Vibrolux (Silverface) doesn't sound good after re-cap

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Stevie, Dec 20, 2009.


  1. Stevie

    Stevie Member

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    I re-capped my '76 SFVR (non-master) as it had the following symtoms:
    1) loss of power (had to crank it to 10 for it sound like what it did at 6)
    2) loss of bottom end (became loose and flappy)
    3) the mids and highs sounded sterile
    4) motor boating

    I replaced the filter caps under the cover with Sprague Atoms using the same values (16uf, 475V) and 7 cathode caps? (25uf, 50V) caps and one bias cap (100uf, 100V).

    Now the bottom is very tight, almost mid-range like. The highs sound choked and slightly sterile.

    Any ideas?
     
    Last edited: Dec 21, 2009
  2. Rocknrollampsinc

    Rocknrollampsinc Member

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    Check all your new connections. Make sure all new/old grounds are good. Check tube voltages compared to the schematic.
    Did you rebias the amp after the recap?
     
  3. twangbanger

    twangbanger Supporting Member

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    Did you replace all the ones under the cover w/ the 16uf value. I believe there are higher values perhaps around 70uf on the first few. also maybe the polarity got reversed on one? Polarity is very important. If you have a voltage chart for the amp it might help to narrow down where the problem is. Good Luck
     
  4. SatelliteAmps

    SatelliteAmps Member

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    They should have all been 16uf under the can.

    I hate to suggest this, but the amp might just be sounding the way it should it. When you recap an amp and do all the bypass caps and bias cap, the bottom tightens up, and highs get more defined.

    Does it have more volume than it did?
     
  5. Stevie

    Stevie Member

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    All the 5 caps under the cover were, and stll are 16uf

    You may be right about the amp may just be sounding the way it should. I purchased the amp used, it was almost 100% original (bias mod and upgraded speakers but the original speakers were included). That means the caps I replaced were 33 years old and my ear got used to the sound this amp produced. Now with the new caps it just sounds so different...although the highs seem choked and the sparkle is gone. Could the tone caps be shot too?

    The volume is now back to what it was, actually a little louder than what is was when I first got the amp.

    Should I be concerned about the plate voltage because according to the schematic there should be 450VDC and I'm getting 413VDC? The bias on the tubes is set to 38mA.
     
    Last edited: Dec 21, 2009
  6. Rocknrollampsinc

    Rocknrollampsinc Member

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    Bias is in the ball park. Wall outlet voltage and rectifier tube will effect plate voltage. Anyway, you are within 10% which is good.
     
  7. soldersucker

    soldersucker Member

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    "Now the bottom is very tight, almost mid-range like. The highs sound choked and slightly sterile.

    Any ideas?"
    What speakers are in there?
     
  8. Stevie

    Stevie Member

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    Good point! I currently have two Jensen P10R (newer, not vintage). They sounded alright with the old caps. I tried the original Oxford speakers and although a little more chimey on the top end, the bottom end was dominant and just sounded bad. I have a Rajun Cajun and was considering getting a Copperhead to go with it or just two copperheads or two Celestion G10 Vintage.

    Oh yeah, one more thing: A big THANK-YOU to all who responded and so quickly too!
     
  9. StratTone

    StratTone Member

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    Sounds like a classic case of "the amp sounds like it is suppose to now". lol I always try and tell people that they may not like the sound of there Silverface once recapped. If the amp is old and not hissing or buzzing I'll leave the old caps in, in the cap pan. (provided there not bulging or leaking visually)
     
  10. AR-305

    AR-305 Member

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    With new caps an amp also seems to respond well to different tubes. I don't recall you're mentioning the age and condition of the tubes in the amp. There are certain brands of 12AX7 that sound darker or brighter in the pre stages. The only time I'll get near those J umpin' J imminy's are when I'm trying to tame down an overly bright amp.
     
  11. Stevie

    Stevie Member

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    Interestingly I tried a different 5U4 rectifier tube from an old hammond organ and it made a big difference in the sound - warmer, thicker, smoother. Who would of thought a rectifer tube would do so much to the tone.

    Tubes were put in less than a year ago and are as follows:
    V1: modern Tung-Sol 12AX7 (I don't use that channel)
    V2: NOS Tesla 12AX7
    V3: NOS Brimar 12AT7
    V4: NOS Tesla 12AX7
    V5: modern Tung-Sol 12AX7
    V6: NOS Mullard CV4024 12AT7
    V7/V8: Groove Tubes #5
    V9: unknown/unmarked 5U4 rectifier

    I'm open to suggestions...
     
  12. StratTone

    StratTone Member

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    The rectifier can make a nice difference in tone especially on clean amps like fenders. I really think you will have to make some changes to the circuit to get the tone like you like now. It is probably just thin because the amp is back to stock form again. You could try a GZ34 instead of the 5u4 and rebias to help get a little of that blackface tone going again.
     
  13. Stevie

    Stevie Member

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    How will putting a GZ34 in help and can it do damage to the amp? What are the differences of the two rectifiers (GZ34 vs 5U4)? I never tried that.
     
  14. SatelliteAmps

    SatelliteAmps Member

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    The GZ34 is a more efficient rectifier, tends to run the voltages slightly higher. Will make the amp have tighter low end, and bit more highs, which doesn't sound like what you are after.
     
  15. Stevie

    Stevie Member

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    I, once again, want to thank everybody who responded back. The SFVR sounds wonderful once again. I swapped the reissue Jensen P10R speakers for an Eminence Rajun Cajun and a Copperhead. Sweet sounds coming out of the baby now! Nice tight lows, chimey mids, smooth highs.

    Thanks Soldersucker for pointing the speakers out.

    Funny though, the Jensen P10R speakers sounded OK when the caps were...shot...but sounded horrible once the amp was recapped. The only drawback is now the amp weighs a heck of a lot more but it's worth it's weight, I assure you!

    Merry Christmas, everyone!
     
    Last edited: Dec 23, 2009
  16. StratTone

    StratTone Member

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    Good deal man! Glad you got it like you want it again. :beer
     
  17. go7

    go7 Member

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    Don`t you just love a happy endings. TGP rescues another amp...
     

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