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.
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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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.
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
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.
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 🙂
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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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.
There is probably no HF load as there would be with an inductive load.
Place a 220nF capacitor across the resistor.
Place a 220nF capacitor across the resistor.
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.
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.
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.
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.
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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It could also be as simple as the non-sparking amp has output jacks wired to short if left unconnected.
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