First PSU Capacitor - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 29th April 2016, 12:19 AM   #1
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Default First PSU Capacitor

Hey All,

I know that all tube rectifiers have a maximum first cap value in the power supply to protect the tube from inrush current at startup. My question is is there a minimum value? I mean that the first cap takes a real beating so a very high voltage cap is best to make it more survivable. But does capacitance also come into it? I have some .068uf 1000 volt caps could they be used with a 5U4G tube?

Thanks, Kevin
__________________
Artificial intelligence is no match for natural stupidity- Red Green
  Reply With Quote
Old 29th April 2016, 12:30 AM   #2
diyAudio Member
 
BinaryMike's Avatar
 
Join Date: Apr 2010
Location: Oregon
If the cap is omitted then you have a simple LC or RC filter, which yields much lower DC output voltage relative to CLC and CRC filters. There's a zone between zero and several uF where DC output voltage increases rapidly with increasing filter input side capacitance, which makes the capacitor's value a critical parameter if you need to hit a target voltage.
  Reply With Quote
Old 29th April 2016, 01:55 AM   #3
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Thanks. So there really wouldn't be much difference between 10uf and 40uf?
__________________
Artificial intelligence is no match for natural stupidity- Red Green
  Reply With Quote
Old 29th April 2016, 02:14 AM   #4
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Location: Upper midwest
Quote:
Originally Posted by mr2racer View Post
there really wouldn't be much difference between 10uf and 40uf?
Depending on the load current, only 10uF for the input capacitor could cause a lot more ripple.
It might work for a tube preamp that needs little current, but a tube power amplifier would need more.
Do you have a schematic?

Last edited by rayma; 29th April 2016 at 02:20 AM.
  Reply With Quote
Old 29th April 2016, 02:17 AM   #5
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
The larger the capacitance in the 1st position, the "stiffer" the supply. SS diodes have a distinct advantage in the turn on surge dept., that enables large caps. to be employed. A big cap. in the 1st position is not without a downside. Check the archives for my posts about "hash" filters.

For a 5U4, insert a CL-130 inrush current limiting thermistor between the rectifier and the PSU filter. You can slightly exceed the data sheet 1st cap. limit, in safety. An additional benefit of the thermistor is a small delay in B+ rise. Remember, directly heated vacuum rectifiers, like the 5U4, turn on almost as quickly as SS diodes do.
__________________
Eli D.

Last edited by Eli Duttman; 29th April 2016 at 02:19 AM. Reason: spelling
  Reply With Quote
Old 29th April 2016, 10:12 AM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by mr2racer
My question is is there a minimum value?
It depends on what you mean by minimum. You can omit the cap altogether (if there is a choke) but then you get more ripple and less DC voltage; fine if this is what you need. A small value for the cap reduces ripple a bit, but can greatly increase PSU DC output resistance (droop); people suggest this to allow for voltage adjustment but in my view it is a bad idea. My advice is to omit the cap, or ensure it is large enough to act as a proper reservoir.

Quote:
I mean that the first cap takes a real beating so a very high voltage cap is best to make it more survivable.
No. It is ripple current rating which matters - assuming the voltage rating is sufficient. Any film cap with sufficient voltage rating will be good enough, and most electrolytics will be good enough too as they are designed for this duty.

Quote:
So there really wouldn't be much difference between 10uf and 40uf?
I don't know what 10uf is. 10uF and 40uF would both work, but you would get less ripple and less droop with 40uF.
  Reply With Quote
Old 29th April 2016, 11:21 PM   #7
diyAudio Member
 
soulmerchant's Avatar
 
Join Date: Sep 2006
Location: Near Zurich
no cap.
  Reply With Quote
Old 30th April 2016, 01:15 AM   #8
12E1 is offline 12E1  United Kingdom
diyAudio Member
 
12E1's Avatar
 
Join Date: May 2012
Location: Near Huntingdon, Cambridgeshire, UK
Quote:
Originally Posted by DF96 View Post
I don't know what 10uf is. 10uF and 40uF would both work, but you would get less ripple and less droop with 40uF.
Or even 10F or 40F...

(Alt-key and numeric pad 2 then 3 then 0 works for me.)
  Reply With Quote
Old 30th April 2016, 02:17 AM   #9
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Eli, With the thermistor's delay do the tubes begin to conduct and thereby limit peak peak startup voltage? Or does the delay not last that long?
__________________
Artificial intelligence is no match for natural stupidity- Red Green
  Reply With Quote
Old 30th April 2016, 03:59 AM   #10
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
I doubt that the thermistor will slow things down enough for indirectly heated types to start conducting fully. The thermistor will tame the turn on surge, which protects the rectifier. As it conducts, the thermistor heats and its resistance drops. That's a negative temperature coefficient (NTC). A voltage drop exists in the cold thermistor that gradually declines, with heating due to conduction.

While a NTC thermistor slows things down for only a few seconds, that's enough time for a SS rectified bias (C-) supply to rise and electrostatically protect power O/P tube cathodes.
__________________
Eli D.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Capacitor to Psu? Nty123 Everything Else 3 19th September 2014 04:46 PM
dodgy PSU capacitor AndrewT Parts 15 8th July 2011 03:53 AM
PSU Capacitor Selection cbj591 Parts 9 9th September 2010 06:20 AM
PSU capacitor selection phreeky82 Parts 3 28th June 2006 10:33 AM
Bipolar PSU capacitor Prune Parts 2 26th July 2004 06:31 PM


New To Site? Need Help?

All times are GMT. The time now is 08:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki