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8 ohm resistor on output sparks tubes
8 ohm resistor on output sparks tubes
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Old 8th October 2019, 08:40 AM   #1
6V6dude is offline 6V6dude  Australia
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Default 8 ohm resistor on output sparks tubes

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.

Last edited by 6V6dude; 8th October 2019 at 08:42 AM.
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Old 8th October 2019, 10:29 AM   #2
DF96 is offline DF96  England
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The 8 ohm resistor is connected across the output, just like a speaker would be? If it was merely in series with the output then it does nothing useful.
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Old 8th October 2019, 11:08 AM   #3
316a is offline 316a  England
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Quote:
Originally Posted by 6V6dude View Post
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
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Old 8th October 2019, 02:06 PM   #4
Printer2 is offline Printer2  Canada
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Try it again with your speaker.
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Old 10th October 2019, 03:12 AM   #5
BJosephs is offline BJosephs  United States
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For sanity’s sake please double check that the resistor is 8 ohms and that when connected to the amp you see close to a short circuit measuring across the 8 ohm resistor. That will confirm that the cable or jacks wired wrong or failing.
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Old 10th October 2019, 10:26 AM   #6
6V6dude is offline 6V6dude  Australia
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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

Last edited by 6V6dude; 10th October 2019 at 10:32 AM.
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Old 10th October 2019, 12:06 PM   #7
mandu is offline mandu  Singapore
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If the tubes spark with the 8 ohms resistor, means the resistor is NOT in circuit or opens on load causing high impedance and induce high voltage across transformer windings. Replace with another resistor.
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Old 10th October 2019, 12:52 PM   #8
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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There is probably no HF load as there would be with an inductive load.
Place a 220nF capacitor across the resistor.
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Old 11th October 2019, 02:15 AM   #9
PRR is online now PRR  United States
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Quote:
Originally Posted by 6V6dude View Post
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.
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Old 11th October 2019, 11:33 AM   #10
6V6dude is offline 6V6dude  Australia
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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.
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