The Immortal Amplifier

[R.G.]

Yeah, the net result of my simulations is that motor tun caps are way, massively, overkill for any normal service considerations. I have not dug deeply into Solen caps, but even ordinary box caps are fine for all but the first filter cap. Film caps with some special-ness should be fine if they can be smaller and still handle the power.

The first filter cap is actually a "power converter" part. It self-heats from the AC ripple current it conducts while filtering rectifier pulses. This self heating can be the major factor in shortening the life of a first filter cap of any kind. That's really what I was after in the sims - what magnitude were the ripple currents, and what did they do in terms of heating? And how much actual current can the prospective film caps take?

The results of this little shallow dive reinforced my feelings about them. Motor run caps are designed for lives of a decade or so with several to many amperes of ripple current running through them, and to dissipate the heat this creates. As a first filter cap, they are being used at very, very light duty, so their lifetimes should be much longer than in the motor running duty they were designed for. I guess I should look at Solen caps from this perspective.

It makes me happy to see some of my ideas leaking into normal practice. The "Immortal Amp" stuff and things like the MOSFET Follies were turn-of-the-milenium things. It takes a while for things like this to get used in such a highly standardized design genre as tube guitar amps.

[didit]

Hello R.G. --

Interesting thread. A less rigorous datasheet based spreadsheet analysis done during COVID lockdown suggested parallel gang of quality "box" film caps would suffice for that critical first stage. Of course no records remain, however after quick dive into Digikey selector and review of this KEMET data sheet that links off of this 8uF part C4AQIBU4800M11J aligns with recollections. For typical 32uF initial filter node, a gang of four nets an ESR less than 2 milliOhms.

Best ..

[Kalamazoo 2]

A 47uF/630v Solen is about 2"x3" according to TubesAndMore specs.
Solen sheet shows 10uF/630vdc to be 26x30 mm.

So, bigger than the F&T's that I used in the Kalamazoo. A lot bigger but I might could have made room. There's plenty of room in the push-pull Epiphone Constellation, but that's a bass amp. It's like an airplane hangar in there.

Solen Fast Film caps

[nuke]

I'm actually OK with electrolytics.

The Kemet PEG124 series are rated for 27,500 hours at 105c. That's kinda-plenty.

https://content.kemet.com/datasheets/KE ... PEG124.pdf

They're not too huge, and plenty good for bulk capacitance and they're not unreasonably priced for axial caps these days.

If they're available in suitable ratings, that's my go-to replacement cap.

Heck, I still have NOS Sprague TVA Atoms in my bin from 2000-2002 that are quite fine. (before they changed how they're made). I wouldn't sell them to anyone, but I'd happily use them in my own projects after a quick check and an hour at rated voltage to make sure they're well within spec. I haven't found any bad ones, despite their age.

[soundmasterg]

I'm actually OK with electrolytics.

The Kemet PEG124 series are rated for 27,500 hours at 105c. That's kinda-plenty.

https://content.kemet.com/datasheets/KE ... PEG124.pdf

They're not too huge, and plenty good for bulk capacitance and they're not unreasonably priced for axial caps these days.

If they're available in suitable ratings, that's my go-to replacement cap.

Heck, I still have NOS Sprague TVA Atoms in my bin from 2000-2002 that are quite fine. (before they changed how they're made). I wouldn't sell them to anyone, but I'd happily use them in my own projects after a quick check and an hour at rated voltage to make sure they're well within spec. I haven't found any bad ones, despite their age.

I had some Sprague Atoms from the same era. I used some in a Sunn that I rebuilt for a friend and after a week one of the caps blew despite my having reformed them on my Sprague Tel-ohmike, and checking them for an hour or so at rated voltage as you suggested. I ended up having to replace all of them with some new F & T's. I kept one to cut open but I try to not keep around or use old electrolytics anymore. I just order them when I need them for repairs so I don't end up having something sit on the shelf.

Greg