Taming the Noise/Hiss

I absolutely love my PRRI; so much that I've thought of buying a Super or Twin Reverb for higher headroom.

However, I've noticed that there is a bit of noise/hiss with tube amps. It's not that bad on the PRRI, but the noise to sound ratio is too great when playing at a similarly low volume on the twin or super.

How can one make these amps DEAD QUIET? Has anyone shielded the living crap out of the chassis or do anything else to cut the hiss, even when the amp is turned up? (think recording)

Nobody?

Tube amps can be notoriously hard to make quiet. You can learn a lot about where it's coming from by pulling tubes one at a time from the preamp forward. If the amp goes silent when you pull V1, you've found the chief source of the noise. But that's where the gain is, and background noise gets amplified too.

I'd replace all of the plate resistors with metal film, or perhaps all of the resistors in the preamp, and individually test tubes for quiet operation.

Guitar amps are notoriously hot-biased, so you often get thermionic noise from the output tubes. Running them cooler may help.

With an oscilloscope, you may spot some ultrasonic oscillation. Sometimes the subharmonics are heard as hiss or dirt on otherwise-clean tones.

You can get some spectrum analyzer software and see where the noise is and what relationship it has to the signal. If it's up high enough, a sharp-cutoff filter stage may help.

A noise gate may help, too.

Anyone try one of these yet?

http://www.sweetwater.com/store/detail/HumX/

I'm with Billm on the metal film resistors. Replacing the carbon plate resistors in the preamp stages (and cathode follower resistors in amps so equipped) makes a huge difference. Replacing those carbons with films might be the best improvement per dollar possible in a classic amp, assuming you can solder.

There are some who think carbon comps do some magic and one has to have them for 'tone'. These same guys will probably tell you that you need to 'break in' your speaker cables. The only tone carbon comps contribute is noise.


I've found that lead dress in Marshalls makes a big difference in the way they hum, if I remember right you want to keep the low signal wires coming from the jacks away from the chassis, which is counter-intuitive to me.. but it works!

For what it is worth I put a aluminum piece under my 5f4 build as it is a head so it was easy to do and yes it did quiet it down some. I still get some noise out of the amp but I suspect the guitar maybe could be sheilded better (Peavey Reactor Tele) mostly as I get the same noise ratio in any other amp I play too.

Anyone try one of these yet?

http://www.sweetwater.com/store/detail/HumX/

I believe the HumX only removes hum from the ground loops that occur when using more than one amp or multiple devices at the same time. It won't do anything to clean up an inherently noisy amp.

Is there hiss with nothing plugged into the input? With nothing plugged in, and the volume turned all the way down, is there still hiss? The answers to those questions may help you locate the source.

Also, try swapping out the input tube with another one.

There are some who think carbon comps do some magic and one has to have them for 'tone'. These same guys will probably tell you that you need to 'break in' your speaker cables. The only tone carbon comps contribute is noise.

Don't be silly--you have to break in power cables, you warm up speaker cables. Plus, you should notice an improvement in the soundstage imaging.

I had similar problems with a couple vintage amps in my guitar room. For some reason (underground cables and power lines) the guitar room was at the nexus of hiss and hum. I ended up treating each as a mutually exclusive problem.

The hiss was remedied by making sure the caps and resistors were up to spec. That's probably not an issue with new amps, but it's critical with vintage amps. Then, to tame the hiss, I started swapping out different value preamp tubes - ones that would adequately fit in the amp's power range but would control the input. This will change the nature of the gain in the amp, but it did work quite well - enough that there was no hiss and only hum.

The hum was another story, and that couldn't be fixed by and external methods (line conditioners, loop erasers, etc. - I even spent big bucks on a Furman unit) - nothing worked because of the frequency interference generated by the lines and cables outside the guitar room. I eventually decided to shield my Tele, and the hum virtually disappeared! From now on, my guitars will have to be somehow shielded for them to work well in the present guitar room. Maybe, if I can do some creative shuffling, I can move the room to a more hospitable location in the house

Traeting the hum and hiss as two different entities worked for me. And I have to thank many here on the TDPRI for their help with my hum and hiss problem.

Dean