el34 Tube Amp hissy fit

[tacoma5050]

I have an el34 powered tube amp (w/four 12ax7s). Last nite after playing for an hour, I noticed the amp now produces a hiss or white noise when no guitar is plugged in or guitar vol is at zero.  You don't hear the hiss until you turn the master vol past 30% and gain past the mid point. As you increase the gain past the mid point, the white noise increases. The sound is very noticeable.

It's not a hum or buzz and it does not seem related to 60hz cycle tone. It sounds similar to this:   http://www.purewhitenoise.com/clips/Pure_White_Noise_Sample.mp3

What are the most likely culprits of this symptom? Caps? tubes?

I am suspecting the preamp section. Could it be a peramp tube? ...if yes, can I just swap them around as a test? Or must I switch in a new one?

Amp Schematic is here: http://schems.com/manu/egnater/egnator_tol50_50w.pdf

[jojokeo]

That's called "noise floor" with many high gain stages and high volume going on. Same will happen when using distortion pedals at volume. Now how much versus how high the amp turned up is is difficult to say unless there in person as to what "normal" is. Are you standing right in front of the amp? Walk away from it and turn facing different directions - does it go away, receded, and get worse depending on posistion? Unplug your guitar from the jack socket, it sould be totally silent since the input's shorted (if using shorted jacks), then leave the plug in and turn up...are you back to the same "normal" level hiss amount or is it more/less??? Is this with a single coil guitar, p-90s, or humbuggies? Are you positive it's properly grounded? Do some of these rudimentary tests/experiments and then go from there if needed.

[tacoma5050]

I hear the noise even with the guitar unplugged and looking at the TOL50 schematic, it looks to have shorted jacks. I am familair to some "niose floor", but this is much louder and the amp never had anywhere near this level of hiss before. I will try to verify tonight that the grounding is OK.

That's called "noise floor" with many high gain stages and high volume going on. Same will happen when using distortion pedals at volume. Now how much versus how high the amp turned up is is difficult to say unless there in person as to what "normal" is. Are you standing right in front of the amp? Walk away from it and turn facing different directions - does it go away, receded, and get worse depending on posistion? Unplug your guitar from the jack socket, it sould be totally silent since the input's shorted (if using shorted jacks), then leave the plug in and turn up...are you back to the same "normal" level hiss amount or is it more/less??? Is this with a single coil guitar, p-90s, or humbuggies? Are you positive it's properly grounded? Do some of these rudimentary tests/experiments and then go from there if needed.

[stratele52]

That's called "noise floor" with many high gain stages and high volume going on. Same will happen when using distortion pedals at volume. Now how much versus how high the amp turned up is is difficult to say unless there in person as to what "normal" is. Are you standing right in front of the amp? Walk away from it and turn facing different directions - does it go away, receded, and get worse depending on posistion? Unplug your guitar from the jack socket, it sould be totally silent since the input's shorted (if using shorted jacks), then leave the plug in and turn up...are you back to the same "normal" level hiss amount or is it more/less??? Is this with a single coil guitar, p-90s, or humbuggies? Are you positive it's properly grounded? Do some of these rudimentary tests/experiments and then go from there if needed.

+1

And cleaning tubes socket maybe ?

[HotBluePlates]

That's called "noise floor" with many high gain stages and high volume going on. ...

The schematic shows 6 gain stages prior to the phase inverter; only one is a cathode follower. Yes, you will get hiss, even if it's mostly amplified resistor noise.

You might get slightly less noise with the loop in parallel mode.

[stratele52]

Tube's plate resistors , if old or cheap can make some hiss.