Resistor on output jack

[Tdale]

I blew an OT once, because someone didn't connect a speaker before they switched the head on.

Would it work to connect a 50W / 8Ohm resistor on the output jack, if I use a jack that openes when the jack is inserted, and closes when it's pulled out (so that the resistor connects to ground when the spaker plug is out, and gets disconnected when the plug is in?

Tommy

[billyz]

Traynor used to use a 200ohm 10w( maybe 20w i don't remember) resistor to ground right off the tip of the output jack. I could not hear any degradation of tone or power. Very safe. Others have used similar schemes. one of the bullet proofing ideas that few use today.

[rooster]

Victoria Amps has used a 470 ohm 2 watt resistor on the output jack. The wattage doesn't have to be any higher - when you think about it - because the OT is not going to really be able to deliver any juice to such a high impedance. Too, remember that the resistor will parallel the speaker and the higher it is, the less it will interact with the speaker - 200 ohms is basically nonexistent, too. Just saying.

I used to use this, FWIW, thought it was a real good idea. But then I woke up one day and realized the primary side of the OT is taking the abuse, and here you need understand your power tubes, when they are going wrong, and not to over fuse things.

And speaking of fuses, a better fix for the problem you experienced is to fuse the center tap of the primary. Try 1 amp for starters. I use 1 amp on my Rocket builds, FWIW.

[Roe]

i'm using hiwatt style output jacks wiring. if nothing is connected, the ht fuse will blow

[rooster]

Roe - Hey, post a pic of that if you can, I hadn't heard of anything special with HiWatt. Thanks!

[martin manning]

Hiwatt speaker jack wiring: http://mhuss.com/SmallBox/page4.html

[M Fowler]

Here is an old Hi-watt gut shot

[rooster]

Mark, thanks. OK, this is just the simple shorting connection arrangement like you would see in a Fender amp. Eh, Roe, how does this blow the high temp fuse? As far as I know it won't. But maybe you have something else that HiWatt used that I cannot find? Really, a HT fuse that would blow if you forgot to plug a speaker in? Now that would be protection even if it would also be a huge pain in the ass if you ran out of fuses on a gig.

[Roe]

Mark, thanks. OK, this is just the simple shorting connection arrangement like you would see in a Fender amp. Eh, Roe, how does this blow the high temp fuse? As far as I know it won't. But maybe you have something else that HiWatt used that I cannot find? Really, a HT fuse that would blow if you forgot to plug a speaker in? Now that would be protection even if it would also be a huge pain in the ass if you ran out of fuses on a gig.

First, it shorts and 0ohm is closer to a speaker load (2-16ohms) than an infinite load. Second, when you start to play it will draw a lot of current and the HF fuse should blow because of that (hopefully before the tubes melt down). the main thing is that the OT should be safe.

[Cliff Schecht]

Mark, thanks. OK, this is just the simple shorting connection arrangement like you would see in a Fender amp. Eh, Roe, how does this blow the high temp fuse? As far as I know it won't. But maybe you have something else that HiWatt used that I cannot find? Really, a HT fuse that would blow if you forgot to plug a speaker in? Now that would be protection even if it would also be a huge pain in the ass if you ran out of fuses on a gig.

First, it shorts and 0ohm is closer to a speaker load (2-16ohms) than an infinite load. Second, when you start to play it will draw a lot of current and the HF fuse should blow because of that (hopefully before the tubes melt down). the main thing is that the OT should be safe.

Yup. Ideally your output transformer is much better off with a short than an open. Something that happens with the open condition is the inductance of the primary coupled with any current will cause the voltages to rise right at the plates. Since the current really has nowhere to flow but into the now massive inductor, as the plates swing even from noise this is enough to start pushing that voltage up pretty quickly. Remember we are dealing with Henries of inductance, even at the typical 100mA of current available from a big bottle output stage and 60 Hz noise into an inductance of 25H gives a voltage spike of 150V. This is taking it easy on the numbers too, bigger OT's have higher inductances and more current available so things can go even quicker on these guys! Once you start trying to play chords it will be over quickly. Ever wonder what that whine you hear is when the OT is running open load? That's the coils vibrating from all of the extra energy they are now having to handle! They quickly die either from excessive power into a saturated core (push-pull OT's can't handle static DC current) or voltage spikes that cause the windings to arc. Either way you're screwed..

Single-ended amps can handle no load conditions better just in that the output transformer doesn't saturate immediately when the energy has nowhere to go. If you aren't using a puny OT or pushing current ratings too hard then you've got a much longer grace period before things go south. Once again though if you start trying to play music you will quickly saturate the OT and lose your output stage.

I've gotten in the habit of never touching the power switch until I first touch the speaker jacks and make sure everything is connected. It's a stupid simple check that can save you a lot of grief. Especially if you are using your favorite piece of vintage iron that would be impossible to replace (cheaply) .

[Structo]

And speaking of fuses, a better fix for the problem you experienced is to fuse the center tap of the primary. Try 1 amp for starters. I use 1 amp on my Rocket builds, FWIW.

Do you mean to fuse the secondary center tap?

[Colossal]

And speaking of fuses, a better fix for the problem you experienced is to fuse the center tap of the primary. Try 1 amp for starters. I use 1 amp on my Rocket builds, FWIW.

Do you mean to fuse the secondary center tap?

Hey Tom,

I think Rooster's talking about the center tap of the OT. This would break the B+ connection to the OT if the secondary shorts. I've seen this fusing arrangement on Komets for example.

[rooster]

Tom - Well, the HT fuse is what I am talking about, found in Marshalls, etc., and it is placed in series between the center tap on the primary side of the OT and B+. It basically is fused to recognize a massive current draw to the power tubes should one of them short. Marshall uses 1/2 an amp in their 50 watters. (See attachment). This said, yes, like Cliff said, if the OT arc'd on the primary side the HT fuse would blow also.

Funny thing, some of us seem to be freaked out by stories of blown OTs. In truth, I've only been involved in one such incident and it was a dinky cheapass Silvertone paper wound OT running questionable black plate 6L6s. I was a kid then and knew nothing about amps so I really have no idea why that happened. But I know the speaker was plugged in 'cause I was playing at the time...

BTW, typically, where this schematic shows the fuse prior to all of the B+, it is seen as a fuse that is placed ONLY between the OT primary and the B+. In other words, the preamp does not have to be fused along with the OT.

[katopan]

I put a 270 ohm 5W resistor permanently soldered across one output jack (always used the 8 ohm jack) on all my amps. Transparent compared to the speaker load as mentioned above, and stops any flyback voltage spikes killing the OT if no speaker is connected.

[Structo]

Thanks for the clarification Rooster.

That sounds like a good idea.