On Using Higher Capacitance Power Supply Caps

[harry]

Hello,

My Carlsbro CS100TC was built around the year 1970. It has the original caps, so I am going to change the electrolytics for safety purposes.

There are 6x 50uF 450V capacitors in there (as shown in the schematic). My question is, could I use 100uF capacitors, or would this be too high a value? I don't mind if the amp becomes very "stiff"; I just don't want it to be dangerous.

http://www.chambonino.com/carlsbro/carlcs100tc.html

Any help / suggestions would be much appreciated!

Thanks,

Harry

[black_labb]

should be fine. its only really an issue when there is a tube rectifier.

[harry]

thanks, black_labb.

should i expect an increase in the HT line voltage if I do this?

[black_labb]

could be a small change, but shouldnt be much.

[jrfrond]

I don't mind if the amp becomes very "stiff"; I just don't want it to be dangerous.

I don't get where the "dangerous" part comes in.

The only issue would be if the voltage rating of the new caps was too low. You can double the capacitance with no problem. It WILL be a tad stiffer, but if you drive the amp hard, it will "thump" in the bass a bit more. This is one of the "secrets" of the vintage Marshall tone over the 5F6-A Bassman. In fact, many amps designed for hard rock and metal will empoly extra filtering for this reason.

[trem]

Would there be any wisdom to increase the capacitance of the first 4 caps? Maybe the last pair that feeds the preamp could be left at 50 mics?

[black_labb]

Would there be any wisdom to increase the capacitance of the first 4 caps? Maybe the last pair that feeds the preamp could be left at 50 mics?

why do you want to up the caps? is it to increase the stiffness or to eliminate hum due to B+ ripple?

i would probably change the 8uf cap after the 22k resistor, this will eliminate some hum if the ripple is the problem, and stiffen up the preamp if the preamp creates some sag.

[tedmich]

Hello,

My Carlsbro CS100TC was built around the year 1970. It has the original caps, so I am going to change the electrolytics for safety purposes.

There are 6x 50uF 450V capacitors in there (as shown in the schematic). My question is, could I use 100uF capacitors, or would this be too high a value? I don't mind if the amp becomes very "stiff"; I just don't want it to be dangerous.

http://www.chambonino.com/carlsbro/carlcs100tc.html

Any help / suggestions would be much appreciated!

Thanks,

Harry

A good idea is having parallel 220-330k, 2-5w resistors (actually 1-2 watts should suffice!) for each of the series caps; this equalizes the load on the caps and also bleeds them down on shutdown. Make sure they're up above the cap and board as they get hot and can hurt cap life. More filtering is better except many guitarists like "sag" and "sponge" from tube rectifiers, low filtering and under-engineered transformers (Mfg like the savings too!). Kendrick amps use higher filtering and kick ass though.

[tbryanh]

The Valve Wizard says: Beware also that unecessarily large amounts of smoothing / filtering can lead to 'ghost notes'- an unwanted effect where a vague, fuzzy echo follows each note played.

http://www.freewebs.com/valvewizard1/smoothing.html

[defaced]

I don't know if ghost notes are affected by preamp design or not, but as an anecdotal point of reference, the Soldano SOL 100 has 220uF of B+ filtering after the rectifier. In a high gain built I'm working I'm currently using about 400uF after the rectifier and then 50uf for the screens and 22uf for each preamp node, and I'm not hearing any ghosting.

[Steve Conner]

I always thought it was the other way round: ghost notes are caused by a lack of filtering. They are new frequencies caused by modulation of the original note with 120Hz and its harmonics.

I once built an amp with all the B+ rails regulated, which is the same as having infinite-sized caps.

[tedmich]

too much filtering = ghosting?

I respect Mr Wizard a good deal...BUT

I have heard this a few times, but also more often the exact opposite argument:

http://acruhl.freeshell.org/mga/main/capacitors.html

maintains that older caps losing their filtering creates “ghost notes”

similar here:
http://www.marshallforum.com/workben...apacitors.html

quote:
Kingsley Deluxe 32, 4 x EL84 amp, maker Simon Jarrett "Ghost notes can quite often be caused by the amps power supply, especially in an amp with small or relatively small value filter capacitors. The sag that occurs when an amp is pushed (which manifests itself as a sort of compression - which we like!) causes a 120Hz signal from the power supply to modulate the actual guitar signal, resulting in ghost notes. Volume always makes it worse. While adding larger filter caps can help reduce it or eliminate it, you may not want to do this to an amp or may not be able to.”

and Aiken Amps concurs: http://www.aikenamps.com/Sag.html

I can't say for sure as I have not done the experiment (yet), but HiFi tube amps go up to ~20,000 uF filtering (admittedly with a very different topology) and suffer no "ghost notes"

My Peavey Bravo will be reborn soon going from 47-47-22-22 uf to 110-110-110-110 uf and if it "ghosts up" I will retract!

PS the parallel equalizing resistors over the series caps is an essential safety practice IMHO

[6267]

I really dig the wizard, but I can't see how too much filtering would cause ghost notes either. 'Course if I took everything I didn't know...

There is a danger in using really, really large capacitance, since an accidental short to human can deliver extra peak current. Although if accidental shorts to human are occuring, you have bigger problems than ghost notes . One of my first tube projects was an AA5 radio with both problems. I believe it was thrown out the window.

That being said, I had 680uF right after a SS rectifier on a prototype that was humming for non-B+ reasons (as I soon determined when it kept humming). It was great though because you could pull the plug and play until the heaters got cool.

[Merlinb]

The preamp will nearly always benifit from larger smoothing caps.
However, you should be cautious about using larger values for the reservoir cap, because it places greater strain on the PT (worsens the power factor). Especially if the PT is an old one, it can cause it to get hot, if it was already a bit undersized (money saving at the factory!). You'll probably get away with 100uF though, but I wouldn't go any higher unless I knew the PT well.

[harry]

Thanks for the replies everyone. Your valuable advice is much appreciated!

I changed the 50uF caps out to 100uF.

I changed a 20+20uF cap for two sets of 2x10uF electrolytics in parallel.

I also changed a 47uF electolytic for 2x33uF electrolytics in parallel.

No hum now. But I am getting a problem as described here:

http://music-electronics-forum.com/s...ad.php?p=99735


Basically when I turn the preamp volume to 7 or 8, with no instrument connected, a very quiet but very horrible high-pitched squealing sound comes from the amp itself... not the speakers, but inside the chassis. Someone suggested it might be one of the filter caps struggling.

I seemed to recall that divider resistors had already been added to the capacitors to 'spread the load'.. however, I don't think it was done correctly. I will try that in the next couple of days and see if it changes things.

Again, thanks for the help.