Noise only on lead channel of a 12AX7 x3 channel Diy pream. need advices.

[N2h2]

Hi,
First I'm apologies for my very bad English but it's not my mother language. Moreover, if I'm asking for help here, it's because all answer found on internet didn't helped.

I'm actually building a 12AX7 all tube guitar preamp and i need some help:

It's a preamp with 3 channel, 1 clean , 1 crunch and 1Lead, using only 12AX7 tube. (same structure as many of modern amplifier with cascaded/paralleled triodes)
I've completely done my clean channel and it work better than expected. Perfect sound for me. Crunch channel is also completely done and just need some improvement to sound a bit tighter.

My pream is build like this:

Clean channel:

~~Filtered input jack (with 39pf cap to ground, resistor in series to filter radio freq etc...) >2 paralleled triodes (clean stage)> 2 paralleled triode (output stage)>output jack~~
Really warm sound, with good headroom, no noise at all (buzz, hiss, hum...) even with high volume (output level higher than my valvking amplifier preamp.) For that stage no problem at all.


Crunch channel:

~~Filtered input jack >2 paralleled triodes (clean stage)>2 cascaded triodes with few distorsion> 2 paralleled triode (output stage)>output jack
In fact it's my clean stage with 2 triode inserted between clean stage and output stage, switched On/off with a 12v electric relay.
It work fine excepted a very small "shhhh" at very high volume, but noting annoying as i will never ever use the volume threshold where the noise is coming as it's almost 3 time the output volume of my valvking with volume knob on 10.

But on my lead channel build like that:
Input jack (also with filter)> 2 paralleled triodes with few distorsion (almost a big crunch)> 2 cascaded triodes (to improve distortion) > Same output stage as clean channel.

On this channel i got a sound which perfectly feet to my way of playing (i mainly play thrash/heavy/death metal) BUT i can't get ride of a really annoying "Shhhhhhhhhhhhhhhhhh" sound even at low volume.

Here is my question, can someone give me advices to reduce that noise?
I don't have this problem on clean and on low distorsion channel, but i really need a high distorsion channel to play metal. any help will be really appreciated


Thanks & regards.

[zambo]

I bet MerlinB knows. He wrote the book on preamps. Literaly. Sounds like a cool project for sure. Please keep us updated. I would think maybe the parallel triodes on the input might be amplifying initial noise to much? Thats purley a guess though. I hope the smart guys chime in  . very interested in this. cheers, Greg

[amptramp]

Some designs use a relatively small coupling capacitor going into the lead channel.  Above the turnover frequency (the low frequency below where the gain drops off), the impedance of the source is in parallel with the grid bias resistor of the input stage.  Below this frequency, you get the noise of the (usually much higher) bias resistor.  Make the input capacitor larger and get the bass reduction later in the circuit, after the first preamp stage.

[N2h2]

I'm currently reading Merlin's book, this is the most useful thing i own to build my preamp as it allow me to go from "bad series tube amp" clean sound to absolutely "perfect" sound form me. If i could meet this guy, i would probably kiss him  .

For coupling caps, i already tried to cut more frequencies on each stages (cut more high and low frequencies) and boost it more on last stage.
But It come with new issues, cutting more high and let pass more low freq get me into the Blocking distortion issue.
Moreover the sound become really flat, with poor dynamic (very cheap sound in fact).

Maybe i should reduce the number of stages for my dirty channel as i noticed that the more cascaded tube you use, the more the noise is loud. but i cannot succeed having a powerful distorted sound with less than 4~5 triodes in series. When i reduce the number of stages, I'm loosing almost all sustain and the amp sound same as if i place a too much sensitive noise gate in the loop of my peavey VK100.

Other annoying thing is the fact i read in this book that using a cathode follower stage in dirty channel could help to get a more distorted sound with less noise, but i failed doing this as if i use 1 cathode follower stage, i got less "shhhh" but  I got an very annoying loud "pop" ("bang") sound  when switching channel (this cathod follower issue is explained in the book, but there are not really explaination to fix it).


Maybe I'm doing something wrong, as the only thing i didn't find in the book is what is the "better" order for coupling gain stage.
I'm not too bad at building preamp but i missing experience as i only build a valvcaster like preamp and a 2x cascaded valvcaster crunch preamp before. but it's my first high distortion try.
So I'll have to ask:
What is the "better" (i know there is no specific rules) order for coupling stages for a high distorted sound?

Is it better to start the cascade with a very (but not good) distorted sound and then placing less distorted triodes to get the sound more beautiful/dynamic, or starting with a little crunchy triode and then adding  a little bit more distortion on each following stages until reaching the desired amount of distortion?

I know an other solution could be using clipping diodes to get more distortion, but I would really like to  get only tube distrtion as it really sound better to me.

Thx & Regards. Julien.

[N2h2]

ok, sorry for double post. I got a really good improvment that reduce noise about 10 time.

I made the mistake to copy my valvking really first input line (between jack and first triode). I found the answer in the book:

"In classic designs, the gride stopper is often positioned befor the gride leak resistor. This form a potential divider and will attenuate the guitar signal somewhat, worsening the signal-to noise ratio."

Damn i could have missed that important thing!
Now i got an acceptable signal/noise ratio to get more stages.

Thx for those who helped (amptramp +++) as it made me read again the "input signal chapter".

It's amazing seeing how series modern high disto amp (Valvking, 5150...) are built, there are mistakes on each stage of the amp, that may explain why there are so much noise when using disto channel.
Sometime seeing these amount of bad deseign make me ask myself why is there "so low noise" regarding the amount of "noise increaser" issue.

Anyway, thx for answer, it was helpful...

Thx & rgds.

Edit:
I decided to mod my valveking thanks to the lesson i got from my mistakes... Wow this is day and night... i'll do that on all my amps as they all suffer from this bad design (in fact looking for many series amps, all are suffering from this bad deseig, not only peaveys !).

[defaced]

On this channel i got a sound which perfectly feet to my way of playing (i mainly play thrash/heavy/death metal) BUT i can't get ride of a really annoying "Shhhhhhhhhhhhhhhhhh" sound even at low volume.

Welcome to high gain cascaded tube amps.  Some are quieter than others, usually because of careful filtering, but they all hiss, it's the nature of the beast.  If you can post a schematic of what you actually have in your amp right now, someone we should be able to suggest circuit adjustments that will help.  

You might also want to check out the SLO Clone forum.  The guys there do pretty much nothing but build and tweak high gain tube amps.  Best high gain resource I've found online. 

[N2h2]

Hey Defaced, you are awsome ! ! !  I found all my answer on slo clone forum !

Noise problem, pop issue on switching...

THANKS A LOT ! ! !

[defaced]

Glad I could help.  So, how does it sound?  That's the important part. 

[N2h2]

yes it sound, and without noise, hum buzz.... but still not completed as I now have to add some option that miss me on other amps:

- Treble bleed for lead.
- Volume boost for solo (as i need it in my band)
- Volume out and in for loops and stereo input for loop return as i have some stereo effect (delay, whammy...)
- after the loop return, thinking duplicate the last input stage, puting the "clone" in parallel to get different sound for each pan of my stereo effects ( maybe some treble and bass boost/cut to adjust panning to my tast).
-add relay in correct place to get footswitchable canal select, volume boost, effect loop on-off. but still not sure about the footswitchable options i need, maybe 1 or 2 more.

Anyway, you are the man! As i didn't know the slo forum and it solved all my issues and more. Your answer was very helpful.

Again, Thx a lot.
regards.