Damage to valve amp when powered up but disconnected to speakers? - 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 15th January 2011, 01:17 AM   #11
diyAudio Member
 
Join Date: Dec 2010
Location: New Hampshire, USA
You want to make sure that you have a proper load on the output while putting a signal through the amp. This will reduce the chance of blowing your output transformer.

Use a non-conductive rod (wood stick) to prod around to see if it changes anything. If you had a ground loop, I would expect it to increase in volume when you crank the amp. Maybe a bad output transformer. Please be careful while doing this.
  Reply With Quote
Old 15th January 2011, 07:03 AM   #12
eeyore is offline eeyore  Australia
diyAudio Member
 
Join Date: Aug 2005
I disgust once, powered on a tube amp with no load. It caused soft sparking sounds from the output transformer and then the main PSU filter resistor when red hot. But after that, connecting the speakers back, everything was fine. Plays very well. However, in this amp that I built, I was able to check everything and make sure each component was still okay, bar the output transformer. From what I was told by the kit maker, oscillations leading to excessive current draw and hence the deistic turn heater. It make have damaged components that were near max spec. But in this case I used well overspecd components. But do have a check anyway.
__________________
DF
http://diy-audio-blog.blogspot.com
  Reply With Quote
Old 15th January 2011, 07:39 AM   #13
godfrey is online now godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
Quote:
Originally Posted by kavermei View Post
But this is only when you're actually playing loud music over the amp while its output is disconnected.
....or if the amp likes to oscillate with no load.


IMHO, damaged tubes or OPTs are likely to cause distortion, but not hum. Miles's suggestion that the hum could be caused by damage to the supply filtering seems more plausible.

[ot]
I once tried converting an EL34 PP amp to an AM transmitter by connecting a tuned circuit across the OPT, and adding a bit of feedback. The resulting oscillation was fierce enough to cause visible sparking on the transformer.

To my surprise the tubes survived the experiment, despite being left to hurt for several minutes while I wandered off down the road with a portable radio to check reception. I don't think the transformer was ever quite the same again though.
[/ot]

edit: oops, I missed eeyore's post. Seems like accidental oscillation is a very real possibility after all.

I can't help wondering why the designer didn't use a Zobel (or something) to make stability less load-dependent. Seriously. It can be done. In solid-state-land, you don't see stickers that say: "Caution, this amp may explode if the speaker is disconnected".

Last edited by godfrey; 15th January 2011 at 07:53 AM.
  Reply With Quote
Old 15th January 2011, 08:56 AM   #14
diyAudio Member
 
kavermei's Avatar
 
Join Date: Jan 2005
Location: Lokeren, Belgium
Send a message via MSN to kavermei
Quote:
Originally Posted by godfrey View Post
....or if the amp likes to oscillate with no load.
Yes, indeed. I overlooked that possibility. Such amps do exist unfortunately.
__________________
Never send a human to do a machine's job. --Agent Smith
  Reply With Quote
Old 15th January 2011, 09:02 AM   #15
eeyore is offline eeyore  Australia
diyAudio Member
 
Join Date: Aug 2005
I think that some of the designers of amps focus on the pursuit of purists and simplicity in design and thus things like zobel, output protection, muting, soft start, etc are often left out. But it does place more onus on the end user to be careful. Each to their own.

But also noticed that some of my tubes also make some noise during start up. Maybe it is the thermal stress and in rush of voltage. Metal - heat - expansion maybe?
__________________
DF
http://diy-audio-blog.blogspot.com
  Reply With Quote
Old 17th January 2011, 10:26 PM   #16
diyAudio Member
 
Horsebox's Avatar
 
Join Date: Jan 2011
Thank you all again for the replies.

There was no evidence of OPTs suffering, and in absence of any means to test them, I've ruled them out.

Several posters have mentioned oscillations. Am I right to interpret that the disconnection of the speakers resulted in an infinite impedance which may have caused large swings in voltage in an attempt to drive current through that infinite impedance? Result being that any large swings in voltage may have taken some components outside their limits and therefore damaged them? I can understand this to an extent but even if volume is zero (i.e zero demand for current)?

Miles Prower suggested checking the filter caps for damage caused by overvoltage. Am I right to assume that these are the caps closest to the power transformer, perhaps directly after rectification (looking at amp circuit diagram I have to hand here)?

Thanks again everyone. I need to know more about what I'm looking for before I take the back off.
  Reply With Quote
Old 18th January 2011, 11:10 AM   #17
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
All circuits have noise. As far as the circuit is concerned, this is the same as signal - it is only us who want to make a distinction. There will always be output, and this can sometimes be fed back to the input. This is what we call an oscillator.
  Reply With Quote
Old 18th January 2011, 12:05 PM   #18
diyAudio Member
 
Join Date: Nov 2010
I have had a situation where an amp came in for repair and it had a slight hum.There were some shorted turns of the primary of the OPT which was enough to unbalance the circuit to make the hum come through, replacing the TX (it was a EMI stereoscope so tx had to be rewound) cured the hum.
  Reply With Quote
Old 18th January 2011, 12:17 PM   #19
Doz is offline Doz  United Kingdom
diyAudio Member
 
Doz's Avatar
 
Join Date: Jul 2010
Location: Sat Down
Quote:
Originally Posted by toprepairman View Post
I have had a situation where an amp came in for repair and it had a slight hum.There were some shorted turns of the primary of the OPT which was enough to unbalance the circuit to make the hum come through, replacing the TX (it was a EMI stereoscope so tx had to be rewound) cured the hum.
Easy to prove then , swap the opt's over and see if the hum goes with it.
  Reply With Quote
Old 18th January 2011, 12:20 PM   #20
diyAudio Member
 
Join Date: Nov 2010
Yup.
  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
Budget valve amp speakers Knoffers Full Range 10 7th December 2010 07:12 PM
Strange damage on speakers swak Multi-Way 8 15th July 2010 05:18 AM
Can Silicon Caulk damage speakers? DeadSpeaker Multi-Way 13 19th January 2009 02:38 PM
newbie question:do i damage the speakers if this is the case? digitaldiy Chip Amps 4 17th March 2005 04:16 PM
may PLOP sound damage speakers? keyser Multi-Way 2 9th March 2005 10:12 AM


New To Site? Need Help?

All times are GMT. The time now is 01:32 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