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Silverface Vibro Champ - Sounds like Do Do!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by drano, May 10, 2005.

  1. drano

    drano Member

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    Bought this silverface Vibro Champ about 1988. Don't have any manufacturing date info for it. The cabinet sides are plywood, not particle board. Tolex is perfect. I'm guessing that the tubes are all original. I never played it much. Don't know if it was played much before that but can't imagine anybody putting a lot of time on one. The speaker has a silver-colored basket with one of those open-sided press-braked frames around the little magnet. On the back of the speaker is ink-stamped the Fender logo with SPECIAL DESIGN underneath that logo. The amp has always sounded like crap. Sounds best with the bass and treble knobs all the way to 10. Tremelo works fine.

    I've always assumed the tone problem was due to the speaker being a cheap POS. Now I am trying to learn how amps work and curious if this thing has any more in common with its big brothers than its brand name. I'm going to try new tubes just to see how much difference that makes. I assume V1 is the the audio channel and V2 runs the tremelo? If so, no point in bothering with V2? And will the rectifier tube have an effect on tone?

    Even if all four tubes do effect tone and I change them all out with new good tubes, if I still get lousy sound and I want to try another speaker, is it 8 Ohms? And are there really any good possibilities out there for a kick-ass 8" guitar speaker?

    Am I wasting my time here? Did the CBS engineers cheapen this circuit down beyond repair with good tubes and a good speaker?

    Thanks
     
  2. John Phillips

    John Phillips Member

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    The amp is biased wrong.

    That may sound surprising, since it's a cathode-bias amp and from what you're saying is probably totally stock, but it is true. Fender built them like that.

    (What probably happened was that in about 1965 they raised the internal voltages to get more power - 6W not 4 - but for some reason forgot to alter the value of the cathode resistor. The result is that the 6V6 runs far too hot, which not only shortens its life but makes it clip far too soon at the forward end of the waveform, resulting in low clean headroom and a mushy, farty distortion. I don't know why no-one noticed, but they built them like that for over 15 years...)

    In order to make it run correctly, you need to replace the 470-ohm cathode resistor with a 1K (preferably 5W wirewound instead of the old 2W carbon), and replace the cathode cap with a 50V-rated type since the bias voltage goes up to around 30V; possibly 50uF not 25 if you want the same bass response. In fact, if they haven't already been done, it may be a good idea to replace all the electrolytic caps in the amp, since they're well over 20 years old now - if it has ply sides it was made no later than the middle 70s (you can date it accurately from the pot and transformer codes usually). It's also a very good idea to fit a screen-resistor, which they left out too - 1K 2W wirewound is ideal.

    You're correct that the second 12AX7 just runs the tremolo.

    The rectifier tube should have very little effect on the tone since in a single-ended amp, the current draw is constant (especially if biased correctly!) and doesn't cause the usual 'sag'.

    The stock speaker isn't all that great... its impedance is 3.2-ohm or 4-ohm. Don't use an 8 - you'll get less power and might even blow the OT (which seems a little more sensitive than on some amps). Weber do make some very nice specialist Champ speakers that you may want to try... I actually got very nice results with a cheapo Peavey Blue Marvel in one once too :).

    Set up right, these are great little amps and surprisingly full-sounding.
     
  3. trisonic

    trisonic Member

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    John's right, he gave me the same ideas and when I eventually had my BF Vibro Champ (same circuit btw) done the difference was incredible. I had a rotten old Utah Speaker in mine as well and swapped it out for an Eminence.

    Jimmy Sommatone did the work on mine (he also did a switchable Tweed thing bypassing the tone stack which increased gain but that's another story and you should hear yours when it is set up perfectly before doing anything else).

    Spend a bit and you'll have an amp that sounds like a $1500 amp.

    Best, Pete.
     
  4. RL in Fla

    RL in Fla Member

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    Tubes make a huge difference also , a different flavored 6V6 or V1 may amaze you . The 5Y3's don't matter (what John said :AOK )
     
  5. TheAmpNerd

    TheAmpNerd Member

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    Wow,

    It has been a long time. The SF VC was my second amp.
    My first one to extensivly mod also. I did the Gerald Weber
    book mods to the amp and still wasn't satisfied.

    I ended up building a pyrmid of caps because I couldn't
    find multisections, and played around with my own
    self styled tweaks.

    I still didn't like the tone/break up and started replacing the cathode resistors and playing...finally settling on a
    950 ohm./5 watt. I did all this by ear (the hard way)
    Well John, now I know why after reading this.

    After the output tranny fried I bought one of the
    Hammond Multitaps that works fine.

    Weber makes a killer Alnico for this amp. : )

    Once they are dialed in, they sound great.
     
  6. rooster

    rooster Member

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    So, with the 1K cathode resistor, what kind of power dissipation is that 6V6 running at? I couldn't imagine more than 7 or 8 watts. Is this a SE amp running at 50-60% idle?

    rooster.
     
  7. Jeff Flowerday

    Jeff Flowerday Member

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    Class A, SE and cathode biased it was probably running over 100% maybe even 110% at idle.

    I'm not sure where the 1K will take it, but John would know, maybe he'll reply.


    I know my 5F1 champ is running smokin' hot but sounds stellar, I have a JJ 6V6 in mine because it can take it.


    Jeff
     
  8. theelectic

    theelectic Guest

    If you increase the cathode resistor, you're lowering the idle current. FYI my SF Champ with the stock 470ohm cathode resistor ran a Sovtek 6V6 (the old, all black inside type) at 22W idle!!! No wonder it ate those tubes for breakfast! Only the hardiest NOS tubes would last more than a couple of months. What I did was to put a 50W, 50V zener diode on the center tap of the PT to lower the plate voltage into the right range (330V). Then with the stock 470ohm cathode resistor, it brought the idle down into a safe range - 13W idle, which a JJ 6V6S handles with aplomb.
     
  9. Reeek

    Reeek Member

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    Hmmm, this all very interesting to me. I have an SF VC that is all but mint. I too installed a Weber alnico 3.2 Ohm speaker. I still have the original Utah in a box though. I haven't been inside the chassis to date it yet (probably 1972-1974 by the Fender logo), but it sounds pretty good to me and I'm also thinking that these mods have not been done to it either so now I'm jonsing to try them. I actually thought about selling it but not sure now.

    I already have world class vintage tubes in it. If anything, it breaks up a tad too soon like was described by John and others above.

    I'm perfectly capable of performing these mods as long as I'm told what to do like this ;)

    BTW to anyone interested, I have a brand new (never used anyway) Weber alnico 8A125 4 Ohm that I would let go for dirt cheap if anyone is interested. It just lays around doing nothing. PM me if interested. I live in Reno, Nevada and would ship it in a real speaker box. What happened is I ordered it for one of my old vintage Custom Krafts and wasn't paying attention to the order form. It was supposed to be 8 Ohm for that amp. My bad :rolleyes:
     
  10. rooster

    rooster Member

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    That's pretty wild. I have a JJ6V6S running SE at 420VDC at the plate, biased with a 360 ohm cathode resistor, and it's running at about 32mA, giving me a little under 14W plate dissipation, which should be right about the center. Breaks up pretty early, but that may be my driver; have to check it out. That's why I was wonderinng about a 1K cathode resistor.

    rooster.
     
  11. RL in Fla

    RL in Fla Member

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    It pays to check em , too . On one I had , the 470's had both drifted (b+ resistor and bias) , one up to around 700 or so , the other down to somewhere in the low 300's .
     
  12. Jeff Flowerday

    Jeff Flowerday Member

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    Your numbers seem a little odd to me. With that plate voltage and that small of a cathode resister, I would think it should be way hotter.
     
  13. John Phillips

    John Phillips Member

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    What you've done there is basically to reverse Fender's "power upgrade" by dropping the B+ down to Tweed level again - so it's not surprising the 470-ohm resistor then gives the right bias, since it did in the Tweeds :).

    I still find it hard to believe that Fender apparently made this mistake, but the evidence is right there...

    You will of course also lower the output power to Tweed level, but there isn't that much audible difference between 3-1/2W and 6W when the 6W is badly asymetrically distorted!
    I'm beginning to suspect something here... the JJ 6V6 is often described as quite 6L6-sounding and very sturdy, for a 6V6. Groove Tubes did use and label a low-quality 6L6 as a 'heavy duty 6V6' at one time, I wonder if JJ are doing something similar? That might explain the colder running too...

    If it breaks up a little too early, it may actually be clipping too soon at the cut-off end, not the forward end, and might need to run a bit hotter.

    The way I found the correct resistor value was basically in the same way as TheAmpNerd, BTW - I simply fitted a test high-power pot (1K-ohm 3W I think) in series with the stock cathode resistor, and dialed it until I got the most clean headroom and most 'balanced' breakup, then measured the total value. You could also use a scope and look for a centered waveform if you have one.

    Make absolutely certain that your cathode-bypass cap is replaced with a 50V-rated one BTW! The cathode voltage will rise to well over 30V with a 1K or more resistor, and the stock cap is only 25V. I didn't notice when I was testing different bias points, and the first thing I did was blow the cap :eek::).
     
  14. theelectic

    theelectic Guest

    Heh heh that was exactly my intent, and the zener worked like a charm :)

    Funny thing is, when I bought the amp (used from a local guy) it came loaded with a metal can 6V6, which worked fine with the stock B+ (around 390V). The metal can tube pulled hardly any current (around 11mA I think) which allowed it to survive, albeit with crappy tone. Thinking it needed the proper tube, I put in a new Sovtek 6V6 (black one) and POOF! it burned up after a couple of minutes. Even the supposedly better 6V6EH wouldn't last too long until I dropped the voltages.

    BTW I suppose it's possible/likely that the JJ 6L6GC and 6V6S share some things in common. Check out the 6L6GC:

    http://store1.yimg.com/I/thetubestore_1858_15381271

    And now the 6V6S (which is spec'd by JJ to 500V max plate):

    http://store1.yimg.com/I/thetubestore_1858_15549271

    To compare, the ElectroHarmonix 6V6EH and its much smaller plates:

    http://store1.yimg.com/I/thetubestore_1859_4059575
     
  15. TheAmpNerd

    TheAmpNerd Member

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    PM SENT
     
  16. TheAmpNerd

    TheAmpNerd Member

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    They are sturdy. I just put some in
    my 50 watt Master Model Type prototype and they
    perform very well after rebias from EL34s. They are
    very similar and give a bit earlier break up, if that
    is what you are looking for.

    Some folks have described the JJ6L6 as a very similar
    to an EL34 in tone.
    John, I wasn't that smart when I first
    started out doing this and did it all the old fashioned
    way...stick one in... solder...test...desolder...stick in the
    next one....solder...test.... Then I learned about the bread boards:)...then learned about pots.:AOK
    What is also illustrated are the different approaches to solving the same problem.

    Wondering, on a scope, how does one identify what a
    balanced break up looks like?

    How does this differ from early break up & late break up?


    Also thinking, you know how the MFG process goes,
    if it ain't broke...don't fix it, or something like that.
    If you look at everything that was happening at that
    time and all the changes, new amps, etc, probably just
    simpler to have one less tranny in inventory.

    AND

    It still worked, so why fix it!

    Like anything, with 20 - 20 hind sight. I think
    everyone snubbed that kid at the cp/m user
    conference...Why would anyone want something
    named dos? We have cp/m.
     
  17. John Phillips

    John Phillips Member

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    Simple - correctly balanced break-up is when the waveform clips at exactly the same time at both cutoff and forward, producing a symetrical distorted waveform. That's also the 'latest' possible breakup since it's the maximum possible clean voltage swing before either side clips.

    On a scope, think of it as the waveform being between a 'floor' (cutoff) and a 'ceiling' (forward clipping, limited by available B+). If the tube is running too hot, the whole waveform is shifted 'upwards' - the amplitude remains the same - so the forward end hits the ceiling and is clipped first, which reduces the clean headroom. If it's running too cold it's the other end which is clipped first - exactly as in half a push-pull Class AB amp, except without the other tube to cover that part of the wave cycle.
     
  18. trisonic

    trisonic Member

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    Eh?

    Best, Pete

    Can you download a pic of the waveform???
     
  19. TheAmpNerd

    TheAmpNerd Member

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    Up late on your side of the pond.
    I think I got it. I'll have to have a look see.

    Have another question regarding hot power tranny.
    Too hot to touch. However it looks like the rest of the
    amp is still with in spec. Anything one can do to
    keep it cool, besides a fan? Perhaps I'll start another
    thread and ask.
     
  20. Jeff Flowerday

    Jeff Flowerday Member

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    Just to throw out my 5F1 JJ 6V6 numbers.

    Plate Voltage = 365VDC
    Current = 48.5 mA
    470 ohm resister

    Works out to a wopping 17 Watts dissipation. Take into consideration the JJ6V6S is suppose to be a 14 Watt tube, most think it's closer to 15 Watt, I'm in the 110%-120% range.

    The JJ is running fine no glowing nothing, she's a very sturdy loud tube.

    I wish I had a scope so I could take a look at it, I'm forced to go by ear. It sounds amazing right now the best it's ever sounded, I had it biased colder for a while there.

    I also mounted it in a Weber 5F1A cab with a Weber 10A125, and she's a little monster tonally now.

    I think rooster has a real cold 6V6 in comparison to mine.



    Jeff
     

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