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Help with High Voltage in Blackface Vibro Champ

Discussion in 'Amp Central Station' started by Josey, Jun 27, 2018.

  1. Josey

    Josey Tele-Meister

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    Taking the advice of some other members I've decided to start a new thread rather than reviving the one I originally started that's nearly two years old now. So as I was saying...

    I really, really appreciate the help and knowledge you guys freely share here. This is definitely my go to site when it comes to research on vintage amp repair and service. You guys helped me bring this little Vibro Champ back to life and I've enjoyed every minute of it. I've performed the basic maintenance such as the recap job including the filter cap can with a 20uF/20uF/20uF 450V as muchxs suggested, as well as replacing the electrolytic caps with one 10uF/25V and two 25uF/25V Sprague Atom's. While I was at it I also replaced the dropping resistors with 2 Watt Allen Bradley Carbon Comp's as well as the V1 plate resistors with 1 Watt ABCC's. Also replaced the original power chord with a 3 prong chord and did away with the dreaded death cap. I also replaced the Cathode cap and resistor with a Sprague Atom 50uf/50V cap and a 5 Watt Wire Wound 1K resistor. These changes definitely brought this little amp back to life in a way that it really doesn't seem little anymore. It has plenty of volume and headroom on tap and the tremolo is oh so sweet. It can really get slow and deep (That's what she said).:twisted:

    I'm really wanting to start recording with this little beauty but despite all the improvements it still has just the slightest of hum, and me never being one to leave well enough alone was wondering if there is something I could do to kill the hum completely. I feel 95% certain the hum has to do with the fact that the B+ voltages still read about 50 volts higher than specifications. My 6V6 is running right at 12 Watts of dissipation so I consider that acceptable but if I could drop the voltage I'm thinking the hum will go away and allow the amp to run cooler and more efficient. I've combed through the archives and found a couple of other threads that dealt with this issue and they were able to tame the hum and drop the B+ voltages by wiring one of two 5 or 10 Watt resistors inline with the First Filter Stage of the Cap Can. I've attached a link to those threads below. Seems like this could be my answer, however in both of those cases they each had the cans with the extra 20uF of filtering wired in parallel for a total of 40uF for the first stage. In my case I would only have the 20uF in the first stage. I'm sure this is a noob question but can I still apply this modification to my amp given my current level of filtering in the first stage?

    http://www.tdpri.com/threads/how-to-kill-hum-in-a-vibro-champ.358724/

    http://www.tdpri.com/threads/67-blackface-vibro-champ-restored.749372/

    Here are some pictures of what I started with and I'll try to get some updated photos of the changes I've made thus far.

     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks for referencing my thread, Josey -- that was a fun project. FWIW mine still has a tiny bit of hum, though it's inaudible once you begin to play. I wouldn't be sure that dropping B+ will clean up hum but others may know more. Dropping B+ in these guys was a fave trick for our friend MuchXS when he was on TDPRI -- but he did it by swapping in a modern PT with lower B+ and IIRC more current capacity. Be aware the resistor method (I don't *think* it depends on your filter size) will generate some heat (yawn) and add some sag (not a bad thing). Again, others may know more. Zeners get rid of the sag factor but are a little harder.

    Do you have a 20/20/20/20 cap can, and is it new? If so you can wire it like mine in the thread you reference. In actuality when you look at old VibroChamps, almost all of 'em actually paralleled the first two filters to make 'em 40-20-20, as the Bronco later showed in drawings, so my best guess is the factory quickly moved beyond the 20-20-20 wiring shown on the VC drawings. Not unusual at Fender, where the factory floor moved fast and the documentation department moved real-l-l slow. Yes, 40uF is slightly overspec for the 5Y3 tube chart, but no one has ever reported all those old VCs and Broncos eating rectifiers for lunch...

    Whew. All that said, I think you've probably got a ton of nice hum candidates in an old amp like that. Dirty input jacks. Drifted components. Funky pots. The brass plate. As discussed at length in the other thread above, a big deal is the single-strand heater dress and the lack of a heater CT, real or artificial. The list goes ever on and on...

    Final note -- I can't see your power wiring well. Do you have a 3-prong cord? How's it wired to the switch, ground, and PT? And is the death cap bypassed?
     
    Last edited: Jun 27, 2018
  3. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    Are you running a Soviet 5Y3? A NOS 5Y3 will drop the B+
     
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  4. Josey

    Josey Tele-Meister

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    Thanks for posting your restoration project King. You did a great job and it's obvious you take pride in your work. I admit I stole a couple of your ideas and applied them to my VC. Can I ask why you put the heat shrink on the lugs of the fuse and speaker jack? I agree with you. The more I've thought about it, the more I'm not so sure that dropping the B+ is going to clean up the hum I have, but I can't help to believe that it's definitely going to allow this little champ to run cooler and be more efficient on the output tube. I'd like to be able to just carry this little VC anywhere with me and not really be concerned with the modern wall voltage of the venue. Just "Plug and Play" sort of speak. I wouldn't mind the extra sag and I don't see how the inline resistor could be any hotter than the cathode resistor is now with my current wiring. So has MuchXS left TDPRI for good? That's a shame. He was a wealth of knowledge and I enjoyed his creative yet informative posts. He shared lots of tips with me when others would only want to lecture me for not being a trained professional.:lol:

    Anyways... the pictures you see are a couple of pictures I took when I first got the amp. Everything there is original and untouched. As you can see it had the original 20/20/20/20 cap-can but in my case they did not parallel the first 20uF's together to get 40uF like I've seen in others. I guess it just depended on who was doing the wiring that day in the factory since there doesn't seem to be any rhyme or reason for when they did and did not increase the filtering in that first stage. However, as I mentioned I've since replaced the cap-can but I went back with just a 20/20/20 at the recommendation of MuchXS himself. So I can't wire it just exactly the way you did with yours. My question is... could I wire it similar to yours but without the extra 20uF of filtering in the first stage, or would I need to change my cap-can again or add another cap to make sure I have that 40uF of filtering?

    I agree it could be another factor causing the hum but adding the resistor to drop the B+ seemed like a good stab to maybe kill two birds with one stone. I've cleaned the jacks, pots, and brass ground plate to the best of my ability. So if one of those is the culprit then it just is what it is and I'll just have to live with it. Yes it could also be the heater dress but I'm not going to fool with that and try to add a artificial center tap or anything there either. I have also added a 3-prong cord and removed the death cap as well. Like I said I'll try to get some more modern photos that show the changes I've made thus far and see if there's anything anyone might notice that I could improve upon. I also have some questions about some of the drifted components and voltage readings I'm getting in certain areas of the circuit. I'm not exactly sure as to what is acceptable and what may be indicative of a leaking cap or out of spec resistor that I might need to replace. I'll try to take some measurements as soon as possible and put it out there for everyone's feedback.
     
  5. Josey

    Josey Tele-Meister

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    Viejo I currently have a NOS RCA 5Y3GT in place and yes it did drop the B+ a bit compared to a Sovtek I tried but still I'm no where near the voltage shown shown on the AA764 schematic.
     
  6. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    50 volts isn't necessarily wrong. In fact, it's probably correct with your rectifier.

    Compare your wall supply voltage to the 112VAC Fender used for their calculations; then allow the 20% variance Fender notes in the schematic AND the 10% parts value variance.

    50 VDC should be well within all those tolerances.

    The one usually missed is the difference in wall supply voltage. Locally I haven't found anything under 120VAC with most at 121 VAC. 121 vs 112 is a 7.5% variance right out of the chute. And just for s***s and giggles - 405 VDC vs 355 VDC (355 plus 50 =405) is just 12.5% higher than what's on the schematic *based on modern wall supply voltage*. Well within 20%

    Unless I missed something, IMO you're looking from a problem where one simply doesn't exist. Buut you can still pring down the voltages if you want - that's all part of tweaking/voicing the amp.

    Hope than helps.
     
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  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    High B+ in 60s-70s Champs is a well-known thing, a given. MuchXS often talked about how Leo loved to punish the 6V6, and in no case more than BF/SF Champs. In this typical post he says,

    The supply voltage in '60s and '70s Champs is too high, usually in excess of 420v....As I've mentioned about 11,000 times, 420v is about 100v more than I've seen on any 6V6 tube data sheet....
    Likewise many Fender schematic voltages are lower than the actual amps probably achieved in reality, even on 110 VAC as noted above. It seems the BF/SF champ is just an extra-hot example. Silverface is right -- if you're not eating tubes, it isn't really a problem.

    IMHO one reason to drop the B+ besides tube longevity (many folks get away with NOS 6V6s, which even Much admitted way out-punch their rated tolerances) would be to lower headroom -- ironic on a VC, maybe, but I use mine as my 'quietest' practice amp and love the way it gets to the edge of dirt about 4 on the dial and stays there until about 7, when it starts to get *real* attitude.
     
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  8. tweedman2001

    tweedman2001 Tele-Afflicted

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    Bear in mind that a single ended amp can have more (normal) hum than a push-pull design. Without knowing how much is "it still has just the slightest of hum" It may be normal and very difficult to eliminate. Best of luck.
     
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  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Sounds like the amp is working fine, although we can't hear how much hum you're hearing. Without getting a new PT and changing the heater wiring, that may be just how it is.

    In my experience, Champs biased to 12W sound okay, but anything from 12-18W is normal for this circuit as long as your particular amp isn't eating/redplating tubes.
     
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  10. Josey

    Josey Tele-Meister

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    Silverface I agree with everything you’ve said. I have other vintage Fender amps that I power with the use of an AmpRX Brown Box which allows me to bring the wall supply voltage, which is normally 121VAC around here, down to as low as 108 if so desired. I usually run those amps right at 111VAC and it definitely gives them a response totally different from 120. The Vibro Champ is no exception and simply sounds better when ran at a lower AC voltage.

    However I was hoping I could tweek this Champ a bit so that I could leave the Brown Box at home but get the same response from it when plugging into the normal +/- 121VAC wall voltages around here. Well that as well as possibly killing the hum. I’m thinking I’ll probably have to re-bias with a different Cathode resistor value if I do install a 5 or 10 Watt resistor in line to drop the B+ value, but my question is whether or not that’s ideal with my 1st stage of filtering which is currently 20uF or would I need to bump it up to 40uF? I imagine just bumping up to the 40uF of filtering in the first stage alone might help with the hum but is it necessarily a must before dropping the B+ values with some value of a large resistor? I hope I’m not making things more confusing.:confused:
     
  11. Dacious

    Dacious Poster Extraordinaire

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    If you still have half the heater wiring running through the chassis it won't help although my drip edge 68 was nigh on silent.
     
  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    The fuse, don't tell anybody, I sometimes work on a live amp (chopsticking, etc) and brushing a knuckle over a hot switch or fuse lug ain't fun. The speaker jack, just for tidy.

    Now your turn. Can you share some post-rehab pictures of your amp? Before you told us those were 'before' pics I had 'drifted components' at the top of my hum list.... I also now see where you subbed in a 3x20 can -- you're right, you do see some that way from the factory.

    If you like how the amp sounds on 110VAC, you might like a dropping resistor. FWIW I didn't have to rebias mine; the mantra on cathode bias is (often) 'it's self-biasing.' A few minutes with a soldering iron would allow you to stop wondering about leaving your 'brown box' at home.

    I'm shockingly weak on theory, but I don't see anyone saying you couldn't add a resistor despite your 20uF first filter. Why do you think it might be an issue?
     
    Last edited: Jun 27, 2018
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  13. Inglese

    Inglese Tele-Meister

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    Quoted, single ended suffer from poor power supply rejection so the only way to reduce hum is by rerucing ripple thus choke filtering or increased cap on HV.
     
  14. Darkness

    Darkness Tele-Afflicted

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    FWIW I have dropping resistors (20watt 470ohm ceramic) in both of my silverface champs. The heat from them is negligible. The voltages are nearly blackface spec at 122-125 AC wall voltage. The amps sound fantastic and don’t eat tubes.
     
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  15. Wally

    Wally Telefied Ad Free Member

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    Ime, a single-ended Fender amp running a 1K bias resistor and 12 watts of plate dissipation will not sound like a Fender single-ended Champ/VC amp. It is biased to the cold side of th8ngs. This will also push the plate voltage up. Put a 470ohm bias resistor in and see what goes on. The plate dissipation will go up and the B+ will come down. Ime, anything below 16 watts of plate dissipation yields a sterile sound in one of these amps. I prefer someth8ng around 18 watts. The best sounding one I have been around was dissipating more than that..... if you are very curious, go down from that 1K in steps while taking measurements and listening at every stop. Find what pleases your ear.....but if you want what a Fender Champ did from the factory, you will end up closer to that 470 ohm resistance. I have done that experiment on that hot Champ I mentioned above....820 ohms of resistance yielding a plate dissipation of 14.7 watts neutered the amp. I put thenoriginal 470 back in, and the little amp was back in action. It had been working that way since 1971,and I feel sure it is still working that way and sounding magnificent today. Fwiw, 85% of max plate dissipation is cold for this amp, ime. It would be hot if it were fixed biased....but a cold Champ is a sterile amp, ime.
    Hum? Paralleling two of those 20mfd sections for the first stage is a step. If you want to go all out, rewire the heater filaments with a twisted pair with two 100 ohm resistors on each leg to ground. This crude filament circuit is a source of hum in these amps.
     
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  16. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    I am far from an expert, but that never stops me.... LOL

    I have an old Twin and after I restored the circuit, my voltages were about 50 watts high in places... But, definitely high everywhere. I had been thinking of doing the zener diode thing, etc when my rectifier tube died (it was a Ruby 5u4a... No more modern rectifiers for me!). In my search for a long time replacement tube, I decided that I might give a Weber Copper Cap a shot and see what happens. In fact, I bought a few of them to try. I was nervous about the difference in heater amperage ratings on some of the tubes that would help me drop the B+. The Copper Cap has no need for a heater...

    The GZ34 equivalent dropped the volts about 3 or 4 volts from the Ruby tube. So, then I tried the 5U4GB equivalent and sure as can be, it dropped my voltages to almost exactly what the schematic calls for! However, I have to now fiddle with the biasing resistor as I the bias is running a bit cool, around 50%, and my bias pot does not have enough range... Gotta replace the bias resistor, I guess.

    I love the way my amp sounds with the lower voltage, but it is not night and day different! I am mostly just happy my tubes aren't cooking the way they were before. It should give me a little lifespan on them.

    All that to say, a 5u4GB or a Weber WZU4GB would drop your B+. However, my twin was dead silent at idle with the tube rectifier and both Copper Caps. So, I'm not sure dropping your B+ would really quiet your amp any.
     
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  17. Wally

    Wally Telefied Ad Free Member

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    The 5u4 will not drop more voltage than does the stock 5Y3 in this little amp. It also draws more current than does the 5Y3. Axis, the 5U4 also draws almost twice the current of a GZ34. It would not be advisable to run a 5U4 tube rectifier in that big 5f8a amp you have unless one carefully monitors the PT. That is the beauty of the solid state rectifier equivalents....they don’t draw that current that tube rectifiers do and therefor are safe in any application.
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    ^^^Wally has been to the mountaintop. Heed ye what is scribed on those tablets!!!
     
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  19. tweedman2001

    tweedman2001 Tele-Afflicted

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    Hear Ye! Hear Ye! Hear Ye! Heed the wisdom of ye Elders or suffer The Pit Of Misery. Dilly Dilly! What ever happened to those commercials? Haven't seen one in awhile. LOL

     
  20. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    They are running a TON of those ads right now. I see them all the time, which is especially impressive for how little TV I watch.
     
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