WhopperPlate wrote:Wow, thanks Glen, I'll take my time and go over every detail.... 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.
Yeah those are 1/2" standoffs. They are the same spec as the RW BOM, although I guess I haven't looked at an original to verify they are the same. Sounds like it is correct though.
Whopper, I know you are not necessarily enjoying this part of the build, but it is really part of what makes a wreck what it is. As Geetarpicker mentioned earlier, his original actually had some of those same type problems that he had to work through. On other threads Geetar Glen has shared about how he locates the amp and the speakers and what his preferred guitar cord is, etc.. That helps us understand these are finicky beasts.... I wonder why Ken Fischer always named them after women? I hope to be wiser with you when you get down to the solution. There are very knowledgeable guys that I hope will chime in, this has certainly been worked through before.
We could make the case that an amp that is built on the ragged edge of instability isn't a good design. From a mass production standpoint that is true and the low production numbers for these amps isn't hard to understand if you consider that there was some degree of tweaking and tuning even for the master himself... he was famous for his never ending twiddling at the workbench. It's the pursuit of Understanding the technical "why he did this" be it crazyness or maybe genius that drove Ken Fischer to build amplifiers on the instability edge that makes this enjoyable - eventually.
between my girlfriend, my guitar, and the express I can barely find time for myself
sounds to me you are takinig care of yourself. Guitars, amps and girlfriend.
The Express is finicky and when she starts acting up it can be a bugger to get through. Especially if your like me I only use current production tubes or old tubes I find locally out of organs so hiss sometimes can be a problem.
between my girlfriend, my guitar, and the express I can barely find time for myself
sounds to me you are takinig care of yourself. Guitars, amps and girlfriend.
The Express is finicky and when she starts acting up it can be a bugger to get through. Especially if your like me I only use current production tubes or old tubes I find locally out of organs so hiss sometimes can be a problem.
Keep at her you will get it working good.
Mark
LOL Yeh that's true, but you've got to take care of them so they can take care of you back. What else is new huh?
I'll be just as patient with this girl as I am with mine.
Well I've got my amp on the bench (cab is on the ground) fired up and running after applying Glen's suggestions and now I have very little oscillation.
Laid glue beads down on the ot primaries, v1b grid, v4 v5 grid wires to flatten against chassis. Lengthened and moved input wire, readjusted bright cap wiring, shortened pin1 plate wire, moved NFB wire closer to tubes, and moved the white bias wire up and away from the NFB wire.
Only with all controls on the bright setting on ten am I getting any runaway oscillation(with or without guitar), but I still haven't properly screwed down the bottom plate so there's still a chance that will fix the rest. Amp is just sitting on top of the plate right now with the plate taped secure in some places.
I am still getting this weird kind of splatty distortion, most noticeable on high sustained notes. Really ugly; worst upon first attack. As the note decays it clears up, obviously gain related. Kind of sounds like ghosting the way it interrupts and modulates the note. Was really bad before the new changes, but now it is better. I had a similar issue with my first amp that I couldn't figure out(one of the reasons why I'm rebuilding her). Maybe chassis vibration?
Either way my hopes are up and this thing feels and sounds so good I'd be nuts to not have patience to troubleshoot. Blooms into harmonic feedback so easy.
Hey Glen, on your amp does the bright cap lead ride along the chassis front panel or through the air?
Charlie,
From my pics of my amp the bright cap lead is in the air a bit, maybe even with the front of the pot but with the washer spacing it back a tad that puts the wire still in the air a little off the front panel.
You could try say a 820, 1k or 1.5k ohm "grid stopper" resistor right on pin 2 on V1. Some folks have used much larger values and maybe could chime in on this tweak. Attach the resistor trimmed to fit right up againts the tube socket terminal, then put the wire lead on the other end basically trimming the resistor leads no nubs. I have pics of an original Express with a 1.5k resistor that I assume Ken put there to dail it in. Apparently a little bit of inline resistance there rolls off the extreme highs just enough to sometimes fix a tendency to oscillate. I'd try that before using any extra shielded wire.
I doubt the bottom plate not being screwed on is the culpret. Having it the chassis lay on the plate is probably good enough to make decent contact. Still if you have the amp on top of a speaker cab IF the speaker is really close to the top of the cab you could have oscillations right off the speaker. Make sure you try the amp with it totally off the cab. That said I can typically play my Express dimed if I want to on top of a Marshall 4x12, tube microphonics usually being the only limitation.
Keep working at it! You should be able to totally dime the amp and have no oscillation problems at all.
Thanks for the advice and words of encouragement Glen! Genuinely appreciated!
Looking at Francesca I've realized I have misjudged several wiring layouts.
The black presence wire is much more direct than what I thought and it seems to not ride the chassis. I also went through and raised all the signal wires up away from the heaters a bit more as well raising and separating the wires by V5 so that they all no longer are making physical contact, but haven't tested any of it out yet for any results.
I'm hoping the distortion/modulation I'm hearing is just a symptom of the oscillation, or from being to close to the emitters. If it's not improved I'll move on to the grid stopper. We'll see tomorrow!
Well I got rid of the whistle/micro-phonics by slapping some dampeners on V1. Now it's easier for me to aurally separate the oscillations from the microphonics. No more whistle with guitar unplugged and amp dimed.
I'm afraid I'm still having trouble nailing the PO. Guitar plugged in oscillates quite early with treble and presence up. Milder settings don't runaway crazy, but they do have the PO modulation issue.
With my guitar cranked and the amp dimed , if I roll back the tone knob all the sustaining interference goes away, but when tone is on like four and five the amp literally sputters and sounds like a fast or slow kind of machine gun fire (depending on the angle of the guitar and tone knob position; motorboating I think is the official term?)
I attached a 1k resistor to v1b but that didn't really help. I'll find a 1.5k or higher to try next.
Tried removing v2 just to see what might happen and I get a surprisingly loud guitar signal at the speakers with amp dimed.
I don't know what I'm going to try next. Since I'm held up I took the time to measure and record all the voltages on this chart.
Still feels and sounds unbelievable so I'm not giving up.
Charlie
You do not have the required permissions to view the files attached to this post.
I haven't built an Express but I have built a few small amps in cramped chassis and without exception, it usually boils down to lead dress.
Wires too close to something they shouldn't be or too long of leads on grids, etc.
I know from reading hear that the Express can be an exorcise in frustration but if you stick with it and try to get your lead dress right and possibly even use some shielded cable here and there, you should be able to get it right.
I've got a scope down at my shop, but I've got my amp at home right now so I'll have to hook it up or bring it home when I get the chance.
I've noticed I've got my bright caps backwards from Francesca, with the 500pf closer to the front panel instead of the 100pf. I don't know how much of difference this would make, but there isn't much other option because I have cut the leads already.
I'm trying my hardest concerning lead dress, Besides the bright switch I feel I have gotten pretty close to Francesca.
I think I'm going to try and remove the bright caps from the circuit entirely and see if the amp still goes wild so I can nail down the source of instability.
I'll be doing lots of testing in that area today. I remember reading a thread
that I can't find now about alternate wiring with the bright caps using a dpdt switch. Any proven knowledge on it's effectiveness?
Also, I still think it seems weird so much signal gets through without v2 installed. I'd figure that the sound would be cut off completely since you're breaking the circuit, no?
Yes, a lot of signal with no v2 is indeed strange. I would try grounding either grid of the pi to determine which side it's leaking into. Then you can check for continuity on that grid and the coupling cap before the grid with all the points on the preamp board, tonestack, and v1 socket. You'll probably find a solder bridge somewhere, like between the plate and grid of the third stage.
Guitars, amps and girlfriend. 
