Determing the reason for mystery? cap in schematic. - 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 10th April 2012, 08:34 AM   #11
diyAudio Member
 
devilsindetails's Avatar
 
Join Date: May 2011
Location: 100th Meridian
Quote:
Originally Posted by Bibliophile View Post
If you have a multimeter, measure between control grid an ground.
Normally you will see 0 Volts. If it is even a little positive, you could have a leaky coupling cap from the driver. What sort of caps did you use ?
OS Sprague Wax Caps, same voltage rating rating and value, but I guess there`s always a chance one of them is not quite up to snuff.

Excuse my inexperience but which pin is the control grid...
__________________
"A problem left to itself dries up or goes rotten. But fertilize a problem with a solution--you'll hatch out dozens." (N.F.Simpson)

Last edited by devilsindetails; 10th April 2012 at 08:37 AM.
  Reply With Quote
Old 10th April 2012, 08:37 AM   #12
diyAudio Member
 
Join Date: Nov 2011
Waxed paper caps were notorious in the past for developing leakage as they aged. Some will disagree but I would use polyester. Control grid is pin 2.

Last edited by Bibliophile; 10th April 2012 at 08:42 AM.
  Reply With Quote
Old 10th April 2012, 08:42 AM   #13
diyAudio Member
 
devilsindetails's Avatar
 
Join Date: May 2011
Location: 100th Meridian
Quote:
Originally Posted by Bibliophile View Post
Waxed paper caps were notorious in the past for developing leakage as they aged. Some will disagree but I would use polyester.
Point well taken...I should be utilizing known good caps...however I used what I had on hand and superficially looked A-OK...how far above the original Uf value can I use safely...I`ve been told no more than 10%...I know I can use higher voltage rated caps (just not lower).
__________________
"A problem left to itself dries up or goes rotten. But fertilize a problem with a solution--you'll hatch out dozens." (N.F.Simpson)
  Reply With Quote
Old 10th April 2012, 08:45 AM   #14
diyAudio Member
 
devilsindetails's Avatar
 
Join Date: May 2011
Location: 100th Meridian
Thanks on the control grid pin...I had looked at pin-out for the EL84 and noticed that it showed g1 and g2.
__________________
"A problem left to itself dries up or goes rotten. But fertilize a problem with a solution--you'll hatch out dozens." (N.F.Simpson)
  Reply With Quote
Old 10th April 2012, 08:49 AM   #15
diyAudio Member
 
Join Date: Nov 2011
Depends on the value of the capacitor and the resistor between control grid and ground or more precisely the time constant of the two. To high on either one can lead to an effect called grid blocking or blocking distortion.
See an explanation here: Blocking Distortion
G2 is called screen grid.
I'm going off line now so I can't respond to any more questions until tomorrow but there are plenty of more qualified people than I here who can help you out.
Cheers for now.

Last edited by Bibliophile; 10th April 2012 at 08:53 AM.
  Reply With Quote
Old 10th April 2012, 08:55 AM   #16
diyAudio Member
 
devilsindetails's Avatar
 
Join Date: May 2011
Location: 100th Meridian
Quote:
Originally Posted by Bibliophile View Post
Depends on the value of the capacitor and the resistor between control grid and ground or more precisely the time constant of the two. To high on either one can lead to an effect called grid blocking or blocking distortion.
See an explanation here: Blocking Distortion
G2 is called screen grid.
OK..thanks so much for your time and courtesy Bibliophile...I can see I have my reading cut out for me...I`think I`ll stick with exact values for the caps for now but ensure they are known good and will try the poly`s as I`ve heard good things about them....Leon
__________________
"A problem left to itself dries up or goes rotten. But fertilize a problem with a solution--you'll hatch out dozens." (N.F.Simpson)
  Reply With Quote
Old 12th April 2012, 12:05 AM   #17
diyAudio Member
 
Join Date: Nov 2011
Thanks for the kind words.
I will be away for several days.
Cheers
  Reply With Quote
Old 12th April 2012, 10:55 AM   #18
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The capacitor ringed in green is for 'tone correction' i.e. HF rolloff. Very common in old radio circuits, as people in the 1950s were not used to hearing much treble. In effect, it lowers the OPT HF resonance frequency. In essence, you have a pair of radio receiver audio stages.
  Reply With Quote
Old 12th April 2012, 01:47 PM   #19
diyAudio Member
 
devilsindetails's Avatar
 
Join Date: May 2011
Location: 100th Meridian
Quote:
Originally Posted by DF96 View Post
The capacitor ringed in green is for 'tone correction' i.e. HF rolloff. Very common in old radio circuits, as people in the 1950s were not used to hearing much treble. In effect, it lowers the OPT HF resonance frequency. In essence, you have a pair of radio receiver audio stages.
Thanks for the input DF...now as this amplifier had a separate pre-amp/radio chassis with treble and bass pots (which I simply removed from the equation) would I be better advised to remove this particular capacitor to restore (raise) OPT HF resonance?

I intend to use this amplifier with only a volume pot and no frequency compensation.
__________________
"A problem left to itself dries up or goes rotten. But fertilize a problem with a solution--you'll hatch out dozens." (N.F.Simpson)
  Reply With Quote
Old 12th April 2012, 06:31 PM   #20
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Probably best to experiment. Treat the capacitor as a fixed treble tone control.
  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
Determing heatsink CorySCline Chip Amps 12 7th September 2011 06:09 PM
Determing current dump from output transistors Arroyo Solid State 3 10th June 2011 02:19 AM
That mystery trem cap jjman Tubes / Valves 6 5th April 2011 01:47 AM
Vote and reason why, can a Cap improve, be a band aid or a waste of Farad? mikee55 Car Audio 12 26th October 2010 02:54 AM
determing speaker power philspeed Multi-Way 0 1st May 2006 12:50 AM


New To Site? Need Help?

All times are GMT. The time now is 05:46 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