How does one harden or Soften clipping in a tube amp? Egnater ie4

[rosssurf]

I was wondering how I might be able to try to modify my Egnater ie4 preamp to try different clipping characteristics. I am interested in experimenting with making the clipping both harder and softer to see what i like. I love this preamp, but It seems to lack a little teeth sometimes. It is an all tube no transistor  hand etched (non pcb) board with 7 12ax7's. It is a dedicated  2 rack space preamp. At this time I do not have a schematic for any of the four independent channels. Perhaps this is an ignorant question to some, but for me i really don't know about this stuff. I am learing more about clipping in pedals and stuff. I have used various diodes from ge to si to LED and understand a little about thier characteristics. I am not sure if this can be applied to a 12ax7 clipping though.  I mean I know i can't use diodes to change this pre , but i was wondering what mods i may be able to do to this tube circuit to experiment with clipping. Any thoughts are welcome.

Ross -  Baltimore

[wavley]

I was wondering how I might be able to try to modify my Egnater ie4 preamp to try different clipping characteristics. I am interested in experimenting with making the clipping both harder and softer to see what i like. I love this preamp, but It seems to lack a little teeth sometimes. It is an all tube no transistor  hand etched (non pcb) board with 7 12ax7's. It is a dedicated  2 rack space preamp. At this time I do not have a schematic for any of the four independent channels. Perhaps this is an ignorant question to some, but for me i really don't know about this stuff. I am learing more about clipping in pedals and stuff. I have used various diodes from ge to si to LED and understand a little about thier characteristics. I am not sure if this can be applied to a 12ax7 clipping though.  I mean I know i can't use diodes to change this pre , but i was wondering what mods i may be able to do to this tube circuit to experiment with clipping. Any thoughts are welcome.

Ross -  Baltimore

Changing the value of cathode bypass caps/resistors will change the gain/bass response of a tube it would help to have a schematic.

[sean k]

My understanding of tubes, preamp tubes anyways, is that it's really about two things that work in conjunction with each other.

Number one is setting the gain of the tube in combination with the bias and number two is then cutting down that output waveform so it hits the next stage the way you want it to ... and how that next one's set up for gain and bias.

If you take the regular fender type circuit with a 100k plate resistor and an .02 cap then a cathode at 820 ohm and a 22uf cap you got a stage set up for a good voltage swing in the region of the datasheet where linearity is at it's best so it's going to be nice and clean.

To get it to dirty up you want to set the bias at a lower or higher voltage so the point it swings on has more chance of hitting the 0V rail and saturating because you can't get anymore voltage or hitting the highest available grid voltage and going into cutoff where no more current is available. Along side this setting of the bias, with the cathode resistor, is how you place the voltage swing with the plate resistor and higher plate resistors will give you a steeper line accross the grid lines and the more difference there is between the swing accross the grid voltages then the more distortion there is of the original signal.

But this can get out of hand very quickly so voltage dividers off the plate cap, which also help to define things, allow you to bring that voltage swing down to something managable again for the next grid.

So the idea, as I understand it, is to get the signal clean and strong, in the first stage or two, then start tweaking it in the direction you want to take with your plate resistors and cathode resistors as well as the dividers. The easy way is to find an amp you like the sound of, maybe a soldano or a mesa boogie etc, try to take away the effect of the power tubes and the OT (and the speaker), which have a marked effect on the sound, and then have a good look at how they acheive that preamp sound.

Usually it's with a lot of stages that just add a little at a time as opposed to one or two easily defined gestures. This can be hard work as it's the sum of little differences that'll get you what you want as opposed to single changes that'll be easy to identify.

Another little thing thats interesting is the use of diodes on the cathode which'll automatically set the bias at .6V on the side of the swing in which you orientate the diode.

But nothing beats reading up on how the tube actually works then making copies of a bunch of 12AX7 grid characteristic charts and drawing lines all over the place.

Check out the AX84 how to pdf and download the pdf's by Norman H Crowhurst. That'll get you started on understanding this kinda stuff and then when you feel ready you be desoldering and adding and subtracting (tweaking) in no time.

Lastly, the biggest problem with this kinda stuff is that it's somewhat problematic to chop and change within the topography of how we mechanically put tube amps together. With point to point and tagboard etc, one of the main thing one tries to acheive is a strong mechanically sound joint between the parts and this makes it difficult and cumbersome to chop and change and so this is why so much effort is put into the design stages to ensure little tweaking is neccessary once all the stuff is together.

Therefore tube amp design is a cumulative effort where whats gone before is changed subtley as time goes on but if one is resonably serious about understanding how tweaking can effect things then a compromise between having something tomorrow thats already had the bugs removed and doing what you actually may think you want, and discovering how to get it, is all about making up a breadboard rig that allows you to make those tweaks on the fly... problematic at best as at such reasonably high impedances one needs solid solder joints.

After reading the Norman H Crowhurst stuff and downloading a whole bunch of characteristics sheets (many years ago) I actually designed my own amp from scratch using 12AT7's and 6AK6's... and it worked... actually works quite well in that it acheived the sounds I wanted... but it's noisy as, mains hum and heaters, unusable actually but it's so solidly built that I just put it aside as it'd be a nightmare to work through and fix it up (I'm lazy) but I learned alot and if I can anyone can!

[Caferacernoc]

Also look at a later Marshall JCM800 schematic. Tube distortion and clipping diodes!