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Silverface deluxe amp hiss :(

Discussion in 'Amp Input - Normal or Bright' started by Bearmont, Dec 6, 2018.

  1. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    hello all,

    Recently aquired a 1970 fender deluxe reverb - it sounds great, cept for amp hiss!

    I tried pulling tubes to see if that was the cause, only the PI would make the hiss go away when removed.

    The hiss while the amp is idling with the volume off sounds as if the volume were set to 7. It’s audible when playing which is super annoying.

    None of my other vintage fenders do this. What could be causing this?
     
  2. evangilder

    evangilder Strat-Talker

    Age:
    52
    129
    Sep 18, 2018
    Ventura, California
    I would guess a bad ground connection somewhere. If you aren't comfortable working on the amp, I would take it to a reputable shop. It may be something simple like a cold solder joint, or a loose tube socket (notorious problem on older Fender tube amps). But without seeing it, it's hard to say.
     
  3. Triple Jim

    Triple Jim Senior Stratmaster

    Feb 27, 2018
    North Central NC
    I haven't had a hiss problem in a tube amp, but I recently had a crackle problem in an old one. It was caused by a bad plate resistor to the last preamp stage before the phase inverter. I'd guess that a bad resistor could also cause a hiss, which is really sort of a steady crackle.
     
    jball85 likes this.
  4. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    Thanks for the responses guys; did some more research and have noticed carbon comp resistors as being a common offender for HISS.

    Looked at plate resistors (the ones in ^ shape coming off the preamp tubes) in my amp (brown colored) and based on the color bands (from left to right: brown, black, yellow, silver) have concluded that they are 100k resistors. Please correct me anyone if that is incorrect.

    They look "old" based on my non scientific examination of the "roughish" exterior of the resistor. Perhaps, this could a place to start!
     
    jball85 likes this.
  5. Triple Jim

    Triple Jim Senior Stratmaster

    Feb 27, 2018
    North Central NC
    100k is correct. Silver is a 10% tolerance, which you probably can't even buy now. Everything will be 5% or better. As a humorous note, some of the resistors in my Silvertone 1484 have no tolerance band, which means they're plus or minus 20%.

    Yes, there's not much to try with your old ones other than changing them to see if that helps, but the outside appearance of the old ones rarely tells you anything.

    Based on your observation that removing the preamp tubes before the phase inverter doesn't help, it makes sense to start with the 100k resistors for the PI plates.
     
    Vic Interceptor likes this.
  6. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    Would replacing the plate resistors with new 100k CC resistors or metal film be best? Would tone be affected by either way be these choices?
     
  7. Triple Jim

    Triple Jim Senior Stratmaster

    Feb 27, 2018
    North Central NC
    Tone is not related, although as usual some will claim it is. I'd stay away from carbon composition resistors. Metal film is a good choice, but anything will be better than failing carbon comps, even new ones.
     
    fezz parka likes this.
  8. rolandson

    rolandson Most Honored Senior Member

    Old ≠ Bad...all the time. But in some cases..like caps, it just might.

    Unusual hiss and/or crackling can be an indication of filter caps.

    If this were mine, and I knew what I was doing, I'd pop the doghouse and inspect them...discharging of course, because I know what I am doing. Maybe I'd even just replace them

    If this is outside your knowledge and experience, there is no shame in having a qualified tech look into it.

    Keep in mind that not addressing a filter cap issue can cause significant damage to the amp.
     
    therevyevot likes this.
  9. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    Understood.

    If the repair fails, what other things should I consider?
     
  10. Triple Jim

    Triple Jim Senior Stratmaster

    Feb 27, 2018
    North Central NC
    I like to take things one step at a time, so if it were mine, I'd change the plate resistors one at a time until I got them all replaced. If that fails, we'll be here to provide more suggestions like @rolandson did above. In the meantime, you might let us know if the capacitors he mentioned look original.
     
  11. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    GUT1.PNG

    Here's a gut shot.
     
  12. dirocyn

    dirocyn Strat-O-Master

    Age:
    43
    656
    Jan 20, 2018
    Murfreesboro, TN
    IF you know how to safely discharge caps, which is necessary for working inside tube amps...

    I suggest you find a schematic for your particular model. Most Fender amp schematics are available online. Then get out the multimeter and start testing components--maybe starting with the plate resistors and then filter caps--until you find what's out of spec. Any visibly leaking caps need to be replaced while you're in there.
     
  13. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    My 1972 Vibrolux Reverb had white Mallory caps, I wonder if this amp originally had those too?

    These kinda look like F&T brand?
     
  14. Triple Jim

    Triple Jim Senior Stratmaster

    Feb 27, 2018
    North Central NC
    The problem is that a noisy resistor or electrically leaky capacitor might not be out of spec when you measure it with a meter.
     
    fezz parka likes this.
  15. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    Germany
    But most always a capacitor makes noises (getting microphonic) when falling apart.
    Besides that it is a clean amp but with a terrible layout and cheapest pcb.
    If you just try to get some gain out of those they act like a diva having soda instead of champagne.
    Don´t get me wrong - Fender is the definition of clean tones in my book (for many of their amps).
    Happy hunting.

    All the best,
    Robin
     
  16. Bearmont

    Bearmont Strat-Talk Member

    72
    Jul 1, 2011
    Bellevue, WA
    PCB?

    Which amp are you talking about?
     
  17. jball85

    jball85 Strat-Talker

    477
    Mar 16, 2014
    East Texas
    Give this a read, and collect some more info, but I'd say carbon comps, filter caps, and lead dress would be my suspects, if you've tried changing out preamp tubes already.

    http://www.geofex.com/ampdbug/ampdebug.htm
     
    johnnymg likes this.
  18. johnnymg

    johnnymg Most Honored Senior Member Strat-Talk Supporter

    Sep 5, 2015
    Central Coast Ca
  19. fezz parka

    fezz parka Duke of DILLIGAF Strat-Talk Supporter

    Age:
    59
    Apr 21, 2011
    Right behind you...
    TJ has it. Plate load resistors. Metal films are great replacements. Look at the circuit and you'll see why PC's aren't really needed here.
     
  20. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    Germany
    PaperCardBoard xD
    It´s an old techs joke here, sorry.
    To be honest Fender never did a good job with grounding layout and noise "free" amps.
    Having said that the JCM800 Marshall is noisy as hell compared to the gain it has (but then it is still a too close copy to the Fender).
    Friedmans are noisy as well, so that´s no excuse.

    All the best,
    Robin