Low volume distortion - not good - Express

[NYC_Blues]

Hi,

So, I know the Express is known for it's ability to clean up w/o losing volume. But, it seems, at least in my case, this means distortion is squashed to match the clean's volume level. Like playing a master volume amp with the preamp volume maxed and the master volume barely on. This is resulting in muffled, static-y, and buzzy distortion if I don't put the amp's volume high enough.

My Express clone distorts at very low volume when I dig in - say 10 o'clock on amp volume, 3-4 on guitar volume with medium output single coil pickup. The hair on the notes I get when I play hard sound fizzy and clipped. With humbuckers I'm not getting total cleans even with the guitar vol down very low and light playing.

The amp sounds better the more the amp's volume knob is turned up.

Is V1 biased too hot or what? I'll check voltages against specs, but in the interim I thought I'd post this question. I'm still a novice when it comes to circuits. Maybe the harmonics/distortion are/is getting squashed at the PI?

Not sure what else to look into in the circuit. I assume Express' sound fine at low volume i.e. it's not a design issue. I've tried different tubes throughout and am biased at 45 mA at 400 v at the plates. I was biased lower (36 mA) before and the problem seemed to be worst.

Any help is appreciated. Thanks!

[PlinytheWelder]

You have something wrong, IMO. Mine is really clean until maybe 11 O' clock on the volume control...

[KellyBass]

Yeah, that's not right at all. Triple check you wiring and resistor values in the preamp (especially around v2) and PI and post some pics if you can.

[PCollen]

Hi,

So, I know the Express is known for it's ability to clean up w/o losing volume. But, it seems, at least in my case, this means distortion is squashed to match the clean's volume level. Like playing a master volume amp with the preamp volume maxed and the master volume barely on. This is resulting in muffled, static-y, and buzzy distortion if I don't put the amp's volume high enough.

My Express clone distorts at very low volume when I dig in - say 10 o'clock on amp volume, 3-4 on guitar volume with medium output single coil pickup. The hair on the notes I get when I play hard sound fizzy and clipped. With humbuckers I'm not getting total cleans even with the guitar vol down very low and light playing.

The amp sounds better the more the amp's volume knob is turned up.

Is V1 biased too hot or what? I'll check voltages against specs, but in the interim I thought I'd post this question. I'm still a novice when it comes to circuits. Maybe the harmonics/distortion are/is getting squashed at the PI?

Not sure what else to look into in the circuit. I assume Express' sound fine at low volume i.e. it's not a design issue. I've tried different tubes throughout and am biased at 45 mA at 400 v at the plates. I was biased lower (36 mA) before and the problem seemed to be worst.

Any help is appreciated. Thanks!

How are you measuring and calculating your power tube bias ?

"The amp sounds better the more the amp's volume knob is turned up."
Most do.....

Just for the record, the first pre-amp stage in the Express is not biased any hotter than a stock Fender, and the second stage is biased colder. A hotter bias would result if you REDUCED the value of the cathode resistor on either stage (like Marshall's 820 ohm Rk on a split-cathode NORM channel). What makes the Express preamp 'hot' is the cascading of the first and second stages , and running the resultant output into the third-stage clipper circuit, as well as the high-headroom PI which allows a strong signal to slam into the power tubes before it starts to break up.

[guitarsnguns04]

pics are always easier for us to help you. If you can post a few close ups of the circuit.

[NYC_Blues]

Thanks for the replies. I'll measure the resistors. I used a Weber bias rite to calculate the bias.

Here's a pic:

[IMG683]http://i257.photobucket.com/albums/hh23 ... nsides.jpg[/img][/img]

[rooster]

NYC - Let me jump in here as well. I wouldn't go 45ma, as most around here run with Glen's suggestion of 40mas. The fact that you said your amp didn't sound so good at 38mas makes me wonder what power tubes you are running? (FWIW, I like the J/J KT77s, they are cheap and seem to have something going for them. NOS Seimens are always good, of course.)

Anyway, where the others are saying there seems to something wrong with your amp, I disagree. I think here it's a matter of determining what exactly is one man's 'clean' and another man's 'dirty'. As far as I'm concerned, the Express is a 'dirty' amp out of the gate, and my comparison would be a BF Twin. See what I mean? And really, both amps are not hi-fi clean.

In my case, and I have some decent tubes in the preamp section, I took a good look at V2. That 150K resistor and the .002 cap are a big deal as far as the balance of the amp goes. For example, if you were to just increase the 150K resistor to 160K, beside the increased gain, you would notice less treble and more bass at the output. Er, but don't forget about that increased gain though - because now it is boosting the bass frequencies - and bass is what typically overpowers an output section. (Eh, if you were just looking for big bass here, you could get a bigger OT. That said, for me, the idea/beauty of the Express is to balance what you have.)

So I might suggest reducing that 150K resistor to something like 135K or so, just to experience the change. (On mine I installed a 50K trimmer in series with a 100K resistor to find what I prefered, typically ended up with 140K or so.) IMO, since this part in the circuit determines the drive/gain and overall frequency of the signal arriving at the PI, I think it's the most crucial element of the Express signal chain in regards to headroom. And really, assuming the plate voltage is typical, its true, all you have here is a cap and a resistor. So maybe look around for some different brand caps, as well, as not all things are created equal. There's even a reference on the AG site here somewhere where different cap values are used and what might then be the appropriate load resistor. In a nut shell, say you went up to .003, you would maybe then drop the load resistor to 100K. See the relationship? Bigger cap/more bass/more distortion so reduce the gain and bass fequency to the PI by reducing the load resistor at V2. Ah, well, this is a crazy and yet cool area of the circuit I think and it takes an ear to balance it. Ken did balance this with the .002 and the 150K - yes - but then was the resistor exactly 150K, and does the cap he used sound like the cap you and I can buy today?

OK, too wordy. Have fun.

[NYC_Blues]

I was initially using Valve Art tubes then changed to some brand new JJ E34Ls to see if this bad distortion issue was related to power tubes. Didn't fix the issue. I'm now running Siemens EL34s. I usually run NOS in all my amps (Mullard, GE 6CA7, or Siemens), but I wait until I know the amp is behaving well before I put in some NOS. I actually started running 45 mA because I saw a post by Glen. Weird, he must have said 40 mA in a later post. At 400 plate volts, seems like 70% dissipation is 43.7 mA.

The amp didn't sound bad at 36 mA, the bad distortion at low volume just sounded worse.

The problem isn't just getting dirt too soon - that dirt is getting squashed/clipped so it sounds bad.

I'm currently running Baldwin/Sylvania tubes in V1 and V2. These are known to be gainier and they made this low volume distortion issue worse. Which seems to make sense.

I still need to measure the resistors throughout and will get back.

[KellyBass]

Are my eyes deceiving me or is R14 (100k) and R15 (82k) reversed in the PI...?

[NYC_Blues]

Wow, very interesting. Compared to the schematic for Kelly, you are correct. I have no idea what the effect is on switching those two. Theoretically, what is the effect?

Anyways, I'll switch 'em tomorrow morning and report back. Still recovering from the ZZ Top show tonight

Good eye!

[rooster]

Yeah, good catch there KellyBass. In a nutshell, NYC, the PI is unbalanced at high volumes using the 10K tail - so the 82K is used to compensate for this.

In your case NYC, if you were really hitting things hard you would have more 'distortion' than if things were reversed. In fact, if you used two 100K resisters here, you would have more 'distortion' at the output than if you used the correctly placed 82K and the 100K resistors. So you were really warping things, NYC. You should remember to reverse the positions of the two power tubes when you make the changes. The tube closest to the tranny was working much harder when the input was pushed than the other tube, as its current draw would have been really peaking, the other not at all.

Oh, I wrote 'distortion' because some people think overbiasing a power tube is 'tone'.... And, again, in your case, you just didn't have both sides running in a balanced way. One tube was distorting much more than the other when the amp was pushed. That may be the best way to say this.

[KellyBass]

Thanks for the good explanation, Rooster...I was curious whether or not that mismatch could cause the problem. This makes me want to open up my "slightly fizzier" xpress and measure those resistors!

[NYC_Blues]

Thanks guys. I went ahead and switched those two resistors and the distortion sounds better. Still not sure why I'm getting distortion on 8 o clock on the amp volume knob...obviously it doesn't sound good since it's distortion at lower than bedroom level. Down to one problem from two Progress!

I still have yet to measure all the resistors. Going to do that in a few minutes.

Output tubes seem to click more on this amp than others. Clicking when going from standby to play and also when turning off power (after being in standby for a minute or so). Not sure if this has anything to do with anything...

[M Fowler]

Going to need more shots of your circuit to review cap stack and power tube wiring as there may be problems there as well as the standby wiring.

Mark

[NYC_Blues]

All resistors look good. Within 2%.

Low vol distortion is most easily coaxed with neck humbucker playing two notes together on the A, D, and/or G strings above the 7th fret. Easiest when playing D and G string at 12th fret at the same time. Going up or down from the 12th fret, the distortion is a little less and less.

What kind of pics should I post?

Maybe has something to do with filter caps? Maybe this distortion is really prevalent ghosting?

I'm running F&T's and have seen some posts mentioning these have had a spotty reputation.