8 ohm resistor on output sparks tubes

6V6dude

Member
2014-09-09 11:07 am
Something I can't quite work out. I'm running 8 ohm speakers from my tube amp. I wanted to take signal directly from the speaker output for recording. So I connected 8 ohm resistor load on the output and turned the amp on without anything else yet connected. But the output tubes (SE amp) started sparkling inside.

It makes no sense to me, 8 ohm is 8 ohm even though actual speakers aren't exactly constant. It shouldn't spark. Any ideas what's going on? I've build few amps so it's not like I know nothing about it. I just never had a need to try replacing speakers with same load resistor.
 
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Something I can't quite work out. I'm running 8 ohm speakers from my tube amp. I wanted to take signal directly from the speaker output for recording. So I connected 8 ohm resistor load on the output and turned the amp on without anything else yet connected. But the output tubes (SE amp) started sparkling inside.

It makes no sense to me, 8 ohm is 8 ohm even though actual speakers aren't exactly constant. It shouldn't spark. Any ideas what's going on? I've build few amps so it's not like I know nothing about it. I just never had a need to try replacing speakers with same load resistor.

Are you sure the resistors are 8 ohms ? I have come across similar issues with unconnected output terminals on pentode amps but an 8 ohm resistor should be fine

cheers

316a
 

6V6dude

Member
2014-09-09 11:07 am
Yes 8 ohm. I've even measured it. Wiring and and output jacks are good too. It's bizarre. I don't want to try it again, bit too risky. I just remembered that sparking looked like when you have an old tube and a bit of carbon (or something) gets loose and gets between the plates. That sparks as well, I've experienced that couple of times, once on the rectifier but on different amps/ old radios.
But I never moved this amp or bumped it to get some dirt loose, I suppose there is a pretty remote chance this was it though. We'll I'm not going to try unless I don't have a choice again when experimenting.
Otherwise the amp works just fine, using it right now - with speakers only off course :)
 
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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
Yes 8 ohm. I've even measured it.....

NO! BJosephs asks you to meaaure the "8 Ohms" *while connected to the amplifier OT*! If really connected, the right answer is "about 1 Ohm". (The OT winding measures about an Ohm on a DC meter; 8r||1r is about 1r.)

If you think it is connected and still read 8 Ohms, it isn't connected, not loading the OT, which sure explains your "sparks".

Sorry if you already got this.
 

6V6dude

Member
2014-09-09 11:07 am
Good idea, didn't think of that. I'll measure it with resistor in. Want to get to the bottom of this mastery.
On the other hand I got another amp I've also build, with that one I can unplug the speakers and nothing happens. Found out by accident where I've turned it on by mistake without speakers connected for a whole day. Even tubes are fine. Similar build but the OTs are different.
 

mandu

Member
2010-05-22 5:27 am
For the other amp you mentioned, Chances are that there was no signal on the amp or there is a small capacitor (0.001uf) across OT to suppress hiss.
If the OT is well insulated, sparks will appear across tube sockets or inside output tubes if driven without speaker connected.
 
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Gnobuddy

Member
2016-03-01 4:10 pm
...another amp I've also build, with that one I can unplug the speakers and nothing happens. Found out by accident where I've turned it on by mistake without speakers connected for a whole day.
To me, that sounds a bit like "I left my 2-month old baby balanced on a bar stool for 30 minutes, and she didn't fall off and get injured. I can leave my baby on a bar stool and nothing happens."

In other words, nothing bad happened the first time. What will happen the next time? :(


-Gnobuddy