Peavey 5150 Mark 2 Hiss/White Noise problem

[The Guitar Colonel]

Hello and Help

I have a Peavey 5150 Mark 2 head.

Problem > When you switch it on there is a hiss/white noise coming through. No guitars are plugged into the input. Volume can be down but the noise is still there. Switching the standby on and off can reduce it.

I replaced the valves, and now the noise is worse. 12AX7s in the pre amp

Help please

Thanks

[mooreamps]

Hello and Help

I have a Peavey 5150 Mark 2 head.

Problem > When you switch it on there is a hiss/white noise coming through. No guitars are plugged into the input. Volume can be down but the noise is still there. Switching the standby on and off can reduce it.

I replaced the valves, and now the noise is worse. 12AX7s in the pre amp

Help please

Thanks

Replace R12, R17, R15, and R13 with a standard red LED.


-g

[Amp Kat]

Guess it's worth a shot as I was going to say put a noise gate in the loop which is what most do and similar to Gary's approach of gating the Cathode when no signal is applied.
Shouldn't that also be polarity sensitive ?

[mooreamps]

Guess it's worth a shot as I was going to say put a noise gate in the loop which is what most do and similar to Gary's approach of gating the Cathode when no signal is applied.
Shouldn't that also be polarity sensitive ?

Well, not similar to my approach. It is the cathode resistors than generates the hiss noise based on the equation :

noise factor = input impedance / gain * TNcRx.

A "noise gate" attempts to attenuate noise ; after the noise has already been generated. Except the "Noise Brake" ; which I developed ; which attempts to attenuate external 60 cycle hum noise picked up by the instrument and induced into the front end of the first gain stage. :]


-g

[Amp Kat]

Right but when you bypass the Cathode resistor with a diode the circuit is not conducting until that threshold diode drop is obtained therefore gating it. When the cathode starts conducting it goes wide open and gain and noise are directly propotional. The polarity of the led should be Cathode to ground so it will conduct as it will block or gate it off the other way. Basically what Peavey does in the JSX and XXX using back to back diodes with a 1 Meg pot calling it a noise gate. While I agree the leds may skew the noise until the signal is passed the 5150 has other noise and buzz issues from the high cascaded gain stages amonst some other issues from high frequency circuit topology that haven't been resolved since it's extistence.

[mooreamps]

Right but when you bypass the Cathode resistor with a diode the circuit is not conducting until that threshold diode drop is obtained therefore gating it.

Just pull the cathode resistors out. It will be enough current to forward bias the PN junction.

Basically what Peavey does in the JSX and XXX using back to back diodes with a 1 Meg pot calling it a noise gate.

It's a worthless approach.

While I agree the leds may skew the noise until the signal is passed the 5150 has other noise and buzz issues from the high cascaded gain stages amonst some other issues from high frequency circuit topology that haven't been resolved since it's extistence.

Perhaps poor grounding. I don't know.. I've never opened up a 5150.

[bluesfreak]

Do you use the FX Loop? If no jumper it with a patch cable as sometimes the switches inside the FX Loop jacks do corrode and can cause funky issues like crackle, hiss, volume drop, etc...

[Amp Kat]

Do you use the FX Loop? If no jumper it with a patch cable as sometimes the switches inside the FX Loop jacks do corrode and can cause funky issues like crackle, hiss, volume drop, etc...

While this does happen with lots of amps and yes the 5150 it's not the cause of the OP's issues. All 5150's and 6505 are like this and it's the nature of the beast. Believe it or not but some really high energy bands want even more noise than it gives as the New Rock Sound like the Mesa produces that tone. It's when players try to get that sustain sweet noiseless distortion is where the 5150 fails. Anywhere past 3 on the pregain control will give you noise,buzz and ocean. Maybe Gary's diodes will help as I haven't tried it but IMO the topology and high frequency networks along with to many cascaded gain stages are the cause. Peavey even elevated the 1st and PI tube to 50 volts using a +/- 24 volt supply to no avail.

[bluesfreak]

He states that it went worse after changing the preamp tubes .. could be a tube problem as well. I had a few 5150 and on all of them I made the bias adjustable and adjusted the poweramp much hotter then the usual 15-17mA => Much better sound and less noise as you do not need to rip the preamp gain open...

[Amp Kat]

Very true and that's part of the other problem is it's basically operating in crossover the way it's biased and needs that hard signal hitting the grids to sound right. If you bias it hotter and crank the pregain it gets real loose. The Return in is very stout and exteremely powerful and clean. So the noise is all related to the preamp section and as you might figure using lower gain tubes does make it sound better in the right spots. Many use 5751's and 12AT's but you have to be careful using the LPS or any other spiral filaments because of the DC present on that 1st and PI tube.

[mooreamps]

He states that it went worse after changing the preamp tubes .. could be a tube problem as well.

If the preamps tubes were old, then less gain , hense less noise per noise factor = input impedance / gain * TNcRx. The ulta low Noise Factor of TNcRx of a standard LED will pretty much dump most all of the hiss content. Good luck...



-g

[MstrB]

But polarity is important with this right, cathode to ground?

[MstrB]

So, did any of you folks try the LED trick and how did it work?