Trying to understand 2nd gain stage

[iammike.music]

I've been reading up alot, but there's one thing that for some reason I can't wrap my head around. Here's where I'm getting lost:

After the first gain stage you could potentially have a voltage swing of 20V p-p (just as an example). What happens when this large swing gets to the grid of the next triode? I understand that it's the grid to cathode voltage that causes the triode to operate, but for small signal tubes this can only fluctuate plus and minus 3 or 4 volts. All I can gather is that the cathode is also getting some of the same fluctuation as the grid, but I'm not sure how!

I guess I'm just trying to understand how you don't overload the tube with the amplified signal you get from the first gain stage.

p.s. reading threads on this forum has helped me learn a ton, thanks for being so wise!

[Enzo]

You mean overDRIVE the tube, and who says we don't?

What kind of test gear do you have? A scope would be great, but even an AC volt meter will work. COnnect a signal to the input of an amp and expl;ore the signal levels at each point. If you guitar puts out 2/10 volt, you most likey won't find 20v at the plate of the tube. Of course 20v p-p is only 7 volts of signal.

In high gain amps, it is easy to ramp up the gain through teh amp, but often you will find voltage dividers through the amp to knock down levels, which adds headroom. That allows added stages for tone shaping, if nothing else.

[Paul P]

Is it possible to drive the following stage too hard and damage its tube ?

Paul P

[Earl]

Keep in mind, also, that a 2nd stage is 'usually' after a EQ of some sort. If the EQ is a passive tone stack, that can really drag down the signal level coming out of the 1st stage Plate.

I've never heard of a pre-amp tube grid getting whacked by over-driving it. When the signal at the 2nd stage grid hits 0 volts (or above) the grid turns from a very large resistance to a very small resistance. The available current from the preceding stage is not sufficient to do any damage.

[kg]

Is it possible to drive the following stage too hard and damage its tube ?

Paul P

generally speaking, at preamp levels, no.

[d95err]

Is it possible to drive the following stage too hard and damage its tube ?

Not really, but you can sure damage the tone. Hitting a gain stage with a too large signal can result in blocking distortion and other uglyness.

There are three keys to good distortion sound:

One - control the gain of each stage - try to generate a little distortion in many stages, rather than a lot of distortion in one stage.

Two - the order in which the stages distort. For good dynamics, the stages should distort last to first. E.g. output tubes first, then phase inverter, then the last gain stage, and so on until the first gain stage.

Three - control the frequency response. In general, you need to cut bass before, and treble after, each distorting stage.

[Synaptic Amps]

I'm not sure I would accept all the above guidelines as maxims but I do agree with the more basic take-home message that good sounding distortion results from the carefully choreographed occurence of overdrive in multiple stages simultaneously.

[Enzo]

Welcome to the forum.

Well, we are always open to a reasonable alternative explanation if you have one.

[Synaptic Amps]

It's the second point involving the order of operations where I take note. I'm not convinced the last to first rule is universally true for all styles of amp design. Maybe it's more true for push-pull topologies than it is for single-ended output stages. It's the latter that's been my principle interest.

Certainly there is a point of taste here. Many people do seem to like the raw sound of a single-ended amp with relatively pure power tube saturation such as the classic sounds of a champ or a skylark driven to the max with hot pickups. To me, however, the occurence of power tube saturation in isolation in a single ended design comes off as too nasty with earsplitting overtones. One solution I've found is to shift the balance so the preceeding preamp stage clips in advance of the power tube.

Now, you might say to me to go back and read rule number one, that the real problem with the champ is that in it's original design it demands too much from a single stage and that is why it sounds harsh. That's fair. You might also say look at rule number 3
and thus argue that the champ isn't cut out as an overdrive amp in the first place because beyond the bypass and coupling caps it lacks the essential tone shaping circuit that an overdrive amp needs to really nail the sweetspot.

Certainly the addition of an EQ circuit and an additional stage of make-up gain before the power tubes are welcome additions to a champ and indeed these are two changes I favor for an improved single ended design. But I've also found that a subtle change in the order of operations in the overdrive cascade can help. By manipulating the preamp (reducing plate voltage, increasing first stage plate resistor, increasing second stage cathode resistor, reducing power tube grid resistor) so that the second stage clips in advance of the power tube I find that a much more pleasant and crunchy overdrive can result. Some headroom is sacrificed but in a favorable tradeoff for quality of tone. An overdrive tone that encourages you to roll up the volume control a little more, dig in a little harder with your attack and push it just a little bit further.

Perhaps my claimed benefit to this arrangement results from the preservation of a sort of symetry in the waveform at the onset of clipping which occurs naturally in a PP output stage but is often lost in an extreme SE output stage. This I would argue is one of the principle benefits gained by following rule #1 above. Again though, some like the daughter some like the mother... If you like your distortion nasty let the output tube clipping preceed preamp clipping. If you prefer a smoother creamier distortion maybe get the preamp breaking up just a little sooner than the output stage - then you can roll it on to a full out nasty power tube saturation when you want to.

[d95err]

The "rules" I wrote are mainly valid for multi-stage preamps with medium to high gain. Especially for heavy rock where you want a tight bass response for heavy riffs.

About the order the stages distort in, I still argue that the order should be last to first. If early stages distort before later stages, you can't overdrive later stages all the way (since distortion will act as a limiter).

The poweramp does not need to distort, but if we have the last-to first distortion order, we can use a master volume to control the balance between preamp and poweramp distortion.

Rules are there to be broken!

[hasserl]

Not sure I agree with your points either, I rather like a well set up two stage preamp into a SE 6V6, or EL84; but these things are subjective. But I share some disagreement with the above "rules". These things are subjective and there's lot's of room for differing opinions. I think it's best to state these things as a matter of preference rather than as fact.

I don't think multiple distorting stages are neccessarily more desireable than fewer. Though that does agree with Carvin's philosophy behind the Quad-X preamp which had about 9 gain stages. But I haven't heard many tone guru's mention Carvin or the Quad-X as a source if inspiration, interesting thought. And while that can be a very cool sound, I think there is also something very desireable in a simple two stage preamp, which is my current choice of amps to use. I also think that having the PI distort before the power tubes is very desireable, and is the most common OD sound so many of us know and love.

[Tubis]

Well, for heavey distortion sounds you might like a 4 gain stage preamp sound and a pretty clean output stage. Then the order that d95err describes is the best way to go in the preamp. Then you get the best dynamics. If the last stage distortes the last it would be a very limited response. A master volume is a must here.