Servicing a 70s silverface: what to do?

Discussion in 'Amp Central Station' started by AJBaker, May 21, 2019.

  1. AJBaker

    AJBaker Friend of Leo's

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    I've had a 74 Vibrolux for a while and I want it to perform at its best.
    When I first got it, it already had a three prong cable, but seemed otherwise mostly untouched. I changed out the filter caps that were blistering as a standard thing to do to old amps, and I made a minor (and reversible) 'blackface' mod by lifting the small treble caps on the 6L6 output tubes.

    What else should I do to get everything I can out of this amp while keeping its basic character? I'm thinking (in descending order) of some of the following ideas :

    - replacing the electrolytic cathode bypass caps
    - add a bias trim pot (see robrob's page)
    - add safety diodes to the rectifier tube (also robrob)
    - Replace the old and possibly tired GE 6L6 tubes (for example with russian 6p3se tubes because they're cheap...)
    - use the ground switch to mod the negative feedback (I'll let you guess where I got the idea...:)
    - changing the brown/navy 'turd' capacitors (some say they're not great, but I'm not too keen on changing them just for the sake of changing them)

    Any ideas on what would be worthwhile?

    Thanks!
     
  2. Nickfl

    Nickfl Tele-Afflicted

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    - replacing the electrolytic cathode bypass caps

    Yes, definitely. If the filter caps were due to be changed these are too. If they are original I'd be shocked if one isn't bad enough to be effecting the way the amp sounds.

    - add a bias trim pot (see robrob's page)
    Yes, thats a good mod, definitely useful on any fixed bias amp. I assume yours has the bias balance control in it? My SF vibrolux had the balance pot converted to a bias adjustment pot, I'm pretty sure Rob has a wiring diagram for doing both in the same amp.

    - add safety diodes to the rectifier tube (also robrob)
    Yes, easy, cheap insurance.

    - Replace the old and possibly tired GE 6L6 tubes (for example with russian 6p3se tubes because they're cheap...)

    Maybe. I'd do the bias mod and set your bias first and see how the GE tubes sound then.

    - use the ground switch to mod the negative feedback (I'll let you guess where I got the idea...:)

    Up to you, but you are getting into true "modded" territory here. As long as you aren't drilling any holes its reversible and shouldn't kill any vintage value the amp has.

    - changing the brown/navy 'turd' capacitors (some say they're not great, but I'm not too keen on changing them just for the sake of changing them)

    No. There is nothing wrong with those caps (unless there electrically is...) and they shouldn't be replaced as a generic tone "improvement" IMO. Any time I see an old amp full of orange drops I think to myself, "I wonder what else the monkey who did that screwed up in here".
     
    Last edited: May 21, 2019
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  3. Milspec

    Milspec Friend of Leo's

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    Changing the electrolytic caps....absolutely, just part of servicing IMHO
    Add Bias Trim pot....absolutely yes, that has made huge benefits to my Silverface amps as you can put it back to Blackface bias specs
    Add Safety Diode....Sounds like an easy thing to do so why not? I haven't done that, but no reason not to really.
    Change power tubes....I wouldn't bother. Play what you have until one goes bad and then experiment
    Ground Switch....beyond my pay grade and experience, so I don't know
    Turd caps....I would leave them there, it is more internet lore than reality
     
  4. zombiwoof

    zombiwoof Tele-Afflicted

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    I would check those GE power tubes on a tube tester, they may be fine. They are great tubes, better IMO than any new production, so I would get as much use out of them as possible. My tube tester has a "life" mode, it gives an indication of the remaining life in tubes, I find it invaluable when buying "pull" tubes from Ebay and such. It's quite possible those GE's have plenty of life left.
    Al
     
  5. Mike Simpson

    Mike Simpson Doctor of Teleocity Silver Supporter

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    Post a picture of the circuit board and the power supply caps under the pan.
    Don't change the coupling caps (brown / navy) unless they are bad.
     
  6. E5RSY

    E5RSY Poster Extraordinaire

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    When I first had my '74 Super Reverb serviced a few years back all the input jacks had to be replaced because they were "sprung". Might check those, too.
     
  7. jrblue

    jrblue Tele-Afflicted

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    I sure wouldn't replace GE tubes with crap Russian tubes -- a major backward step -- unless the current tubes meter bad. Swapping out the "turd" caps is an excessive and devaluing mod unless they are out of spec. Lifting the treble caps is a good and easily reversible mod. The other stuff sounds fine to me though I'm another one who is not familiar with the ground switch/neg. feedback mod or whatever advantage it is supposed to offer. My personal approach is to either leave an amp stock except for reversible mods and retubing, or to go big and do a mod/rebuild job. I dislike half-modded amps. Who knows if the modder had a clue?
     
  8. AJBaker

    AJBaker Friend of Leo's

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    My apologies, here are some pictures (before I changed any of the caps)
     

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  9. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    Replace the white Mallory bypass and bias caps. The blue and brown caps in the amp are fine. I'd leave them alone. As for the power tubes, if they bias closely to each other use them. If they are have drifted it is time to replace them. I know others will disagree but I'm not a fan of bias balancing and prefer matched tubes.
     
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  10. Doctorx33

    Doctorx33 Tele-Afflicted

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    Yeah. I did that years ago to my silver face bassman heads. I was young and dumb.

    Slightly less dumb presently.
     
  11. Mexitele Blues

    Mexitele Blues Tele-Meister

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    Consider retensioning the tube sockets. Mine were quite loose by the time I got around to servicing.

    Did you replace the bias supply caps?
     
  12. Dacious

    Dacious Poster Extraordinaire

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    Jack plug and tube socket retensioning (make sure caps are discharged) is a simple task - just make sure there's no arcing on the tube sockets.

    Lube of all the pots.
     
  13. mgreene

    mgreene Tele-Holic

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    Dont mess with the tubes unless they sound bad - like someone said above, new production tubes will likely be a step backwards. If you know someone with a tube tester, you can see how much life they likely have left - although preamp tubes tend to go bad (for whatever reason in that particular tube) pretty quickly.

    That gold box cap by the roach is certainly after market - it looks like an ELNA (euro brand). Would be interesting to know what it is doing - I assume stopping "ticking" from the vibrato circuit.
     
  14. theprofessor

    theprofessor Friend of Leo's

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    This is all we can really hope for, right?
     
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  15. AJBaker

    AJBaker Friend of Leo's

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    Thanks for the input!
    I'll look into replacing those electrolytics. Looks like I need a few 22uF/25V caps and one 47uF/50V.

    I hadn't noticed that gold cap, interesting...
     
  16. schmee

    schmee Poster Extraordinaire

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    - replacing the electrolytic cathode bypass caps Yes
    - add a bias trim pot (see robrob's page) Just make it adjustable fixed bias ie: Blackface. There ae a couple resistor changes that go along with this.
    - add safety diodes to the rectifier tube (also robrob) why?
    - Replace the old and possibly tired GE 6L6 tubes (for example with russian 6p3se tubes because they're cheap...) If they are fine leave them.
    - use the ground switch to mod the negative feedback (I'll let you guess where I got the idea...:) You can, but I have never found that mod of much use really. You might find adding a mid control there more useful.
    - changing the brown/navy 'turd' capacitors (some say they're not great, but I'm not too keen on changing them just for the sake of changing them) How does it sound? if good leave it.

    -Change out the cap on the little bias board, go a little higher voltage.
     
  17. AJBaker

    AJBaker Friend of Leo's

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    Here are some fresh photos of the inside.
    Looks like I've got two 80uF, one 5uF and five 25uF to replace before I do anything else.

    I wonder why 80uF caps are in there when the schematic calls for 50uF? I'd be fine going down to 50uF, right?

    Is it normal that three leads from the output transformer aren't in use?
     

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    Last edited: May 23, 2019
  18. AJBaker

    AJBaker Friend of Leo's

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    Looks like some resistors were also replaced at one point.

    Strange that those things were done, but no-one thought of changing the very obviously bulging filter caps..
     
  19. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    I'm sort of confused.
    I thought the SF Vibrolux used 16uF caps for filter caps. I don't see 50uF on the AA279 circuit. Unless you mean for the bias circuit I reckon -
    and the Power Transformer, not Output Transformer is the one with three wires taped off and heat shrink added. My best guess is that it's a transformer used for different countries - having the capability to accommodate 50hz, 60hz - 100v, 120v, 220v, 240v etc.
     
  20. AJBaker

    AJBaker Friend of Leo's

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    There are two caps going to the bias pot marked 80uF 75V.
     
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