KT88 Bias Adjustment - 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 28th February 2006, 12:34 AM   #1
batinal is offline batinal  South Korea
diyAudio Member
 
batinal's Avatar
 
Join Date: Feb 2006
Location: Seoul
Default KT88 Bias Adjustment

I have the problem with my KT88 PP stereo integrated amp to set bias voltages of KT88 of which scheme as below;

If I set Vk to the same value by adjusting the pots for both tubes, I have audible hum so I cannot set them to the same value but different values, I.e. 0.410v and 0.435v, etc. Can anyone tell me what I have done wrong? Those tubes are said to be pairs.

Thank you very much in advance.

Kay
Attached Images
File Type: gif kt88_bias.gif (30.6 KB, 564 views)
  Reply With Quote
Old 28th February 2006, 12:57 AM   #2
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Kay,
Two questions.
Are the tubes semi-new (good quality) at least and matched?

Have you checked the resistors to make sure they are the same value (measure them)?

-Chris
  Reply With Quote
Old 28th February 2006, 01:27 AM   #3
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
This one I think is simple - The 10 Ohm cathode resistros are NOT equal. The NO HUM condition is when the idle currents are matched.
The fact that one side reads 0.41V abd the other reads 0.435 just means that the resistors are 5% different in actual value.

Might I suggest that you replace the 10 Ohm resistors with 1% 0.5W metal film. This will mean that the voltage readings for the same current will be within 1%, Plus the low power rating of the resistors means they will act as a fuse and blow up in the event of a tube short, thus protecting your output transformer.

Lesson: Check the tolerance of the components you are using and be aware that even if they start out at 1% tolerance they may move ourside that tolerance value if subjected to abuse.

Cheers,
Ian
  Reply With Quote
Old 28th February 2006, 04:15 AM   #4
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Sounds like tubes may be matched on DC conditions but not on AC signal gm. I found this out with 6550B's.
Don't overlook the hum may originate in the previous stages to phasesplitter, i.e unequal ripple rejection in either phasesplitter output to to rough B+.
Is the neg bias clean and C16/17 in good condition ?
You don't show any neg feedback loop. I presume you don't have any , so this will make the output stage matching doubly sensitive to mismatch and even a mains transformer in proximity will be noticeable.

Perhaps you should post the rest of the circuit...

richj
  Reply With Quote
Old 28th February 2006, 12:47 PM   #5
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Ian,
I had a customer come in with the same problem. One Chinese tube was defective. I was thinking tubes because the elements were aligned poorly. Some new outputs solved it.

So it can happen!

-Chris
  Reply With Quote
Old 2nd March 2006, 12:40 AM   #6
batinal is offline batinal  South Korea
diyAudio Member
 
batinal's Avatar
 
Join Date: Feb 2006
Location: Seoul
Thanks for great tips!

I will check on all the possibilities you mentioned.

Sorry, I am still working on making full schematic of the amp which came to me from China w/o it.


Best regards,
Kay
  Reply With Quote
Old 19th May 2006, 04:37 PM   #7
diyAudio Member
 
Join Date: May 2006
Quote:
Originally posted by gingertube
This one I think is simple - The 10 Ohm cathode resistros are NOT equal. The NO HUM condition is when the idle currents are matched.
The fact that one side reads 0.41V abd the other reads 0.435 just means that the resistors are 5% different in actual value.

Might I suggest that you replace the 10 Ohm resistors with 1% 0.5W metal film. This will mean that the voltage readings for the same current will be within 1%, Plus the low power rating of the resistors means they will act as a fuse and blow up in the event of a tube short, thus protecting your output transformer.

Lesson: Check the tolerance of the components you are using and be aware that even if they start out at 1% tolerance they may move ourside that tolerance value if subjected to abuse.

Cheers,
Ian
Good one Ian,

I think i will tri that on mine too
  Reply With Quote
Old 20th May 2006, 05:16 AM   #8
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Why use 1% tolerance stuff for tubes ? In earlier days even 20% was quite justified and only a few resistors used at 5% at critical locations. looking at the other posts regarding matched tubes from vendors, tube checks are usually done at 250V. At the more usual 400V in amps, variables will result. One might notice in AB, the tube quiescent conditions and low power class A may be well matched but when running on demand into class B there can be quite a cathode current mismatch between pairs.

With output transformers, the standard EI stampings provide a very small but inherent magnetic gap between laminations. Compare the inductance values with a toroid of same performance and with a continuous iron sheet trany how little current it takes to saturate the core. Various current balancing circuits have appeared but I found them all effected the sound quality. All in all I find the basic transformer the best solution to tube variations and that little inherent magnetic gap keeps the iron B/H curve more linear and touchhigher to saturate.. to the advantage of tube operation.

richj
  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
Fixed Bias adjustment alexmoose Tubes / Valves 9 12th June 2009 02:32 PM
BOZ bias adjustment help rtate Pass Labs 16 11th July 2008 07:04 PM
Aleph-x bias adjustment bach2800 Pass Labs 31 30th October 2006 10:03 PM
Bias adjustment for power amp? Help please cdl Solid State 11 28th August 2003 03:22 PM


New To Site? Need Help?

All times are GMT. The time now is 04:21 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