What makes an amp clean up well when rolling back?

[leadfootdriver]

I couldn't word the question right due to the constraints of the subject line.

But what specifically in the amp design makes it inclined to cleaning up nicely when you roll back on the guitar volume?

Wouldn't it clean up even better if you used lower plate resistors?

[JamesHealey]

the express does this so well because it compresses at higher gain levels and the compression lifts when the volume is rolled down.

This is due to the power string design, high primary impedance and gain staging design.

it's more than one contributing feature of the design.

[leadfootdriver]

THanks for the reply.

I think I sort of understand this. So, the high primary impedance of the OT puts a heavy resistance against the power tube plate load, thereby compressing the output signal? Would this be considered transformer saturation?

I tried to search out how to select the proper primary impedance for an OT for a desired effect, but I really could get a clear answer that I could understand. Especially from a tube data sheet.

Does the JJ EL84 datasheet give a figure to calculate the PI for an OT?
http://tubedata.milbert.com/sheets/163/e/EL84.pdf

[paulster]

It's also because the various stages of clipping in the amp (power amp, pi and preamp) all occur over a very narrow range of input signal, so you can be playing clean but your picking dynamics can push a note into heavy clipping, for example.

The power supply compression that James has already mentioned is what helps to minimise the volume difference between playing clean and distorted, and really allows you to do this, whereas with another amp you could simply find that your clean tone was too quiet with the volume backed off.

It's a triumph of design.

[wallybob]

I'm having a hard time understanding your comment of "power supply compression." I can understand this in the context of a lightly filtered B+ power supply like many old Fender tweed amps where waveform peaks will naturally be limited (compressed) by PS sag. But the TW has a PS that's stiffer than most amps out there and this effect would not be as apparent. Please explain.

[paulster]

The Express power supply isn't that stiff. It's well-filtered in terms of ripple (but then not massively), but it has that nice 1K dropping resistance between plate and screen, and then 18K2 for the PI and the 9K1's for the preamp, so under load the power supply sags substantially.

It's been well documented here before, with Glen even posting the voltage readings from Annie under idle and heavy load to illustrate the point.

[geetarpicker]

I truly believe the power supply is a big part of it. Perhaps even the PT adds to it, having a certain amount of sag to it also. These amps also have a very nice clear clean tone, and maybe the lack of a cathode follower stage, lower B+ overall but with a hot bias, and careful attention to how much breakup each stage has contributes to this effect. The result being a nice clean tone, but once clipping occurs it takes off quickly but controlably like it has a built in brickwall limiter. Also the input impedance might contribute a bit of sparkle when the guitar is backed off, possibly due to lack of series resistance. On a different note I can definetly say my speakers, being old G12M Celestions (or two a slightly lesser extent my Scumback M75s, as they seem just a tad dirtier overall) add to the clean to mean range, perhaps due to the extra natural compression these have. Nice pots (and 50s wiring in Gibsons) can also enhance the ability to work the amp from the guitar.

[Nigel Tufnel]

There's noteworthy things posted above already and here's what I have to add in a not so scientific manner. I "think" it has a whole lot to do with the tone stack right off of the first stage soaking up a bunch of potential gain and every stage after that being more or less wide open straight into the PI. When the volume on your guitar is up high it's plenty to smack the crap out of the later stages and when you lower it it's just enough attenuation to keep things clean. The bright cap especially in the 500pf position boosts treble like crazy and keeps things nice and clear when you roll off and the whole sum of the design lets the top end saturate along with everything else when you wind up the guitar's volume. <---- Wrecks for dummies......like me

[PCollen]

the express does this so well because it compresses at higher gain levels and the compression lifts when the volume is rolled down.

This is due to the power string design, high primary impedance and gain staging design.

it's more than one contributing feature of the design.

It's because you are providing LESS SIGNAL into V1, it's as simple as that. Roll the volume down on a log pot, and you are at 50% signal level at 7-8 on the dial. Same with picking dynamics: pick hard = more string vibration amplitude = more signal level, and v.v if picking soft. Works just like the gas pedal on a car.

[JamesHealey]

Thats the basic of the PCollen for sure!

I understand this entirely, but the Express goes from clean to distortion without a big volume change like other amps.

Try going from clean to distortion on a cranked Super Lead, theres quite a volume difference compared to an Express for instance.

Thats what I was trying to explain in my post.

[geoffrod]

I couldn't word the question right due to the constraints of the subject line.

But what specifically in the amp design makes it inclined to cleaning up nicely when you roll back on the guitar volume?

Wouldn't it clean up even better if you used lower plate resistors?

not sure if you have found this thread but there are some good explanations/ opinions in it.
http://ampgarage.com/forum/viewtopic.ph ... light=mean
cheers
geoff

[PCollen]

Thats the basic of the PCollen for sure!

I understand this entirely, but the Express goes from clean to distortion without a big volume change like other amps.

Try going from clean to distortion on a cranked Super Lead, theres quite a volume difference compared to an Express for instance.

Thats what I was trying to explain in my post.

James: While not an Express, I have a Komet 60 preamp board I built (using only 100K on the V2 plate) coupled to a Marshall JTM45 (KT66) power section, and I have noticed what you describe. I attributed that to the V3 clipper feeding the PI, as once you get beyond the clipping threshold of this stage, a slight input signal level reduction can make a noticeable difference in degree of breakup.

[Scumback Speakers]

I'm going to throw in another factor that may have been overlooked in this discussion...audio taper guitar volume pots.

Recently I bought a used LP, and when I tried it with my Express clone, it didn't clean up at all until the guitar volume knob was down to about 3.

I tried it with one of my other LP's, which cleaned up by 6-7.

Checked the pots on the recent LP I got, and it had linear 500k pots instead of audio taper 500k pots.

Now it cleans up like the other LP's I own.

[geetarpicker]

Very true AND "audio taper" pots vary from brand to brand, and even era greatly. I measured some old Centralab 500ks and when they are on 1/2 they are only at about 60k, and when they are on 2 they read only 12-16k. Point is they come on very slowly, and I've yet to find a modern audio taper pot that comes ON as gracefully. I gave up running modern pots (even well respected repros of the old ones) and now have the old pots in both of of my historics (R7 & SG/LP) and of course my old original LP. For some reason these days 250ks seem easier to build with a smooth taper and I can live with the modern 250ks, but the 500ks seem much trickier and are a nut yet cracked IMHO. Also on a nit picky note the old pots didn't have dampening grease in them at all. So, as long as they are clean and corrosion free they turn like butter. First thing I typically do with modern pots is to wash out the grease w/Deoxit, but on some pots this isn't possible and still doesn't fix the taper only the ease of turning.

[redshark]

Pots are VERY critical here. Glen did you ever check on the PRS pots? supposely they designed their pots to be a copy of the old centralabs and so did Hamer I think. I would like to know if they achieved their goal?