Blue Tube Red Plating = Help! - 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 15th January 2011, 06:43 AM   #1
diyAudio Member
 
wushuliu's Avatar
 
Join Date: May 2008
Location: Los Angeles
Default Blue Tube Red Plating = Help!

I have an Onix SP3 5881 P-P tube amp. Worked fine, then I decided to add some film caps to output caps on PS, and replace the coupling caps. Nothing major. When I powered it up one of the power tubes started glowing blue and then 15 seconds later started red plating. I checked the bias with the pots provided on the amp: the other 3 tubes were .30-.4v, the red plater was 10v(!) (the recommended is 1.15v setting for this amp). Swapping tubes didn't help. I took off the bypass caps on the PS and issues remains. The preamp, driver, phase and 3 other power tubes seem fine. Any ideas on troubleshooting?

I made no rearranging of components, just popped bypass caps on and then off so I'm at a loss.

Here's the schematics from another thread:
Attached Images
File Type: jpg Onix SP3 Amplifier Schematic.jpg (833.6 KB, 253 views)
File Type: jpg Onix SP3 Power Supply Schematic.jpg (289.4 KB, 242 views)
  Reply With Quote
Old 15th January 2011, 06:55 AM   #2
diyAudio Member
 
wushuliu's Avatar
 
Join Date: May 2008
Location: Los Angeles
Pics
Attached Images
File Type: jpg int1.jpg (511.9 KB, 248 views)
File Type: jpg onix.jpg (129.0 KB, 227 views)
  Reply With Quote
Old 15th January 2011, 07:40 AM   #3
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Since this happened after your intervention, the most probable thing is that YOU make a mistake !

Are output tube still red when you remove the 12AU7 ?
If yes, hunt for a short that killed the -B supply.
If not, suspect your new link caps.

BTW they don't need to be so big

Yves.
  Reply With Quote
Old 15th January 2011, 08:23 AM   #4
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Those coupling caps really need some sleeving on their leads. With all those leads snaking around, they're just aching to short against something.
  Reply With Quote
Old 15th January 2011, 08:36 AM   #5
diyAudio Member
 
Mr. Zenith's Avatar
 
Join Date: Aug 2009
Location: Central NC
I hate to be the one to tell you this, but I'm afraid that what you've witnessed is what Tubelab calls the "Red Glow of Death".

According to this site, you appear to have an outdated version of the schematic:

Quote:
This circuit appears to be of an earlier version of the SP3 since it only shows one bias adjustment pot per channel, but we know that there are 2 pots/channel--one per output tube. So, they must have reconfigured the biasing circuit. to allow for biasing each output tube individually. The circuit was further modified so that the cathode of each output tube is now grounded via a 30 ohm resistor (in the diagram, the cathode is connected directly to ground).
Given this information I can only assume that the 30 Ω resistor is used for bias measurement. If this is indeed the case then the 0.3 to 0.4 V measurements on the other three tubes correspond to a cathode current of 10 to 13 mA, meaning that they're operating at near cutoff (total dissipation of around 3 to 4 Watts each).

The tube measuring 10 V is drawing 330 mA or so cathode current - causing a combined grid and plate dissipation of around 117 Watts!!! Depending on how long the tube was in that state, you'll be extremely lucky indeed if something hasn't melted inside, like grid wires and such. Even worse, drawing that kind of current through circuits not designed for it can easily blow expensive parts - like output transformers.

It's tough for me to go much further without knowing exactly what parts were replaced or bypassed, or without a revised schematic showing the actual bias circuit used.
  Reply With Quote
Old 15th January 2011, 08:39 AM   #6
diyAudio Member
 
wushuliu's Avatar
 
Join Date: May 2008
Location: Los Angeles
Quote:
Originally Posted by Yvesm View Post
Since this happened after your intervention, the most probable thing is that YOU make a mistake !

Are output tube still red when you remove the 12AU7 ?
If yes, hunt for a short that killed the -B supply.
If not, suspect your new link caps.

BTW they don't need to be so big

Yves.
I reflowed the solder on the PS joints where I had the bypass caps and also tightened down that socket - it was somewhat loose. No red plating so far, but some of the tubes do have some blue glow now which I don't recall previously. I will re-test tomorrow and proceed from there.
  Reply With Quote
Old 15th January 2011, 08:42 AM   #7
diyAudio Member
 
wushuliu's Avatar
 
Join Date: May 2008
Location: Los Angeles
Quote:
Originally Posted by Mr. Zenith View Post
I hate to be the one to tell you this, but I'm afraid that what you've witnessed is what Tubelab calls the "Red Glow of Death".

According to this site, you appear to have an outdated version of the schematic:

Given this information I can only assume that the 30 Ω resistor is used for bias measurement. If this is indeed the case then the 0.3 to 0.4 V measurements on the other three tubes correspond to a cathode current of 10 to 13 mA, meaning that they're operating at near cutoff (total dissipation of around 3 to 4 Watts each).

The tube measuring 10 V is drawing 330 mA or so cathode current - causing a combined grid and plate dissipation of around 117 Watts!!! Depending on how long the tube was in that state, you'll be extremely lucky indeed if something hasn't melted inside, like grid wires and such. Even worse, drawing that kind of current through circuits not designed for it can easily blow expensive parts - like output transformers.

It's tough for me to go much further without knowing exactly what parts were replaced or bypassed, or without a revised schematic showing the actual bias circuit used.
I only had the tube on for 15- 20 seconds at a time. Yes it was damn hot and i figured that tube was drawing a ridiculous amount of wattage, but luckily there seems to be no damage to the rest of the amp so far. Bias is now 2v on the problem socket and 1.3 on the other I tested. Off to bed before I do more.

Last edited by wushuliu; 15th January 2011 at 08:47 AM.
  Reply With Quote
Old 15th January 2011, 08:50 AM   #8
diyAudio Member
 
wushuliu's Avatar
 
Join Date: May 2008
Location: Los Angeles
Quote:
Originally Posted by wrenchone View Post
Those coupling caps really need some sleeving on their leads. With all those leads snaking around, they're just aching to short against something.
Yeah, I intend to do that though there's more room around them than it seems in the picture.
  Reply With Quote
Old 15th January 2011, 01:03 PM   #9
diyAudio Member
 
Rundmaus's Avatar
 
Join Date: Aug 2005
Hi there,

the inner cap on the right side of the picture seems to have its lead in contact with the metal can - possibly shorting it (temporarily?). As written before, the leads should be insulated!

Greetings,
Andreas
  Reply With Quote
Old 15th January 2011, 02:24 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
I assume the new coupling capacitors are much larger than the old ones. They will have much more stray capacitance to nearby circuit nodes. The result could be ultrasonic/RF oscillation around the phase splitter/driver stages, due to capacitive positive feedback. This may explain the blue - RF can do this to a valve. One option is simply to use normal good quality caps instead of giant boutique items. Another option is to see if you can insert a grounded insulated screen around the caps.

I had similar trouble with a 5-20 phase splitter, due to large coupling caps to the outputs. A screen made of a zigzag of stiff copper wire solved the problem. It stopped the input to the phase splitter from seeing the output.
  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
Blue glow in tube. Is the tube dead or not? Pentium100 Tubes / Valves 37 22nd October 2009 06:45 PM
Tube glows red hot, blows fuse damagefactor Tubes / Valves 17 24th February 2008 07:42 PM
fisher sa-16 very red tube and? robertleo Tubes / Valves 2 18th June 2007 09:38 PM
WTT: Blue LEDs for Red & Red/Green LEDs CaliforniaBob Swap Meet 2 9th June 2003 02:58 PM
Blue glowing tube Gabster Tubes / Valves 24 27th September 2002 04:08 AM


New To Site? Need Help?

All times are GMT. The time now is 01:14 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