New Express build; first time post

[Jackie Treehorn]

Ok, did you revise your presence pot since your initial pictures? The ground to the stainless steel pot looks questionable to me. One of the pi grids is pretty close to ground already, so I wouldn't expect grounding it to have much impact.

[geetarpicker]

I do notice some signal (not alot though) coming out of the speaker if I dime the amp with V2 pulled. Same goes for my original and clone. Sounds kind of like an am radio. Hard to verbalize "some" signal, but some is normal.

[M Fowler]

Here is the DPDT switch.

[WhopperPlate]

Thank you everyone!

Glen, yours sounds comparable to mine, more or less as loud as my clock radio on ten. I think I should not worry.

Good eye Jackie! I reflowed that solder joint and I think it might've had something to do with the problem, but I'm not sure how much at this point as I have to give my ears a brake from all this back and forth listening and I need to do more testing. I'm having a hard time decerning microphonics with oscillation right now. I can turn it up louder with the tone controls cranked before it starts with the feedback/oscillation.

Thanks for the DPDT diagram. I ask because I don't have much more lead to work with on the bright caps, but with this config it would be easier in many ways. Is it any less/more prone to instability compared the traditional method?

BTW I saw a pic in the build guide with the bright caps arranged on the opposite side next to the input jack (in the pot wiring section). Made me curious, wouldn't this be more unstable?

I'm not too experienced with this section of the amp yet, but most things would seem more stable than the traditional method, just from the looks of things.

Charlie

[Jackie Treehorn]

Ok, did you revise your presence pot since your initial pictures? The ground to the stainless steel pot looks questionable to me. One of the pi grids is pretty close to ground already, so I wouldn't expect grounding it to have much impact.

Quoting myself, On further thought, I think pin 2 is probably ok since by grounding at the pin not the coupling cap, the grid was at dc ground, too.

With V2 removed, I can generate max of 74 db of AM type tone with the first four controls dimed and the presence at 0. With all controls dimed, I got up to 84 db. These measurements are through a Greenback Marshall 4x12. I'm not sure what weighting I used; I measured with an iPhone App. The presence had a pretty significant impact so if you removed the NFB, it might make it seem worse.

It must be some capacitive coupling from the second stage to the phase inverter, since the tone and volume controls affect the sound.

[WhopperPlate]

84db, that makes sense to me.

Charlie

[WhopperPlate]

So I'm still working out the bright cap layout; what is proper in a express, the 500pf closer to the front panel or the 100pf?

Francesca shots show what seems to be a big 500pf closest to the front panel and going to the bottom tap on the switch and the 100pf to the top. Layout in the file section shows the opposite. Is this correct, or just a Francesca thing?

Also, does anyone know how stable is the DPDT switch config?

I'm going wait to install the switch and caps back in until I can troubleshoot things without them.

Charlie

[geetarpicker]

Mine has the big cap closer to the front panel, and unique to my amp I have a 500pf and 50pf not a 500/100. To be honest though I always use the 500pf setting which on mine is toggle switch downward. Mine just uses a SPDT switch. If your amp oscillates with the knobs all dimed I doubt it has anything to do with the bright caps as the volume pot when dimed ends up bypassing the caps for the most part. Pulling the caps may make it oscillate less at lower volume settings, simply because of the more limited high end. Still I think if you get these amps dailed in correctly the bright cap adds to the clean to mean potential, and helps give clarity to the cleans. My original amp sounds almost muddy without the bright switch engaged.

The way you have your bright caps in the first pic is very close. Just straighten the yellow wire coming off the center switch terminal so that it runs straight over to the right then up. Bend your cap leads so they are somewhat pulled towards you just enough so the yellow lead can run straight over from the switch center pin, then runs upwards in front of the other wire the 90 degress up, then 90 degress to the right over to the center terminal of the volume pot. Mine looks similar to Francesca near the caps, but the two PVC wires more or less run right next to each other one behind the other. Also in my amps (compared to Francesca being opposite) my caps run to pot terminal 3, and the center tap of the switch wire to pot center terminal that wire being behind the other and therefore closer to the panel. This has been mentioned before as something that was different in Francesca vs my amp and other later ones but I think the BOM already covers this.

One though though is are your wires underneath the preamp board (B+) tight against the board? Probably makes no difference. That said in my original you can get under there with a mirror and tell Ken used a little bit of superglue to keep the wires down tight against the underside of the board. Come to think of it my original also has superglue keeping the two wires from the PI to the output tubes tightly in place, and even the shielded input wire has a little glue there too in the corner edge. I also did this in my clones and I guess it was Ken's way of just making sure those wires never moved. Lead dress under the boards may not be critical but it's something to think about.

Another detail that probably means nothing is my amp has the input jack rotated so that the shielded lead exits closer to the chassis, instead of up in the air towards the ground plate.

You do seem to have a different power supply cap setup. Are you doubling up caps to get the values more "correct?"

Have you checked your pots to make sure the values are truly "correct"?

Overall it's a very nice looking build. The trick being to figure you why it's not happy. Sure "looks" like it should work fine!

[redshark]

Glen, just one question. You only play your amp with the bright switch on max setting as we all know.
Then why did Ken design the amp with also the mild setting and no bright switch?
What would be the application of having 3 degress of bright if you think only using one is correct?
Did he did this because he had in mind that not everybody wanted to take a 4x12 everywhere and that using other cabs like a 2x12 or 1x12 could use other levels of brightness to sound right?

[rawnster]

Don't want to derail this thread at all. But, redshark, looking at your amp, and reading all of the tweaking going on, forced me to look at my express build again. I noticed that my V1 wiring wasn't exactly like the Francesca pics, or similar to what Glen has mentioned about his amps. So, I shortened a few wires here, and repositioned a few wires there, and dang if it didn't get a little quieter. The hissing level lowered a bit. Very cool. I know this has been mentioned over and over, but lead dress is so stinking critical. It reminds me of my dad when he used to take me fishing. I wouldn't catch anything, and he would always reel in a few. He said my problem was that I wasn't holding my mouth right. My southern dad must have known something.

Anyway, great looking amp, redshark!

[redshark]

Well Rawnster, the pics of my express guts are not in this thread, this is whopperplate's which by the way is a good looking build.
I know, you know and everybody knows that to build a express is really challenging because of the stability issues. Lead dress not only affects the stability but also the noise levels and overall tone of the amp, how dark or bright, yadah yadah....is never ending how the amps changes as changes are made. if I look at my amp now and compare to the gut shots I took when i fired it up is very diferent.

Still I think that Glen's suggestions are very interesting and the hint that the amp needs to be dark in order to be fine tuned but I would love to know according to Glen why did Ken use 3 degress of brightness? If the amp was gonna sound good only with the 500pF then why not wire that direct and have the switch removed to avoid instability? there must be a reason why and my guess has to do with the speaker cabs.

[Jackie Treehorn]

So I'm still working out the bright cap layout; what is proper in a express, the 500pf closer to the front panel or the 100pf?

Francesca shots show what seems to be a big 500pf closest to the front panel and going to the bottom tap on the switch and the 100pf to the top. Layout in the file section shows the opposite. Is this correct, or just a Francesca thing?

Also, does anyone know how stable is the DPDT switch config?

I'm going wait to install the switch and caps back in until I can troubleshoot things without them.

Charlie

I think that's wise to get the amp stable then stick the bright switch back in. I'm going to make a radical suggestion to try and isolate the source of the instability. Usually, it's the second gain stage, but things like motorboating seem more drastic and might indicate a problem later in the amp. I suggest tacking in a 500 pf or so snubber cap on the second gain stage to absolutely stabilize it. You can stick it across the plate resistor, or from plate to cathode. Leave the grid resistor on. I think you said it was 1.5k? If the amp is then stable, you know you have to do the usual maneuvors on that stage. If it's still motorboating or oscillating with the snubber, then you'll have to take a look at the grounding and lead dress around the power amp, sort it out, then remove the snubber and see where you are.

[rawnster]

Hey redshark, yeah, my bad on the screen name. Yours was stuck in my head for whatever reason.

Yes, WhopperPlate, your amp looks great.

what a numskull I am.

[redshark]

No problem Rawnster, here is the pics of mine.....

https://tubeamparchive.com/viewtopic.php?t=10129

as I said, many things had changed.....regardless lead dress but if you wanna point something in mine that seems wrong maybe I haven't figured it out yet!!

[WhopperPlate]

Thank you all for your help and comments.

Glen, your comments are so very helpful. Thank you.

Rawnster, I love your analogy. The non sensical always seems to make sense (uh, does that make sense )

Jackie, I'm going to do some more testing and will proceed with your plan if I haven't mellowed her up enough.

Talk to you all later,

Charlie