Express Cathode Bypass Capacitors

[Littlewyan]

As you may know I've changed the capacitors in my Express recently from 715P to 6PS with fantastic results. Looking at my Cathode Bypass Capacitors that I've used they are the brand 'Forever' (never heard of them, cheap Taiwan caps maybe) and seem to be quite a bit smaller than the caps that Mr Fischer used on original Wrecks. I'm leaning towards ordering some Sprague 25uF 50V Caps to replace them, anyone ever done this and noticed any difference?

I know some cheap Taiwan Caps are made in a way to decrease the caps in physical size and will measure the correct value using a meter but as soon as you put high voltage across them you no longer get the specified value. Though this tends to only really affect the larger mains filtering capacitors they make.

FYI, the caps in the amp at the moment are 22uF 25V which is the value Ken used but they are noticeably smaller than the caps he used.

[David Root]

Never heard that about "cheap Taiwan e-caps" before...cannot figure out how that would work.

On the bypass caps, I'd be more concerned about the no-name brand than their size. All modern caps are smaller than the stuff Ken used 30 years ago.

Those black Sprague bypass caps have been associated by some users with unexpected failure in operation. I use them and have not yet experienced that myself, but the reports are out there.

An alternative would be the newer Sprague 30D type, I use these too and have not seen any problems posted with these. They are made in USA, right on the cap. The smaller values tend to be up to 50% above nominal value but above 15 uF they seem to be closer.

[Mark]

I think you'll find almost all modern caps are smaller than the vintage caps, I think they have a lower ESR too. This may or may not effect the sound of the stage. I'm inclined to think a change in tone won't be heard. The other consideration is the tolerance of the caps K F used and what was the typical value?

An example is the Gibson Historic pots, they are 500K with a 20% tolerance, the pots I received were between 420K to 450K. Not 400K to 600K.

[ToneMerc]

An alternative would be the newer Sprague 30D type, I use these too and have not seen any problems posted with these. They are made in USA, right on the cap. The smaller values tend to be up to 50% above nominal value but above 15 uF they seem to be closer.

Also know as the Sprague TE series, that's my "top shelf" cathode cap choice, followed by the Vishay MAL series.

TM

[Littlewyan]

I may leave the capacitors for now. I am going to possibly be rebuilding the Express after Christmas, might be getting a Pacific Output Transformer and installing that means I'll have to take out the main component board to unbolt the OT. So at the same time I'll be rewiring the amp with the correct wire (mine is horrible thin stuff) and may replace the bypass caps as well.

Thanks for the responses guys

[beasleybodyshop]

I replaced my cathode caps with those Sprague ones you mentioned. I did find a tonal improvement over the radioshack nichicons that I had used.

But i found the best difference i could make was to tack another 150K in parallel with the voltage divider after the 2nd gain stage. Now its not as wooly sounding and more controlled.

[Littlewyan]

Yeah but thats changing the circuit. I'm interested in tonal changes when changing parts that are supposed to be the same spec. I think I will give the sprague caps a go when I basically rebuild the amp. I have read in a few places about differences between electrolytics but have always assumed that the big differences will be between mains filter caps and not cathode bypass caps.

[rp]

Hard to hear any diff after 5mf, though never tried a (sensitive) TW and never with a big sealed cab. If you run it through the tone stack calculator the curve hardly changes after 5mf. So, for fun try a 4.7-5mf / 100V film cap. Not saying it's better or even good, just something to do after all the eggnog's gone.

[Mark]

There are 10uF film caps available and these should be the best caps to use as there is no ESR with these caps and they'll last longer than electros.

When tweaking my Champ clone I used 5uF caps and I could hear the difference between 5uF and 22uF, but it wasn't a big deal, I was more or less looking for "liveliness" from the amp.

[Littlewyan]

As tempting as that is I'm actually trying to move closer towards KF's Trainwreck Recipe. Like I said I may be rebuilding the amp after the new year as there are a few things that I wish I'd have done differently when building this amp, but thats for another thread really.

Merry Christmas btw

[Littlewyan]

Just to let you guys know I have ordered new capacitors and in the end I went for Vishay AHT 22uF 63V Caps. They are supposed to last 8000h when running at 125 degrees so should last a lifetime as Cathode Bypass Caps. I was going to go for the Sprague TVA Caps but they didn't quote how long the capacitors should last and actually mentioned very little in the datasheet :S.

Here are the caps I ordered: http://uk.mouser.com/ProductDetail/Vish ... 2fm0aDw%3d

[donzoid]

Hi Wyan,

I've been lurking this thread, and I'm curious about the comment you made early on about the 6PS being better than the 715P's.

Can you share what the benefit was to your ears, as I have been considering doing this to my Express clone also. Did it have a smoothing effect, or a midrange shift etc. Thanks!

[Littlewyan]

When I was using the 715p Caps the amp was quite harsh sounding with so much clarity that there really wasnt anything to the distortion. Changed to 6PS and the high end has been rolled off slightly and there is now a real nice grind to the mid range. Still a very bright sounding amp but its no longer harsh. Still needs work but the caps definitely helped a lot. I should point out that I replaced the unbranded Silver Micas with Cornell Dubilier Silver Micas.

Hope this helps

[donzoid]

It does help...thanks!

What I did (first) to get rid of my ice-pick-y issue was install a cut control. That is quite helpful when I have to set master volume down at all, as it gets more and more fizzy.

Then before our last gig, I changed the 500pF cap to 360pF...but after running it without the cut (I have a bypass switch) all night, it lost too much of its bite. I already have orange drop polyester's in there now but like you, have been considering a different OT such as the new one RJ had made by Merc. I feel like I'm far away "enough" with the MV and cut control, so those are gonna come out in favor of a VVR.

Thanks again,

[Littlewyan]

Have you got a thread showing pictures of your build?

Edit: I just found your old thread with photos of your build. If you want help with it start a new thread, post recent gut shots of the amp and tell us what transformers, speakers and guitar you're using. A simple wire change can make/break these amps.