Fender '73 Pro Reverb fizz on top of notes (scope images included)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by driesp, Jan 31, 2015.

  1. driesp

    driesp Member

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    Dear all,

    I have a problem with my pro reverb that I can't solve. It's a '73 silverface that I had blackfaced a year ago, it got full service;

    - New filter and cathode caps
    - New tubes (tried alot), now V1 pulled, 5751 in V2, NOS mullard 12AX7 in reverb driver, tung sol 12AX7 in reverb recovery and a NOS RFT 12AT7 in the PI. Power tubes are Shuguang 6L6WGS
    - New power tube sockets
    - New orange drop coupling caps
    - New plate resistors
    - Solid state rectifier (like it more than with the 5U4G rect)
    - New 1k5 power tube grid resistors and 470ohm screen resistors
    - Blackface bias circuit
    - New Jensen neo tornado speakers

    But since they the day I got the amp there was always a kind of fizz and buzz on top of the notes when the amp is pushed into distortion. I like the overall tone a lot with all the mods, but it is just that fizz that really annoys me. I can hear it too with on the normal ch. It seems that higher bias improves it a bit, but then the idle current is too high.

    I recently tried to re- route the grid wires of the power tubes because I suspected oscillation but that did not made any improvement. Tomorrow I'll add some pictures of the inside of the amp. Has anyone other ideas?
     
    Last edited: Feb 10, 2015
  2. swiveltung

    swiveltung Member

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    bacon frying? or fizzing?
    Those are tough. I would suspect pretubes and speakers first.
    If the bias is really affecting it... you might try different power tubes.
     
  3. driesp

    driesp Member

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    Thank you for the answer!

    It is like a trebly fizz on top of the overdrive that doesnt belongs there. I tried several sets of power tubes and pre amp tubes. I work mostly with an attenuator (aracom) to bring it down to bandlevel volume. It is a really loud amp.
     
  4. slider313

    slider313 Silver Supporting Member

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    1) Put a tube (12ax7, 5751) back in V1.
    2) Your using a 12ax7 Mullard in V3 as a reverb driver? Replace it with a 12at7/6201
    3) Neo magnet speakers may or may not be part of the "fizz" sound.
    4) The 5U4 has softer clipping properties than diodes.
    5) Most blackface/silverface Fender amps tend to have a "raspy" sound when pushed into distortion. Does it sound the same when it's turned down for cleans and a pedal is used for OD?
     
  5. driesp

    driesp Member

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    I actually liked the 12ax7 more in the reverb driver. But I will replace this one!
     
  6. pdf64

    pdf64 Member

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    Did the fizz become apparent when the amp was blackfaced?
     
  7. driesp

    driesp Member

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    Can not remember this really, but I think it was always there. It was also a bit the reason why I changed out certain components, but it never got fixed. Not that the amp doesn't sound good now, it is just that added fizz that makes it unpleasant.

    I also think the B+ is a bit high though (in the 485V range), but I didn't like the sag with the tube rect, and the fizz was also present (but maybe got worse)
     
  8. Greentone

    Greentone Member

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    When you say "overdrive", amp cranked or with a pedal of some sort?
     
  9. driesp

    driesp Member

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    Amp cranked, I always use it with my aracom attenuator, I don't really like overdrive pedals, except maybe for a tube screamer to add a little gain but it's gets fizz too then.

    Edit; here are some pictures of the board:

    [​IMG]

    [​IMG]

    [​IMG]
     
  10. driesp

    driesp Member

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    I noticed also an PI unbalance, when measuring the power tube's current striking an low string, the most far right tube draws a lot more current than the one closer to the PI, it stays when the power tubes are switched. Idling current is now 32mA with a B+ of 485V, and when a low note is played the one tube goes up to 65mA while the other one goes max to around 55mA.
     
  11. driesp

    driesp Member

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    I put again a tube in V1 and tried the normal ch, it seemed like the normal ch was ok and did not have the fizz.. I probably made a mistake then. So it seems only to happen on the vibrato ch, I made a soundclip of it;

    - First I'm playing the vibrato ch where you clearly hear the buzz, then I plug into the normal ch what sounds much better.

    http://soundclick.com/share.cfm?id=13040089
     
  12. ScioBro

    ScioBro Supporting Member

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    swap the the right most two 12AX7 tubes V1 and V2.
    does fizz change to normal channel? Then it is your V2 tube
     
  13. driesp

    driesp Member

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    No it doesn't help, but I still noticed some fizz on the normal ch when the treble was raised. I swapped the RFT 12AT7 with a tung sol 12AX7 in the PI and most of the harsh fizz got away. Too bad for such a nice tube :( maybe it's more hi-fi use intended.
     
  14. pdf64

    pdf64 Member

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    I wouldn't worry about the power tube current imbalance when driven thing just yet.
    Rather the high, stiffer VB+ may be more of an issue for the amp.

    The Pro only has a smallish 35W OT, but still with a primary of around 4K.
    With that VB+ it's more like a SR, which has a much bigger OT, and so the power tubes may be pushing Pro's OT past its limits.

    What is the heater voltage, eg 6.3Vac?
    The first thing to establish is whether the line and the PT primary are best matched.

    It may also be useful to assess the issue with a 5U4 rectifier in place, as that may help to keep the OT power within its capacity.
     
  15. driesp

    driesp Member

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    I set the voltage selector to 230v so i got the best heater voltage (6,4v now).
    With the tube rect, the sound is more mushy, and I dont like that I guess. Maybe the stock filter spec is too low to prevent ghosting?
     
  16. Blue Strat

    Blue Strat Member

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    Try the tube rectifier again as a test. I'll bet the SS rectifier is what's causing the fizz. Also try a 5AR4 which will be a compromise between the SS rectifier and the 5U4.
     
  17. driesp

    driesp Member

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    It might be true about the over saturated OT. When the volume is pushed to 10, it really oscillates and saturates (esp. when the bass is set high), listen at the clip. I'll try to bias it correctly with the 5U4G again.

    http://soundclick.com/share.cfm?id=13040154
     
  18. pdf64

    pdf64 Member

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    That sounds horrid, I suspect there's several things wrong, some positive feedback mechanism, maybe a vibration induced dodgy connection, bad B+ cap.
    Does the amp gain increase or decrease if the power amp global feedback loop is opened?
    What's it like if a different, physically separate speaker cab is used?
    Have you tried some different 'known good' power tubes?
     
    Last edited: Feb 1, 2015
  19. Structo

    Structo Member

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    It could be the screen grids having too much voltage on them.

    You can try larger screen resistors than the 470R.
     
  20. driesp

    driesp Member

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    Today I indeed tried to disconnect the NFB, it got worse. Overall the amp sounds good when not pushed too far, but from the moment it gets into distortion things start to happen. I think it is always present, because if you set the treble high you hear it on lower volumes too. I also put some (older) sovtek 5881 tubes in it today and it did not change much.

    I have a sf vibrolux here too from a friend that left it here for rehearsals, but it is still silverface spec (330k and 47k PI resistors instead of the 1M's and 87/100k couple). It has a lot less gain but actually sounds better. When I play my amp over his speaker cab (eminence ragin cajun) it sounds better, but it are just darker speakers that hide the fizz I think. But noticed too that this vibrolux has a lot less bass (you can put it over 4 and still sounds good). Mine actually has too much, and maybe this creates too much flub?

    edit: I tried again the 5U4G rect back and set the bias good (40mA on 434V) but to my ears it just sounds worse, the amp distorts faster and is not tight at all. I checked all the caps and they look good on the first sight.

    Probably a scope and a signal generator would help a lot here isn't it? I'm thinking about buying a digital scope, might be the right moment now.
     
    Last edited: Feb 1, 2015

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