Clean up on Express

[sepulchre]

Hello all,

The purists here will tap their feet and shake their heads at what I'm doing, or trying to. Sorry about that but I'm kind of stuck with what I've got.

What I've got is pretty close to an Express. The power supply is a little different but I've managed to get the voltages in line with the UR12 schematic. All the values along the signal path are the same as that schematic with very few exceptions: there's 820R on the grid of V1a and a 22k grid stopper at the front end, V1b. Also, I replaced the 47pF balance cap in the PI with a 500pF one. The rest is stock.

It sounds pretty good - Very loud - but it doesn't really clean up with the guitar's volume knob. For testing I put a master volume in it between V2 and the PI. When it's turned down I can tell what's going on in the preamp and it is this: cranking up the regular volume yields some distortion. I'll have to put my scope on it but I'm thinking it's in the second stage just after the tone stack and not in V2. I will have to verify that with the scope. Note that the distortion is not the nice sort, in fact it's pretty rough.

From everything I've read it seems that the overdrive in these amps occurs in the power tubes and some in the PI. When I take the master out it certainly does roar but backing off on the guitar volume doesn't quite clean it up - there's still some kind of rough crap going on.

I don't have NOS tubes - JJ gold pins are all I could manage at this point. But that shouldn't cause V1a to be getting pushed too hard.

Any ideas? Am I foolish to be trying to make this attempt? It's so close . . .

Tia,
Ken

[leadfootdriver]

What kind of pickups? I can get bell cleans, but only with a single coil.

Regular JJ preamp tubes aren't really the best for this amp. Everyone seems to run long plates in these amps. The JJ ECC803 is good, so are EI's, and Sylvania's.

What do you run the amp volume at? I run it at noon for a full fat distorted tone. But better cleans can be had at 10 or 11.

[sepulchre]

I play a single coil Strat - no humbuckers.

I just ordered a JJ ECC803S for slot one. Wish I could afford NOS or at least some fancier models but Xmas for the grandkids takes precedence.

I usually run just over noon. Guess I'll try backing that off a little but I wouldn't be surprised if the problem is still there. However, it's worth a shot and I'll report results.

Thanks!
Ken

[Reeltarded]

I'd bet ya I have a couple tubes laying around here that have more good kharma than I really need... maybe not, I have been a mixed bag of tricks!

Merry Christmas to you and your grandkids. PM me an address and thanks again to the fellas who helped me out when I was out of options myself.

Merry Christmas, Amp Garage peeps! Thanks so much for all your help!

Ho! Ho! Ho!

Yeah, this is a public display of affection.. stop blushing!


Miles

[sepulchre]

Thanks, Miles! I'm sure your mixed bag is tipped in the right direction.

Merry Christmas to you and yours, and to everyone here as well! This is certainly a fine group of folks.

[Tillydog]

What I've got is pretty close to an Express.

Is the NFB stock?

Andy

[Richie]

Change the feedback resistor to 47k or 27k.
Change the 150k resistor to 68k or 100k, and change the .002 there to .0039 or .0033uf
you could also use a temporary trim pot in place of the 150k resistor, and play the amp with the other changes made. Then use the trim pot to fine tune in the value you like or where you like the sound/gain of the amp.

[jeff12]

Allasandro PEC 1meg volume pot will help open up the clean range of the volume control.
Glen was kind enough to help me figure that out on my amp, not sure what you have in your amp. If you don't have one it may be worth a try at some point.

[dartanion]

Change the feedback resistor to 47k or 27k.
Change the 150k resistor to 68k or 100k, and change the .002 there to .0039 or .0033uf
you could also use a temporary trim pot in place of the 150k resistor, and play the amp with the other changes made. Then use the trim pot to fine tune in the value you like or where you like the sound/gain of the amp.

I have gone as low as 47k in place of the 150k and you can also decrease drive to power tubes a few different ways.

[Richie]

Change the feedback resistor to 47k or 27k.
Change the 150k resistor to 68k or 100k, and change the .002 there to .0039 or .0033uf
you could also use a temporary trim pot in place of the 150k resistor, and play the amp with the other changes made. Then use the trim pot to fine tune in the value you like or where you like the sound/gain of the amp.

I have gone as low as 47k in place of the 150k and you can also decrease drive to power tubes a few different ways.

Tube swap type to value tweaks in different sections can all have an effect on the amp. And yes the pot taper/ramp/sweep/law or whatever you want to call it effects the amp.
Yep as they say, everything matters..

[geetarpicker]

A few things come to mind.

Is the amp possibly oscillating? Even a mild oscillation can cause very odd clipping, and or a clean tone that is garbled. The oscillation can be causing the output tubes (or other stage) to distort way before the guitar signal itself does, causing a weird modulation of the squeal mixed with clean guitar. Even if you can't hear the oscillation, you can sometimes see it in how bright the blue glow is showing in the output tubes. You might look in a dark room to see if moving the knobs on the amp with no signal effects the blue glow. An ultrasonic oscillation will make the tubes light up more strongly and get hotter, even if you can't hear the notes! That said you could even have an oscillation that is occurring no matter how you set the knobs. Anyway, any oscillation will hurt the tone of the amp especially the clean tones.

If the output tube are really underbaised the sound won't clean up very well. Seems my amps run well between 35-48ma or so per tube. Wide range I know, but for example down at say 20ma my amps don't work well at all.

Make sure your output tubes are pretty close to matched. If one tube is really weak it can clip way before the other, causing the transition into dirty to be much more gradual, in effect it will seem to break up much early than it should.

The guitar pots need to have a very slow taper, as it can be quite hard to find the sweet spot for cleans with short throw quick taper pots. That said, 250k pots aren't too hard to find with decent tapers these days, but 500ks are much harder to find. The stock 250k Fender pots these days are decent, and the latest era Gibson Historic pots are good for 500k.

Also even though a 500pf cap on the PI will mellow the amp out, it does seem to muddy up the clarity a bit and can inhibit the clean sound a hair.

On my amps the clean to mean range seems the greatest with the full bright switch ON (500pf), though at that point it takes other measures to tame the highs a little. Longer cords, darker speakers, using the tone control on the guitar, and mellower tube choices for starters. I have more recently done some nice recordings with my '85 Express with the bright switch off, however that does seem to make the clean to mean range a bit more limited as the clean doesn't quite have the chime to sound as clear as it could. In other words it's a compromise when it comes to the bright switch and how it affects the way the signal clips in the circuit. Still my latest Youtube video comparing my '85 and '89 Express was done with the bright switches off, thought the amps still clean up well but with perhaps just a tad more volume loss on the cleans compared to using the amps with the switch on.

Hope that helps!
GK

[sepulchre]

Sorry for the delayed response. I sleep days and work nights. Fortunatelt my job affords me the opportunity to be online while I work. So...

@Andy: yes it has the stock NFB. I tried to make it as close to original as I could except for the changes I noted above.

@Richie: I'll check out the various resistor values. I think I've got a 100K trim pot I could try there. That will change the cutoff of the HP filter. And I have a ceramic 0.002uF to try the Trudy mod so I'll see what that does too.

@Jeff12: The Allasandro pot looks cool. I'll see about getting one . . . after the first. It's got an Alpha in it now.

@Dartanion: Not sure I want to decrease gain to the power tubes. I want it to have that roar, but I will if I must.

@Richie: Don't really have many tubes to swap. Reeltarded has been kind enough to send me a nice 5751 to try. When it gets here I'll report the results, but looking at 5751's data makes me think it may be the answer to the distortion I'm getting; a little less gain than a 12AX7 (iirc amplification factor = 70). And yep, everything does indeed count.

@Geetarpicker: I didn't get a chance today to check it out with a scope but oscillation is one of the things I will look for. I've got a fairly decent layout, if a bit ugly from changing parts so many times (this amp has evolved). But the gain stages are well separated, no parallel signal wires and I used a LOT of coax. Still, I will check it on the scope. There's not a bright blue glow to the tubes, no more than my other amps really.

Yes, I need to check the bias on the power tubes though I don't think they're too far off. I had them at about 44ma before I started messing with the power supply and the HT hasn't changed that much.

I got the tubes from a guy in Denver, tctubes.com, who does extensive matching testing. He asks about plate and screen the voltage and OT impedance, etc. to best match the customer's needs. I don't believe he's blowing smoke so I think they're okay.

I did a treble bleed mod on my guitar a few of years ago and I may have gone a little overboard. It sounded okay with the amp I was using back then but since I've built a few amps it seems a bit much. I don't recall the values I put in it but it deserves a revisit. I believe it's a stock Fender 250K pot and I do like the treble bleed but I'll try the standard 100K/.002uF values and see how that does.

I didn't mention it before but I don't have a bright switch installed . . . yet. I'll have to pick up an on/off/on switch at RS or somewhere. So maybe that will help. Like Richie said, everything counts.

Thanks a million to all you guys for the help, info and suggestions. I've got lots of good stuff to try now. I'll let you know how it goes.

Ken

[Reeltarded]

Until you get a switch just throw a bright cap on the volume. You want it clean and you will love that.

[sepulchre]

That's kind of what I had in mind after reading what Glen said. I didn't do it before because it's a really bright amp . . until Cranked. I'm learning.

[Reeltarded]

Always better and smoother rolling off a little at the control than leaving it almost as dull as you would like it.. sort of. Chimey is good if it isn't brash.