tube amp being weird -- any ideas - 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 8th March 2008, 06:15 PM   #1
diyAudio Member
 
soundbadger's Avatar
 
Join Date: Sep 2007
Default tube amp being weird -- any ideas

just finished building a single ended kt88 stereo amp and it works fine for a while then one channel goes very load and the other very quiet. it is consistent i doing this i can turn it off leave it for a while then turn it back on and get perfect stereo sound for a while. i am confused.
any ideas
  Reply With Quote
Old 8th March 2008, 09:51 PM   #2
diyAudio Member
 
Join Date: Oct 2006
Location: Los Angeles
Hey soundbadger, what are you driving the KT88's with? A schematic and/or layout would be helpful. It is possible that the problem lies in the driver stage. Also go ahead and double check all your voltages and currents. Make sure nothing is shorted. I'm no expert, but I know how frustrating it can be to troubleshoot an amp.....
  Reply With Quote
Old 8th March 2008, 10:27 PM   #3
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Yes, check the voltages and currents while it's working well and again when it's not, to see what is changing.
  Reply With Quote
Old 8th March 2008, 10:32 PM   #4
diyAudio Member
 
Join Date: Feb 2005
Location: Gelderland
if both channels use a single supply, especially if tube rectified,
it might be that one of the channels`s tubes and/or bias gets runaway.
what biasing method is used?
what value grid bias resistors are used?
resistor possibly going open when getting warm?
but just guessing now..

do you have and will post a schematic?
  Reply With Quote
Old 9th March 2008, 12:53 PM   #5
diyAudio Member
 
porkchop61's Avatar
 
Join Date: Aug 2004
Location: Mansfield, Connecticut
I believe he's building Mikael's amplifier, is that right soundbadger?
I've built this amp myself, as well as many others, and it's a very simple design.
I'd do as Ray_moth suggested and take some voltage readings. I have voltage measurements on my page if this helps. It's got to be something simple.
Glenn

Mikael's KT88 SE amp

Glenn's KT-88 build
__________________
"Imagination is more important than knowledge"
  Reply With Quote
Old 10th March 2008, 03:01 PM   #6
diyAudio Member
 
soundbadger's Avatar
 
Join Date: Sep 2007
sorry have been away,,
yes it is Mikael's amplifier i am building.
i shall take readings tonight to try and see what happens when the problem starts to occur.
  Reply With Quote
Old 10th March 2008, 04:40 PM   #7
diyAudio Member
 
pmillett's Avatar
 
Join Date: May 2005
Location: Dallas (but I am not a Texan!)
Beware using a 470k grid resistor on the KT88. I've had modern KT88's draw enough grid current that 470k is too high. The result is that the bias point goes weird, sometimes even oscillates.

Just for kicks solder a 100k in parallel (it may hurt LF response) and see if the problem goes away.

If that fixes it you might need bigger coupling caps to restor LF response.

Pete
  Reply With Quote
Old 10th March 2008, 06:15 PM   #8
diyAudio Member
 
TubeHead Johnny's Avatar
 
Join Date: Jan 2005
Location: The emerald city, Seattle, WA.
The great thing about stereo is you can always compare the voltages between the working/non-working channels. It's possible that one of the KT88's are defective.
__________________
"No man really becomes a fool until he stops asking questions" - Charles Proteus Steinmetz
  Reply With Quote
Old 10th March 2008, 09:43 PM   #9
diyAudio Member
 
soundbadger's Avatar
 
Join Date: Sep 2007
THE BIAS VOLTAGE APPEARS TO DROP ON ONE CHANNEL WHEN THE PROBLEM IS occurring.
(sorry caps off now)
the bias resistors are also pretty hot, i measured them about 70 degrees centigrade , they are 12W resistors which i believe should be fine.
i am guessing that either one of the bias resistors or one of the bypass capacitors is not good.

the pre-amp stage seems fine even when the problem is occurring.
  Reply With Quote
Old 11th March 2008, 07:09 PM   #10
diyAudio Member
 
soundbadger's Avatar
 
Join Date: Sep 2007
i changed the bias resistors for ones of greater power and changed the bypass caps, still the same problem after about 10 mins.
  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
Ideas for SE amp with this tube Pentium100 Tubes / Valves 13 14th December 2011 03:21 PM
Weird old power transformers (filament, SS and tube) - $7 each! pjanda1 Swap Meet 6 29th March 2010 05:17 PM
tube hum in a 6SN7 Aikido - is this weird? cbutterworth Tubes / Valves 16 12th March 2007 05:52 PM
Ideas for a tube that heavily distorts ? jim Tubes / Valves 8 3rd July 2006 03:32 AM
Need ideas for a music only tube sub. J.k.Wilson Subwoofers 4 18th September 2005 04:50 PM


New To Site? Need Help?

All times are GMT. The time now is 03:04 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