New Express build; first time post

[RJ Guitars]

Whopper, great job!

I was looking at those photos and I kept thinking I should know this builder, those parts all looked really familiar but I didn't recognize the name until Big Jim chimed in.

Thanks for the kind words. This is a good forum and there are lots of really kind and knowledgeable guys to lend a hand and give everything a 2nd look.

Of course, the inevitable question - how does she sound? I'm curious about the usual stuff, especially if you get any hum or hiss and what it takes to conquer it. That always seems to be the challenge that I read about on the Express builds.

Anyway, nice job and it was my pleasure to work with you.

rj

[WhopperPlate]

Heh, Heh, Hey RJ! You deserve high praise. Thanks a ton for the kind words my friend!

Fired it up checked the voltages, found out standby switch was upside down (doh!) and the input wire was wired hot to ground (doh!). After all that the amp fired up and....holy shit.

This the most touch sensitive amp I have ever played. Leads and legato jump out and bounce like a high gain diode pedal or something, except you don't even need all the gain. At 11oclock this amp is saturated and screaming, after that it just gets better. It even stays tight enough to tackle some metal like crunch.

I used JJ EL34's and Tungsol 12AX7's. Tungsol sound great, but one of the JJ's red plated. Switched the tube around and the redplate followed the tube so and I had to swap the pair out with some old Tungsol EL34B's. No more red plating.

The hum and hiss weren't noticeable, but I am having some hf oscillation with the treble, volume or presence cranked (kind of like a whistle at times, and a little scratch on the highs), but I don't have a head case yet and I only laid the bottomplate over the amp and haven't screwed it in place yet.

Besides those issues, this amp was blowing my bands mind. My other guitar player said it was damn near perfect and very forgiving, and I have to agree. I have always had a hard time finding the proper combination of crunch, tightness, and smoothness that was as playable as it was versatile. The search might be over.

All I need to do is get a cab and stabilize these oscillations and I'm all set.

Charlie

[M Fowler]

A lot of the problem in an Express is centered around the presence circuit in my experience.

For cheaper 12AX7 tubes, not going with NOS, the new JJ Gold pin tubes are rated very high for gain and low noise.

[KellyBass]

Very Nice build! They're loads of fun, aren't they!?

I think you'll find that even slight changes (components, wire type, length, lead dress) around the volume, treble and presence pots will make a huge impact on highs and can help in taming some of that whistling.

I had one that, no matter what I tried, I couldn't get her to stop whistling with the highs cranked. A 510pf polystyrene cap in place of the 500pf silver mica and now she behaves real nice. I can run the treble and presence at full and the bright cap on and no whistling at all! Very crispy, though...

Have fun!

[FYL]

A lot of the problem in an Express is centered around the presence circuit in my experience.

A marshallesque version works IMO much better (25KL pot, 100n in series, 4K7 in //).

[KellyBass]

A lot of the problem in an Express is centered around the presence circuit in my experience.

A marshallesque version works IMO much better (25KL pot, 100n in series, 4K7 in //).

I like that idea! I've got the parts...I'm gonna give that a try.

[WhopperPlate]

Thanks for all the tips! I've always love the Tungsol reissues. Whatever I throw them in usually sounds and feels great. I have never had much luck with JJ 12AX7's; always a bit bland, kind of compressed and weird IMO. Never tried the gold pins though. Anyone ever try the gold pin Tungsols?

I spent all last night compiling all the notes I could find on the forum about lead dress and stabilization. After all that and hearing some of the nightmare scenarios (and having experienced some nightmares with my first build myself) I think I'll be able to reign in the beast with slight adjustment of a couple leads.

As recommended by my readings I'm going to start by separating the yellow/blue grid wires a little more, moving the v1b input lead away from the volume pot casing, and possibly applying a little more sealant or hot glue to the bright switch area (Noticed I didn't cover up as much as I should have).

I read in one posting that the bright caps are prone to instability if they don't have good insulation between each other, so now I'm concerned if I let them get too close together when they were glued.

Oh well, I'll take it one step at a time see what works. I'll try to have the voltage chart up before my bands dress rehearsal tonight.

Thanks again everyone!

Charlie

[geetarpicker]

My original Express would oscilate if the output transformer primary wires were to far up in the air as they passed under the power supply board. With them kept down against the chassis all has been well for 21 years and counting. My OT primary wires have a few twists to them and run closer to the power resistors, as in several inches away from the back panel. You might work with your routing of your primary wires. Mine come off the output tubes and route under the sockets, but then take a turn upwards towards the front panel and then go under the power supply board just above the 4 power resistors in a row. Seems the bright caps in my original and two clones are right next to each other with no problems.

Nice looking build!

[WhopperPlate]

Thanks geetarpicker, I will heed your advice.

As well, thanks for the compliment; very much appreciated!

Charlie

[WhopperPlate]

Well, no luck today. Applied everyones suggestions ; shortened and moved ot primaries away from rear of chassis based on Glen's comments, adjusted lead dress all around. Swapped preamp tubes back and forth. Still problem is not fixed.

Sometimes it sounds worse with a much more ugly clipping than before. I'm wondering if the tubes are mainly to blame? When I tap V1 I can hear pretty much the same oscillation frequency. I'm thinking I might need to order some select tubes.

I'm also thinking about throwing a shielded wire on v1b to see what problems it kills, but I almost feel like it's a cop out.

Anyways, more chop sticking tomorrow!

Charlie

[geetarpicker]

I'm just guessing here based upon lead dress on yours compared to my original '89 Express. Here are some observations of the puzzle so far.

1) On pin 1, V1 my wire there is up in the air and quite short and doesn't lay down on the chassis.

2) The wire coming from the two bright caps on mine runs behind the wire off the switch center tap, meaning the lead off the caps is closer to the front of the chassis than with the wires laid the opposite of what you have. The wire coming off the center contact of the switch goes between the two bright caps and then runs in front of the wire connected to the caps which is closer to the chassis. Hope that makes sense.

3) On mine the presence lead is a little more direct and runs right next to the output tube socket as opposed to 3/4" of an inch away or so as you have it.

4) It seems your standoffs may be shorter, and that would throw off the lead dress a tad too.

5) My white bias wire runs along with the rest of the B+ wires up high, you have it running down lower and close to the presence wire which may be causing some interactions.

6) On mine the shielded wire off the input jack is long enough that it sits into the outside corner edge of the chassis along at least a few inches of the wire.

7) It also appears your OT is farther to the right than mine, perhaps by about an inch. That would make the primary wires longer, and in my experience those wires are critical in placement.

8) On mine that wire that ties the bottom end of the big 25w power resistor to pins 6 of the power tubes is connected on top side of the power supply board and runs up in the air all the way to the tubes. Not underneath the board and running down near the primary OT wires or chassis.

9) Another thing, make sure your pots are tight. If the treble or mid pot isn't making perfect contact with the chassis things can make lots of racket, squeals and extra noise. You might even clean the pot washers and the inside face of the chassis near the pots with rubbing alcohol then reassemble and see if that helps. I've had this issue happen both with my original and one of my clones. It seems especially when you have a wood front panel one is apt to not tighten things much to not mess up the delicate panel, but things still need to electrically make decent contact.

10) Same goes for the input jack as it's only grounded by it's mounting nut.

11) IMHO you shouldn't need any shielded wire except the input lead. What kind of hookup wire did you use? In my experience some wire is darker or brighter and that might make the signal more delicate to control. Still I doubt that is the issue here.

I've of course had microphonic tubes from time to time, but I've NEVER had a tube actually cause run away oscillation issues. I really don't think better tubes are going to cure that.

12) You are running a bottom ground plate?

Just brainstorming, maybe something here might help?!

[M Fowler]

Good information Glen.

[Structo]

I am by no means knowledgeable about the Express amp but having read a lot of threads about them, I recall that many times symptoms of instability were reduced if not eliminated by installing a cover plate on the bottom of the chassis.
Of course, get your lead dress adjusted as Glen suggested but then try a cover plate to see if it helps.

[KellyBass]

Thanks, Glen!

I'm going to print this out big and hang it on my wall!

[WhopperPlate]

Wow, thanks Glen, I'll take my time and go over every detail.

I already tried tightening down the pots real good, but that didn't seem to solve anything, but I didn't go as far as rubbing alcohol.

Still haven't attached the cover plate via screws but I have been sitting the amp on top of it during operation.

I'm not sure about the standoff height in comparison to the original; I got the parts from RJ, maybe he can chime in.

Anyways, awesome thank you.