Q about clean boost vs another tube stage

[daz]

This is something that puzzles me and i want to know why it is. I have found that i can generate a lot of gain in tube amp's pre section and i have successfully done so. But it seems that with every amp i've owned or built, no matter how much gain the pre has, using a clean boost does things that cannot be recreated in cascaded tubes stages, and i was hoping someone could explain to me why this is.

First of all, you can drive the input to the point of having as much gain as could ever be used with a SS clean boost, and not only is it totally void of blocking issues that you'd get if you added another tube stage in front of an already high gain pre, but the tone is so much tighter and fluid. I have for years used clean boosts with a ton of amps and w/o exception every one of them responds like the ultimate extra tube stage but with endless gain and more fluid tone than any amp i have heard can generate with it's own hi gain pre. It took me forever to finally reach a pre design that allowed the amount of stages i was using to sound great and with no blocking. yet during the entire time (months) that i was tweaking and experimenting with no success, i could at any moment add a clean boost to the front with a pedal and achieve great gain with no blocking and the tone would improve greatly even at the points in time where the amp's tone was very mediocre or even bad. Why is this? What is it that makes a clean boost work so well that can't seem to be achieved with another tube stage?

[cbarrow7625]

I think part of it has to do with driving the first stage from a nice low output impedance. That really tightens things up in the first stage. I had the same experience recently when I was building a complex tube buffer stage (0 gain). It had a nice low output impedance and it just made the amp come to like & sound so much tighter and robust.

Not sure about how you are avoiding the blocking distortion but I would love to hear someones's take on that.

Chris

[daz]

So if thats the case, what are some tricks one might use to create that tight gain in the first stage of a high gain pre if even possible? (i suppose it isn't since i never played an amp like that)

[defaced]

Cathode follower as the first stage maybe?

[daz]

I don't thing a guitar could drive it could it?

[HTH]

I don't thing a guitar could drive it could it?

a guitar will drive a MOSFET buffer (emitter follower) which is similar in theory (I think), so why not a tube?

actually, screw the tube stage, just build a MOSFET EF into the front end of the amp.

[cbarrow7625]

Sure it can Daz. Its just voltage, there is no "drive requirement". What comes in to the grid goes out the cathode slightly lower in amplitude.

Some of the Fender reverb units & amplifiers with the reverb unit built in to the first stage (which model is that, I can't recall off the top of my head - it's one of the "newer" models that came out in the 90's) have a cathode follower as the first stage. You just have to use a 2M/2M voltage divider (B+ to ground)on the grid to set the bias for the cathode follower and use a DC blocking capacitor on the input.

It is possible, but I don't think that having a simple cathode follower at the input is going to give you the same kind kind of impact. I think what would work better, believe it or not, is a grid leak bias amplifying stage followed by a cathode follower with a generous amount of feedback keeping the gain of the stage very low. This is a simplified version of the stage I was using.

This arrangement decreases the output impedance even further,and like the solid state pedals you like, produces only moderate gain (instead of zero gain) with a fairly linear clean boost with no AC phase shift or LF cutoff from the cathode bypass cap. Yes, local feedback in a tube amp is generally frowned upon and much maligned. But in a situation like this where you are looking to duplicate the things you like about solid state boosters, this fills the bill. Think of it as an effect at the front of the amp & not really part of the gain structure.

You don't have to use the grid leak bias arrangement. That is just a personal preference of mine for this. Also, take the feedback point after the coupling cap to the "1st" stage. It will save you a cap and help minimize any phase shift through the feedback loop.

[daz]

Thanks. I wasn't so much asking in hopes of finding a way to do anything, more just a curiosity i've had for many years about why a tube stage can't duplicate to sound/feel of a pedal in boosting signal strength.

[Enzo]

Why can't a humbucker sound like a single coil? They don;t sound the same because they are different things. Tubes have different curves than transistors and op amps do. The voltages the circuits run on are different affecting dynamics in various ways. Tubes are voltage based devices while transistors and ICs (which are made of transistors) are current based devices. Internal capacitances differ between tubes and solid state. And on and on.

[Chuck H]

I've done pretty well with building "boost" stages into the front of amps. The first mod I ever did was to add a master volume to a 1959 Marshall. It worked, but I wanted more. So I used to boost the guitar signal with a Boss GE-7 EQ pedal with a little roll off on the top and bottom, bumped up a little in the mids. It was a killer tone. When I started doing more mods I wanted to get away from using the pedal so I cascaded the channels on the Marshall. After much tweaking I was able to get it to sound as good or better than the pedal. So this has been one of my basic starter designs ever since.

More recently (he, he. 20 years later) I built a Trainwreck/BFish/18 watt kind of thing. I was really happy with the tone but again I wanted more gain. I knew which stage I wanted to push harder was the one following the effects loop. So I pugged in my old GE-7 again. With the eq flat I bumped the gain about 4.5db. Bingo, golden singing sustain. I installed a switch to disengage the effects loop and boost the driver for that circuit and is sounds identical to the pedal in the loop. The final design will have the increased gain AND the effects loop, but this one is my personal prototype so It's cool the way it is.

Point is...Don't assume there is only one way to do things just because the path isn't clear. On the Marshall the tone was slightly different. But not worse for it in any way. On my new design, even with the gain boosted in that circuit the output impedance was still low enough to make any difference between boosting the driver and boosting with the pedal unnoticable. Maybe you'll want to experiment with a cathode follower after a "boosted" input stage to lower the impedance. It would mean adding a tube to the design unless you already have a spare channel you can omit or an unused triode floating around. But it might be just what you want.

Chuck

[Steve Conner]

I once built a high-gain pre with a cathode follower as the first stage. I liked the tone, but I ended up modifying it to be another gain stage, for that extra push over the cliff

In another homebrew, I just used a standard Fender/early Marshall preamp, and bolted an EF86 onto the front as a boost stage. With the gain turned up full, this causes blocking distortion and gets really ratty and snarly, not at all the modern saturated high-gain feel, but I wanted a more old-fashioned tone from the era before amp designers knew about blocking distortion To keep it under control, I added a switchable tone shaping network to remove some of the low end. It stops the preamp from just imploding when you hit it with a neck humbucker.

I think the difference with the clean boost does indeed come from the low output impedance. This allows it to drive the grid of the first tube as hard as possible, without generating blocking distortion at that grid. A tube stage can't do that without charging up its coupling capacitor and going into blocking.

A while back, a long-time member called Ray Ivers published a zener mod that effectively cancels out blocking distortion, by clamping negative grid swings. If my theory is right, adding Ray's mod to the grid of the first tube after your added boost stage would give it a character more like Daz's clean boost.

[Chuck H]

A while back, a long-time member called Ray Ivers published a zener mod that effectively cancels out blocking distortion, by clamping negative grid swings. If my theory is right, adding Ray's mod to the grid of the first tube after your added boost stage would give it a character more like Daz's clean boost.

I absolutely remember that. It started as a discussion of the Paul Ruby implementation to fix buzzy 18 watters. Ray liked the idea and had experimented with it on a Marshall and posted his results. Then someone asked "can you do it to a preamp tube" and the thread morphed from there. I'm sure it's in the archives, if it isn't still in the regular history here. It wasn't that long ago.

Good call Steve. And I thought you suffered silicootie phobia with respect to guitar amps

Chuck

[6267]

It's easy to get a nice low input by reversing a high-impedance microphone transformer...these were often designed to bring a ~200 ohm microphone *up* to 20k or so for the high grid input impedance of tube amps. By finding the right one, you could put the guitar on the 20k or so side and the amp's first grid on the 200 ohm side. Tada, and now you can use balanced inputs too!

[allsavy]

One of the nicest clean boosts I've heard recently was the ZVEX super hard on pedal... local feedback one transistor type of thing that has a wopping 5 meg output impedance!
Brings old weak magnet pups back to life and is very transparent. This would seem to counter what was stated about low impedance earlier in this post.

[cbarrow7625]

Actually it has a 5 meg INPUT impedance. That means it doesn't load down the pickup very much. Although they don't publish the OUTPUT impedance, for a typical JFET circuit it should be relatively low (like less than 5k). That is far lower than a typical tube plate output impedance.

High input impedance & low output impedance are (IMHO) the requirements for a great sounding boost. No wonder you love the SHO.