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Please help to identify a stock component in a SFDR

Discussion in 'Glowing Bottle Tube Amp Forum' started by trouserpress, May 14, 2018.

  1. dunehunter

    dunehunter Tele-Meister

    Age:
    64
    330
    Jul 14, 2011
    Larkspur, CO
    Unfortunately, I don't have much to compare it to; however, mine seems to have more hum that hiss. Although, when I started my little project (ended up converting the amp to AB763), it did have some noticeable hiss. I had attributed that to the grid stoppers but haven't, as yet, followed up and replaced those originals. And now, after my various other replacements, that hiss has all but vanished.
     

  2. schmee

    schmee Friend of Leo's

    Jun 2, 2003
    northwest
    Again... the hissing is very often all those 100k CC resistors on the board.
     
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  3. trouserpress

    trouserpress Tele-Meister

    Age:
    55
    427
    May 4, 2015
    Leipzig
    Right Schmee, that's why I already got rid of 6 of those. After that the normal channel went from moderate hissy to DEAD quiet and the vibrato channel went from very hissy to still-a-bit-too-much hissy.

    Couple of days ago I followed the signal with a signal tracer (again) and I found that a big proportion of hiss begins within V2 right between pin 7 and pin 6. Because i did a lot of tube swapping before with virtually no effect on the hiss I was beginning to suspect the tube socket of V2.

    On Monday I ordered some new sockets from TubeAmpDoctor. At the weekend things might clear up (or not).

    Hey Dunehunter congrats to your hissless amp - I want that too! If there's any noise left at all please tell me what's the difference (ratio) in hiss level between your vibrator channel and normal channel. I've read there is supposed to be not more than twice the noise in the vibrato channel.
     

  4. dunehunter

    dunehunter Tele-Meister

    Age:
    64
    330
    Jul 14, 2011
    Larkspur, CO
    Hey @schmee...Do you mean the 100K load resistors? Just curious.
     

  5. dunehunter

    dunehunter Tele-Meister

    Age:
    64
    330
    Jul 14, 2011
    Larkspur, CO
    I'll look into it and get back to you.
     

  6. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    The vibrato channel has an extra gain stage, so more hiss is not unlikely. Check the 3.3Meg/10pF network. This has potential to add a bit of noise.
     
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  7. trouserpress

    trouserpress Tele-Meister

    Age:
    55
    427
    May 4, 2015
    Leipzig
    Thanks JDOxO, but I've already replaced those two components during my first go at noise reduction a couple of months ago. That resistor seemed to be microphonic but obviously did not add to the hiss.
    Yesterday I replaced the mystery cap with a new orange .02 cap and the ticking (it is rather an intermittend hissing) got a little better - maybe as good as it possibly can get. Nevertheless I still got approximately 10 times more hiss in the reverb channel than in the super quiet normal channel and I feel there is more room for improvement.
    New tube sockets came today and I'm about to install one at V2.
     

  8. trouserpress

    trouserpress Tele-Meister

    Age:
    55
    427
    May 4, 2015
    Leipzig
    Happy end: I brought the noise level of the second channel down to a point where it won't bother anyone anymore. This is what I did:

    - installed a super low noise 7025 -Mullard-like tube from Tube Amp Doctor
    - tightened the grip on all tube sockets along the reverb signal chain (I just had not been in the mood for replacing the sockets; maybe someday later)
    - resoldered all connections to the tube pins thoroughly (did that before but only in a half hearted manner)
    - replaced all of the remaining 100k carbon comp resistors in the tremolo and reverb section with 2w metal film resistors.

    All of the above added to improve the hiss situation wheras moving wires cautiously with a chopstick in the life amp didn't do anything good.

    I think the amp is nearly as good as it can get.
     
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  9. schmee

    schmee Friend of Leo's

    Jun 2, 2003
    northwest
    Sorry, I didn't see this. Yes the Plate load and Slope resistors in the pre section.
     
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  10. dunehunter

    dunehunter Tele-Meister

    Age:
    64
    330
    Jul 14, 2011
    Larkspur, CO
    Thanks!
     

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