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giving up, starting over

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Feb 2, 2021.

  1. Jewellworks

    Jewellworks Tele-Meister

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    I've got 68:1 on V1, and 70:1 on V2. Which is more than what you get in a 5F1. I could knock it down, but I also want more drive after the Bone Ray, cz it's not enough as-is.
    I have another tap on my PT w 550, rather than the 650 I've got on it now. That'll lower the gain....
    I'm also seriously considering changing the entire tone stack to something simpler (5F2A style, or AA764 Champ style) because it weighs it down so much. But I really have to get the nasty squealy gone first.
     
  2. Jewellworks

    Jewellworks Tele-Meister

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    Lowered my plate resistors to 100k (from 150k) and changed V2 cathode resistor to 1.5k so it's center biased. Getting gain of 60v on both V1 and V2. Only 16.8W on the 6L6, down from 18.5. Bypassed the tone stack, and I still have the nasty squealy when cranked. :mad: It seems somewhat less so? But still there for sure.
    At some point I put a 47k grid stopper on V2, but it didn't make any difference.
    So weird...
     
  3. Jewellworks

    Jewellworks Tele-Meister

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    I tried all 3 of these, with no difference whatsoever.
    I put the 100pf on V1 and tried it. Same old nasty squealy. Kept it in, inserted a 500k pot as a grid stopper on V2, which turning it up (higher resistance) made it much worse, took it out, then I set a 50k pot at 10k before putting it in as a grid stopper on the 6L6, and ... No difference at all.
    This is all AFTER I lowered the gain to 60, as used in a 5F1 Champ, which is basically what this thing is now that I've bypassed the tone stack...
    No NFB, no cathode bypass cap on V2.
    I even thought maybe the 1M volume pot wasn't allowing any grid leak after I turned it up all the way, and put in a parallel 1M grid leak on V2. Nada. Still nasty distorted and squealy when cranked.
    I'm baffled. And frustrated.

    Ive got 1 last hope. Ive been using a mix of PtP wiring and tag strips for all my components. I think my lead dress is fine, but I read on another post in this forum that lead dress can be an issue for squeels. I ordered a turret board and when it gets here, I'm going to lay this out just like Leo did for a 5F2A, and hope for the best. I really can't imagine how that would make that much of a difference, but hey.... Nothing else works.
     
  4. Jewellworks

    Jewellworks Tele-Meister

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    I'm also beginning to believe there there is something inherently non-musical about the distortion in a 6SF5. it just sounds nasty. I'm about to gut this thing in it's entirety and put in Noval sockets in the Octal holes and put in a 12ax7 and abandon my whole "vintage" thinking on this Amp. This has really been frustrating and disappointing
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I suppose the 6SF5 tubes could be the source and not all tubes are equal. The octals are known to be more micophonic as well.
     
  6. NTC

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    How about using 6SL7's? You can keep the octal socket that way, though you still have to rewire most of it.
     
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  7. Jewellworks

    Jewellworks Tele-Meister

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    Definitely an option. Dual triode. What to do with the 2nd Octal socket?
     
  8. NTC

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    Maybe use one extra triode as a cathode follower to drive your tone control. Perhaps use a 6SN7 in the first gain stage and parallel two triodes. Just some suggestions.
     
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  9. Jewellworks

    Jewellworks Tele-Meister

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    I already have a 6J5, which is a single triode. Looking at the data sheets, they look pretty much identical, with a low mu of 20, and similar high plate current and grid values. Use that as V1, to first half of 6SL7 triode cathode follower, to the tone stack, to the other half... Hhmmmm. We might be on to something
     
  10. Jewellworks

    Jewellworks Tele-Meister

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    Actually, the 6SF5 works great in V1. Probably leave it...
    I assume the 6SL7 has a more musically appealing distortion? This is a known thing?
     
  11. Jewellworks

    Jewellworks Tele-Meister

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    Hows this for a preliminary schematic?

    dECO v7.jpg

    havent figured out voltages or load lines yet, but i want to ask if the basic bones look good? then ill move on to a layout.

    V1 to gain control, to V2 Cathode follower to Bone Ray tone stack, to V3, to 6L6. no NFB.
    particularly, on the cathode follower... i have a .01/1M before the tone stack. seems like i need to keep the DC off the tone controls, and the 1M is to add a little separation. is this necessary?

    thanks
     
  12. NTC

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    The 1M probably isn't necessary. You already have a 2M ohm path through the tilt control. Leave it in the layout but try the circuit without it.

    Make sure you don't exceed the heater to cathode voltage on the 6SL7.
     
  13. Jewellworks

    Jewellworks Tele-Meister

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    data sheet says 90v. so... heaters are 6.3, so that means ... ... what does that mean? ive never measured heater to cathode
     
  14. NTC

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    Raise the heater center tap referencw voltage. I have never done it, but a lot of threads around here have talked about it.
     
  15. Jewellworks

    Jewellworks Tele-Meister

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    ive got the heater center tap landed at the cathode of the 6L6, which raises it 470 ohms.
    i also measured (just for reference) the Heater to Cathode voltage of the 6SF5's and V2 is sitting at -22v. so im sure it'll all be good
     
  16. Jewellworks

    Jewellworks Tele-Meister

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    finally got my 6SL7 in the mail yesterday, and i had already changed my circuit from a 6SF5 to 6SL7, using the schematic shown below.
    this is my first time using a cathode follower to drive the Bone Ray tone stack. i triple checked my wiring, and its all good. -results? not good...
    my humming is worse than before, but that could easily be my grounding and layout. but aside from that, im not getting as much overdrive and "good" distortion as i was expecting, and when playing hard, it breaks up all nasty sounding. no squeal, but not a very pleasant distortion. like its right on the edge of clean, but not very good.
    i took measurements, and noted them on the schematic below. i plotted the load lines and im center biased on all the tubes. im running a little hot w/my plate dissipation on the 6L6 (19.8W)
    when i inserted a probe to measure my heater to cathode voltage, i got a horrendous noise out of the speaker. static and squeal and just horrible. i did a quick measurement at pin 3 of the 6SL7 (before the 150k resistor) and got "0". the noise was too much and turned it off before i could measure pin 6.
    that said, my heater C/T is landed at pin 8 of the 6L6 and elevated with the 470 ohm cathode resistor, so im "guessing" im not exceeding the 90v H/C limit on the 6SL7.
    any ideas? why is this lil amp kicking my ass?
    dECO v7W_2.jpg
     
  17. gigante

    gigante Tele-Meister

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    I have no help for you, but look forward to hearing opinions from people who do.
     
  18. NTC

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    This may or may not help - I am not a tube circuit designer.

    Look at the cathode follower circuit in a tweed Fender (I am looking at a 5E7 Bandmaster). The grid of the follower is dc coupled to the gain stage before it (no coupling capacitor). You have the follower grid tied to ground with the volume control. You need another gain stage between the volume control and the follower.
     
  19. Jewellworks

    Jewellworks Tele-Meister

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    hhhhmmmmm... where do i put the volume control? can i put that after the cathode follower, before V2B, to replace the 1M grid leak....

    so direct couple the plate of V1 to the grid of V2A, no coupling cap, then Cathode follower off V2A, tone stack, volume, grid of V2B

    dECO v7W_3.jpg
    like this?

    all that DC doesnt kill the grid? 180+v? i need to read up on Cathode followers...
     
    Last edited: Feb 23, 2021
  20. NTC

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    Yeah, kindvof like that, but ideally you would have another gain stage at the front end, the the volume control, then the gain stage-cathode follower pair. Look at a 5E7, 5F4, or a 5F6A schematic that have voltages. The 5E7 shows 140V on the follower's grid and cathode. But it also shows another gain stage before the volume control. A 5F4 is the same, a 5F6A is similar.
     
    Last edited: Feb 23, 2021
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