Express Capacitor Choice

[Littlewyan]

Has anyone actually used OD 715p Caps in an Express? I've read a lot of posts from people stating that these are big no no but has anyone actually tried them? I have them in my build and find it very pleasing to the ear. I must admit that using the 500pF Bright Cap is too much even for my Les Paul but it even then the amp doesn't get ear piercing.

[rooster]

The basic argument is that polypropylene (715) is faster than polyester, the Ken choice. Speed in this case = brightness/edginess.

[Jackie Treehorn]

The 716P is my general go to cap, I'm not as familiar with the 715. They need some time to burn in, they can sound a bit constricted at first, but after 10-20 hours they sound great. They're a transparent cap, so they don't screw with the sound as much as a polyester.

The interesting thing, is that if you watch the videos Glen posted on youtube of Ken discussing what's special about his amps, he says they're based on two technologies: guitar amps and hifi. He then specifically mentions polypropylene and silver mica capacitors.

I've always thought of the Express as more of a hifi amp. In the hifi world, you generally go for modern, high performing parts. They're not seeking out caps from the 50's or 60's amps, as it's well known they hold back the amps. That's how I feel about the Express. It's not like building an old tweed circuit where you're going for a bunch of slop, I feel the Express should be fast and raw with a lot of harmonics. I may never be as good an amp builder as Ken, but I can certainly use better parts as a crutch! So, for what I like, polypropylene is better. People seem to focus so much on the high end, but a polyester cap can be pretty deleterious to the mids and lows, too. If I'm trying to choose a part to soften the top, I'd much rather use a fat sounding resistor like a carbon comp or a capacitor like a Russian Paper in Oil. Those parts don't lose the detail or smear the mids as bad as some of the polyester caps.

The Mallory PVC's, though, actually sound pretty close to a polypropylene. They have a slight roll off of the highs. They're pretty good sounding caps. The dielectric is not the whole story. The brown Xicon polypropylene sounds rolled off like a polyester, for example. But, I don't find the PVC to be "magical" or essential. I would prefer a 716P.

Three approaches I'll try in an Express to deal with some extra top from not using polyesters are...
1. Local negative feedback on the second stage. I'll run the plate and cathode wires to the board parallel which rolls off a touch of top end and stabilizes the amp. You can actually control the amount.

2. I'll use one polyester to get the chewiness in the mids. It only takes one polyester to get what I like from them; I don't see any advantage in the Express to using polyesters everywhere. I suggest the .002.

3. Parallel caps. If you bypass a coupling cap with a smaller cap, you can have the sound of the smaller cap on just the highs. I like to do this with good sounding ceramics, so you get the clarity of the polypropylene with the grit and softening of a ceramic on just the highs. This is actually a hifi trick. I've got an old pair of Snell speakers from the 70's and the capacitance on the tweeter is made up of two large electrolytics of equal value, then bypassed by an orange drop and then another wima!

Also, switching the silver mica to polystyrene softens the top, too.

Sorry, long post!

[Littlewyan]

Don't be sorry that was extremely informative! I'm wondering about the 500pF Mica as it does add a lot of high end but there just doesn't seem to be much clarity when using it. Which is odd as you'd think the clarity would increase! It might be a bad cap as I had one from the same batch in my Marshall and the sound with that cap was so clear.

[KellyBass]

Try a 500pf polystyrene cap in place of the silver mica. To my ears, they're much less grainy and harsh.

[rooster]

Jackie, "1. Local negative feedback on the second stage. I'll run the plate and cathode wires to the board parallel which rolls off a touch of top end and stabilizes the amp. You can actually control the amount."

I get the visual of what you are suggesting, I think, correct me if I have it wrong. Looking at a 12AX7, you parallel the wires from pin 1 and pin 3, or pin 6 and pin 8, as they return to the board and the plate resistor/coupling cap (pin 1 or pin 6) and the cathode resistor/cathode bypass cap (pin 3 or pin . - two wires - and somehow this effects the top end entering the coupling cap attached to the plate creating negative feedback and which thereby stabilizes the amp? You're saying there is a formula here and based on how close the two wires are, either more or less high end is rolled off/more negative feedback is presented to the circuit?

[Jackie Treehorn]

Yes, that's it exactly. The distance between the wires (closer is more) and the amount of area that they're parallel determine the amount of capacitance between them. There is a formula here, http://en.wikipedia.org/wiki/Capacitance, but you can measure, too. There's not a ton to work with, of course.

[rooster]

Jackie, this? http://en.wikipedia.org/wiki/Parasitic_capacitance

[Jackie Treehorn]

Yes, that's a good one. I tend to think of that as unintentional coupling. Something to keep in mind though, that positive feedback coupling is to be avoided most of the time!

[vedat]

Special thanks to @Jackie Treehorn, the knowledge and experience you shared has to be appreciated. Now coming towards the first and basic question of this thread. Comparing it with an image, sometimes, we need to see vivid images with sharp and bright colours and sometimes, the artistic value of an image lies in the subtle blur with a few sharp and focused areas. The choice of this cap is much like that, people don’t usually recommend it because it provides a very rapid response which is edgy and irritating at times but in other scenarios, it might be preferable, depends on what you think.

[Littlewyan]

To bring my Express closer in line with the original I want to change the caps out and possibly even the wiring (emailed RJ for a price). First of all I'm still using the 715p Caps with values such as .022uf and .0022uf and 470pf caps instead of 500pf caps. The amp sounds nice but the high end is a bit piercing at times and for use of a better word a bit glassy. So I figured changing to 500pf caps with give a tiny increase in mids and using 6PS Caps will roll off some high end. I'll also be changing to .02uF and .002uF Values which will make a small difference, depending on the caps I get. Technically a .02uF with 10% tolerance could actually be a .022uF.

I want to change the wiring as the wire I used was 22AWG that is 1.2mm thick. Its thin, weak, doesn't look nice and i dont trust it (insulation breaks so easily). Do you guys reckon I'll hear a difference once I've done all of this? I know I could use local negative feedback like Jackie suggested but I think that would kill harmonics as well.

I should add that I'm playing the amp through a 2x12 Cabinet which I realise doesn't have as much bass but I still think the amp needs some adjustment. May be able to try it through a 4x12 on Sunday at my band practice to see.

Edit: Also Vedat I apologize I didn't realise you posted on here until now. Great explanation there. I feel like my Express is too transparent, needs a bit more character which I believe these modifications will give me.

[Clyde]

Has anyone ever done a blind test to really try and differentiate the difference between caps? I know it's easy to hear things when you know what you're listening to and rely on general internet knowledge of what people say things sound like, but have you really listened with your ears only? Just wondering.

[Littlewyan]

I would like to do a recording before and after but I just dont have the equipment. It does have to be a blind test though as like you say when you've been told what to expect you listen out for it. Sometimes I wonder if you just hear what you want to hear. At the end of the day though as long as you feel better about your tone thats what matters.

I've ordered the 6PS Caps and 500pf Silver Micas. I can't wait for the silver micas as my current 470pf cap tends to bring in a lot of noise when its switched on. I just assumed its normal but we'll see.

Edit: Got an idea, I have a spectrum analyzer that I could use. I'll do before and after readings.

[Littlewyan]

To update this thread: Changing the caps made a big difference to the sound. The results are here in another thread:

http://ampgarage.com/forum/viewtopic.php?t=26526

I didn't post them here as it wasn't just a Trainwreck thing but more of a general technical post.

[BLT]

Tone is so subjective, most of the time I don't detect tonal differences, except in side by side comparisons. What I notice more than anything is responsiveness. Some of my Fenderesque amps have been dead in my hands. When I started building Wreckish type amps, my playing started to represent what I was trying to express, especially in the EL84 amps. My next bulid will be with 716s, so maybe I will revisit this thread.