Changed a cap and lost some bass...? - 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 25th October 2007, 01:14 PM   #1
hnb2907 is offline hnb2907  United Kingdom
diyAudio Member
 
hnb2907's Avatar
 
Join Date: Aug 2007
Question Changed a cap and lost some bass...?

Hi folks,

I've been tinkering around with my Chinese amp, and have replaced a 100nF 400V MKP1841 Metallized Polypropylene Film Capacitor on the 6J8P frontend. I've dropped in a 100nF 630V of these as a replacement http://wduk.worldomain.net/acatalog/SONIQS_SAX_range.pdf

Now, the mids have more clarity, the treble is bright (maybe too much), but the bass seems almost none existent.

I've put the old cap on a component tester at work, which claims it is 98pF, 4ohms@100hz, 0.25ohms at 1khz, 0ohms at 10khz.
The new cap measured the same way claims it is <0.25ohms at all those test frequencies. Maybe the different internal resistance of the cap at 100Hz is the reason?


As I'm a beginner here, the question is....

Looking at my circuit diagram, the cap changed is C1. Would replacing both R1+R2 with, for instance, 15Kohm each change the time constant of the R/C network to roll off at a lower frequency? I'm thinking that 1K5 to 15K could produce a noticable change of 1 decade. Then through trial and error find the correct resistance? Maybe I should be looking at the 8K2 resistor?

I'm basing this idea on the idea that the cap is somehow causing AC negative feeback onto pin 6 of the 6J8P, and that C1+R2 or C1+(R1//R2) are creating the low frequency cut off.

Maybe someone could give me a quick explanation of what's going on around the 6J8P and how to tweak it...?


Cheers,
ChrisC.
Attached Images
File Type: jpg el34.jpg (84.7 KB, 269 views)
  Reply With Quote
Old 25th October 2007, 01:48 PM   #2
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
If you increase R1 and R2 to 15k, you will have 15k in series with the cathode and the bias will be so high that the tube will probably not be able to conduct. Your problem is almost certainly C1, which bypasses grid 2 to the cathode and is fairly critical. If its value is too high, it could cause LF instability because of the negative FB loop. If it's too low, the symptom would indeed be weak bass and the middle and high frequencies would be emphasized. The replacement cap you've fitted may be defective in some way.
  Reply With Quote
Old 25th October 2007, 02:35 PM   #3
hnb2907 is offline hnb2907  United Kingdom
diyAudio Member
 
hnb2907's Avatar
 
Join Date: Aug 2007
Quote:
Originally posted by ray_moth
If you increase R1 and R2 to 15k, you will have 15k in series with the cathode and the bias will be so high that the tube will probably not be able to conduct. Your problem is almost certainly C1, which bypasses grid 2 to the cathode and is fairly critical. If its value is too high, it could cause LF instability because of the negative FB loop. If it's too low, the symptom would indeed be weak bass and the middle and high frequencies would be emphasized. The replacement cap you've fitted may be defective in some way.

Thanks for the sanity check - I'd completely missed thinking about the bias!

I'll have a check of the new caps on the meter. Is it more likely that the difference in internal resistance is causing this, rather than them being faulty?


Cheers,
Chris.
  Reply With Quote
Old 25th October 2007, 03:26 PM   #4
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Quote:
Originally posted by hnb2907

. . .Is it more likely that the difference in internal resistance is causing this, rather than them being faulty?


Cheers,
Chris.
I can't beleive the internal resistance could have such effect !
In your first post, you say having measured 98pF ? Typo ?

Yves.
  Reply With Quote
Old 25th October 2007, 04:45 PM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Try a different 100nF (0.1uF) cap in that location, it could be that you don't like the subjective change in tonal balance that the new capacitor gave you. (Other brands may sound different depending on construction.)

Even (good) cheap amplifiers are often built with combinations of components chosen through careful listening to work synergistically with other choices made in the design. It is quite possible that the substitution of a better cap (or one that at least measures better) has unmasked a design compromise somewhere else in the circuit. Or it could be a bad sounding cap...

I would also measure the frequency response of the entire amplifier, and you will probably see it is reasonably flat down to at least 50Hz.
Do this with an 8 ohm resistive load, with an output voltage at 1kHz of say 1.0Vrms - sweep the frequency down to 20Hz and up to 20kHz.
Note that 707mVrms represents the -3dB points in the response curve. (You need to use a meter with better than 20kHz bandwidth or a scope.) Note that this is the small signal response, but should not be far off the mark for typical listening levels with efficient speakers. (125mW)

Finally I assume you meant 98nF, not 98pF - this would certainly result in rather poor low frequency performance..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 25th October 2007, 06:25 PM   #6
hnb2907 is offline hnb2907  United Kingdom
diyAudio Member
 
hnb2907's Avatar
 
Join Date: Aug 2007
Quote:
Originally posted by kevinkr


I would also measure the frequency response of the entire amplifier, and you will probably see it is reasonably flat down to at least 50Hz.

I did this a few weeks ago before I made any changes at all to the amp, and it was flat from around 55Hz to about 27kHz. Had planned to pick up my signal generator from my workshop on my way home tonight, went in to collect my snailmail, and still forgot to bring it home. Doh.

Quote:
Finally I assume you meant 98nF, not 98pF - this would certainly result in rather poor low frequency performance..
indeed - 98nF - another one of those days - as you can probably tell?!

My current plan is to check the frequency response and then put the original caps back in, to make sure the sound returns to how it was. I've got a collection NOS 6SJ7's of various ages/manufacturers on the way to replace the Chinese 6J8P with. maybe I'll go back to the caps another day....

Anyway, I'm still having fun learning a bit more about all this - thanks for your help eveyone...

C.
  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
Yamaha cr-620 bass is lost in one channel YAKI Solid State 8 13th December 2008 06:26 PM
New Member Lost my Bass in Speakers...HELP singring Multi-Way 1 15th November 2008 10:27 PM
lost bass response Wagener Solid State 4 31st January 2007 08:06 AM
I wonder if (Bl) is changed. haggy Multi-Way 0 7th April 2003 10:55 AM
How would you like DIYaudio.com changed? Jason Everything Else 21 27th August 2001 10:07 AM


New To Site? Need Help?

All times are GMT. The time now is 03:36 PM.


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