Advice on this old brown super

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Mar 13, 2006.


  1. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    So yesterday I popped for this old '62 6G4-A Super amp. I'm selling my Twin to finance it...

    anyhow, it sounded great in the store, love especially the phasey three-tube vibrato.

    Upon getting it home, however, the vibrato started "ticking" when i got the speed above about 3. Didn't do this in the store. I'll probably take it back and see if they will pay to have someone repair it for me (but I hope they don't send me to some amp butcher...).

    The amp is amazing though, sounds really killer. I think it just needs a little touching up.

    Upon opening it up, there are some stickers in it saying it was serviced in 1990, probably a filter cap job. They are still holding strong as the amp is dead quiet. NONE of the electrolytics on the board (are they called cathode bypass caps?) have been changed though, they are all the old Astron 2-in-1 types. Furthermore, this amp must be pre-blue mallorys, as it has all those yellow Astron caps too... see pics.

    I swapped all three vibrato tubes and it's still making the low-end thumpy motorboating type noise when the vibrato is on. Is it probably those old crusty caps?

    Also there's some weirdness going on around the rectifier tube... looks like there are about 4 diodes strung across it and a bunch of resistors.... see pics. Does this mean the tube rectifier is bypassed and is doing nothing but loading down the filament current from the PT?

    FINALLY, the least important thing... this isn't really technical but it looks as if someone has painted the cab at some point, maybe black or grey, then blonde, then someone did a fair job of cleaning all the paint off but there's still a lot of it there. I think cleaning the rest off would be a fun project, any ideas? I'd leave it as-is before getting it re-covered, but I'd like to clean some of it off if possible! Any thoughts? Some nice cigarette burns too... this thing is trashed but man does it sound good!

    Thanks for any tips.
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  2. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    I'll take a shot at it...

    Caps on the board -- at least one has had one section disconnected and replaced (silver can next to the brown Astron).

    Weird rectification -- can't quite tell from your picture what the diodes are connected to. Strong possibility that the tube is there for decoration these days. For fun you can pull the rectifier tube and see if the amp still operates. The large metal film resistors are probably to bring B+ back down into spec with the solid-state rectification. Probably used pairs to handle the wattage, though not sure why they ran two pairs from the AC side.

    Tremolo - all kinds of odd things can happen with this trem circuit. The typical Fender ticking that you hear about in the LDR-based trems doesn't really happen here (because there's no neon bulb firing to make the ticking pulse). I'd go through the oscillator portion of the circuit and make sure all components are within spec. Drift & funkiness in the three phase shifting ceramic caps is pretty common. Looks like the low-freq side of the input freq divider has had some caps changed, so there's another place to inspect. At some point someone has changed out the 8.2M resistor between the oscillator cathode follower and the intensity control, so it'd be interesting to know what value of component was installed.

    Oh yeah... how's the humidity at home? The inside of the chassis looks a bit crusty in spots -- there's always the possibility that the move from a dry store to a humid home has caused the tag board to soak up some moisture and create cross-talk paths. Also, and this is a long shot, if there's crap on/in the trem footswitch jack it can disturb the oscillator.
     
  3. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    THanks wakarusa. I'll inspect it. I do know that the store was humidified (I saw the humidifier going) and I keep my humidity at 50% at home (I also have a humidifier) which seems pretty standard. I'll open her up again and check the component values.
     
  4. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    haven't opened her up yet, but putting a high-quality replacement phase inverter tube has lessened the problem somewhat. There was a piece of Sovtek trash in there... Also replaced V5 with a UOS rca 12AT7 (80s, nothing special) and it improved the range and sound of the tremolo greatly.

    Also, suspicions confirmed: Rectifier tube is just decoration! :)

    Need to un-do that mod in my opinion.
     
  5. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    Interesting... V5 is supposed to be a 12AX7, though (as you've seen) a 12AT7 or similar will work. FWIW, V3, V4, and V5 make up the trem circuit with V3 serving as the oscillator, 1/2 V4 as a split-load phase inverter (the other half is unused), and V5 as a gain/mix stage to put the low and hi frequency trem signals back together and a final boost before the PI.

    I'd go back to the tube rectifier too, but more out of nostalgia than anything else. Brown Supers are one of my all-time favorite amplifiers (I also have a '62).
     
  6. WailinGuy

    WailinGuy Member

    Messages:
    1,268
    Joined:
    Aug 30, 2003
    Location:
    Silver Spring
    If you're concerned about reliability and gig-worthiness (as opposed to owning a "museum piece"), I would highly recomment replacing all of the electrolytic caps on the circuit board, along with all of the yellow Astron coupling caps.

    It is common for electrolytics that old (around 45 years old!) to be either completely dead or getting very close to failure.

    Those old yellow Astrons are foil and wax paper dielectric caps; they are very prone to leaking DC voltage, especially in humid conditions. (The wax paper absorbs moisture and loses its insulating properties.) I don't know if any of those are contributing to the tremelo ticking issue, but keeping them in there is asking for trouble in the long run. Leaky coupling caps can create noise problems as well as wreaking havoc with the intended bias points of the tubes, usually causing bad tone or reduced gain or both. I predict the amp will perform and sound better after the Astrons are replaced.
     
  7. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    :jo

    Sorry. I put a UOS 12AX7 (not AT7) in v5.

    For what it's worth, I agree with you. This amp is awesome (put an old Silvertone 12AX7 in v2, a little microphonic but not enough to be noisy (just enough to sound sweeeet) and a better phase inverter and it's killing! I'm thinking that it beats my 65 deluxe hands down (which sucks because it's 20-20% heavier!)

    The low-end whump-whump only happens when the intensity is at max and the speed is between 3 and 7. It's been greatly reduced with the tube swaps. Back the intensity off a bit or put the speed out of that range and it goes away completely. Could this be in the realm of "normal?" Turning the bass tone control down won't dial it out (doesn't affect it at all).
     
  8. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    Will anyone second this? The thing sounds really good to me right now so i would be biting me nails if I were to do a wholesale component replacement like that. THe electrolytics I can see for sure but will anyone second Jim's opinion on the Astrons?

    What would you recommend replacing them with, orange drops?

    For the record the amp is dead quiet right now, no unsavory noises other than tremolo ticking. Turned all the way up with no guitar in both channels are dead quiet.
     
  9. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    I'd really have to hear it to know "normal". At extreme settings the tremolo in these is very strong and phasey. Thinking maybe the tremolo phase inverter is clipping at high intensity settings to give you the "whump whump", but, again, would really need the amp on my bench to figure it all out.

    Of course there's always the doctor's advice:
    Patient: "Doc, it hurts when I..."
    Doctor: "So don't do that"

    Also just noticed the leather handle in the pictures. If you go to the trouble to clean up the exterior any more, these originally had the brown rubber "dog bone" handle.
     
  10. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    thanks "doc."

    This thing is ragged out on the exterior. There are a few extra holes where it looks like various and sundry different handles have been on this amp :) Also plenty of cigarette burns and evidence of two or three different colors of paint.

    What's your opinion on the yellow Astrons?
     
  11. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    I'll agree with Jim's description of how wax paper/foil caps behave. Whether to do wholesale replacement or "replace 'em as they fail" is a more personal choice. If you intend to gig the amp on the road and don't carry a tech with you, reliability becomes more important than nostalgia and originality. If it's to live in your basement (or go out for the occasional gig) and you don't mind the occasional visit to your tech (or your bench) for check-ups AND the amp is behaving itself, then there's no rush.
     
  12. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    Yeah I am a full-time pro player but this amp will probably live at home and go to the studio mostly. The light weight of the nonreverb deluxe (33 lbs) is way too enticing for NYC gigging. This one weighs in at about 45 therefore I'm betting the deluxe gets the nod 99% of the time! P.S. the amps volume control does not affect the volume of the ticking in any way.
     
  13. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    Another fine amplifier. I've got a '64 that sounds better than any Deluxe Reverb I've ever heard :)

    Wouldn't expect it to. My money says your trem issue is either "normal" or something going on in the oscillator, the trem phase inverter, or an interaction between the two.

    Just so we're clear... that circuit has two phase inverters. A long-tailed pair that feeds the power section and a cathodyne ("split-load") that applies the oscillator output to the musical signal. We're talking about the latter here.
     
  14. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    cool. Thanks for all your help. Yeah now that I have an old reverb unit I don't need the twin anymore (it's a 68) so i'm selling it. Between this super and the deluxe I've got it pretty covered I think, maybe i'm old-fashioned but I love old fender amps, all the channel switching hi gain stuff is not for me.

    I might give my tech a call and have him give this amp a once-over with me. Thanks again.
     
  15. mr coffee

    mr coffee Gold Supporting Member

    Messages:
    1,514
    Joined:
    May 3, 2004
    Location:
    uranus major
    with those big ass resistors also on the socket .. are they running to ground? [​IMG]

    Is that an original PT or someones attempt a dropping some hivoltage?

    yellow astrons have been around since the tweed days..
     
  16. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    Those resistors indeed go to ground. PT looks original.
     
  17. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    Took this amp apart tonight to work on getting some paint off the cab (goof off rules!)

    Now I may just by waaaay ignorant here but.... how does one adjust the bias in this amp? I see no bias pot at all! (???)
     
  18. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Newnan, GA
    A 6G4-A Super doesn't have an adjustable bias, though it isn't too hard to install one.

    I tend use a rig that replaces the bias circuit tag card with a 6 lug terminal strip loaded with the new circuit. Just works out that the spacing for the screw holes for the terminal strip and the original card are close enough to work

    You may have to adjust the value of the 43K resistor to suit your amp (variations in the bias supply tap voltage dontcha' know). The 25K thing is a small timmer potentiometer with the wiper tied to one end. Anything that you can mount on a terminal strip will do. You can also use larger value caps (I tend to use 22uF), but if they get too large then the circuit takes a while to charge up -- problem being that the slower the charge, the longer before C- settles out and, if you don't let the amp sit in standby for a bit when you turn it on, the longer the power tubes sit with no bias. Here's the setup:

    [​IMG]


    Edit: Almost forgot -- resistors are 1 watt or better and don't forget that the electrolytic caps are installed with the '+' marked end to ground, since, in this case, ground is actually the more positive point.
     
  19. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    Interesting. Thanks Todd. I think soon I'm going to take this thing in, add the adjustable fixed bias, get it checked out for leaky Astrons, replace the electrolytics on the board, get the rectifier put back to tube, and maybe even a filter cap job. Not sure but there is a sticker inside saying it was serviced in 1990 which would be 16 years ago which would mean if that's when it was done last they're about due anyhow... might as well while I'm in there.

    I think that amount of work is a little bit beyond my scope at this point. I want it done right.

    Thanks for your help.
     
  20. brad347

    brad347 Member

    Messages:
    4,798
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY

Share This Page