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

[SoulFetish]

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.

When you say "ordinary box caps" are you referring to the DC-Link types which are designed for smps/inverters?
For instance, a Vishay MKP1848C series cap rated at 60uF/600V is rated for 17A RMS and an Ipeak rating of 840A.
It is rated for 105C, but obviously lives it's best life around 70C, with the voltage and capacitance derated as temperature increases.
Am I overlooking something in the datasheet to suggest it is unsuited for a reservoir capacitor?

[R.G.]

When you say "ordinary box caps" [...]
Am I overlooking something in the datasheet to suggest it is unsuited for a reservoir capacitor?

No, quite the opposite. Pretty much, DC link or whatever box style caps are fine for anything further in the RC dropper chain.

The only caveat on any film cap being used for RC dropper chain duty is the RMS current through the cap's effective ESR making for self-heating. DC link caps are rated for high currents, and will be fine - loafing in fact.

[shane]

Hi RG
I Don't know about anyone else, but I would seriously like to hear more about your work on simulating a tube rectifier. Sound like an incredibly useful thing.
Cheers

[R.G.]

@Shane: I'll dig it out and post it. There was a thread on Music Electronics Forum that went to several pages on the topic, but MEF has been down for some time now, so I can't just point you to it.

[shane]

That would be great, thank you RG. I'll look forward to that.
I haven't been on the music electronics forum for ages, it seemed hard to access properly for sometime now. Pity id like to have read that stuff!

[R.G.]

MEF seems to be having an existential crisis. tboy apparently hosts the whole thing on his own hardware, so he has to keep both going. Rumors say he has health and other problems.

I hope it's backed up.

[soundmasterg]

MEF seems to be having an existential crisis. tboy apparently hosts the whole thing on his own hardware, so he has to keep both going. Rumors say he has health and other problems.

I hope it's backed up.

Tboy was hosting the MEF forums on his own server in his garage for decades. Worked fine except for when he got some health problems and had to be hospitalized, and then after he was released he had some continuing issues that made it so he couldn't continue to maintain the server the way he had. Some of the regulars on MEF took over the keeping it running operations such as updating software etc., but the guy doing that is apparently in EE school and had some problem with updating something recently which is when the site went entirely down. From what I heard in a thread here, he had messed up some update and was working to fix that, but was in the middle of finals. Was expected to be fixed over a month ago with no word on any updates. MEF was one of the best forums out there for tube and solid state amps, guitar pickups, pedals, etc., with a wealth of knowledge documented going back 20 years or more, and a lot of really good and experienced members such as our esteemed R.G. Keen, Enzo (until he passed), Merlin, for example. Hopefully it is able to come back and not disappear permanently like the Weber forum did.

Greg

[shane]

I really hope it gets back up and running again, it was a fine forum. Fingers crossed for it's return.
You know I had no idea that Enzo had passed, man that's sad. I'm so behind with events.
Cheers all.

[soundmasterg]

I really hope it gets back up and running again, it was a fine forum. Fingers crossed for it's return.
You know I had no idea that Enzo had passed, man that's sad. I'm so behind with events.
Cheers all.

Yeah a lot of the folks on the forums are getting old. Enzo passed early last year I believe. PRR and Sluckey on the Hoffman forum have stepped away to do other things in the last 6 months but they're both up there also. With all the facebook groups, the traffic on the forums has died WAY down, and some forums have gone away like the Weber one. It is too bad really as forums are a better source for the information because it is all saved and is searchable, but the facebook groups just scroll down and it's gone. The MEF was the first forum I got into for tube amps, before it was the MEF...I think around maybe 1997 or so. Tboy when I asked him to graciously picked up the Sunn forum and the pickup forum when they were going to be going away unless someone took them on, and he's run a fantastic site for decades.

Greg

[martin manning]

It is too bad really as forums are a better source for the information because it is all saved and is searchable, but the facebook groups just scroll down and it's gone.

I agree. FB is pretty much worthless for the kinds of things discussed here.