Tube amp issue - 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 23rd June 2015, 01:41 AM   #1
diyAudio Member
 
Join Date: Nov 2011
Default Tube amp issue

I just received a music reference rm-10mkii from a seller with the right channel not working. I want to be sure that I am not wrong with this and accuse the seller.

I took the amp out, plugged in the tubes, hooked up the speakers and preamp. Before I used it I turned it on, let it sit for 20 minutes, and then went to bias. This is the same procedure I did the last time I had a rm-10 and everything was fine that amp was fine.

I biased the left channel and when I tried to bias the right I got zero reading on the meter. The tubes fired up and there was not plates glowing red. I then switched the tubes around, per the sellers suggestion, and still no reading. I played music and did not get any sound out of the right speaker.

What could it be? If it was the fuse blown wouldn't the tubes go crazy hot and burn out? If it is the output transformer wouldn't both channels be dead?

I have not heard back from Music Reference yet.

Any insight would be much appreciated.

Last edited by Enochrome; 23rd June 2015 at 01:44 AM.
  Reply With Quote
Old 23rd June 2015, 02:03 AM   #2
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Location: Upper midwest
Quote:
Originally Posted by Enochrome View Post
If it was the fuse blown wouldn't the tubes go crazy hot and burn out? If it is the output transformer wouldn't both channels be dead?
Sounds like the output tube screens of the bad channel aren't getting any voltage, so there's no plate current.
Maybe a resistor or fuse in the screen supply has opened. A tube could have arced or shorted to cause this.
Do you have a schematic?

Last edited by rayma; 23rd June 2015 at 02:15 AM.
  Reply With Quote
Old 23rd June 2015, 02:50 AM   #3
Koonw is online now Koonw  Malaysia
diyAudio Member
 
Join Date: Apr 2013
Location: Miri, Sarawak, Malaysia
You got a problem in bias circuit for right channel, your left channel ok? Then you can compare and trace where the negative bias voltage disappeared with all tubes unplug.
  Reply With Quote
Old 23rd June 2015, 02:54 AM   #4
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Location: Upper midwest
Quote:
Originally Posted by Koonw View Post
You got a problem in bias circuit for right channel, your left channel ok?
Then you can compare and trace where the negative bias voltage disappeared with all tubes unplug.
If there were no bias, the output tubes would be glowing red.

Last edited by rayma; 23rd June 2015 at 03:01 AM.
  Reply With Quote
Old 23rd June 2015, 03:08 AM   #5
Koonw is online now Koonw  Malaysia
diyAudio Member
 
Join Date: Apr 2013
Location: Miri, Sarawak, Malaysia
Quote:
Originally Posted by rayma View Post
If there were no bias, the output tubes would be glowing red.
Oh, ok fine I hear you wrong, sorry.

You got zero reading on the meter (which measured the cathode bias current for auto-bias the tube ) but no plate current, then anode (plate) circuit is opened, it could had opened after the auto-bias opened so better check both.

Last edited by Koonw; 23rd June 2015 at 03:22 AM.
  Reply With Quote
Old 23rd June 2015, 05:07 AM   #6
diyAudio Member
 
Join Date: Nov 2011
Thanks for the assistance. I will look into a new fuse, if that is not it I will return to the seller to repair because it was sold in this state.

Last edited by Enochrome; 23rd June 2015 at 05:15 AM.
  Reply With Quote
Old 23rd June 2015, 05:51 AM   #7
AllenB is offline AllenB  Australia
diyAudio Member
 
Join Date: Oct 2008
It would probably be best if you plan to take it back. By the way these often have only one fuse. There is reason to believe it isn't blown.

Finding the problem shouldn't be hard if you have a meter. You should be trying to measure current through each device rather than just bias voltage.
  Reply With Quote
Old 23rd June 2015, 03:24 PM   #8
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Location: Upper midwest
Quote:
Originally Posted by Koonw View Post
You got zero reading on the meter (which measured the cathode bias current for auto-bias the tube )
but no plate current, then anode (plate) circuit is opened, it could had opened after the auto-bias opened so better check both.
It's possible that if each channel has an individual power supply or plate fuse,that this is open. This would be a less common situation, though.
  Reply With Quote
Old 23rd June 2015, 04:21 PM   #9
Koonw is online now Koonw  Malaysia
diyAudio Member
 
Join Date: Apr 2013
Location: Miri, Sarawak, Malaysia
Quote:
Originally Posted by rayma View Post
It's possible that if each channel has an individual power supply or plate fuse,that this is open. This would be a less common situation, though.
I try to explain why you don't have red hot tube, simple because

1) auto-bias has failed due to broken wire, faulty component or loose dry joint

2) Then it has zero voltage on the grid so EL34 draw a lot of current and blew something usually a fuse, resistor, even OPT. So if you replace the components only without also check the faulty circuit, you end up same situation.

But sometimes fuse will just blow when try to adjust bias current of a misbehave tube.

Last edited by Koonw; 23rd June 2015 at 04:24 PM.
  Reply With Quote
Old 24th June 2015, 02:05 PM   #10
M Gregg is offline M Gregg  England
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
1) auto-bias has failed due to broken wire, faulty component or loose dry joint
Auto-bias<<<cathode bias with fixed resistor or CCS. Or control circuit.

Fixed bias <<negative voltage on the Grid adjustable to set bias idle current.

Mixed bias<<fixed cathode resistor with adjustable fixed bias negative voltage on the grid. (Shared bias)



Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  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
Tube issue or SSE? mpri Tubelab 1 26th February 2014 02:21 AM
Grounding issue between turntable and tube amp steffner Tubes / Valves 7 23rd February 2012 05:34 PM
Issue w/ DIY tube amp build bubba_b Instruments and Amps 7 9th January 2012 07:35 PM
RE: Fisher 200 tube amp fuse blowing issue movingcoilguy Tubes / Valves 5 5th November 2010 05:07 PM
Blue Voodoo Tube amp issue sasij Instruments and Amps 1 24th July 2008 11:51 AM


New To Site? Need Help?

All times are GMT. The time now is 02:14 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2