap, through. Better this than fighting with them, as they come through the chassis smack under the center of the board.

    I drilled one last hole in the chassis, near the AC strain relief, to mount a double-keps-nutted terminal for the AC ground.

    I've procrastinated as long as possible, but now it's time to drop the board in. I have NO IDEA how I'm going to hook all this stuff up in such a tiny space. I'm breaking for dinner now, but when I get back, I may have to connect most of the flying leads on the tube side, before installing the board.

    This really had better work.


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  3. D'tar

    D'tar Friend of Leo's

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    Thats 20#s in a 5# bag right there!!!!
     
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  4. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I been busy. I took one last look underneath, and then hopefully hid that stuff forever.

    I pushed, pried, angled, and connived that board, trying to keep track of the wires. I've checked several times, but still have this fear that I'll realize one wire got buried underneath. I think I'd cut the chassis apart before removing the board at this point.

    The 5/8" nylon standoffs that came with the kit were crap. One was stripped on arrival, and they only sent enough as if for a 6G15, without all that extra crap on the end. I tried to find a few more, couldn't, and just bought a full set of what I could find, which were aluminum. But... mine are 3/4" tall, not 5/8". I figured it would give me a bit more room to move the power supply wires around underneath, as I tried to get them around the edges of the board. But that 1/16" was almost my undoing. the lower edges of the tube sockets are just a wee bit below the board.

    I feel like a watchmaker. A tube-watch maker.

    I want skinnier fingers, and about two more inches long. I want skinnier cutters and pliers. I want a third hand (see fingers spec).

    Right quick I abandoned my neat tube socket wiring diagram, which would be perfect for a BF/SF, for instance. Part of the problem is lack of space, and part that I've never worked on a tweed-style chassis before.

    My lead dress looks terrible, to me. I figured the main thing was to get the connections made - any way I could. Then I tried to shift a few wires here and there, running plates and grids at right angles. I guess if it's noisy but otherwise working, I may move a few.

    I like doing this stuff, but really, this part sucked. I get the chassis size is to make it look like a 6G15 - same cab, etc. Who cares?! Another couple inches would have been very helpful.

    Some of you might get a chuckle out of that dog turd .047 uf that I pulled out of some silverface. It tests within spec, and I needed one more for the returns isolation scheme. Huge sucker.

    Anyway, here's the damage. Comments and suggestions welcome. Next up, power supply, switch, ground, pilot, fuse.


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  5. clintj

    clintj Friend of Leo's

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    Mercy. For as much as is stuffed into that chassis, that looks pretty darn good to me. The real test is when you get tubes in and start running up pots to check functions.

    That looks like a stretch limousine version of a 5F1. :)
     
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  6. moosie

    moosie Doctor of Teleocity Silver Supporter

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    This PT comes with a heater center tap. According to Gehring, instead of grounding it, I can run it to pin 8 (cathode) of the 6K6 to minimize heater noise.

    This is the first I've heard of this. Good idea?
     
  7. clintj

    clintj Friend of Leo's

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    Does that have a bypass cap on it? If not, I wouldn't use it. That connection needs to see an AC short to ground to stay quiet.
     
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  8. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Thanks.

    Yes, the cathode has a 25uf e-cap in parallel with a 1k / 5W resistor.

    So, the cap allows AC to pass, providing the needed ground connection.

    But why does connecting it here, vs ground near the power supply, quiet the heater circuit?
     
  9. clintj

    clintj Friend of Leo's

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    I'll defer to Merlin on this one. The bleed he talks about is from some electron flow being attracted to the filament vs the anode (heater-cathode leakage current), and that bleed will follow the AC heater voltage causing hum to carry over. Screenshot_20171216-195656.jpg
     
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  10. moosie

    moosie Doctor of Teleocity Silver Supporter

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    OK, I have read that several times before. Sometimes I think I understand it, but mostly not quite. Typical Merlin :lol:

    Thing is, my DC ground is already elevated. If you refer to my diagram (the one I drew, in post 25), all DC grounds, AND the filament ground, return to chassis ground via a hum-loop-block circuit (on it's own mini-board far left).

    So, it would seem that it doesn't matter whether I connect heater ground to the cathode, or to the star point at the H-L-B.

    I struggled to understand why Gehring would do this, but then I remember that he added the H-L-B afterward. Perhaps his cathode comment was from his first iteration, and he forgot to mention it's no longer important.
     
  11. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Here's a detail of the area in question:

    Screen Shot 2017-12-16 at 10.23.41 PM.png
     
  12. clintj

    clintj Friend of Leo's

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    How elevated do you expect your ground network to be? The idea of the heater elevation is to create a difference in voltage between heater and cathode that basically drowns out the AC noise signal. The power tube cathode is usually plenty high enough to do this, but if the ground network is only raised a volt or two it may have little to no effect.
     
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  13. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Max elevation will be 0.7 V. At that point, the safety diode pair will open, to avoid a large build up of potential, and to allow the fuse to blow.

    So, I need to to both, looks like. Easy enough to do, I'll attach the heater center tap to V3, pin 8.


    I am about ready for testing. My only other build was an Allen kit, totally paint by numbers. Worked first time, so I didn't learn anything, except how to follow directions.

    I haven't thought through a start up plan yet. Any thoughts welcome. Or links to a good reference.

    For example, does it make sense to test the AC loop before connecting the secondary, since there's no standby?

    Then, all pots on zero (intensity on 10 I believe), and ... no tubes?

    Key measurements to consider?

    etc...

    I've already done the visual inspection (dozens of times), and most of the continuity testing.
     
  14. King Fan

    King Fan Poster Extraordinaire

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    Wow, Moosie, that thing is beautiful. Truly a work of art. Nice!

    +1 for Clint, Merlin, and the elevated heater CT. I did it on my first amp (not really knowing why, just cuz it had the CT and smart folks said to do it) and it's so quiet it scares me.

    For startup, tough one -- that gorgeous creature goes way beyond an amp, like the whole giant ground loop potential and its associated safety issues. Still, that makes me think you might be able to adapt Rob's startup, especially around the testing of safety and circuit grounds. In addition, I recently found Mr. Luckey's startup / in-build testing sequence to be a *ton* easier to follow than the Ruby method everybody cites, and a *ton* more useful than my old "use a bulb limiter and cross your fingers" method. Even though you're past the in-build stage, you might be able to use most of his steps.
     
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  15. keithb7

    keithb7 Friend of Leo's

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    I’m following along quietly. It looks great based on how much is stuffed in that tweed chassis!

    Do you have a lightbulb limiter Moosie? It offers peace of mind. It’s especially helpful with the thought of a wire getting hung up, while stuffing it all in the chassis.

    My start up goes along these lines. Amature for sure:

    I’d probably start with the continuity test setting on the DMM. Spread the schematic and layout diagram out. Grab a hi-liter and trace out every connection. Check all for connections to ground. If good, proceed with light bulb limiter and no tubes.

    If good I insert tubes. Test with lightbulb limiter still. If good go straight into wall at 15A and power it up. Then the real testing begins.

    I like to check all b+ points first. Every where. One hand in pocket here. Clip-on spring loaded DMM leads are nice here. Comparing voltages to literature as I go. Problems are addressed as I go, often with help from members here!

    Seems stable? A quick little test playing ensues. Bias gets tweaked. Shunt method, I prefer. Then every connection gets a love tap with a chopstick. Unplug. Drain caps. Clean up some leads. Touch up a few joints. Play like mad.
     
    Last edited: Dec 17, 2017
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  16. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Thanks guys. @King Fan, I hope to eventually think of this unit in the exalted terms you've been using: "gorgeous creature", "work of art", and something about distant peaks of the Himalayas as seen from the delta plains, if I recall (that one nearly scared me off). But for now, it's the Cranky Porcupine, fer sure. :D

    In addition to ignorance, laziness, and hubris (is anything left?), I deferred all powered testing because of the ship-in-bottle cramped space. I couldn't have got the board (back) in place with he switch, fuse, and pilot installed, for example. I guess I could have assembled, tested, and disassembled portions, but that would seem to eliminate the benefit, and ask for more problems.

    I do have a lightbulb limiter, and plan to use it.

    I don't have any extra fuses, so we may be in for "smoke and a long wait" while I order a big bag full. Hmm, that piece of wire is the right length... :twisted:

    I have done all the continuity testing, as I went along. Including and especially all power rails and returns, because they're all under-board.

    Every component has been double-checked for value with a meter, as it was placed on the board. I only had just enough parts, and there are none left over. That's always a good sign, though they could all be in the wrong places... The board is laid out with two rows of eyelet pairs, and I had this recurring nightmare (awake and asleep) that I'd shifted one set of them, by one eyelet. So EVERYTHING would be completely wrong. I should probably check that one more time...

    I did start with a known good layout, but I completely redrew, and rearranged everything, mostly because I knew from the start that I wanted to do a multi-star ground, sort of. Just the act of thinking through the whole circuit very carefully, and drawing every detail, has helped immensely. Honestly, I don't know what many of the components do in those positions, and I'd like to, but at least I'm not just following along.


    I'm good with safety working in a live chassis, though I'll need to tweak my chassis holder for this little guy.

    My schematic from Weber / Gehring does not include voltage readings. I'm hoping to get readings similar to those in the 6G15 and 6G12. Here's a schematic someone (Gehring? Hoffman?) cobbled together.

    Fender 6G15 + 6G12 Combined - schematic.gif


    Re biasing the 6K6, I've never biased a cathode-biased amp before. Need to do some reading...

    And I still need to read Rob's and sluckey's startup lists...
     
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  17. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Figuring I might never get this far, and you wanted no association, eh? Make no eye contact with the crazy man... :lol:
     
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  18. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I built a small amp with the same 6K6-driven, 6G15-derived reverb, and with the Weber PT I think you're using, for what it's worth.
    Last I had it open, my numbers were (I'll try to include just the relevant ones)

    B+1 336 (maybe lower than yours due to higher current demands -- I'm running 1 6V6, 1 6K6, 1 12AT7, and 3 12AX7)
    B+2 331 (this is the node from which my reverb driver is supplied)
    6K6GT plate 322V (same Weber 022905 transformer, I think it's 7K:4R, so 14K:8R tank?)
    screen 328V (I think I have a 470R-2W screen resistor here, but it seems there isn't enough current through it to bring the voltage down below the plate voltage)
    cathode 26.5V
    Stock cathode resistor 1K ohms
    Vpk = 322-27=295V
    Ik = 26.5 mA, Ip = 25 mA
    PD = 7.4W, 87% of max

    Your build looks great!
     
    Last edited: Dec 17, 2017
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  19. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Thanks a lot!
     
  20. moosie

    moosie Doctor of Teleocity Silver Supporter

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