Vibro Champ with bad Hum and dirty Tremelo Problem, Please help!

Discussion in 'Amp Tech Center' started by American_George, Mar 2, 2013.

  1. American_George

    American_George TDPRI Member

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    Hello everyone.
    I'm American_George from Bamberg, Germany and my second build is driving me a little crazy!
    Here's what I did, the why: becaus I thought I could: Gutted my Champ 12 because I wasn't using it and didn't like the distortion playing (where I usually am) at home. I did sucessfully build a weber 5f2 Champ and she (they're always she's) is dead quiet and sounds good, day or night! But I was missing reverb and that (very sexy) tremelo. If I was going to build both into an amp I would have just gone to a princeton reverb and there is not a lot of room in the champ 12 chassis. So a Vibro Champ was the logical choice, right?
    So I made a eyelet board choosing individual Caps for the power supply. Bought a Weber (not classy but they do work) multitap PT and a new (didn't need it but decided to use it) OT. Orange drops, TAD caps not the whole nine yards but close enough. I don't want to get into Amp Mojo just get her up and running. Any Amp I can buy is probably better than my playing anyway.
    Now there is often the problem with too much amp mojo reading and I think I fell into that trap... My power supply started at the 640 VAC center tapped output, I was getting high readings and decided to change over to the 600 VAC tap and got things well in range, with good readings across the mesuring points.
    I did manage to kill my 6V6: either with too much V or my 6V6 wasn't healthy to begin with but I'm running a 6L6 now and sounds good, except for the terreble Hum and the tremelo sounds (for no better way to describe) dirty and at minimum speed too fast. Also the tremelo has a loud put-put-put in the back ground.
    Due to too much mojo reading I had filtered my PT at 16-16-8-8 the first stage was added as suggested, and was soon changed to 32-32-16-16 (all Caps now added parrallel)
    A few wiring mishaps (you can do a bunch of errors and it will still work, not good but will work) Now we're finally getting to my problem!
    Hum: V1 and V2 removed 6L6 just bearly making any noise, I could live with it. Info Filament twisted well away from every thing and CT grounded. The OT is probably to close to the PT but no more room. I am using (again room) full-wave rectifier and with a load resistor a LED instead of the Fender light bulb, again no room.
    With V1 and, or V2 installed, all knobs at Zero, Hum pretty loud. Turn Volume up, Hum increases to 11 O'clock then it's as quiet as church mouse and at around 1 O'clock begins to be really loud. It makes no difference which imput I use. At maximal Volume the Hum is almost as loud as the guitar signal.
    The trebel and bass pots add a little noise but I think that's to be expected and react a little with each other. Maximal signal is all Pots at max. I don't see the Tone stack robbing anything, but at this point what do I know? The Amp is loud and so is the Hum. By the way I'm almost sure it's 100 Herz (50 cycles here in Germany)
    The Tremelo sound is just dirty. Probalby a results of the Hum getting into the signal. At minimum Speed, max. Intesity the amp isn't so loud (to be expected) the effect is there but nothing to write home about. With speed almost off, Intensity just on you can hear just a little put-put in the background, I think that is expected, right? Turning speed and Intensity up a little the speed just putters up and the puttering is very loud.
    What do I do? Monday I wanted to go and buy some new caps at least 22 UF for stages 2 and 3. But I've added an extra filter stage and it's running 32 UF as well as the first stage. Will just changing stages 2 and 3 to the traditional values have that much impact?
    I know I can slow the tremelo down by exchanging one 0,01 UF with a 0,02 Uf which I had originally had and not really noticing an improvement. So.....
    Where did I go wrong or am I looking in all the wrong places?
    I would be really thankful for any thoughts on the matter.
    Thanks in advance from Bamberg, Germany
    American_George
     
  2. bwacke

    bwacke Tele-Meister

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    American_George,

    Welcome! It sounds like you have a grounding issue in your preamp section, but since you say installing EITHER V1 or V2 causes the problem it is a little confusing. You may have a power supply issue or two problems in that case. Try working on the preamp first with the tremolo tube removed so that competing complications don't confound you.

    A few suggestions to get started:
    1. Measure the plate voltages of the 2 sections of the preamp 12AX7 (pin to ground)
    2. Measure the 2 cathode voltages of the tube (pin to ground)
    3. Re-solder the connections to all of the cathode and plate circuit components
    4. Don't forget to hit the volume and tone pots and associated components
    5. Make sure the cathode bypass electrolytics are connected in the correct polarity
    6. Re-solder all the power supply electrolytic cap connections
    7. Measure down the chain of power supply caps
    8. Check the all power supply cap polarities

    Good hunting...

    Bob
     
  3. Telenut62

    Telenut62 Poster Extraordinaire

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    You should be able to start isolating this buzz easily by eliminating stages of amplification.
    Take two clip leads and ground the end of each.
    Now clip the other lead to lug 2 of the first preamp tube and do your test... no difference?
    Leave it and clip the left over clip lead to lug 7 of the first preamp tube... no difference...?

    Remove one from the tube socket lug and clip it to lug 2 of the second preamp tube socket.

    Heaters have a CT?
     
    Last edited: Mar 3, 2013
  4. American_George

    American_George TDPRI Member

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    Hi Amp techs...
    first off thank you for quick replies....
    I'll be checking things again on Monday.
    Today I did some replacement of Input, Volume grounds, as well as removing a ground bus on the back of the pots with good results. The Hum (100 Hz) is way down.
    I want to change the Tremelo und Preamp power caps to 20-22 uF typical for this amp (right now they're at 16uf) before taking further steps.
    I did look at my wiring for the hundreth time and discovered that I had wired the Neg. FB into the ground at speaker out instead of speaker +. NF wasn't connected at the time but you can look at these things every day and never see a mistake.
    I want to mesure the filament voltage, but how exactly? With the tubes in and mesuring to ground I get voltages that continually change 4V, 2V, 1, 3V on each side. Is this normal am I doing something wrong?
    I'll be getting back on Monday with (hopefully new and positive) results.
    Thanks
    American_George
     
  5. bwacke

    bwacke Tele-Meister

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    Glad to hear you are making progress.

    If your filament AC measurement is changing, it points to a missing ground of the center tap of the filament winding. Even if you have checked it 1000 times, go for 1001. The center tap really only reduces hum and does not affect the power to the filaments, so you may find it will help your problem. Filament voltage can be checked by CAREFULLY placing your meter probes on the filament tabs of the tube sockets. It is very easy to look away at the meter and have the test probe tip slip off the filament connections on to more interesting points that spark and pop. The safest way to do the measurement is with a clip lead placed on the socket before you turn the power on. But, of course, nobody actually does this. :)

    With the center tap grounded to the chassis, each of the other two filament leads should read about 3.2 to 3.4 VAC. (I.e. half the total filament voltage of 6.3 VAC)


    By the way, don't be too "shocked" if the increased capacitance is hardly noticeable.

    Bob
     
  6. American_George

    American_George TDPRI Member

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    Hi Tele-Master and Friend of Leo's,
    Thanks for the help!
    Hum is almost totally gone, here's what I did: simplified the grounding sceme to Leo's original, that helped a great deal, no Mojo grounding scemes, just simple follow the dots.
    Then becaus I was getting wierd readings from filament voltage, I capped the CT from PT and replaced it with 2X 100 Ohm/ 3 watt resistors, wow did things get better (thanks for the reading tips) with no Hum I'm happy.
    I didn't add (because the shop didn't have them) any more Caps to the power supply. It's ok as is.
    The tremelo is working much better. I changed all the 0.01's to 0.02's and it slowed down to the point where I can use it. With speed, and intensity turned up I still get unpleasant brumm-brumm-brumm in the background but tuned in to something useful it works. If you have any really good ideas to improve this send them along, for the time being I'm going to play it the way it is.
    I will change over to a 6V6 when I get one, the 6L6 it works fine though. It'll be a drop in.
    Thanks for the help so far.
    "The fish are where the fisher is and the fisher is where the fish are, but not at the same time."
    "Guitar beginners buy gear, musicians make music"
    "Amp builder beginners buy parts, Amp techs understand parts"
    The quotes are all from me to you!
    greetings from Bamberg, Germany
    American_George
     
  7. American_George

    American_George TDPRI Member

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    Location:
    Bamberg, Germany
    Vibro Champ Build

    Hi Bob,
    before I sign off on this project I do have one or two questions:
    In the original the Intesity pot is 25K's I'm using 50K does this mean maximal Intensity is half way across that Pot?
    Can you use Caps (the three in the tremelo that is) that are rated less than 400 V. I don't see high voltage readings in the schematic?
    How come some builder just need the 0,01's others 0,02 and still others need o,47 caps to slow the tremelo down? What is interacting here? and if I want to further slow it down which (looking at the layout) should I change first? Or doesn't make a difference?
    I'm adding some pictures hopefully you'll be able to look at them in my Picassa Album.
    https://picasaweb.google.com/ggborrelli/TurnedAChamp12IntoAVibroChamp?authkey=Gv1sRgCJLFyrSjz7an3gE#5851894435005736530
    Once more thank you
    American_George
     
    Last edited: Mar 5, 2013
  8. bwacke

    bwacke Tele-Meister

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    American_George,

    Congratulations on solving your hum problems! Well done!

    "In the original the Intesity pot is 25K's I'm using 50K does this mean maximal Intensity is half way across that Pot?"

    Using a 50K pot in the tremolo intensity can be a potential problem (pun intended). The pot and the cathode resistor (68K) of the tremolo cathode follower form a voltage divider which feeds tremolo signal to the cathode of the second preamp triode. With the 50K instead of 25K pot, more voltage from the tremolo signal may be fed into the preamp than its circuit can accommodate. This is probably the reason for your "brumm-brumm-brumm". I recommend you do not keep the 50K pot by itself in the circuit.

    You can either:
    1) replace the 50K pot with a 25K pot or...
    2) place a 47K resistor across the two outside terminals of the 50K pot which will bring the total resistance down to about 24K.

    If you choose to use the parallel 47K resistor, the scale on your 50K pot will not be symmetrical. I.e., at the middle of the wiper travel you will have 18.26K instead of 12.5K. You may hardly notice this nonlinear response.

    "Can you use Caps (the three in the tremelo that is) that are rated less than 400 V. I don't see high voltage readings in the schematic?"

    My advice is to use 400 V caps in the tremolo oscillator circuit. You might get away with 250 V caps, but why risk the failure? The plate circuit of the tremolo oscillator has about 170 V on it when idling. The oscillation peaks may be 10 volts higher, so you can experiment with 250 V caps if you have some lying around, but when you find a value that works at the frequency you like, buy 400 V caps for reliability.

    "How come some builder just need the 0,01's others 0,02 and still others need o,47 caps to slow the tremelo down?"

    This oscillator circuit uses the capacitors and resistors in the plate/grid feedback circuit to charge and discharge into the grid. The resistor and cap values as well as the plate, cathode and grid voltages all contribute to the frequency of oscillation. The capacitance requirement will vary according to the resistors and tube type in the circuit. You already discovered that changing from 0.01 to 0.02 slowed the oscillation period. Ideally, the two 0.01 caps should both be changed to the same higher value and the 0.02 caps should be raised by the same proportion to give the same phase shift and minimize the distortion of the oscillator, but it isn't too critical. If it sounds OK and oscillates, it's good. Experiment a bit.


    I was able to view your amps on Picasaweb. They look great. I hope you have a lot of fun and learn a lot (more) with them.

    Best regards,
    Bob
     
    Last edited: Mar 6, 2013
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