A dumb question about coupling caps. - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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 15th August 2003, 03:44 AM   #11
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Yeah, before the 5687 is conducting, voltage will surge to 400V. A 300V rated cap would work since most are rated 150% or so for whatever time, but a 400V is easy enough to come by.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 15th August 2003, 06:50 AM   #12
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Thanks everybody. I think that a 630 volt cap is going to be the ticket there.

G
__________________
Gavin
  Reply With Quote
Old 18th August 2003, 10:56 PM   #13
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
> isnt the 15k resistor dropping 150V at 10mA? Therefore, only 150V is seen at the coupling cap.

At turn-on, the B+ rises quickly to 300V or more. The tube sits there stone-cold, unconductive, for 5 to 10 seconds. C3 sees 300+V in series with 15K+330K= 345KΩ.

Assume the unloaded power supply is 300V. (I suspect it will be over 400V, which makes things worse.)

Because of the resistance, the cap charges to 300V "slowly". But not as slow as a tube. In 0.114 seconds it will charge to 63%, which is 189 volts. By 5 seconds when the tube is starting to conduct, the cap will be around 295 volts.

Over-voltage damage to a cap is instant, unlike tubes where a slight overvoltage just shortens life.

Most caps have some margin on voltage rating, so the cap may not die the instant it hits 201 volts. However if it does die when you put more than rated volts on it, you have no good cause to complain.

The old "metalized" caps had non-uniform (rough paper) insulation. Some parts of a "200V" cap would break-down at 100V, other parts at 600V. The metal was VERY thin, so if you got a break-down in a high-current cuircuit, the small area of metal around the weak-spot would burn-away and become useless. "Self-Healing". At the factory, they would blast the caps with the rated voltage (blowing-out the weak areas) and then check for minimum capacitance. So if you put 250V across a 200V 0.1µFd cap, it would quickly become a 250V 0.09 or 0.08µFd cap. 400V surges could burn-away half the metal and leave you with 0.05µFd. In lowest-price radios, this was acceptable. In "good work", you don't want to trust any "self-healing" scheme.

In many circuits, the worst voltage surges are not in-operation but at turn-on. Sharpen your pencil and your mind and watch what happens on paper as the circuit comes to life.
  Reply With Quote
Old 19th August 2003, 12:39 AM   #14
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
In many circuits, the worst voltage surges are not in-operation but at turn-on. Sharpen your pencil and your mind and watch what happens on paper as the circuit comes to life.
Absolutely...there's more to it though...I often find people calculating caps under static conditions forgetting what happens when an AC signal is superimposed...

The turn on surges can be lessened by using tube rectifiers and ...they're so much easier on components, aren't they?

So, now you know why grandma's radio lasted forever...no excuse why your tube amp shouldn't last forever...

And it can sound great all along...

Cheers,
__________________
Frank
  Reply With Quote
Old 19th August 2003, 04:40 PM   #15
diyAudio Member
 
Join Date: May 2003
Location: Morton, Illinois
Default Capacitor size

Good question G.

I think you made a good choice deciding on a 630 volt cap. Value or size of cap will depend on how the other components "sound" in the design and brand of parts used, but .33uf does seem like the minimum value I would use.
  Reply With Quote
Old 26th August 2003, 03:39 AM   #16
diyAudio Member
 
Join Date: May 2003
Location: Morton, Illinois
Default DC coupling vs cap

It seems interesting to me that some designs use DC coupling, which seems to go much lower than cap coupling, yet caps are restricted to value size. Maybe this should be a new string, but I find that interesting. What do others think?
  Reply With Quote
Old 26th August 2003, 08:06 AM   #17
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Positron,

I see your confusion: DC coupling=infinite value cap? Not quite.
DC coupling: A transient can cause grid current to flow. Once the transient is over, the circuit is almost immediately* back to normal.
AC coupling using oversize cap: A transient causes grid current to flow, causing the charge across the cap to rapidly change (due to the lower grid impedance under these conditions). Once the transient is over, the circuit takes a finite time for the cap to restore it's original charge.
The smaller the cap value, the faster the circuit normalizes.
This is one reason that transformer coupling is favoured by some of us over CR coupling.

Cheers,

Almost immediate* assumes a perfect power supply.
  Reply With Quote
Old 26th August 2003, 08:09 PM   #18
diyAudio Member
 
Join Date: May 2003
Location: Morton, Illinois
Default No confusion

I see where you are coming from, no confusion there, but I was thinking of what the string was initially, mainly concerned about, bass response. With such a small time constant, are we going to get flat bass response vs DC coupling? Just a question.

I was aiming at another point too; in general how the size of the cap affects the sonic qualities before grid current flows. I could have been alot more clear. Sorry about that.

Steve
  Reply With Quote
Old 26th August 2003, 08:17 PM   #19
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Hmm,

Looks like I was the one a little confused with the subject
So, in answer to your question, there is no difference in bass response between DC coupling and an infinitely large coupling cap.
As for cap sonic qualities, that's a contentious issue with many opinions. I just try to choose the best quality cap (of the right kind) that I can afford.

Am I closer now?

Cheers,
  Reply With Quote
Old 27th August 2003, 01:29 AM   #20
diyAudio Member
 
Join Date: May 2003
Location: Morton, Illinois
Default Understand

I understand DH. I could have been more clear. I was thinking of DC vs, say the .47uf cap.

As you mentioned, infinite value cap under perfect conditions should sound like DC coupled. But it seems even a 1uf cap starts to color the sound, regardless of brand, even if just a little.

Wonder also about the field each layer of "foil" pesents to other layers when charging and discharging, since current is flowing through the foil.

Any thoughts?
  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
Dumb question on Coupling cap ratings... john65b Tubes / Valves 9 18th October 2008 01:26 PM
no coupling caps ray bronk Tubes / Valves 2 20th May 2008 07:10 AM
Pre-amp coupling caps Bill Fitzpatrick Parts 15 4th March 2005 11:54 AM
What value for the coupling caps? roibm Parts 12 23rd March 2004 05:58 PM


New To Site? Need Help?

All times are GMT. The time now is 08:55 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2