My SExpress is finally done

[KellyBass]

Well, I wouldn't say done...it's making some beautiful sounds. I've got lots of tweaking left to do. But, all in all, I'm very pleased with it so far. I've always been intrigued with trying to capture some of that "express thang" in a single ended format. While I can't say that this amp nails it completely, for obvious reasons, the "clean to mean" via the volume knob is amazingly good!

The tone stack is in the first gain stage and the stage(s) values are very similar...so far. I wanted lots of usable volume, so I decided to go with the kt88 and I'm glad I did...it really rips! At the moment it's cathode biased.

There have been some proposed schematics posted on here and some great discussions that helped me a lot. Thanks especially to Dartanion, Xtian and Bigbeck.

I've got lots of cleaning up to do before I post any gut shots...

and I plan to post some clips as well.

[surfsup]

Nice. I must admit i have been considering changing the preamp in my HO to a TW preamp. Your build is inspiring. Do you mind sharing a schematic of what you did?

[JamesHealey]

did you make a fake PI or did you use the Pre without the PI?

[dartanion]

Looks great! Can't wait to hear some clips.

By the looks of it, with 2 12ax7s, you did a driver stage to mimic a PI.

[RJ Guitars]

Nice work Kelly... can we look forward to more details and some sound clips.
? Would love to understand more about how to get the Express behavior in the lower powered amp format.

rj

[Zippy]

Would love to understand more about how to get the Express behavior in the lower powered amp format.

rj

Hmmm... SE KT88 = "low" power???

[JamesHealey]

Yeah it's hardly low power lol..

I imagine that single KT88 is going to be as loud as an Express with 6V6s for sure! and even possibly as loud as one with EL34s!

[KellyBass]

Yeah! I don't know what I was thinking!

This thing's a beast!

Thanks for all the nice comments!

It's fun amp to play, The harmonic content is amazing...definitely SE. Maybe a little too much content?...maybe a little odd order sneaking through? Not sure yet. I need to record it.

Yes, at the moment I've got it set up with a 1/2 PI...and I guess I'll keep it that way. I like the way it behaves, but it's tempting to try it straight in since I've had good luck with that as well, in a SE el34 amp that I love.

I've still got some tweaking to do and a bit of chilliness that I'd like to get rid of. When I get some of this sorted out, I plan to draw out what I have and post it. I'm very anxious to get everyone's more experienced (than myself) opinions and make some improvements on the design. I just sort of went for it...and borrowed, I think, every trick in the SE book to build it. But I'm sure there's a lot that I missed.

I'll get some clips up when I can get my shreddin' buddy over here. He never turns down food.

[dorrisant]

Very nice looking amp... I have been mulling over a big-bottle SE Express. Very interested in how you warm it up... Could you possibly take some before and after clips. Just curious... what brand of caps did you use?
Tony

[KellyBass]

Very nice looking amp... I have been mulling over a big-bottle SE Express. Very interested in how you warm it up... Could you possibly take some before and after clips. Just curious... what brand of caps did you use?
Tony

That's a great idea, Tony. I will do that...starting with what I have now, record before and after each adjustment and note the amp changes and amp/guitar settings. I'll use my strat and a reverend humbucker guitar (that's on loan to me at the moment). You guys might have to endure some "bass player on guitar" though!

As far as warming it up, I plan to play with the voltages a bit and switch out some components in the signal chain. I used a LOT of spare, crappy stuff in this build. I wasn't sure if it would even work that well. Now that everything's cool, I'm gonna invest in some better sounding stuff and see what happens.

I think especially critical is noting the volume settings on the guitar, as clean-to-mean-without-tone-loss is part of what we're after here...and this amp really does achieve this. To the extent that a PP express does? Not quite sure yet. Maybe the recordings and comparisons will help. I've also got my PP express clone, to record and compare it to.

The extent that this SE amp is able to achieve some of the clean-to-mean thing lends some credence to the first stage tone stack and hot/cold (or cold/hot...can't remember off hand) biasing of the gain stages as being a critical component of the xpress thang. I'm not sure how much the faux PI adds to this, but from what I've read here, the xpress PI is a critical part of the magic. Maybe Dartanion can chime in and expand on this...If I remember right, he's the one that suggested trying it.

I used 4 47uf Sprague atoms and no choke in the power supply....cause I had em'. I also hung a seperate 10uf cap on the KT88 screen supply for additional filtering and stability...and cause I saw it in a HiFi book and it seemed like a good idea. It really seemed to help stabilize the wacky screen voltages I was getting. Maybe not, though. I often make the mistake of making many changes and not testing between...so I don't end up knowing for sure what solved it. Bad habit.

[dartanion]

Using the full PI retains the characteristic of clipping in the PA > PI > Preamp. I am sure you can setup a single driver stage to sound similar with some experimentation.

If I am reading your post correctly, you used 47uF filter caps for the preamp stages? That's a bit much for these stages. I would wager some warmth would be added by backing these down to 20uF or even lower to say 10uF.

[dorrisant]

Glad to inspire, if at all possible. I have been experimenting with some express preamps with the fake pi. I have had a log of complements on how it works outside of the normal push-pull configuration. BTW... the sozo mustard caps (the regular ones... not the hand-made type) sound great in the ones I've built. I have been reading about how the premium sozos are even better... I will have to see for myself sometime.
How big of a PT did you end up using?

[KellyBass]

If I am reading your post correctly, you used 47uF filter caps for the preamp stages? That's a bit much for these stages. I would wager some warmth would be added by backing these down to 20uF or even lower to say 10uF.

You know, I had wondered about that...how would using these higher values effect the overall sound and feel of the amp. I must say, I'm intrigued, and fairly in the dark, about PS filter caps and how they contribute to the tone of the amp. I can see how they can effect feel. I'm going to revisit that chapter in Merlin's book. I've got a bass amp build planned that specs 2 800uf caps in C1 and C2 of the PS! They are huge! But the designer insists that they are a critical component in his design. "Gotta get down to those lows" he claims. Mostly I used these 47uf's because I had em'...and they stack up nicely. Dart, If I was to pull, say one of the 47's and replace it with a 20uf, which one would you consider to be most critical. The one that feeds the pre?...or earlier in the chain?

Dorrisant, I'd like to see your conclusions on the PI experiments. I used a Edcor XPWR107 PT. It's 600VCT @150ma. I used the 20watt SE OT that doberman sells. I'm using the 2.5k tap. Thanks for recommending the sozo's. They are going to be part of the "warming up" of the amp. OD's are in their now, and I can't wait to pull em' out.