Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

JCM800 2204 Phase Inverter issue and other
JCM800 2204 Phase Inverter issue and other
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 Yesterday, 02:21 AM   #71
turk 182 is offline turk 182  Canada
diyAudio Member
 
Join Date: Oct 2012
i still think there's a problem in the bias supply and leakage in the phase inverter caps ( yes i know they are "new and recently replaced")
the thing i'm finding strange is when your pre is at twelve o'clock and the master is fully clockwise your B+ is at 396 (sagged) but your bias voltage has increased by nearly -20 volts, not sure how that's possible... the output tubes must have supressor grid leakage. in fact can you confirm that the output tube socket wiring is correct that the screen grids are common to cathode and the suppressor grid 1k's are ok and tied to B+? at this point with all the red plating that these tubes have done they may be cooked!
has the B+ line been checked for oscillation? and have the main filters been replaced?
the other thing i would look at is the feedback line off the transformer are you certain it from the proper winding and the right phase to produce negative feedback ( if it' wrong and positive that could be what's behind the red plating with increased gain)
  Reply With Quote
Old Yesterday, 07:34 PM   #72
SimoS is offline SimoS
diyAudio Member
 
Join Date: Aug 2018
Quote:
Originally Posted by Koonw View Post
I saw other people fixing the red plate by changing out bias capacitors C15 and C20, 10uF each. You can connect another 10uf cap in parallel to see it's doing anything. So a larger cap will reduce bias voltage drop for same B+ sag.
I replace both the Bias capacitors (I suppose you indicate C19 and C20) and also the diode D1 and the 220K resistor R30. No changes.

Quote:
Originally Posted by Koonw View Post
From the images I saw one tube clipping earlier than the other, so the B+ sag cause a drop in bias voltage some more to the other, making it even lower and hence more tube current and red plating. The tube that clipped earlier has higher transconductance or Mu than the other, unless the PS output is asymmetry. If you have function generator you can check waveform symmetry with triangular waveform before and after output stage.
Below you can see the image with triangular waveform before (below) and after (above) the output stage.
20181016_205130IO.jpg

Quote:
Originally Posted by turk 182 View Post
i still think there's a problem in the bias supply and leakage in the phase inverter caps ( yes i know they are "new and recently replaced")
I haven't new caps at home. Probably tomorrow I find them and try a new replacement

Quote:
Originally Posted by turk 182 View Post
in fact can you confirm that the output tube socket wiring is correct that the screen grids are common to cathode and the suppressor grid 1k's are ok and tied to B+? at this point with all the red plating that these tubes have done they may be cooked!
I triple checked the wirings and I quite sure they are correct. Below a couple of picture of V4 and V5.
20181016_210408V4.jpg
20181016_210516V5.jpg
I suppose the tubes are not cooked because I don't let them the red plate for a lot of time. When red plating arise i reduce immediately the volumes.

Quote:
Originally Posted by turk 182 View Post
has the B+ line been checked for oscillation?
What do you mean for oscillation? there's a little of variation but within the range of +/- 1Vdc when the amp is warm

Quote:
Originally Posted by turk 182 View Post
the other thing i would look at is the feedback line off the transformer are you certain it from the proper winding and the right phase to produce negative feedback ( if it' wrong and positive that could be what's behind the red plating with increased gain)
In the image below you can see the input signal to power tubes (below) and the feedback signal (above). They are opposite so I suppose it's correct.
20181016_205433INF.jpg
  Reply With Quote
Old Yesterday, 07:39 PM   #73
SimoS is offline SimoS
diyAudio Member
 
Join Date: Aug 2018
Quote:
Originally Posted by SimoS View Post
I forgot to say that on the scope the image with triangular waveform after (above) the output stage was oscillating a lot.
. Unfortunately is not possible to show with picture but it seems to go up and down like a slower wave across the positve and negative peaks.
  Reply With Quote
Old Yesterday, 09:25 PM   #74
turk 182 is offline turk 182  Canada
diyAudio Member
 
Join Date: Oct 2012
yes that's oscillation try increasing the horizontal time base.

top trace in the first pic in post 72 shows it

Last edited by turk 182; Yesterday at 09:29 PM.
  Reply With Quote

Reply


JCM800 2204 Phase Inverter issue and otherHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Marshall JCM800 2204 clone, need help Rudd3r Instruments and Amps 2 8th January 2017 03:21 AM
RCA phase inverter? KTMrida4life Car Audio 5 19th September 2015 09:33 PM
JCM800 / 2204 style build hum / buzz plea for help paul525715 Instruments and Amps 14 17th July 2012 08:41 PM
JCM 800 2204 PT issue Dr.R Instruments and Amps 5 2nd March 2012 06:39 PM
Phase inverter ThSpeakerDude88 Tubes / Valves 37 29th August 2007 03:15 PM


New To Site? Need Help?

All times are GMT. The time now is 03:14 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki