BF Super Reverb hiss...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SouthernShred, Feb 14, 2008.


  1. SouthernShred

    SouthernShred Member

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    With the help of a couple of visits to Todd Shock (Wakarusa) and the application of my ham radio homebrew experience, I've been continuing the tune-up/mod of my '66 Super Reverb (player condition, repro cab, hodgepodge speakers, etc) that Todd started about 2 months ago. After getting rid of the snap crackle and pop with new plate load resistors, I've made a few reversible mods to my liking and it's been sounding awesome...I've noticed that the amp has has a high frequency hiss, not a hum, and that hiss increases when flipping the bright switch on (vibrato channel)...I'm definitely going to test all the preamp tubes when I get home, but since this was on my mind this morning at work, I figured I'd post it here. FWIW, I've split the V1/V2 shared cathode and lifted the normal channel from the circuit at the PI. I replaced the power tube sockets recently as well. I can post a complete list of things I've changed if that would help.

    Shannon
     
  2. Blue Strat

    Blue Strat Member

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    Plate load resistors, preamp tubes and almost anything (resistors and capacitors) in the signal path can add noise. If reducing the volume control setting reduces noise, the problem is before the volume control.
     
  3. SouthernShred

    SouthernShred Member

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    Yeah, it's been a few hours since I played it, but the hiss doesn't increase with the volume control. Plate load resistors are new 1w carbon film. Cathode bypass caps are new 25uF / 25v sprague atoms
     
  4. Blue Strat

    Blue Strat Member

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    V2 12AX7 is a suspect and so are the R's and C's between the volume control and the phase inverter. Also, check to see if the reverb control has any effect on hiss volume.
     
  5. SouthernShred

    SouthernShred Member

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  6. SouthernShred

    SouthernShred Member

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    I take that back, the volume control completely affects the hiss/hum and actually goes microphonic/feedback laden over 7 on the volume knob...gonna test some preamp tubes
     
  7. SouthernShred

    SouthernShred Member

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    well, I know where to look...not the preamp tubes, but I have an idea...
     
  8. Blue Strat

    Blue Strat Member

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    Sounds like V2 is suspicious.
     
  9. SouthernShred

    SouthernShred Member

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    I tried several known, good 12AX7's in V2, but I'm going to check the cathode bypass caps on V2 as well
     
  10. Blue Strat

    Blue Strat Member

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    Clean the tube sockets if you haven't already. One bad tube pin connection and all bets are off.
     
  11. SouthernShred

    SouthernShred Member

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  12. Wakarusa

    Wakarusa Member

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    At first blush I'd check the plate load resistors you installed. Could just be a noisy one in the batch or (maybe?) a touch of excessive heat from the soldering iron. Also, check over all of the work you've done to make sure you haven't disturbed a ground or left a lead flying in the air. Finally, you can try taking a wooden probe (we call 'em spudgers, but a chopstick will do) and see if moving leads around changes the noise -- just in case you disturbed lead dress while you were in there.

    PS: finally found the time to get my BFSR back in shape. When you wanna cut heads? :)
     
  13. SouthernShred

    SouthernShred Member

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    I'm going to go back over the plate loads tonight or tomorrow. I could have easily cooked one that I installed with my old soldering station...all the other work was done with my handy dandy new Hakko 936, temp controlled within the Kester 245 melting range... Thanks for the help Todd, I'll get right on it :D

    BTW, I think I need to start ordering parts in bulk if I intend to make this my hobby...piece mealing stuff through multiple sources is for the birds...

    in terms of your PS, I should have some free time to make the trek up to your place within the next month, we can have some cold ones and you can look over what I've done if you're so inclined...
     
  14. Wakarusa

    Wakarusa Member

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    I can pretty much guarantee I've got enough parts on hand to build several super reverb circuit boards. It's nothing to drop some resistors and caps in an envelope. I'm probably less expensive too.

    You're always welcome to share a beer :AOK.
     
  15. SouthernShred

    SouthernShred Member

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    Thanks Todd!

    http://picasaweb.google.com/shannondew/SuperReverbIssues

    Here are some pictures as she stands right now...I reduced the hum completely by using a spudger and fiddling with the lead dress, BUT the white noise hiss is still there...I fired it up with V1 and V6 in...no hiss...added V2, no hiss...added V4...hiss...Tried a ton of 12AT7's and the hiss remains...I redid all the plate load's tonight and lifted each cathode bypass out of the circuit one by one trying to isolate if it were cathode leakage. For purists looking in horror at the non-original state of the amp...don't worry, I have all the parts I removed/replaced...but it's a "player" condition amp and I intend to play it, not sell it for a profit...anyway.

    BTW, the amp currently sounds great, but the hiss...not sure if it's just something typical that all amps like this deal with or what...

    Below is a history of my changes...

    '66 Super Reverb History

    -New Filter Caps (not done by me)
    -New Bias Cap (not done by me)
    -New Power Tube sockets (ceramic)
    -.1uF Coupling caps replaced with Orange Drop 6PS .1uF equivalents
    -Plate load resistors replaced with Carbon Film equivalents
    -Cathode bypass caps (25uF) replaced with Sprague Atom 25uF / 25vdc
    -Midrange pot on vibrato channel changed from 10K to 25K
    -Vibrato circuit disconnected at Intensity knob
    -Split V1/V2 cathode
    -New 1.5k/25uF cathode bypass for V1
    -Changed 250pF ceramic cap in vibrato tone stack to 250pF silver mica
    -Changed 820 ohm/25uF cathode bypass to 2.7k / 25uF cathode bypass on V2 pin 3
    -Changed V2 pin 8 cathode bypass to 2.7k/.68uF
    -Changed .02uF preamp coupling cap to .04uF 6PS Orange Drop
    -Lifted 220k resistor at PI to remove normal channel from circuit
    -Replaced 100ohm resistor with 47ohm
    -Replaced .001uF ceramic cap on pin 2 of PI with .002 6PS Orange Drop
    -1ohm 2w resistors added between pin 8 and ground of both power tube sockets (bias test points)
    -Biased to 38.4 mA
     
  16. soldersucker

    soldersucker Member

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    You may have to start reversing some of those mods one by one to find it.I would start with the cathode spitting you did in the pre amp myself.There are alot of gain mods(vibrato disconnect,split cath,25 k mid pot etc..)my Supers(4) are somewhat hissy at thier best and i bet they would be even hissier this far from what Leo intented.my .002 .Good luck.
     
  17. SouthernShred

    SouthernShred Member

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    I reversed the cathode split, normal channel removal and changed the Silver Mica 250pF back to ceramic last night to no avail. I don't mind reversing it, it sounded great stock as well. Just fun to play with things.

    After getting to really crank it and play it this morning, I'm not sure it's any more hissy than it was before I modded it.
     
  18. Wakarusa

    Wakarusa Member

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    Not sure if there's confusion or a typo, but...

    V1 + V6 = no hiss says the normal channel is behaving.

    When you add V2 with V4 still removed you wouldn't hear any change since V4 is downstream of V2. When you then add V4 and get hiss (I'm assuming V3 is still out) it suggests the problem is in the vibrato channel, but not necessarily associated with the reverb loop. Which makes me wonder why you're swapping 12AT7s.

    You can verify that the hiss isn't from the reverb loop by either lifting the lead where the loop returns to the main signal flow (one side of the 3M3 resistor, may be easier to disconnect at the pot) or by just turning the reverb control all the way down.

    If you confirm that the hiss isn't the reverb loop, then you're likely culprit is the 3M3 resistor. When this one goes wonky I'll replace it with a 1W carbon film or 1/2W metal film (lower noise by far than the 1/2W CC that's in there), replace the 10pF bypass cap with a silver mica, and also replace the 470K/270K dividers on the reverb return (because it's easier, in my opinion, to replace these than to work around leaving the original parts there -- otherwise you've a devil of a time getting the 3M3, 10pF, and lead out all back into the same hole without damaging the 470K/270K pair).
     
  19. SouthernShred

    SouthernShred Member

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    Thanks Todd. I'll shoot you an email and some moolah if I need some parts.
     
  20. SouthernShred

    SouthernShred Member

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    fwiw, turning the reverb all the way down changes nothing in terms of hiss.
     

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