Really Bright!

[Cliff Schecht]

Something that isn't mentioned often is how easy polystyrene caps are to melt with an iron. It's usually not a problem if you are quick and methodical about how you solder them (i.e. have it stably mounted before soldering). It's easy to tell if you've melted them too, the cap will look physically melted (the clear insulation layers deform easily under heat). This is something I've learned from melting a lot of caps and eventually tracing these out as the problem. Guess I have a heavy soldering hand .

[M Fowler]

I use the 510pf polystyrene caps that turretboards.com sells.

[PlinytheWelder]

I have a polystyrene cap from Watts Tube Audio in C2 on my express. These high voltage PS caps are hard to find although Mouser still sells low voltage Xicons.

To the OP.... What speakers are you using?

[rooster]

Yeah, Mark mentioned that you reduced gain with one swap and now I will mention that by swapping the 500pf for 250pf you are making the amp brighter at the same settings, as well.

So stay with the 2.7K and stay with the 500pf I think.

As to the amp being overly bright, the first thing I would do is try this. Replace the first 25/25 cap with a 250uf/25VDC version. I suggest this because, by doing this swap, the amp should be MUCH fatter. Your gain should also increase at the same time. ...So, if you do this and the amp REMAINS bright, replace/return the cap with the original 25/25 value and move on to the .002 cap. Here you just need to try another type. I will suggest the Sozo mustard version as I use this and it seems consistantly good. ....OK, did this change the bass content? If not, next replace the second 25/25 cap with the 250/25 version. ....Did this increase the bass and gain????

Whew, a lot of swapping but you need to find out where the bass is being shunted.

Another cap to swap is the .1 cap that joins to the 10K resistor (..and the 1M resistor at the PI on the other end) that goes to the Presence control? (No, not the .1 coupling cap that takes the signal from V2 to the PI, but the other .1.) IN FACT, are you sure this is connected? It seems like a hard thing to miss but a cold solder joint on this part is not good. ....BTW, did your PI voltages read out like the build guide shows? This would be a good thing to check. Actually, check this cap first? If you have buggered it - as in one end is not making contact in the circuit - this could be your problem.

OK, that's all I got.....

[billyz]

Maybe post some pics too. Someone might see something. a few other ideas . What taps did you use on the OT ? Polypropylene or Polyester Caps? Audio Taper Pots ? I know these are pretty unlikely, just trying to help.
I have built a few wrecks and none were bright. Even though I used Teflon stranded wire.

[Gibsonman63]

Thanks for the suggestions guys. I have all of my build pics under "Meet Lori Pearl". I need to post some updates, but I haven't done anything major. I did take your advice, BillyZ and used a non-grounded support for the end of my ground bus farthest from the input jack.

I ordered most of the parts directly off of the BOM.

The amp does sound awesome. It is hard to describe, but it seems to have nice mids and a throaty bottom, but the treble is a bit piercing and I have to run the bass and mids almost dimed and pull way back on the treble to get a balanced sound. Also, I noticed that it is really quiet when I am not playing compared to my Marshall 800s and the noise rejection is much better. I was even able to test it without turning my flourescent work light off, something that makes the Marshalls hum like crazy.

I have band practice tomorrow, so it will be a day or so before I can tweak.

[billyz]

I must have missed it but, What speakers are you using ? Cab? Guitar? Pickups etc.

[billyz]

I don't know what effect it has but I see you ran your plate wires very long and close to the chassis and your heaters up on V1. I would do the opposite. I lay the heaters down to the chassis and keep all grids, plates and even cathodes up away and at right angles where they cross each other.
At the very least I would shorten your Plate leads using the shortest path same with Grid leads.

Not a wreck exactly but you get the idea.

[KellyBass]

I think the fact that your build is quieter than your jcm800 is a very good sign!

The polystyrene I used is the 510pf. While I've still got plenty of glassy treble, it seems much less painful than with all the silver micas I tried...even lower valued ones. Even the "turn everything up to ten" client said it sounded "prettier".

[rooster]

Gibsonman63 - OK, that last post suggests to me that something is very wrong with your amp. The traditional hiss of an Express cannot be missed in a quiet room, and, excluding RF interference, the Express is every bit as noisy as an 800 series Marshall if not more. ??

I have to say, too, that it always confuses me when someone says something like 'the amp sounds awesome' but then follows that with 'but there is too much treble so I have to really adjust the knobs to compensate'. ??? See why this seems a huge contradiction? How can you amp sound 'awesome' when it is not behaving like a typical Express? No, a correctly running Express sounds awesome - and with all the knobs on 12 o'clock. Sure, a particular guitar and room might cause you to adjust you knobs a bit, but I can't even imagine turning my bass knob past 1 o'clock with any of my Strats or Teles.

Well, egads, I hope you figure this out. Going back to something I said earlier, did you check your voltages against the voltage chart? Right now I have to think your PI is fubar so get to the voltage chart and compare your voltages here at least. ...Er, of course I am assuming that you have a decent speaker or two, but, well, this may not be the case? Well, its true, a crappy speaker can wreck an Express so I hope you have something decent here. That said I don't know of many speakers that would play/sound that good with the bass and mids turned to 10 on an Express!

[M Fowler]

Gary did a great job on compiling voltage readings see this section.

https://tubeamparchive.com/viewtopic.php?t=8326

[Gibsonman63]

Thanks again guys.

I haven't checked all the voltages on the tube pins, but here is what I have so far:

Mains - 120.3VAC
Secondary - 601VAC
B+1 - 406VDC
B+2 - 395VDC
B+3 - 320VDC
B+4 - 307VDC
B+5 - 297VDC

Power tubes are biased around 42mA and 44 mA. I forget which one was which.

I have a 1 X12 closed back cab with a V30 type speaker and a 4 x12 closed back cab with V30s.

I am playing a '79 SG, completely stock.

The Marshall is an 85 Split channel. It is not too loud on the clean channel, but the gain channel hums a lot. The TW has almost as much gain, but without the hum.

BTW, I also play through an '79 JMP 100W head w/ Master volume. Same cabinets and guitar, but not some much intense treble.

I know the "but" is always confusing. The TW, does sound nice once I get it adjusted. I have never played or heard a TW other than recordings and videos, so maybe I have a limited basis of comparison.

I am not a purist by any stretch of the imagination. I have vintage Fenders for that. One of my main motivations for building my own amp was to be able to tweak and modify it to my liking, so if my final design ends up devaiting from a "true" Express, I am not going to lose any sleep over it. I am trying to get a few hours playing time in before each tweak just to keep me on the right track. I will back up and record the voltages from all of the tube pins and then go from there.

Thanks again guys for all the help on this project.

[redshark]

Ok just a couple of things.....

For what I've heard around here the V30 might add to the brightness of the sound.
Are you playing the amp with bright switch full? (500pf)

Try this....

Set all the knobs at 12 noon except the presence at 9 and the vol at 1pm
No bright switch....

If it's very bright with this settings you might wanna check lead dress on the tone stack.

[M Fowler]

to be able to tweak and modify it to my liking, so if my final design ends up devaiting from a "true" Express, I am not going to lose any sleep over it.

No wonder you ignored all my posts.

Your like me build a TW but tune it to what you want and for your own purpose, very good, Ken would have liked that.

Just go through the voltages for your own reference on the preamp tubes and you should be getting close to where you like the amp.

Mark

[Jackie Treehorn]

Change the 10k/470 tail/cathode resistors to 39k/1k. That'll give you a lot fuller lows to balance out the tone.