1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

5F6A Hum...

Discussion in 'Amp Tech Center' started by MrCoolGuy, Aug 25, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Let’s try Blencowe and get the phase inverter return current flowing away from the previous stages and input jack. Try jumper here. Apologies if you already tried it.

    CB21C0D7-7107-4D53-B404-B10490A2AEDD.jpeg
     
  2. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    That’s why they pay @D'tar the big bucks.
     
  3. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    Oh man! Good catch. I didn't see that.
     
  4. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    Ok so jumper between filter 3 and 4 ground. And leave the ground to preamp or to power ground?
     
  5. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Sorry that was a half solution. Also need to lift one end of the power amp ground to chassis. See red X below. Leave the connection to the preamp ground bus.

    6CE1A901-12B0-409B-B63A-2E599BB2E122.jpeg
     
    Lowerleftcoast and D'tar like this.
  6. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    I’m assuming your HT+ center tap is wired directly to the negative B+1 terminal like Rob has it.
     
  7. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    Yes it is. So leave it there, and ground all four caps to the preamp side?
     
  8. D'tar

    D'tar Friend of Leo's

    Posts:
    3,074
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Lol! Paid by the hour.... Maybe we can stretch this out a few more pages
     
    MrCoolGuy likes this.
  9. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    It worked!
    I had tried it the other way, with all 4 caps grounded to the power amp side, but i never tried all 4 grounded to the preamp side. If you think about it, this is closer to how fender actually had it, with the filter caps grounded directly to the doghouse cover.

    So the hum is gone. There is more hiss than the amp used to have, but I can deal with that... unless yall have any ideas.
     
    JamesAM, andrewRneumann and tubegeek like this.
  10. tubegeek

    tubegeek Friend of Leo's

    Age:
    60
    Posts:
    3,879
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    Two hundred posts.

    :eek:
     
    JamesAM, andrewRneumann and D'tar like this.
  11. D'tar

    D'tar Friend of Leo's

    Posts:
    3,074
    Joined:
    Jan 11, 2013
    Location:
    WNY
    andrewRneumann and MrCoolGuy like this.
  12. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    Yeah... I know. Hey, I'm still learning.
    And guys like you, @Wally , @D'tar and @andrewRneumann have taught me a few things. Thanks!
     
    andrewRneumann likes this.
  13. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    So, this is a Blencowe idea? To ground those filter caps to the preamp side? I'm not sure I see the logic there.
     
  14. tubegeek

    tubegeek Friend of Leo's

    Age:
    60
    Posts:
    3,879
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    I'm just amazed it took so many stabs to get it.
     
  15. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    Yes, and you're only seeing half of it. I did a ton of reading and experimenting on my own too.
     
    D'tar, andrewRneumann and tubegeek like this.
  16. D'tar

    D'tar Friend of Leo's

    Posts:
    3,074
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Sooo, how about that hiss!!!??? LOL

    :lol::D:D:lol:







    Too Soon?
     
  17. D'tar

    D'tar Friend of Leo's

    Posts:
    3,074
    Joined:
    Jan 11, 2013
    Location:
    WNY

    From Robrob site


    unified ground bus that is only grounded at the Normal Low input jack. The power transformer high voltage center tap is connected directly to the B+1 filter capacitor negative terminal to minimize hum. The typical split-bus ground sends all the preamp return current through the chassis. This ground scheme doesn't flow any current through the chassis which any electrical engineer will tell you is a good thing.
     
    andrewRneumann likes this.
  18. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    HA! Awesome! Sorry, I only do hum, I don't do hiss.

    Kidding aside, you might start by replacing that 10K in the power supply and the 100K load resistor V2A.

    I think I read the OP when it came out, but decided to work on something else. Then it got to 100 posts or so and I thought they must be off on some tangent related to who knows what. Then it got to almost 200 and I finally gave in and had to see what the heck was going on.

    You are most welcome sir! And in all reality, you are providing the learning experience for all of us. I learn so much from this forum!

    Did you change the ground scheme when you changed the caps? It was quiet before, so I'm not sure you still don't have an actual problem somewhere. What we might have done here is just got the hum away from V1 and V2.

    The Blencowe scheme for grounding is based on the idea that the grounds are all connected in one big daisy chain. It is connected in order from least sensitive to noise (power amp grounds) to most sensitive to noise at the far end (input valve). The current in the ground bus/wire starts out as a trickle coming from V1A and slowly builds as the grounds from V2, PI, and power tubes are added in. Think of it like a river that starts as a small stream and ends up as a huge river. The huge river end is the HT+ C.T.--and that makes sense, it's the power supply and eventually all current has to go into that wire. Why do it this way? The more current in the ground wire, the more noise voltages are built up. If we ground V1 to "mouth of the Mississippi" where the current is high, we are bound to have noise issues. So we ground V1 at the headwaters in Minnesota where it's nice and still. Ideally, there should be no current flowing in the chassis. There should be no current flowing between the ground bus/wire and the input jack ground... that connection is simply to provide a reference voltage so that the ground and chassis are both at the same potential.
     
    D'tar likes this.
  19. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    @D'tar
    Lol
    Not too soon!
    So I replaced that 10k resistor and that 8uf cap. (Before I fixed the hum)
    Hum was still there, so I put back the Sprague 8uf cap. The new 10k resistor melted. So now I put another new resistor and a new 8uf cap.
     
    Last edited: Sep 11, 2020
    andrewRneumann and tubegeek like this.
  20. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

    Age:
    37
    Posts:
    476
    Joined:
    Jun 16, 2020
    Location:
    Fort Worth, TX
    Yes, actually I did.
    I didn't mention it because...
    So when I first built the amp, I made a mistake by looking at 2 layouts. RobRob's has first 3 filter caps grounded to the power amp side, last 1 grounded to the preamp side. Weber's has all four grounded to the power amp side. So, accidentally, I connected the negative terminals of all 4 filter caps, but still had them ground twice, to preamp and power side. Obviously that created a hum. I did some chopsticking and messing with ground wires. I found that when I disconnect the ground from the power amp side, hum gone. Amp sounded great. Fast forward a year to when I decided to relocate the caps. I figured since I was installing new caps and a doghouse, I would do the grounding the right way. That's when I ran into the issue. I didn't know it was ok to have all 4 grounded to the preamp side, so I didn't think it was worth mentioning. I know better now.
     
    andrewRneumann likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.