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Power supply cap values and how they effect tone?


 
5/22/2000 10:01 AM
Mark Abbott
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Power supply cap values and how they effect tone?
I've heard some techs say they don't like 100uF filter caps (supply to O/P tx) as they make the amp sound stiff.  
 
Yet others say increase the cap values as it increases bass response. I have very briefly experimented with my Bandmaster, I used alligator clips to add an extra 20uF cap to the preamp supply, I couldn't hear any increase in bass response.  
 
I have also noticed the Vibroking has in places twice the capacitor values as "classic" vintage amps it looks like, what is Fender's (Zinky's) idea here, more bass, tighter sound?  
 
I tried out a Vox AC 30 reissue (with Vox blues speakers)a couple of weeks ago, people on various bulletin boards said they sound stiff, drenched in starch would have been a more accurate accessment.  
 
Okay, so what gives with these filter caps?  
 
 
 
Thanks  
 
Mark
 
5/22/2000 3:47 PM
Gil Ayan
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quote:
"I've heard some techs say they don't like 100uF filter caps (supply to O/P tx) as they make the amp sound stiff.  
 
Yet others say increase the cap values as it increases bass response. I have very briefly experimented with my Bandmaster, I used alligator clips to add an extra 20uF cap to the preamp supply, I couldn't hear any increase in bass response.  
 
I have also noticed the Vibroking has in places twice the capacitor values as "classic" vintage amps it looks like, what is Fender's (Zinky's) idea here, more bass, tighter sound?  
 
I tried out a Vox AC 30 reissue (with Vox blues speakers)a couple of weeks ago, people on various bulletin boards said they sound stiff, drenched in starch would have been a more accurate accessment.  
 
Okay, so what gives with these filter caps?  
 
 
 
Thanks  
 
Mark  
"
 
 
Mark, adding some more capacitance to the filter section will decrease the impedance of the power supply, and since it is primarily capacitive in nature, less impedance means that more lows will get through.  
 
I don't know that clipping a 20uF in parallel to an existing one in the preamp will make much of a difference... Basically the first filter cap, which is the one that affects the power tubes, will make a big difference. Small amps like the old Deluxe Reverbs used 16uF there, later DR amps used 32uF there... More modern amps use two 100uF caps in series for a total of 50uF there, and will sound bigger, and some amps have two 220uF in series for a whopping 110uF capacitance. The amp will be extremely fast an punchy, so at low volumes it may sound "stiff," but if you need to play loud, the bottom end will not crap out like when you dime an amp with a flimsy power supply.  
 
Gil
 
5/22/2000 4:18 PM
kg
quote:
"some amps have two 220uF in series for a whopping 110uF capacitance."
 
 
just for comparison, the baga has a filter cap bank of eight 500 mic @ 500 WVDC LCR caps. they are run in series/parallel for a total effective capacitance of 1000 mics at up to 1kv of voltage.  
 
DON'T try this with a tube rectifier!  
 
the bass is very controlled and powerful though, and hum is minimal even at full tilt.  
 
ken
 
5/22/2000 7:32 PM
Chris Harden
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The only real disadvantages I can see when using larger amounts of power supply smoothing is the cost and size of the smoothing capacitors, providing you are using SS rectification, as allready mentioned by Ken. I know it's not a direct comparison, but on my SS Marshall, I found that when I increased the size of the smoothing capacitors, the transient response (attack) became far better, most noticable when playing ultra-clean (as I often do).  
 
I have been lucky to be given a quantity of 600uF @ 250V Capacitors. They're not new, but have been reformed, and seem to work fine. I'll probably use 6 in series-parallel to give 400uF @ 750V. The only problem is that each one is 1 1/2 inches in diameter and 3in long.  
 
Chris
 
5/22/2000 7:34 PM
ken gilbert
chris, forgive my asking, but why do you need 750VDC voltage compliance? what kind of b+ are you looking at?  
 
kg
 
5/22/2000 7:56 PM
Gil Ayan
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quote:
"chris, forgive my asking, but why do you need 750VDC voltage compliance? what kind of b+ are you looking at?  
 
kg"
 
 
I think I would go the same route if the B+ was too close to 500VDC. Most Plexi style mains transformers will run the B+ up there, so when the amp is on standby one is lilely to exceed 500VDC. I shoot for 700VDC ratings in the first two filter banks in those cases...  
 
Gil
 
5/22/2000 9:44 PM
Chris Harden
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I'm using a transformer with a 0-400V secondary, which will give me 550V or so. I can't afford to buy another transformer at the moment, but running EL34's at 550V shouldn't be a problem.  
 
It does re-animate me if I touch the wrong part of the power supply!  
 
Chris
 
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