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Old 26th June 2011, 03:54 PM   #1
manos82 is offline manos82  Greece
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Default ecc83 guitar pre-amp hum

hi there,
i need some help, i built a EL34 amp with ECC83 pro-amp for my quitar.
at the 1st level (the ecc83) i`m try to built an equalizer (bass&treble) and there is hum.

the input is the A point
if i move the input on the 2nd gril (pin 7)of the tube, the amp play`s perfect,without hum.

i bielive that i got some kind of feedback betwin 2 levels of ecc83.
how can i remove it....
any suggestions?
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Old 26th June 2011, 04:21 PM   #2
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I presume that you mean that the input is at the grid of the first valve (not the plate) when you are getting hum.

There is something strange about that circuit. Is it really a 5.6k plate load resistor on the first triode. Seems way too low for a 12AX7. And where does that capacitor on the grid of the first valve go? Some kind of global feedback?
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Old 26th June 2011, 05:37 PM   #3
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There should be no global feedback in circuits which contain bass-treble controls.
The hum problem might be due to ground loops or a poor layout in the tone control circuit. If the wire runs are long, shielded cable is absolutely necessary because the tone control circuit impedances are high.
A full schematic of the preamp would help.
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Old 26th June 2011, 06:57 PM   #4
manos82 is offline manos82  Greece
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there ...all the schematic view...

about the ground ... at the equalizer nothing grounded....how i got hum from this...
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Old 26th June 2011, 07:03 PM   #5
manos82 is offline manos82  Greece
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shielded cables are already used.

something i didnt said ,i `m not use pcb for the equalizer. just wiring the compoment on ponts.
and are as short as i can.
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Old 27th June 2011, 03:58 PM   #6
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looking at this schematic it is not correct for a guitar amp.

input impedence must be stabilized first. the volume knob should be after the first tube stage. this looks like a hi-fi amp the way the input is set up.

a very good example is :

http://www.drtube.com/schematics/vox/ac501976.gif

and

http://www.drtube.com/schematics/gibson/ga-1rt.gif

and

http://www.drtube.com/schematics/marshall/jtm45tr.gif

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Old 1st July 2011, 12:08 PM   #7
manos82 is offline manos82  Greece
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my problem is at the R=470K nad C=47p...

by the time i remove the resistor and the capacitor and sortcircit the line between the potentiometers, hum is gone.

that because some frequencies are cut.
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Old 1st July 2011, 12:37 PM   #8
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If you bypass the first stage cathode resistor it will help reduce hum.

But for a guitar amp you would be better to choose a different circuit that is designed for guitar rather than hi-fi.
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Old 30th July 2011, 01:24 PM   #9
manos82 is offline manos82  Greece
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i want an explanation.
at the ecc83 (v3) we got an 4.7 k ohm potentiometer and a capacitor .

for what reason is there.?? adjust the voltage of the cathode? and how many volts must be?
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Old 30th July 2011, 01:34 PM   #10
manos82 is offline manos82  Greece
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hi again,
at this guitar amp we got an ecc83 ( place v3) , 2 resistors serial connected and a potentiometer .what for is that (the potentiometer).

any explanation the reason of the
existence?

adjust anything?
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