Mbira

From what I can tell higher capacitance = less ripple.  The drawback would be higher cost.  I understand that higher capacitance high voltage caps were a lot more expensive bak then.  Is there a benefit in those older amp circuits (like fender amps) to using 22uF caps instead of higher values?  Does it do something to tone to have a little more ripple? 

When designing tube circuits these days is there any benefit to sticking with the lower capacitance? 
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


ruffrecords

Re: Capacitance choice for power filter caps in tube circuits
« Reply #1 on: July 05, 2020, 03:39:23 PM »
If you are using tube rectifiers in your power supply then the size of smooting capaitor you can use is limited because of the limited inrush current to tube can handle. So in that case you will still need to use the old 22uF values. Check the datasheet for the rectifier tube you are using.

On the other hand, if you use semiconductor rectifiers they can withstand much higher inrush currents and so you can use much higher values. A very large number of SMPS for home computers use 470uF 400V capacitors so although this is a large value at a high voltage they are not very expensive. Using several of them in a simple CRCRCRC HT filter can produce extremely low values of ripple and noise.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Gus

Re: Capacitance choice for power filter caps in tube circuits
« Reply #2 on: July 05, 2020, 03:39:37 PM »
What type of tube amps?

With guitar amps the first two matter for "feel" and smaller can be "better".

Low ESR can hurt a tube rectifier and cause issues with solid state rectifiers.

Mbira

Re: Capacitance choice for power filter caps in tube circuits
« Reply #3 on: July 05, 2020, 09:13:18 PM »
Thanks guys. I'm generally looking at tube rectifiers for now. 

If you are using tube rectifiers in your power supply then the size of smooting capaitor you can use is limited because of the limited inrush current to tube can handle. So in that case you will still need to use the old 22uF values. Check the datasheet for the rectifier tube you are using.

Ok, so let's say a 5AR4.  Here's a datasheet: https://drtube.com/datasheets/5ar4-amperex1958.pdf

Am I looking at the "Maximum Peak Current per plate"?  750 mA?

I see it says "Maximum Capacity (condenser input filter) 60uF"   so is that saying 60uF caps are the biggest you can safely use? 

Reading a 5u3 datasheet:
http://www.tubebooks.org/tubedata/hb-3/receiving_tubes_part_1/5Y3-GT.PDF

I think this is saying max 20uF cap?  But I can use a 10 henry choke? 

In any case, I'm just starting to learn how to read triode tube characteristic charts.  I clearly have a lot more to learn to be able to also make sense of the rectifier charts.

Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

trobbins

Re: Capacitance choice for power filter caps in tube circuits
« Reply #4 on: July 06, 2020, 12:37:11 AM »
Mbira, it is worth your while finding some technical tutorials, and forum threads, on this topic as there can be a bit of learning to do, especially if you aren't cloning an existing design exactly.

The datasheets provide a simplified design example path to take, but even then you need to know your power transformer specs,  and your load requirements.  So the design example in the 5AR4 datasheet uses a 60uF capacitor input filter, and you have to work through whether that example suits your power transformer and load.  You need to work through understanding what the transformer winding resistances mean, and whether your application aligns with a DCV and up to a certain max DC current.

If you can get your head around all that, then there are more detailed design paths to take that assess the other 5AR4 ratings, and usually include using a simulator like PSUD2 to assess a variety of performance issues.  Few people go down this path as it can get technically detailed.

Bobby Baird

Re: Capacitance choice for power filter caps in tube circuits
« Reply #5 on: July 07, 2020, 10:25:19 AM »
I cloned a fender vibroverb 2x10. The original called for 5 x 16uf filter caps. With low capacitance there was some ripple creating noise, but the sound was sag and loose. When you hit your lowest e string the sound would sag off because the amp needs more on demand energy to push those speakers. This type of sound is perfect for blues as it gives you a feel of tappering off notes that cry out. Not so good for percussive sounding genres like metal. Adding more capacitance will stiffen up the sound making the attack more immediate and percussive, also won't decay as fast. I replaced the first 2 filter caps with 25uf to have 50uf on the rectifer tube. 60uf is max at which point can cause arching in the tube. A blown rectifer! 50 uf made the sound to stiff no feel. I thought I would like it, but the sound was drastically different from the 2 x 16uf filter caps. The modern reissue uses 5 x 22uf caps. I just ordered 5 x 20uf caps to rebuild the filter section under the doghouse. This will put the sound in line with the fender amps from the blackface era. My amp being a brownface caps that large were hard to come by at that time, but, the sound is uniquely vintage. I'll have some 5 x 16uf and 2 x 25uf 450 volts sprague caps for sale if anybody wants them. Hopefully with this setup. I will have the best of both worlds.

Bobby Baird

Re: Capacitance choice for power filter caps in tube circuits
« Reply #6 on: July 07, 2020, 10:30:42 AM »
More Guts.

Bobby Baird

Re: Capacitance choice for power filter caps in tube circuits
« Reply #7 on: July 07, 2020, 10:32:48 AM »
If you want the real deal cabinet and you live in Austin go to Armadillo Amp Works.

Mbira

Re: Capacitance choice for power filter caps in tube circuits
« Reply #8 on: July 07, 2020, 09:04:30 PM »
Thanks guys.  I had always thought that sag simply came from using a tube rectifier.  I understand now that it's the relationship of the cap and the tube. 
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

 

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