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tube preamp makes rushing noise at speakers

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Hey guys,


I've been working on this tube preamp build for a few months now, with the help of a friend. I finally finished it and noticed a rushing noise coming out of the speakers. We haven't been able to figure out how to get rid of the noise.
The noise seems to be coming from the mosfets. I double checked the vero board where the mosfets are but i don't see any mistakes. I also checked to make sure the zeners are orientated correctly and they are. The mosfets make noise with nothing connected to the gates. I.E. when the 6SN7’s have been pulled so that only the B+ is present. I've pasted the schematic hopefully you guys can see it. Does anyone have any advice?
Z
 

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A bit of back story here, I built this phono preamp by Eli Eli Duttmann's phono preamp voltages
so we thought to reuse the fet secton the preamp. In the phonostage there's no issue with a rushing noise.

painted, I haven't tired replacing the trimpot yet


Mark Tillotson, r12 and R16 are metal film i believe. the only cc is the 510r



6A3sUMMER. You and Eli seem to agree that R13 and R14 would probably work better between the the zener and the gate. Perhaps that would help with the noise?
 
Most probably you have build a high frequency generator. To tame these, the grid stopper resistors must be connected directly to the gate input of the MOSFETs (inductance must be minimal). Even a short piece of wire in between may create a small inductor forming a high frequncy resonsnt circuit with FET and other parasitic capacitance. In addition, it could be helpfull to have a small blocking cap from the drains to ground (also connected to the drain pin with short leads) as the power supply connection also forms an inductor to the next blocking cap.
 
aboos



My grid stop resistor 510r is 3 holes away from the gate.
I still haven't trimmed the excess board strips. Perhaps this is the problem leading to some sort of inductance ? My plan was to trim once finalized.

What value small blocking cap would you recommend from the drains to ground?







Most probably you have build a high frequency generator. To tame these, the grid stopper resistors must be connected directly to the gate input of the MOSFETs (inductance must be minimal). Even a short piece of wire in between may create a small inductor forming a high frequncy resonsnt circuit with FET and other parasitic capacitance. In addition, it could be helpfull to have a small blocking cap from the drains to ground (also connected to the drain pin with short leads) as the power supply connection also forms an inductor to the next blocking cap.
 
I would think that if its oscillating try a couple of 510R in series with C3 and C6. The other comment is that there is not much current going through the mosfets (1ma) to drive RCA or XLR outputs. I would suggest some TO220 mosfets and say 10ma. Do you need U7 you could drive U8 directly. The other thing is stripe board may not withstand 250V between strips. You may have breakdown causing the rustling. If you use stripe board remove one track between anything above 75V. You can do this easally with a soldering iron and a pair of fine nose pliers. Another thing that can cause this a dry joint such as on a plate resistor.
 
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UL track spacing regs at 250VAC is 1.5 or 2.5mm I forget which.

My own experience if veroboard valve circuits is that it is very easy to have a slow or fast short. I try to leave a track width and arrange B+/HT to be spaced from other tracks.

Where isolation are cut using a spot face cutter, take care of minute strands of copper, as well as board substrate dust. Rinse with IPA after soldering and a paintbrush to work the dirt along the tracks but not across them.

Pay attention to the lead dress and smoothness of the solder joints on the underside of the board, there shouldn't be any points, and if there is, the solder can be reflowed to smooth them.

I was also using DIL sockets cut to length, for the wire ended valves I was using. In particular a problem is the clearance around 0.1" pitch at the solder side. The DIL sockets themselves never once arced or noticeably tracked, even at 300V - but the solder joints arced over, tracked, and carbonised the board to eventual failure. The issue is no different on FR PCB.
 
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Gregas,

You said:
"the grid stop resistor 510r is 3 holes away from the gate".

But . . .
Is the grid stopper the Only thing that is connected to the gate?
In other words, did you re-wire the grid stopper and zener so that the
grid stopper resistor is Between the zener and the gate.

The zener is quite capacitive. And, that capacitance varies as the voltage across it changes.

Copper strips that are side by side are capacitive to each other (and so are wires).
Copper strips are inductive (And so are wires).
Copper strips to Drain; copper strips to Gate, copper strips to Source.

MOSFETs have lots of bandwidth and are very sensitive to any surrounding circuitry that is resonant.
Resonance is inductance and capacitance acting together.

Two copper strips that are side by side form an air transformer.

Oscillator types that can be created by the parasitic C and L of wiring (or copper strips) are:
Colpitts (capacitive dividers), and Hartley (tapped inductors, auto transformer, or transformer feedback).

MOSFETs are far more sensitive to "wiring" and copper strips (that are actually L and C), versus Audio Tubes sensitivity (such as 6SN7).

Copper strips are commonly used to make UHF preamps with JFETs, and copper strips are commonly used to make UHF power amplifiers with RF MOSFETs.
 
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