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How to kill hum in a Vibro Champ?

Discussion in 'Amp Central Station' started by Vito, Oct 27, 2012.

  1. Vito

    Vito TDPRI Member

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    I have a blackface Vibro Champ (date code PI, 1966, September) that has always had a low-level 120 Hz hum for as long as I can remember. The hum is constant, and is independent of the volume setting, or even whether there's any input. It hums even at idle, with nothing plugged in. It's not a loud hum, but it's audible, and it makes the amp unsuitable for recording at low (clean) volume settings. The amp is stock (no mods), with factory wiring.

    I have just replaced all the tubes, so the 5Y3GT rectifier isn't a suspect. I'm more inclined to suspect the original GE "can" filter capacitor (it's a 20µF/20µF/20µF/20µF, 450VDC, with two sections wired in parallel to make 40µF). After 46 years, some leakage is likely. In fact, there are other problems with the amp that new tubes didn't solve, so I'm going to replace all the capacitors anyway.

    I'm seriously considering disconnecting the can cap and substituting individual individual 500V electrolytic caps for filtering. I've been advised that it might reduce the 120 Hz hum somewhat, but I've also seen posts (elsewhere) that say the hum is an intrinsic result of the power supply design, and even new filter caps won't eliminate it entirely.

    I'm committed to the recap job anyway, but I'm curious as to whether anyone knows if it's possible to eliminate the 120 Hz ripple entirely — I mean short of completely redesigning the power supply.
     
  2. DavidP

    DavidP Friend of Leo's

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    My battle with hum with a SFVC...

    first, are you sure its 120 hz or 60 hz hum? (The latter sounds like a fretted B note on the A string to me). I expect that you have one side of the filaments referenced to chassis ground vs. the twisted pair configuration. If so, I'd start with that, and there are a lot of how-tos to do this. I'd replace the cap can with one from Amplified Parts/ Antique Electronic Supply as they are drop ins, but beware that you will need a big iron (80W minimum with chisel tip) to do this, otherwise you may not get a solid solder connection and that can also introduce hum from insufficient grounding (guess how I found that out...). I've done both, along with metal oxide 2W dropping resistors and 1W metal oxide plate resistors, and now mine is very quiet.
     
  3. printer2

    printer2 Poster Extraordinaire

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    If the heaters are not part of the problem as outlined above then the filter caps are the place to start given their age. It is true you will effect the response of the amp if you increase the filtering but I think it is a good tradeoff. You can only go so large on the first stage otherwise you are stressing the rectifier tube.

    One thought would be to put a resistor in series with the first cap, brings down the voltage a bit but then again the amp is probably running higher on today's line voltages compared to when it was made. After the first stage, the screen capacitor can be beefed up, this really helps out with the hum from the power tube. The first stage hum may be more from the power transformer. I have not worked on a Champ myself (take that under consideration) but I have played with a Champ type amp I built myself.
     
  4. Vito

    Vito TDPRI Member

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    Thanks for your reply, DavidP.

    Yes, I'm sure it's 120 Hz .

    Just to be sure I'm not hearing the first harmonic of a 60 Hz fundamental (which the little 8" speaker might not be able to reproduce with great volume in an open backed cabinet), I plugged the output into my Tremolux cab, which has two 10" speakers. There's no 60 Hz there; it's definitely 120 Hz hum. Actually, that makes sense considering the way the 5Y3GT full-wave rectifier works. It seems most likely that what I'm hearing is 120 Hz ripple coming off the rectifier tube.

    Perhaps I should clarify that you're not talking to an amp genius here. I have a decent knowledge of physics but only a rudimentary understanding of amplifiers in general, and relatively little knowledge of Fender tube amps in particular. So your statement...
    ...doesn't mean anything to me. I'm guessing that what you're talking about has something to do with the transformer leads that power the 5Y3 rectifier, but if I'm mistaken, that just goes to show how little I know.

    All I can tell you is that the amp has factory wiring. It has never been modified. Except for a couple of differences between my blackface amp and the Fender/CBS AA764 schematic that is ubiquitous on the Internet (on my amp, the first filter cap node is 40µF, not 20µF; and there's no 330pF cap between pins 5 and 8 of the power tube), my amp is otherwise wired exactly the same as shown on the schematic...if that's any help.

    As for the dropping resistors and plate resistors, I guess I can figure out what those are, but there again you're over my head as regards their effect on the hum. The new can capacitor you mentioned — now THAT makes some sense to me. If my existing 46-year-old can isn't doing its job, then it makes perfect sense that a new can would improve the filtering. I guess I won't know for sure until I complete the recap job.

    Thanks again.
     
  5. Vito

    Vito TDPRI Member

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    Yeah, that's pretty much what I figured. I probably should get that done first and then see what happens.

    I still haven't decided whether to use individual caps or replace the can. If I go with the can, I had intended to use a CE Manufacturing 40/20/20µF 500VDC can. But I'm intrigued by your idea of slightly attenuating the first stage voltage and "beefing up" the second stage. Maybe that resistor in series with the first stage filter cap would make the power tube run a bit cooler; it has always seemed to me that it runs very hot—much hotter than the 6L6 power tubes in my other Fender amps.

    ...er, when you say "beef up", I assume you mean increasing the capacitance of the second stage filter cap, right? I guess I can do that pretty easily just by adding another capacitor in parallel with the second stage tap on the can.

    Thanks!
     
  6. printer2

    printer2 Poster Extraordinaire

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    Yes you can easily try out increasing capacitance to the second stage by just wiring a capacitor in parallel.

    From your reply above you pretty much have the maximum value of capacitance that is prudent in the first stage without any modification (other than replacing the old cap). I just looked up the schematic for the amp and I see that Fender already increased them as compared to the Tweed amps. The new power supply capacitor may be all you need.
     
  7. DavidP

    DavidP Friend of Leo's

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    More on filaments...

    we all have to start somewhere so here goes...

    If your VC is stock I expect that you have one of the two filament wires going to chassis ground. There's a pair of green/heater filament wires coming from the power transformer and I expect only one first connects to the pilot light and the other to a chassis ground connection -- likely a tab attached to one of the transformer mounting bolts. Then a single green wire travels from the pilot light to the power tube, and you will have a wire from power tube pin 7 and preamp tubes pin 9 to chassis ground. This arrangement is different from the VC's big brothers and sisters which have both filaments running to the pilot light and then a pair of wires (twisted together) that run to the power tube and preamp tubes. I won't go into further details at this point -- this should enough info to let you determine the wiring scheme in your amp.
     
  8. DavidP

    DavidP Friend of Leo's

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    Oh, the can cap...

    Yes, the silverface AB764 layout/schematic lists a 40-20-20uF can capacitor vs a 20-20-20 in the AB763 model. I dunno why Fender did this, but assuming the 40uF section is the first cap hooked up in the power grid, it exceeds the max input capacitance of a 5Y3 rectifier. Nothing will blowup but it will stress the tube over time and ultimately reduce its lifespan (on the other hand, it will likely tighten up the low end!)
    I'd replace the old cap can with a 4-section 20-20-20-20uF one -- you only need to use 3 sections for stock hookup but will now have an additional one in reserve to either add a filtering stage (you can also drop voltage if required by bridging a resistor) or parallel two together to achieve 40uF. Its only a few bucks more.
    Now if you're relatively new to all of this hocus-pocus, remember that the cap can will hold a charge in the 300-400 volt range even after the amp is turned off. So, here's the usual cautionary note -- YOU NEED TO DRAIN THE CAPS BEFORE DOING ANYTHING WITH IT!!
    Good luck!
     
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  9. Vito

    Vito TDPRI Member

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    OK, thanks. Now I'm with you.

    You've described it correctly. One of the green wires from the power transformer secondary is grounded to a lug on a transformer mounting bolt. The other goes to the pilot lamp, and then to pin 2 of the power tube, and to pins 4 & 5 on each of the 12AX7s. And yes, power tube pin 7 and pin 9 on each of the preamp tubes are wired to chassis ground.

    So, you mentioned "the twisted pair configuration". Is that a modification to the stock wiring configuration — one that will reduce the hum?
     
  10. DavidP

    DavidP Friend of Leo's

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    On resistors...

    Ok, I should have read your post above more carefully -- here's some clarification/information on the respective resistors you're wondering about...
    The two larger (1W) 1K and 10K resistors on the LH side of the board are part of the power supply grid -- they may be fine but take a lot punishment and I always replace them with 2W metal oxide (flameproof) resistors of the same value. Towards the RH side of the board, you will have two 100K resistors going to pins 1 and 6 of the first preamp tube on the right -- these are the plate resistors which also take a lot of abuse. There's an ongoing debate on the merits of retaining those original carbon composition resistors vs. replacing with metal film/oxide, and I don't want to fan a flame on this one, but IMHO the metal film/oxide are quieter and I don't notice any loss of 'mojo'. Oh, I also replace the original 1/2W value of these two 100Ks with a 1W -- bigger resistors run cooler!

    That's it, that's all, & that's for sure -- carry on!
     
  11. DavidP

    DavidP Friend of Leo's

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    Re: twisted pair filament wiring...

    Yes, it is -- some find the reduction minor, others find it substantial. Please Google this mod as its not just twisting wires together -- I expect no filament center tap (green/yellow) coming from the PT so you will have to insert a virtual center tap off the pilot light connection. Without launching into another lecture, read up on it first and ask any questions afterwards...
     
    Last edited: Oct 27, 2012
  12. Vito

    Vito TDPRI Member

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    Aaaccckkk...I got out of sequence with my replies, but no sweat.

    OK...first off, I might not have made it clear, but there are two discrepancies between what's in my stock '66 blackface amp (which is an AA764, according to the sticker inside the cabinet) and what is shown on the standard AA764 schematic that shows up everywhere on the web. The discrepancies are:
    1. The first stage filter capacitor is 40µF, achieved by wiring two sections of the General Electric 20µF/20µF/20µF/20µF (450VDC) can in parallel. The AA764 schematic shows that the first stage filter is 20µF. I'm certain that the 40µF is not a modification. The factory-installed cloth wire is stripped back and soldered through two lugs on the cap, and the wire is dressed in such a way that it looks like it was done that way at the factory.
    2. The schematic shows a 330pF cap between pins 5 and 8 of the power tube, but there's no such cap in my amp. My understanding is that the 330pF cap was necessary in the silverface amps because of other changes CBS made in the amp, which caused high-frequency parasitic oscillations, but the pre-CBS amps supposedly don't need it. In any case, mine doesn't have it, and it sounds OK without it.
    So, in this pre-CBS AA764, someone at the factory decided that the first stage capacitance should be 40µF, for whatever reason. I've read elsewhere that some blackface ampmeisters believe that the 40µF filter creates too high an inrush current on the 5Y3 rectifier, and that makes sense to me. So I like the idea of getting a 20/20/20/20 can and using only three sections to start with. As you say, I can always add a section later if needed.

    As for your cautionary note...thanks; I'm well aware of the potential for zapitude from charged capacitors, but it never hurts to be reminded. ;)
     
  13. DavidP

    DavidP Friend of Leo's

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    330pF cap...

    Yeah, I pulled that cap from my silverface with no ill effects -- but I also cleaned out all of that crappy CBS lead dress wiring which I expect is why they put it in there...
     
  14. printer2

    printer2 Poster Extraordinaire

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    So I just did a quick simulation in Duncan's Power Supply Designer

    http://www.duncanamps.com/psud2/index.html


    And with a 20uF cap on the input it says you get 13VAC ripple, with a 40uF cap you get 7.5VAC, which makes sense. Now since you have separate 20uF sections I stuck a 100 ohm resistor in between the paralleled caps and fed the output transformer from the second cap. The ripple dropped down to 4VAC. Not bad for adding a 1W resistor to the amp. You do loose a little in your DC voltage to your output transformer though, after all nothing is free. You loose 5V, not a bad tradeoff as far as I am concerned. Your rectifier tube is happier and you get a quieter amp.
     
  15. Vito

    Vito TDPRI Member

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    Wow, printer2...thanks for that! Power Supply Designer looks like a handy tool, but it appears it's Windows only, so I snooped around and found a similar tool for Mac OS X. Cool! :cool:

    Anyhow, I'm not going to worry about losing a few volts on the supply if it will reduce the hum. I'm assuming that cutting the amplitude of the ripple voltage almost in half will also cut the amplitude (volume...as in loudness) of the hum by a corresponding fraction.

    This idea is sounding better and better the more I hear about it. And it gives me plenty to play with. YOW! I'm going to order the caps and a few resistors and have some fun. :D

    Thanks again!
     
  16. Vito

    Vito TDPRI Member

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    The "twisted pair+center tap" modification

    Hi David:

    By way of follow-up, I've done some preliminary research on the "twisted pair modification". Apparently, it requires creating a "virtual center tap" using a 100Ω resistor between the pilot lamp and chassis ground if there's no center tap on the power transformer. However, there is a yellow/green wire on my power transformer (although it has been clipped and capped with some electrical tape), so maybe that's the center tap. There's enough of it sticking out that I could connect to it.

    I have more to learn about it, but I think I see how it works. Essentially, it's giving the heaters their own dedicated circuit path, rather than having the circuit completed through the chassis ground, which I imagine can pick up all kinds of noise. I guess it saved Fender a little bit of wire to use the chassis as a conductor for the heater circuit, but I don't see how it saved them any time; they still had to solder the power tube (pin 7) and preamp tubes (pin 9) to ground. It seems to me that it would have been just as easy to run the separate wire from the center tap, especially if it made the amp quieter. Go figure...:rolleyes:

    Interestingly, in the process of researching the twisted pair thang, I've also found some information about changing the stock two-wire (ungrounded) power cord to a three-wire (grounded) cord—something I've been meaning to do. Apparently, it can eliminate the need for the .047µF, 600V capacitor between the fuse block and the chassis. I snickered when I saw that the unofficial name for that capacitor is "the death cap". :lol:
     
  17. Tele-phone man

    Tele-phone man Tele-Afflicted

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    I have a book by Gerald Weber called "Tube Talk for the Guitarist and Amp Tech". This is a compilation of articles he wrote for Vintage Guitar magazine over the years. One early chapter is called "Reducing the hum in your single-ended amp". He describes how to change your heater supply to reference a 40V point rather than ground, which changes the DC potential between the heater supply and the plate enough to reduce heater induced hum. The mod assumes that your amp already has a twisted-pair heater supply circuit where neither lead goes directly to ground, but rather both legs see a 100 ohm resistor to ground. Gerald describes using a 270k ohm resistor from the plate supply connected to a 27k ohm resistor to ground, creating a 40V (roughly) reference point. Then move the common point of the two 100 ohm resistors from ground to this new 40V reference. This biases the heater supply to 40V+ rather than to ground.

    I've not ever done this mod, so I can't vouch for it, but I trust Gerald Weber to know more about tube amps than I ever will, so I would look into it.
     
  18. Vito

    Vito TDPRI Member

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    Thanks, I'm still researching it, but it does sound like it should reduce the noise, and it seems that there are many who have done the mod who agree.

    I'm definitely looking into it. Thanks again!
     
  19. backporch guy

    backporch guy Tele-Afflicted

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    sledge hammer?:D
     
  20. DavidP

    DavidP Friend of Leo's

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    Baby steps...

    Hey Vito: Looks like you've got a number of options to pursue here -- I'd do them incrementally to see the net effect of each, rather than a bunch at once (easier to troubleshoot if one gets messed up!)
    IMHO, cap can and filament rewiring would be #1 and #2 respectively on my list.
    Let us know how it goes!
     
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