Getting the most distortion out of a single tube

[SnickSound]

Humor me here :)

You can get a LOT of distortion with just an opamp and a couple diodes. But what if you want to use a single tube running on proper high voltage, and no diode clipping at all.

Knee jerk reaction is to get the most gain out of the first triode (say a 220k plate, 1.5 or so cathode, bypassed with say 1uF to keep the mud out of it), then run that into a cold clipper stage (unbypassed 10k Rk, 100k Rp). But turns out... it's not THAT gainy (some would disagree, but fact is it's less distortion than say a Guv'nor pedal).

Before I resort to diodes, how would you address this situation?

What I'm tempted to try is a MOSFET boost before the tube, since that can run on B+. It's still cheating a bit, but less so than clipping diodes.

[John]

I *think* you want to have unbalanced bias on both halves, thereby encouraging early distortion with less gain. IOW, the reverse of when you're biasing the tubes for the most clean amplification, or "headroom". Offhand, I'd try increasing the cathode resistors to maybe 4.7k instead of the normal 1.5k (assuming 100k plate load)


 

[plumcrazyfx]

A Rangemaster driving a 5C1 (relatively low gain by itself) will give plenty of gain for anything short of metal.

[ac427v]

I have been surprised at how little actual gain comes from a cold clipper stage with a 10k cathode resistor. That design really needs the third stage to make it work. But changing the 10k to a 2.2k makes a ton of distortion! Don't give up without trying that first. If you want even more, add a .68 or 1uf bypass cap.Craig

[SnickSound]

A Rangemaster driving a 5C1 (relatively low gain by itself) will give plenty of gain for anything short of metal.

Well sure, if we add pedals into the equation, sky's the limit.

But that's not what I'm asking.

[SnickSound]

I have been surprised at how little actual gain comes from a cold clipper stage with a 10k cathode resistor. That design really needs the third stage to make it work. But changing the 10k to a 2.2k makes a ton of distortion! Don't give up without trying that first. If you want even more, add a .68 or 1uf bypass cap.Craig

You are correct indeed. I breadboarded my circuit and started playing around with it. Warm-biasing the 2nd stage gave me tons of gain, but not that much distortion per se. This circuit is feeding an AB763 front end which serves as the base clean tone. By putting a pair of warm-biased stages before it, I do get tons of overdrive due to hitting the AB763 front end very hard... but... never been a fan of how these sound when "boosted" this way, a bit buzzy. The first triode plate-feeds a tone stack which has a very low impedance. Whatever the reason, it never sounds good IMHO when trying to overdrive such a stage.

So now I'm thinking... add a cathode follower to that first AB763 stage. Now I can hit the first stage as hard as I want, the follower will take care of feeding the tone stake. DC-coupling it gives us that cool compression on positive peaks that Marshalls have, which warms things up.

And if I do that... that leaves me with another half of a 12AX7 to play with. Run that as a new 3rd stage for the distortion circuit and cold bias it.

What I end up with... is very similar to a SLO100 or Dual Rec or some iteration of a hot-rodded Marshall: 2 gain stage, a cold clipper, DC-coupled cathode follower (and its driving stage), tone stack, and voilà!

[jjasilli]

But what if you want to use a single tube running on proper high voltage, and no diode clipping at all.

It isn't up to the circuit designer; you've already been defeated by the tube designer.  The tube will behave per the loadline.  With a "proper" plate voltage the tube will generate plenty of undistorted signal swing, unless you exceed the input signal voltage for that loadline.  This will always be more than 1V, given a "proper" plate voltage.  Tubes were designed this way on purpose.  No guitar signal will be able to meet or exceed that (without a boost).

OTOH, with a very high or very low plate voltage, the loadline will be shifted toward one side or the other of the tube's performance chart. This will take the tube out of its comfort zone.  Now a guitar strength signal of less than 1V can cause either the top or the bottom of the waveform to distort.

[John]

I've often wondered about the feasibility of introducing distortion by using a tube diode, for instance the 6AL5 found in Heathkit vtvm's.

[ALBATROS1234]

ive only been doing this a couple of years and my first amp build were single ended one preamp tube and onepoweramp tube with solid state rectification. in my limited experience though i was initially trying to do just that get one preamp tube to kick the power tube in the tail and rock. i found it very hard to balance the circuit though. i could get plenty of gain for the crunch but there was always a sacrifice in that in order to get that gain you get alot of unacceptable noise as well as driving the tube into oscillation(even more and inherently worse sounding noise). to me you have to shoot for moderate gain and use a clean boost to push the input of your tube or an old school type tube driver/tube screamer which is adding a gain stage on the front end. i am learning that for my purposes getting gain from a tube amp works better if you spread the gain over more stage and use each stage to add its own flavor,kind of like the cold clipper you mentioned. it seems to be about impedance if the driver/preamp is sending a ton of gain but the impedance is too highe for twhat the power tube wants to see blammo noise.oscillation. plus i find that the more tubes you use it smooths out and balances your overdrive. in a two tube amp each tube is giving 50% of the tone albeit depending on the volume and gain structure that 50%is not perfect or constant. but it seems to me as well as making for a better balanced circuit more gain stages add tonal and textural variety especially if you bias them all in different ways. idk. to each his own i guess, i can only go by my results and the mental data from my experimentation. i have never built an exact clone of any amp i slap a basic deign together then i alligator clip in different values for resistors and capacitors until i am happy. the one i am working on now is starting to sound really sweet ended up as a 7025 set up like a 12ax7 in a marshall v1 feeding a half of a 12au7 into a 6v6 and this think grinds nice and hard, i can get 70s AC/DC tones to a guns and roses or tesla raw bluesy 80s metal sounds. this is just my opinion and i dont really know exactly what youre after but i thought i would share my experience trying to do what i think you are wanting to do.

[SnickSound]

I've often wondered about the feasibility of introducing distortion by using a tube diode, for instance the 6AL5 found in Heathkit vtvm's.

There's a thread somewhere in her about... he wasn't very successful.

For what it's worth, I just spent time on the bench playing with LEDs and diodes. Made it sound like a very complicated Tubescreamer.

From all my experimentation so far, the best a single tube can do is push the next tube harder. But the next stage is an AB763 front end, meaning a plate-drive tone stack. It sounds too buzzy when driven hard, so I'm gonna put add a cathode follower anyway, which means I have an extra triode to play with so I might as well put it towards making the distortion circuit better. 3 triodes feeding a DC-coupled cathode follower dual stage... that's a recipe for hot rodded Marshall tones.

[SnickSound]

to me you have to shoot for moderate gain and use a clean boost to push the input of your tube or an old school type tube driver/tube screamer which is adding a gain stage on the front end.

This is what I've always done... but when you think about it, why can't the clean boost be another tube stage? So far noise and oscillation has been under control even though the amp is partially on breadboard.

[jjasilli]

So long as the single tube is a double triode, then the first stage can overdrive the second stage.