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Tube Rectifiers & Solen Caps


 
11/16/2004 10:07 AM
Glenn
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Tube Rectifiers & Solen Caps
I've seen it here numerous times that Solens have an in-cct. equivalent capacitance of twice their stated value (e.g. a 10uF Solen performs like a 20uF any other capacitor brand). Tube rectifiers, for reasons I can't recall, cannot handle large capacitances hanging off of them. So, my question is:  
 
Can you use the Solen cap as a filter at the value indicated by an old schematic (and 'sneak' in extra capacitance and cheat physics), or must one halve the value (the schematic calls for) when using a Solen in place of anything else? TIA!!!  
Glenn
 
11/16/2004 11:06 AM
MKB
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"I've seen it here numerous times that Solens have an in-cct. equivalent capacitance of twice their stated value (e.g. a 10uF Solen performs like a 20uF any other capacitor brand)."  
 
Has this effect been explained? Did they mean half the ESR or some other parameter? How can the cap measure 10uF but work like a 20uF?  
 
This has my mind spinning a bit...
 
11/22/2004 2:16 PM
moocow Capacitance Loss
Electrolytics become less effective at high frequencies but a film cap like the Solen will retain its capacitance. I looked at a spec sheet for an electrolytic and it lost 40% of its capacitance at 1kHz compared to 120Hz. A 20uF capacitor of this type would look like a 14uF capacitor at 1kHz. A film cap retains its capacitance and it can act like a larger electrolytic, but only at higher frequencies.  
 
In real life, the capacitance loss doesn't seem to cause any problems. This is because at higher frequencies, the capacitance doesn't need to be large in order for the frequencies to be filtered out. For example, suppose our 20uF capacitor did a good job of filtering out a 120Hz signal. At 1200Hz, we only need 2uF worth of capacitance to get the same amount of filtering that we had at 120Hz. The above example shows we should have at over 10uF at 1200Hz so we should be ok. Another way to think of it is higher frequencies are easier to 'filter', so the loss of capacitance isn't much of a problem.  
 
Still, some people like to add a film cap in parallel with an electrolytic. The film cap doesn't need to be very big to ensure filtering is good even out to radio frequencies.
 
11/22/2004 3:16 PM
Shea
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quote:
"Still, some people like to add a film cap in parallel with an electrolytic. The film cap doesn't need to be very big to ensure filtering is good even out to radio frequencies."
 
 
Would that film cap be effective at filtering out switching noise from solid-state diodes?  
 
(I realize it's disputed whether that noise has any audible effect in a guitar amp, but anyway ...)  
 
Shea
 
11/23/2004 2:55 PM
moocow
The hi-fi tube amp guys think RF is a problem. It can be measured, but I'm not convinced it can be heard in a blind test. It seems they prefer to use special diodes or snubbers in the power supply circuit to kill high frequencies. The paralleled capacitor idea dates back many decades to the days of ham radio, so I would imagine it's worth a try in a tube amp. I did speak to one amp builder who claimed it made a difference, but I guess I'm too lazy to try it for myself. But if I were interested in high-gain amps I'd probably give it a try and see if it helps any.  
 
One warning I read about the paralleled caps was that the small cap could interact with the internal inductance of the electrolytic cap. This is supposed to be prevented by making small cap not too 'small'. I really haven't found any reference that tell how to avoid this problem, so it's just one more reason why I haven't given this a try.
 
11/16/2004 12:13 PM
neil
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Re: Tube Rectifiers & Solen Caps
"I've seen it here numerous times that Solens have an in-cct. equivalent capacitance of twice their stated value (e.g. a 10uF Solen performs like a 20uF any other capacitor brand). "  
 
 
 
Never heard that one. Solens will take up at least double the chassis space for the same uf's of a Sprague Atom, though.  
 
 
 
"Tube rectifiers, for reasons I can't recall, cannot handle large capacitances hanging off of them."  
 
 
 
When you power up the initial draw to charge that first capacitor (in a cap-input or pi power supply) may exceed the rectifier's ratings. Choke-input power supplies don't have this problem. They do, however, have a greater voltage drop.  
 
 
 
Rectifier tube data sheets list the maximum capacitance that tube can reasonably handle. Here's one for the gz34:  
 
 
 
http://tdsl.duncanamps.com/link.php?target=AB2534CB  
 
 
 
The number 60uf was figured for a gx34 running at max. At lower voltages, you could use more capacitance safely. That being said, 60uf is enough for most applications.  
 
 
 
As far as Solens v. electrolytics: I've used both in power supplies in both guitar and hifi amps. IMO, I don't think they are worth extra cost and real estate when compared to Sprague Atoms. I don;t usually hear the difference, and the one time I did, I went for the Sprague 'lytics. (Believe me, I was all set to like the Solens after spending all that money.)  
 
 
 
If you are looking to add additional filtration to a power supply, I would suggest adding another stage: another cap and choke in front of your first cap. The voltage drop from the choke won't be too too great.
 
11/16/2004 12:46 PM
jaysg
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source for larger Solens?
quote:
"As far as Solens v. electrolytics: I've used both in power supplies in both guitar and hifi amps. IMO, I don't think they are worth extra cost and real estate when compared to Sprague Atoms. I don;t usually hear the difference, and the one time I did, I went for the Sprague 'lytics. (Believe me, I was all set to like the Solens after spending all that money.)"
Thanks for the advice. Nonetheless, I'd like to experiment with a set someday. I've tried to find a US source but haven't come up with one. Angela seems to have pass cap values, not filter caps.
 
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