Low Level Hum in My Fender Tube Amp

Discussion in 'Tube Audio' started by tube-a-lou, Jan 7, 2018.

  1. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,354
    Hi all,

    I've been working on this for a few weeks but have run into this little problem low level
    hum. I have a original Fender 1953 Deluxe amp it uses the infamous 6SC7 tubes (preamp).
    So maybe the RCA's I'm using are noisey but I'm not sure. I did change all the electrolytic's
    three F&T's and two Sprague's are new. I also changed all the coupling caps to some old blue
    mallory's which I think are still good, I did have them in another amp and there was no
    problem's. I'm just stuck, can it be the 240K resistors I put in, can resistor's cause hum or
    just crackling, maybe the three F&T's 22uf are not strong enough, what do you guy's think?.

    Thanks
     
  2. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,541
    Location:
    San Francisco, CA
    Heater lead dress on early Fender amps is notoriously bad. Start there.
     
    Bill Ferris and KentTeffeteller like this.
  3. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,354
    Okay that's a start, with the 6SC7 tubes there is only one wire going from one tube two the other
    the 6V6's I know I could twist.

    5B3.jpg
     
  4. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,541
    Location:
    San Francisco, CA
    For the best hum rejection, you want both leads to be twisted tightly **right up to the socket**. And either ground the center tap of the heater winding on the PT, or make a virtual center tap out of 2 220 ohm resistors. You might get even better hum rejection by floating the heater supply on top of the output tube bias voltage (5B3 is cathode bias, right?)
     
  5. drew_t

    drew_t Well-Known Member

    Messages:
    653
    Before you start tearing up and redoing the heater wiring on a vintage amp, you might want to unsolder the 6.3 volt power transformer wires, and connect a 6 volt lantern battery to the heater string. If you still have the hum with the tubes lit up by DC power, you'll know to look elsewhere.
     
    GYMusic and thorpej like this.
  6. Alan0354

    Alan0354 Super Member

    Messages:
    2,970
    Location:
    Bay Area
    What do you mean by working on it? I hope just changing the filter caps. You don't and should not change the coupling cap, this is part of the original circuit that make the value of this antique amp!!! I would just change the big filter caps and NOT touching anything else. You should not change the resistor.

    If you want to play with the circuit, buy another cheap amp and play with it and leave this one alone. This is precious. I design and build guitar amps, I bought a 73 Pro Reverb, I still kept the body of the circuit untouched. I use it as reference for clean sound I design my amp against. Your's is a 53, 10 times more valuable. I would first PUT BACK EVERYTHING EXCEPT THE FILTER CAPS. Then see whether you fix the problem. Leave it alone no matter what the result is. take pictures along the way so you know how to reverse it back if necessary.

    You have a hand built amp with wires and torrid board, you might wired it wrong. Resistor and coupling cap is NOT going to give you hum. It must be something wrong you did. That's the reason I strongly discourage people changing out things that are not broken.........particular on this precious amp. I won't be surprised you already drop the value of your amp by quite a bit.


    All that said, post the schematic and post a picture and show what part you changed. My first guess is something you did to the filament wiring and the two resistors that connect from each filament wire to ground.

    DO NOT modify the amp, it DID NOT have hum before, you did something to the amp. try fixing it, not changing it. You start modifying the amp, pretty soon it will not worth anything. You want to play, sell the amp and you have enough money to buy like 10 new amps for you to tare apart.
     
    tube-a-lou likes this.
  7. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,541
    Location:
    San Francisco, CA
    Indeed, that’s good advice.
     
  8. solderjunkie

    solderjunkie AK Subscriber Subscriber

    Messages:
    44
    Wait, weren't you just advising him to modify the heater wiring?



    I've owned a dozed 50's tweeds in addition to many other vintage amplifiers both instrument and hi-fi and rarely had to change a cap in any of them. I have an original Tremolux that still has the original filters and is quiet as a church mouse and sounds like angels singing. This mindless urge to tear the guts out of a perfectly good and rare piece of music history astounds me, much like the modern mythology and fixation on "lead dress" making for a quieter and better sounding amp.
     
    Last edited: Jan 8, 2018
  9. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,541
    Location:
    San Francisco, CA
    He should fix it only if it's the problem, obviously.
     
  10. solderjunkie

    solderjunkie AK Subscriber Subscriber

    Messages:
    44
    Might be a bit too late to go back to Eden. Unsoldering the original connections alone cut the value in half.
     
    tube-a-lou likes this.
  11. Alan0354

    Alan0354 Super Member

    Messages:
    2,970
    Location:
    Bay Area
    ...............................
     
    Last edited: Jan 8, 2018
  12. Alan0354

    Alan0354 Super Member

    Messages:
    2,970
    Location:
    Bay Area
    ................................
     
    Last edited: Jan 8, 2018
  13. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,354


    When I received the amp, it was in pretty poor shape, no speaker, no output transformer. When I went to look into
    the amp it looks like Ceasar Diaz worked on the amp back in 89 and changed out all the 250K resistors to 220K
    (more headroom?). When I did change the filter caps with F&T I put it on and the static and cracking noises were
    real bad so I removed the original coupling caps which seem to be better but the low level hum still remained. It's funny
    when I turn up the mic loudness control to 12 the hum goes down but the hiss goes up.
     
  14. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,354
    It is, let me just say it was pretty much worked on a lot before I received, it so value wise it's
    much half value as stated above. I did put in a set of original Blue Mallory's from another
    sixties amp I had. This is how I received it there was some work done on it.


    P1060236.JPG
     
    Last edited: Jan 8, 2018
  15. solderjunkie

    solderjunkie AK Subscriber Subscriber

    Messages:
    44
    Wow. Yes, that really was a basket case. I have found that the original layout in these old Fender amps really works so not necessary to rearrange it at all.
    The first thing I usually do before I begin trouble shooting an old amp is go through and liberally spray all of the connections, pots, jacks and tube sockets with a quick drying electronics cleaner such as Deoxit or CRC.
    There have clearly been a lot of connections disturbed in your amp, so I would check every single ground connection on both the chassis and board as well as their connections to the pots, input jacks and tube sockets.
    I have found the majority of the time the origin of hum and noise originating from tubes. Mismatched power tubes in particular, although not quite as common in a cathode biased amp, will created hum,.You've obviously got to have another set of tubes to check this. Once tubes are ruled out I move on to the electrolytic caps..then signal caps being last. Check the bias resistor spec and it's ground as well.
     
    Last edited: Jan 10, 2018
    Bill Ferris likes this.
  16. Alan0354

    Alan0354 Super Member

    Messages:
    2,970
    Location:
    Bay Area
    Anyone mention about matching the two output tube? If the tube is not matched, it will hum. I don't know this old amp, Fender had balance adjustment in their Black Face and Silver Face amps. You adjust the pot after you change the tube......by adjusting the hum out.

    So before doing anything drastic, check that first.

    I am not sure about hifi tube amps, I can tell you for guitar amp, matching tubes are way way over rated. Just have a balance pot and I can blind fold and put in two random tubes ( of cause the same tubes like 6L6 or EL34 etc.) and adjust the balance to get rid of the hum and it's ready for gigs.
     
  17. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,354
    I've been doing a bit of work on it, I did have all old Blue mallory's in the amp but I decided
    to try four Orange drop 418P's in some of the positions. It did clean up the sound a bit, but I
    do want to try these new Jupiter Caps in it, they suppose to be modern copies of the old
    Astron caps but for now I'll leave it. I did clean out all the controls and it's a bit quieter which
    is good check and resolder some connection, I'm beginning to think it might be these 6SC7
    tubes or the old 60 year old wiring. I mean the hum doesn't get in the way of my guitar playing
    it's just low in level. I did redo the grounding wire putting it on one of the screw's that hold the
    transformer and screwing it down good, I'll keep at it and check the wiring to see if anything
    is amiss.


    fullsizeoutput_5ece.jpeg
     
  18. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,354
    Update! I did find a wiring mistake. I was going though the amp checking the wiring from
    the board to sockets and found at the 6V6 tube there a wire that runs from the point
    in between the two 10K resistors to pin 4 which is the 280 volt point. Mine was coming
    from the right point which is 250 volts point, so I changed it to the other point. So I think
    I was starving my 6V6's a bit. If you check the layout you could see it.
     
    Last edited: Jan 12, 2018
  19. drew_t

    drew_t Well-Known Member

    Messages:
    653
    Those are 10K (10,000 ohm) resistors, not 10Meg (10,000,000 ohm). The "M" in the schematic is the outdated abbreviation for 1,000, based on the Roman numeral system - - M for thousand, MM for million.. Anyway, it looks like you have the correct value (brown-black-orange) installed. Did moving the wire to the correct junction get rid of the hum?
     
    tube-a-lou likes this.
  20. knockbill

    knockbill Addicted Member

    Messages:
    6,910
    Location:
    SE PA
    Hey Lou,,,
    That's a pretty nice looking amp, and I like the tube choices... Not sure I need another clone build,,, but this one really intrigues me!! Already found specs and cab dimensions, and I have a turret board and txs that would work,, may be forced to build this one also!!!
    Are you keeping all these amps you've posted here? If so seems you have quite a collection!
     
    tube-a-lou likes this.

Share This Page