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In Reply to: RE: Thanks! nt posted by bjh on April 10, 2014 at 17:55:52
Many times guys will rebuild a vintage amp or preamp and they mistakingly order the same value filter cap(s) that came with the unit many years ago.
While this can surely work,it doesn't address the two major problems that exist today that didn't exist many years ago.
Number one,the digital age of computers,plasma TVs, and digital everything else,has put a lot of noise on the power lines and RF transmission in general,that it gets into our equipment thru the power supply.Having a generous amount of quality and abundant filtering,can help suppress noise that comes into the amp or preamp.That is very important for quiet backgrounds and to be able to hear those little nuances we frequently mention.
The second problem is the dynamics in the listening material we have today as opposed to when the amp was originally made. 30db to 40db dynamic range was about the norm in those days where today its 90db to 100db on average.That type of dynamic range puts a heavy demand on the power supply caps where those under rated stock values in amps like the Dynaco ST-70 or Dyna MK3, simply cannot keep up and supply the needed energy for the circuit demands today with high peak power ratings that could be 300 to 400 watts out of a 60 watt amp.
Filter caps do more than just filter ripple out of rectified AC.They have to store energy and they have to supply energy to the load on demand.This brings us to those monstrous computer grade electrolytics mentioned that have an ESR rating thru the roof.This presents a couple major problems..Number one is,dielectric absorption is very high and basically much of the stored energy stays trapped inside the capacitor across the dielectric so that it's not really usable. There is capacitance that gets used but it much of it is wasted because of the cap's inefficiency.The other problem with these large computer grade caps is,since they feed the audio output devices,whether it be tubes or transistors,the audio signal is riding on that line which is an AC voltage in itself,and a portion of the signal can be filtered out thru those big caps with high esr..This is why those caps have to be bypassed with a polyproplyene cap(s) so that the big cap sees a lower ESR and doesn't absorb the audio signal that is riding on that line,or at least very little of it.
Dennis Hadd and Roger Hebert use a lot of those big CG caps and I have one of Roger Hebert's early amps and preamps here,based on the MFA design.I know Dennis still uses those big CG caps but I don't know if Roger does anymore.Now you know why I like poly film caps for filtering and energy supply,even if I have to use the original values because they are very fast,very efficient,and the ultra low ESR doesn't damage any part of the circuit.
Charge up one of those 4500uf@500v computer grade caps..Then take your screwdriver and short across it and see how many times it takes to fully discharge the cap.You will see the stuck and wasted energy I'm talking about.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/10/14Follow Ups:
Mikey,
On the Valve Wizard article, it says: "When the reservoir capacitor charges up with each input cycle, it draws a large current pulse from the rectifier and power transformer. This is known as the ripple current, and the larger we make the capacitor the larger it will be...this is why you should not use a ridiculously large value capacitor, as the increased ripple current would put a great strain on the rectifier and power transformer. "
So, regarding vintage tube amps and their power supplies, is there any data that tells us how large the capacitor (film) should be that replaces the electrolytic capacitor? For example, in my Lafayette KT-550, the schematic shows a 2 x 200uF electrolytic capacitors as the "reservoir capacitors". In replacing these electrolytics, I have used 2 x 110uF Claritycap TC film capacitors. Smaller in uF value, but film instead of electrolytic. In the back of my mind, I always wonder if I should get 2 x 200uF film capacitors. This would go against the recommendation in the quote from the article, but aligns with your comments about needing more filtering given our modern day noisey AC.
So, which is better? Lower uF film capacitance or higher?
Pat
In a tube amp with a CLC filter and tube rectifier, the cap before the inductor should be limited by the rectifier's ratings, for example, I believe a 5AR4 is only supposed to see 30 µF. There is some wiggle room for more capacitance with lower than maximum plate voltages and design center margins, but the first cap should not be arbitrarily large (certainly not in the hundreds of µF.) In contrast, the reservoir cap (after the choke) can be much larger. I don't know how this changes for the second cap in a CRC filter.
Incidentally, some of those CG caps may have poor reliability; google the "capacitor plague."
I usually put a 35uf poly off the GZ34 but you can go 50uf with no problems.I don't want to discourage people from using electrolytics because there are many fabulous ones out there like the Panasonic ED and the F&T and the JJ are all fabulous caps and I use tons of them.I like to use polyfilm caps for filtering as much as possible but it's not always practical because of size restraints.
Honest amplification is better than excessive 2nd order distortion anytime.
...discharging a cap with a screwdriver is a totally unwise thing to do. It's dangerous and the cap can be damaged unless it was designed for rapid discharge.
Most caps will survive being shorted, but discharging p/s caps is so easy, why do it? I made up a discharge tool that's easy: a 100Ω 10W resistor with leads on both ends; 1 end has a probe and the other has an alligator clip. Clip to the chassis and use the probe on each cap for a few seconds and it's all done. Or use the method at the link.
WW
"A man need merely light the filaments of his receiving set and the world's greatest artists will perform for him." Alfred N. Goldsmith, RCA, 1922
hello!here is a tube amp without cap, only wire:
http://www.jogis-roehrenbude.de/Leserbriefe/Frank_Bloehbaum-Amp/NoRNoC.htmregarding the first cap before the choke: in the radiotron design handbook
there is a chapter about the power supply, the best is no cap, because than you have a constant current without a ripple drawn from the rectifier, because the choke can store energy. There are also pictures which explain this relation. cheers Armin
Edits: 04/17/14
Steve
Of course it is and it's not good for the cap..I keep a 30uf poly around just for that purpose to demonstrate just how strong the snap is coming out of the cap as opposed to many electrolytics.
I should have said to do this on an old cheap computer grade electrolytic if you want to experiment..You can get those for next to nothing at hamfests..It was simply a way to show people how bad those caps are.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/11/14
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