signal on b+???

[12afael]

I have some noise on my engl clone preamp .
I put a osciloscope on b+ to see the ripple , there is not ripple but if I play the guitar I can see the signal on the supply, is that normal?
I think the filters should tied to ground all signal isn`t? maybe my filters have a esr too high, but I put a 0.47uF poliester cap in parallel with the first tube filter and the signal is still there.

I have oscilation on the first stage if I don`t tied to ground the input or if I don`t have a guitar conected. that is positive feedback ! from the supply???

thanx

[Enzo]

If your caps are new, I doubt ESR is the issue.

First, where is the system ground, and where is the scope grounded? Don't ground your scope near the speaker jacks. Ground it to the ground end of the cathode resistor in the preamp.

Then what else shares the same B+ node with the first preamp stage. Ideally the first stage or two should be decoupled from the rest of the B+. What sort of decoupling did you use? Where are those decouplers grounded?

Yes, if signal gets on the B+ rail, it can cause feedback.

[12afael]

Hi Enzo, thanx for your help, I have 3 ground points , one for the power supply, one for all cathode grounds , and another for controls(pots) and input jack.

I put the ground scope on the chasis.

then what else shares the same B+ node with the first preamp stage. Ideally the first stage or two should be decoupled from the rest of the B+. What sort of decoupling did you use? Where are those decouplers grounded?

I have a 2k, 60uf decoupling filter for the first two stages(first 12ax7), the filter cap is 10/15 cm far from the first stage, you can see it on the picture is the red cap near of the 12ax7. It is grounded on the bolt of the 12ax7.
the 2on and 3rd stage have a 47uF cap through a 47k R and the main filter is a 94uF filter (two 47uF caps) .

http://img177.imageshack.us/my.php?i...scn20942or.jpg



http://img163.imageshack.us/my.php?i...scn20959qr.jpg


12afael

[Enzo]

Ground the scope to the bottom of the cathode resistor and see what you get. I may be wrong, but I worry about ground return currents in the chassis being part of the problem.

[12afael]

the position of the scope ground didnīt make a difference.

I found the noise on the anode of the first stage. the cathode , grid and b+ are clean.

today I divide the ground, a star ground for the first tube and one for the second it reduce the noise a little, but it is still there.

I worry about ground return currents in the chassis being part of the problem.

what can I do about it? time ago I put just wires for ground and it work worse.

[Satamax]

the position of the scope ground didnīt make a difference.

I found the noise on the anode of the first stage. the cathode , grid and b+ are clean.

today I divide the ground, a star ground for the first tube and one for the second it reduce the noise a little, but it is still there.


what can I do about it? time ago I put just wires for ground and it work worse.

12afael, sorry to be harsh, but clean your wiring first imho, that bundle of wires running all parallel and astray ought to be bad. When you've done that after you might not have oscilation anymore. May be use fresh tubes too, old ones can be microphonic and give you trouble. As well, the two or three Engl schemos i've checked have a 10ĩF cap decoupling the first triode from the 3 following ones, that might tell you something.

Bye;

Max.

[12afael]

Max. I know mi wiring is not pretty, but long wires carry the same signal or out of phase signal. I keep far the wires that are in phase. the oscillation does not worry too much, it only appear when I don`t connect my guitar.

about the decoupling filter , here are a couple of schematics, they have a 10uF for the first two triodes. it does not have sense to decouple just the first stage since the anodes of the two first stages are out of phase.
anyway 60uF are better than 10uF.

http://www.blueguitar.org/new/schem/misc_amp/

12afael