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Thread: super reverb amp clone issues

  1. #1
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    super reverb amp clone issues

    Hello

    I have a noise issue on a clone
    It came in to me with a note saying just dont sound right, the owner did not make it
    I replaced 3 pre amp tubes and power tubes
    Bias cap and pot, input jacks as faulty
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    On test i found there to be too much noise on both channels
    but the vibro channel is worse
    I have been on this for a few days on and off and im loosing my mind
    If you remove tubes except power and pi all quite
    If you remove v1/v2 the noise goes away

    I have does the tubes again
    checked ht and voltages all correct ref the schematic, apart from the noise its working well reverb and vibro channel all good

    Cant see hum on the rails
    Have checked heaters for noise and fitted 2 x 100R to ground off them to see. still the same
    Added some screened wires on signal lines
    It looks like its fairly well been put together
    weber tx's and correct part no
    Im thinking lead/wire address, looking at the layout all looks ok

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    20190207_142029.m4a
    The scope/audio is on vibro channel up full with nothing in the inputs and scoped from output speaker jack
    I don't know if the audio is working for you, not for me, Im new and ain't sure if i can add audio files
    Im hoping you experienced guys can look and listen and let me know what you think
    mr a

    any more info you need just shout

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    Last edited by Arbutt; 02-07-2019 at 09:41 PM. Reason: adding more info

  2. #2
    Supporting Member nevetslab's Avatar
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    In looking at the lead dress, following the output from the 47nF cap off of V1 pin 6, I see it's twisted with the power supply line that feeds the two preamp tubes. I'd unbundle that and maybe even use a shielded cable to get it to it's 220k resistor where it sums with the output off of V4 pin 6 (via it's cap and 220k resistor).

    I can't make out the cap value on the bias pot, though it looks small. I know the 65 Super Reverb re-issue doesn't show a second bypass cap off of the bias pot, but I'd make that one at least 47uF/63V, or even 100uF/63V. The solder joints on the first ground (blk wire from the input jacks, I presume) together with the buss wire from the cathode resistors/bypass caps of the two tubes looks like it needs to be done better. Not to crazy about the soldering on the bias pot case, where the resistor, electrolytic cap and wire come together. Looks like a cold joint.

    We haven't seen the back side of the front panel pot wiring yet.

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    Thanks for that. The cap on the bias pot I added an extra 10 uf as I could see a little ripple. The main cap is 100uf 100v. Fitting the 10uf made no difference. I will look at it tomorrow and do the checks and changes.
    I'll let you know. Arbutt

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    Don't forget the joker g1's Avatar
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    So it is a hum that you are trying to track down?
    60Hz or 120Hz ?

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    Supporting Member loudthud's Avatar
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    I think there is a ground problem. The two red wires from the cap board ground to the lug near the pots. The red wire that goes to the left most filter cap should ground at the same place as the Red/yellow stripe wire from the power transformer (I don't see where that is grounded).

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    Hello people. Been busy today but I have made some progress. Loudthud was on the money. I have removed the 2 red wires and add one and grounded on the power tx. It so much better channel 1 is great still too much noise on vibrato. But that is reduced also did a few changes that nevetslab stated. Can anyone point out to me the best ground points. Many thanks arbutt

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  7. #7
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    Quote Originally Posted by Arbutt View Post
    ...Can anyone point out to me the best ground points. Many thanks arbutt
    'Teach a man to fish' http://www.valvewizard.co.uk/Grounding.pdf
    From that, the red 0V / circuit common / ground wire from the reservoir cap is a super contaminating transmitter, it should be connected directly to the red/yellow wire at the chassis tab by the PT.
    Referring to this build as a clone seems to me a misnomer, a 'clone' being an exact copy, ; the wiring arrangements here bears little resemblance to a real BF, look at the heaters for example and absence of brass behind the control pots for chassis 0V connections.
    Hence G1's unanswered query regarding the hum frequency.
    I think it's 'not ideal' for the red 0V / circuit common / ground wire from the g2 node cap to share the 0V / circuit common / ground chassis used for the preamps.

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    Hi pdf. I'm not sure on the g1 question.. As I'm more self taught... I have removed the centre tap from the heaters and connected it to the same as the 0v from dog house to the power tx. The pots do have a copper plate tight up to the chassis. Thanks for the download link. I will read tonight... I the amp ain't perfect. I have made a few myself without any noise issues. As I followed the layout. Thanks

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  9. #9
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    Quote Originally Posted by Arbutt View Post
    ...I'm not sure on the g1 question..
    Get the hum trace displayed on the scope, count the number of divisions per cycle and note the time per division; f = 1/t

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    Lifetime Member Enzo's Avatar
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    Here is my secret ever so simple 60/120 test:

    TOuch your finger to the tip of your scope probe. Set the sweep speed to get one full cycle or maybe two full cycles. That is 60Hz. Now probe your hum signal. If it has the same number of cycles on the screen it is also 60Hz. if it has twice as many cycles, it is 120Hz. I do this all the time.

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  11. #11
    Supporting Member loudthud's Avatar
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    This page from the Hoffman site should be helpful.

    Link: https://el34world.com/charts/grounds.htm

    It doesn't show the best possible ground, but it will work well enough for a Fender style amp. It shows what you might call a Marshall style with a wire soldered across the back of the pots, but the Fender brass strip does the same thing. The Fender style eyelet board will have more points to ground, but they should all go to the brass strip or the ground lug near the preamp. The main thing that makes this work is the ground for the preamp filter caps should go to the preamp ground, not the ground near the power transformer where the main filter caps go to ground.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Many thanks for all your help. It's now sorted. Mostly ground issues . And few lead address. I've learnt a few things about ground placement in the last few days. arbutt

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