Help appreciated - strange hissing noise

[guitarz_dave]

My amp (Fargen Blackbird 40 watt, 2 6L6's) has a strange hissing noise. It is quiet when on but no chord is plugged in. However, once I plug a chord in (with or without a guitar), then the hissing noise comes on and stays. This seems to increase as I turn either the volume, reverb, treble or middle knobs. I am going from a PRS McCarty and Gibson CS336 via Evidence Audio cable direct to amp (no effects).

Here are some things I've tried: swapping cables (to a difference Evidence Audio cable, George L's, and Quantum), changing which electrical outlet in the house I use, rotating the socket placement for each matched pair
of preamp tubes (two each of 12AT7 and 12AX7), using an external cab instead of the speaker in the combo, swapping positions for the power tubes, disengaging the reverb, plugging in my Korg Pandora instead of the guitar (still hisses).

Please let me know any suggestions you may have. (I appreciate it).

[guitarz_dave]

Thoughts, comments, suggestions, etc appreciated

[SatelliteAmps]

Sounds like an issue in the preamp section. Could be a lot of different things inside the amp. The next step would be to open the amp up and check things out. Might be time to go to a tech.

[mooreamps]

Originally Posted by guitarz_dave

My amp (Fargen Blackbird 40 watt, 2 6L6's) has a strange hissing noise. It is quiet when on but no chord is plugged in. However, once I plug a chord in (with or without a guitar), then the hissing noise comes on and stays.
Please let me know any suggestions you may have. (I appreciate it).

bad cord, perhaps ??? The input impedance of the first gain stage is quite high, and very sensitive to electronic signals coming in the front end. and I've had input cables go bad on me. At least, swap the input cable out with a known good one, to remove that as suspect.

Also as was pointed out, I'd look at that first gain stage in the pre-amp, perhaps a grounding problem as well.

-g

[Structo]

A lot of amps use a switching or shorting jack on the input.
What this means is when there is not a cord plugged in the input is shorted to ground. Thus no noise.

When I have had noise in the past, be it a louder than normal hiss or other noise it was the preamp tube responsible depending on if it was a single channel amp or two channel switching channel.

Tubes are always the first suspect so try swapping V1 with a kown good tube.
If that doesn't cure it, go on down the line of tubes substituting each tube until the culprit is found.
All amps hiss to a degree, especially if the volume is turned up.

If that doesn't solve it, it's time to open the amp and if you aren't qualified to do this take it to a tech since the insides of an amp can kill you if you are not experienced for this type of work.

[guitarz_dave]

Thanks guys. I'll see a good tech, as I know my own limitations

[guitarz_dave]

I took it to Val King who did a thorough cleaning, etc. While there is no noise while operating it out of his studio (and other places) there IS noise when playing it in my (older) house. I have grounded outlets, but still get the hissing noise. Any other suggestions?

[eugenedunn]

is this hissing or 60-Hz hum? It's pretty hard to escape 60-cycle hum especially with single coil guitars.....

Does the sound go away when you turn off the reverb? I've been in situations when the reverb circuit causes hiss.

[guitarz_dave]

Originally Posted by eugenedunn

is this hissing or 60-Hz hum? It's pretty hard to escape 60-cycle hum especially with single coil guitars.....

Does the sound go away when you turn off the reverb? I've been in situations when the reverb circuit causes hiss.

Humbuckers (Gibson CS336 and PRS McCarty)

Tried turning off the reverb and unplugging it, but still get the hiss.

I think it is something with the power from my house, not the amp, as the amp does not hiss outside my house. It is grounded power. I need some device that would stop this

[VikingAmps]

Sounds like a text book example of RF interference. You probably live closer to a radio station or other RF generating device than the other places it doesn't hiss. If you feel up to the task sometimes the cure is as easy as moving the 68k input resistor to right on the tube socket. Proper grounding scheme is also a big help but that would require more extensive rework.

[guitarz_dave]

Originally Posted by RedMan

Sounds like a text book example of RF interference. You probably live closer to a radio station or other RF generating device than the other places it doesn't hiss. If you feel up to the task sometimes the cure is as easy as moving the 68k input resistor to right on the tube socket. Proper grounding scheme is also a big help but that would require more extensive rework.

I live within a mile of an electrical company substation. Maybe that is causing it? Anyway, how do I move the 68k input resister? I am inexperienced with electronics, so maybe I should have someone do this for me (ie, a tech)?

Also, "how" does the RF interference get solved by moving the 68k input resister?

[VikingAmps]

Putting the input grid resitor right on the tube socket filters any RF picked up before it. If it's on the jack the input wires are still susceptible to RF. After the first gain stage your impedance is usually much lower and the miller effect helps keep out the RF. All you have to do is unsolder the resistor from the jack, resolder the input wire to the jack tip, unsolder the wire on the tube socket (usually pin 2) and solder one end of the resistor on the socket and the input wire to the other end of the resistor. That usually cures it. I don't know why so many amp manufacturers copy Fender's mistakes.

[banjoze]

guitarz_dave, did RedMan's solution work?