PP KT88, Need help! - 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 12th November 2012, 08:30 AM   #11
diyAudio Member
 
Join Date: Aug 2012
When hum and voltages change so dramatically with just movement of wires, my guess would be that there are some bad connections.
  Reply With Quote
Old 12th November 2012, 09:34 AM   #12
jazz is offline jazz  Netherlands
diyAudio Member
 
Join Date: Dec 2001
Location: leiden, physically that is...
Hi,

Looking at the picture there seems to be a cathode bypass cap missing on the most left socket.

Joris
__________________
oh, what a beautifull day
I wanna go out and play!
  Reply With Quote
Old 12th November 2012, 09:46 AM   #13
diyAudio Member
 
Join Date: Aug 2008
Quote:
Originally Posted by EEApprentice View Post
DeathRex: Yeah, I've been using Morgan Jones' Valve Amplifiers almost as a Bible haha.
Building Valve Amplifiers, by the same author, should also be in your library, very good reference for construction

Jacob, the Phase Inverter gets DC reference directly from the previous stage (so it is fixed biased), so no use for a grid leak. It does still need a grid stopper though...

Last edited by costis_n; 12th November 2012 at 09:53 AM.
  Reply With Quote
Old 12th November 2012, 06:08 PM   #14
diyAudio Member
 
aardvarkash10's Avatar
 
Join Date: Mar 2007
Location: Auckland, NZ
amongst all of hte comments I'd add that your grounding scheme (if it follows the schematic) is almost designed to induce hum. Check out the very good article on grounding schemes in hte articles section of the site.
__________________
"Folks, you can't prove truthiness with information. You prove truthiness with more truthiness. In a process known as truthinessiness." - Stephen Colbert, The Colbert Report
  Reply With Quote
Old 12th November 2012, 07:30 PM   #15
diyAudio Member
 
Join Date: Sep 2012
aardvarkash10: The actual grounding is not like the schematic, I have everything running to one main star ground near the back and center of the chassis, I thought that would eliminate ground loops.

jazz: Yeah, shortly after posting the picture I noticed my KT88 caps kept coming off.

DeathRex: You were right! The resistor values for my cathode bias are wrong, I used the equivalent resistance seeing by the caps (used to calculate the cap value) instead of the correct cathode resistance. I've already ordered those new resistors.

dgta: Because of the resistor thing, I'm going to wait until I get those DC values since some of the component values are wrong.

There is one channel that is actually reproducing music, however the bias for my KT88s on that channel is 55 [V] and 48 [V]. The music is not very loud and a bit distorted.

The other channel still has that LOUD, low frequency hum. I've tried disconnecting coupling capacitors going from the output towards the input. I've found that if I disconnect the input to the KT88s, the hum is gone. If I disconnect the input to my driver after the phase splitter, the hum is gone. If I disconnecting my input from the pot to the pre-amp before the phase splitter, the hum is there. So this hum seems to be originating between my pre-amp and phase splitter.

I rewired that entire section and the hum is still there....
  Reply With Quote
Old 12th November 2012, 08:25 PM   #16
diyAudio Member
 
Join Date: Dec 2005
Location: london
Hi EE,
I have a pair of push pull kt 88 monoblocks. Your assertion above is correct. The thing to do now is try using the non humming channels front end to drive signal to the phase splitter on the humming side to assert that it is the pre at fault, and then try to figure out the difference between them. Use screened wire, seeing as how you will be linking over lots of circuit that can add hum. I am sure you will have a bad connection somewhere- it is very easy to do when you use the same colour cable everywhere- I have rectified a few diy'ers errors (and others who ought to know better) due to this.
__________________
Bernie.
  Reply With Quote
Old 12th November 2012, 09:16 PM   #17
diyAudio Member
 
Join Date: Sep 2012
bernification: But the non-humming channel still has problems. The bias for those KT88 is way above what I calculated and they are mismatched.

Last edited by EEApprentice; 12th November 2012 at 09:38 PM.
  Reply With Quote
Old 14th November 2012, 01:35 PM   #18
diyAudio Member
 
Join Date: Dec 2005
Location: london
Hi EE,

one problem at a time. Try to eliminate the source of the hum first.

Then check all your cathode bias resistors and caps, and the pinout of your kt88s.
It sounds like one amp has a good balanced output. Check to see if both amps cathode resistors correspond, and then try to swap valves and see if the values match.

Between both amps it sounds like you have at least one working amp. Should be easy to add them together and then cross reference to get both running.

Good luck.
__________________
Bernie.
  Reply With Quote
Old 14th November 2012, 05:18 PM   #19
diyAudio Member
 
Join Date: Jun 2011
Before I have to run off I would like to say that construction techniques are important.

orientate tube sockets so the heater windings and leads are away from signal lines.

keep the parts mechanically stable by putting them between terminal strips then wire to the socket.

and also briefly looking at the schematics stay away from cathode bias unless you want to tweak the design to offset the uneven signal loading and amplitudes. use fixed bias method like a -40V supply adjustible type through 100k-220k resistors. like this:

Click the image to open in full size.
  Reply With Quote
Old 16th November 2012, 03:31 AM   #20
diyAudio Member
 
Join Date: Sep 2012
So, I took the main advice and completely re-did the wiring, the only ones I didn't de-solder and re-soldered are in the power supply (middle section), the rectifier tubes and some heaters.

Also, I made DC measurements at several points for both left and right channel.

I have attached all this info on the post. (Original post has the component list and circuit diagram with the calculated values)

After re-doing the wiring and changing the resistor values for all cathode bias (turns out I had the wrong values). THE PROBLEM IS STILL THERE!

Left channel has CRAZY LOUD HUM.
Right channel dead quiet. Also before changing the wiring I unplugged the humming side and the right side did play music but it was a bit quiet and distorted.

Anyone has any advice? I thought for sure changing the connections would fix it, now I'm wondering if I have a busted tube or even OPT?

Thanks again to everyone who's taking time to help out.
Attached Images
File Type: jpg IMG_3207 - Copy.jpg (651.9 KB, 128 views)
File Type: png LEFT_DC.png (41.0 KB, 110 views)
File Type: png RIGHT_DC.png (41.4 KB, 104 views)
  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
kt88 PP rectifier. ruben alstad Tubes / Valves 7 5th September 2012 10:07 AM
Which KT88 PP and which Transformers? Dave S Tubes / Valves 66 8th January 2012 05:37 PM
Kt88 Pp barossi Tubes / Valves 2 24th December 2008 06:55 PM
KT88 PP suggestion jarthel Tubes / Valves 13 27th June 2007 03:10 AM
My new pp KT88 design baggystevo82 Tubes / Valves 41 16th May 2005 02:02 PM


New To Site? Need Help?

All times are GMT. The time now is 10:42 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