Rob Robinette Deluxe Micro Mod with Blackvibe Tone Stack?

Discussion in 'Shock Brother's DIY Amps' started by Dbsoundman, Aug 1, 2020.

  1. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    Thinking about the LND150, I realized I may have missed something following the AC coupled cathode follower design. My schematic doesn’t have the 1M resistor as Rg, there’s just a direct feedback path from the source to the 1k resistor at the gate. I’m wondering if that’s my problem...but hoping it’s not because that will be a real pain to add in.

    Heres a reminder of what I’ve got:
    496DB7B4-DAC7-40B5-8F0E-CB0A0B43F80C.jpeg
     
    sds1 likes this.
  2. mrriggs

    mrriggs Tele-Meister

    Age:
    41
    Posts:
    227
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    That's definitely a problem. You need that resistor.
     
    sds1 and Dbsoundman like this.
  3. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    If I have time tonight I plan to rewire to add it. Just have to see if I actually have a spare 1M resistor around, then I have to relocate a few connections to accommodate it.
     
    sds1 likes this.
  4. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    OK, did the mod, so now the gate is fed by the 1k resistor as it was before, but now there's a 1M resistor going from the "input" of the 1k resistor to the source. I didn't take measurements before the mod, but I still don't think I have very much gain. Don't get me wrong, I'm VERY pleased with the way the amp sounds, I just can't get the preamp to break up using a Strat with the volume (not master) turned up to 10.
    21-05-12 21-18-07 3256.jpg

    I sent my schematic to a friend of mine who is a much better electrical engineer, I'm going to see what he has to say.

    Meanwhile, I did some of my own measurements with a scope...definitely not losing gain through the LND150, the top trace is "before", lower is "after", using a 400 Hz test tone. It's a little hard to get a good picture of a "moving" scope trace but you get the idea; basically no loss from input to output. I measured the input at the 1k gate resistor and the output at the source terminal.
    21-05-12 21-32-08 3259.jpg

    Also, I had mentioned some hum issues, it occurred to me that the hum had to be transformer related since I could hear it in my headphones the second I turned the amp on, way before the tubes warmed up. I loosened one side of the output transformer and slid it back and forth and found the "null" where I got the least hum. I'll drill a new hole this weekend hopefully. Still have a little bit of hum, but I'm getting a little tired of chasing it, and once again it happens as soon as I turn on the amp so I think I may just have to live with it. When I build my next amp, I'll likely chose the Fender-style power transformers that are oriented vertically, I have a feeling that makes a huge difference.
    21-05-12 21-17-09 3255.jpg
     
    sds1 likes this.
  5. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    @robrob does the B+2 voltage have a significant impact on the gain available in this circuit? I used the "smaller" transformer that outputs 190VCT, and when I measured my B+ voltages today I found that B+1 is 290V, and B+2 (which is used by both tubes) was 240V. I put a second 15k resistor in parallel with the one in the circuit between B+1 and B+2 to raise the B+2 voltage, and it came up to 260V; I haven't yet had a chance to see how that affects the audio gain.

    EDIT: checked the grid bias on both sides of the 12AY7, with the bypass and negative feedback switches in the "normal" 5E3 circuit positions (what I've been using so far), I'm seeing about -1.9V of bias on the first half of the tube, and about -1.8V on the second half, which seems to be spot on based on the data on this page. I also noticed that they spec the B+ voltage at 231V, which means I'm actually slightly higher than the original design at 240V. So now I'm not really sure what I need to check next...
     
    Last edited: May 13, 2021
  6. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    Talking to myself again...

    Did more scope analysis while taking a quick break from work, and what do you know, I'm losing a LOT of gain across the TMB tone stack.

    This is the output of the LND150 vs the output of the tone stack (input to the volume pot) using the same scaling on the scope - you can barely see the tone stack waveform!
    21-05-13 14-03-58 3264.jpg

    Adjusting the scale on the scope, now you can see the second waveform, but note that the scaling is way different; 0.1 volts per division, whereas the input is at 0.5 volts per division! So I'm losing like 80% of my gain in the tone stack.
    21-05-13 14-04-21 3265.jpg

    Is it possible to change the circuit on the second half of the 12AY7 to make up this gain? Or should I look at providing a boost via the LND150?
     
  7. mrriggs

    mrriggs Tele-Meister

    Age:
    41
    Posts:
    227
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    Why not use a 12AX7 instead of a 12AY7?
     
  8. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    Funny you should say that, I JUST tried that. Played my strat with a 12AY7, then switched to the 12AX7...a very VERY moderate increase in gain. Not really noticeable.

    But, in other news, I found that when I use an actual speaker, I can't hear ANY hum, so I'm thinking my headphones (audio technica studio model) are just a little bit TOO sensitive. I can live with that!
     
    sds1 likes this.
  9. sds1

    sds1 Tele-Afflicted

    Age:
    44
    Posts:
    1,759
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    Yo! Been lurking a lot, not a lot of responding. You're doing pretty great by yourself!

    You might try lifting the tone stack (desolder 6.8k to ground), if you suddenly have a ton more gain you will know you are barking up the right tree, you can also hear how much gain might be available otherwise.

    And then I guess the question is whether the LND150 is doing its job or not?

    EDIT: Also, where are you probing for "LND150 output"? Is that at the source, or at junction of 680R/100k? Probe both and kindly report back the difference in signal amplitude if any.
     
  10. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    @sds1 didn't mean to imply you weren't helping, I just have a tendency to reply to my own thread a lot more than other people...and keep editing my posts!

    I probed the LND150 at the junction of the 1k resistor and the negative feedback from the source as the "input" test, then I put my second probe at the junction of the 680R, 100k, and negative feedback to the gate for the "output". Note that I now have a 1M ohm resistor between gate and source, which I didn't have initially. With this setup, I get very close to unity gain across the LND150, which is what I would expect from an AC coupled cathode follower - the cathode follower isn't supposed to have any gain, it just provides buffering to prevent the tone stack from loading down the 1st preamp stage.

    Given the fact that the LND is configured as an ACCF, I'm not sure I can get any actual gain out of it without rewiring it, and at that point it might not work properly as a buffer. I'm going to do a little more reading on the Blackvibe amp design to see if there's anything special to the buffers and gain stages used in that circuit since that's where I got this tonestack design from.
     
  11. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    I think I'm still barking up the wrong tree.

    Unless the source impedance (Zsrc) is way off between this Duncan Amps simulation and my schematic, then the result I saw on my scope (0.1v out for every 0.5v in) is perfectly accurate - if I calculate the dB loss ratio from those two voltages using 20*log(Vout/Vin) I get about -13 dB, which is just about exactly what the simulator shows here.
    2021-05-13-1620939039_739x491.jpg
     
  12. mrriggs

    mrriggs Tele-Meister

    Age:
    41
    Posts:
    227
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    Your Zsrc is way off, 38k is the output impedance of an unbuffered gain stage. The output impedance of your buffer should be around 1.3k [if I remember correctly].
     
  13. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    89CC52FF-7DB7-443D-8F40-ADA947B83013.jpeg 19592E8F-8217-4CB4-87F3-47059BAC9084.jpeg 2DD0AFCE-4106-4130-8EA5-E6D667BD31D6.jpeg Well...

    I fixed it! The Hammond 269 transformer has two primary taps, one for 115v and one for 125v. I had always assumed that I’d want the extra boost from running at 115v to get higher B+ voltages, though it does also boost the heater voltage. On a whim, I tried 125v, and combined with the 12AX7, it sounded AWESOME. I’m not sure it has traditional 5E3, but it’s not exactly a traditional 5E3 anyway so that’s not a fair benchmark.

    Ill get into more of the details later, but I’ve found that the TMB tone stack interacts very interestingly with this circuit. The mid control is almost like a presence/drive control...the more you turn it up, the more overdrive you get, without actually making the sound too “middy”. Granted this is my evaluation playing with headphones, we will see how it turns out with the speaker connected.

    I think I’ll call it “Deceptacon”...
     
    zeppelinofled69, sds1 and awasson like this.
  14. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    In that case, I'm still basically on par with the simulator, using an impedance of 1.3k boosted the gain across the tone stack a bit, but there's still a dip down to about -13 dB between 200Hz-1000Hz or so.
     
  15. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    Here's some basic recordings I made just now with the voice recording app on my iPhone. Not perfect but I think you get the idea. Now that I have a guitar amp I promise I'll actually practice more :lol:
    https://cloud.tastefordanger.xyz/index.php/s/PjA2ExxRQeQWtAx

    One interesting thing about this amp is the mid control really changes the amount of gain/overdrive you get. If I turn the mid down, I don't so much notice a major tone difference as I notice a lot less overdrive. The bass tone control makes a small difference, but I think with the 8" speaker it really doesn't amount to much. I do like being able to control the treble separately, so overall I think the 3 knob tone stack was worthwhile, though it didn't quite turn out as I had expected.

    The bright switch doesn't seem to make a difference, but that may be a mechanical issue with my switched potentiometer; at one point I was refitting it to the front of the chassis and something weird happened, I noticed the potentiometer was actually kind of bending for some reason. I reversed it, but after that it became really hard to pull out the bright switch and the potentiometer itself had a lot more physical resistance to knob turning. I don't really feel like the bright cap would really give me something I needed though, so I'm OK with it.
    Now I want to build a micro bassman or something like that!
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,378
    Joined:
    Dec 29, 2012
    Location:
    United States
    The typical blackface mid resistance is 6.8k or a 10k mid pot. The 100k mid pot allows you to dial out most of the tone stack load which boosts the signal considerably and will obviously increase overdrive downstream. One mod you might want to consider is place a 6.8k resistor between the mid pot and ground. That way when you dial the mid control to minimum you get standard Fender mid response. It makes it easier to find "normal" mids with a 100k pot.
     
    Dbsoundman and sds1 like this.
  17. Dbsoundman

    Dbsoundman TDPRI Member

    Posts:
    74
    Joined:
    Dec 10, 2014
    Location:
    Maumee OH
    Very interesting. My favorite sound on the amp currently is with the mid at 6-7 out of 10, treble at about 6, and bass around 6. The bass control hardly makes a difference and the treble difference is subtle but still noticeable. Basically I’ve dialed out most of the tone stack I rigged into this build! Oh well.

    I plan to build a bassman micro next, as designed except for the pre-master volume as I don’t have enough room on my chassis. I’m very interested to see what I think of that tone stack now!
     
    robrob and sds1 like this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.