# no speaker load



## vadsy (Dec 2, 2010)

How long will a tube amp last if the ON switch is flipped without a speaker load plugged in? If things are working fine after this little problem is caught early are any of the components worse off then before?


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## greco (Jul 15, 2007)

I put up a thread about that topic a long time ago.

Wild Bill replied with a long response. Basically, you should be OK.

I'll try and find a link to the thread.

Found it:
http://www.guitarscanada.com/amp-bu...output-transformers-time-needed-blow-one.html

Cheers

Dave


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## WCGill (Mar 27, 2009)

Being basically forgetful I do this all the time, nothing to report. If you don't have any signal going to the amp, there will never be an issue, and probably none with signal. Output transformers are much more robust than you think.


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## dcole (Oct 8, 2008)

Always double check that you have a speaker connected. I usually ohm out any new speakers I connect to my amps since I am one of those guys that will crank it to hear the sound if I don't hear it when the volumes down. So, when testing a new amp/speaker combo, keep the volume low and use the low E string strummed open to minimize any possibilities of frying something. At least the primary inductive reactance will be lower than playing the 12th fret on the high E.


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## jimt11 (Jun 6, 2012)

I was considering installing a permanent resistor across the output of an amp I am "planning". I was thinking something like 160 Ohms, but I have no idea if this would be a good plan, or how to calculate the appropriate resistance. What do you do?

Jim


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## greco (Jul 15, 2007)

jimt11 said:


> I was considering installing a permanent resistor across the output of an amp I am "planning". I was thinking something like 160 Ohms, but I have no idea if this would be a good plan, or how to calculate the appropriate resistance. What do you do?
> 
> Jim


Jim...this won't really answer your question, but I built a dummy load with a 1% tolerance 4 ohm Vishay power resistor rated for 75 watts, IIRC. I put it in a small enclosure and ran a 1/4 inch plug to it.

This was for a '64 blackface BandMaster head that I used to own and I frequently forgot to plug it into the cab when setting up to jam. At that time, I was so afraid that I would damage the amp that I decided to leave the dummy load plugged into the amp when I wasn't using it...just in case. 

Ironically, I never forgot to connect the amp after I built the dummy load.

Cheers

Dave


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## vadsy (Dec 2, 2010)

Thanks boys!This is all very confidence inspiring. The reason I'm asking all this is I wentaway for a bit and had my brother watch my place. He plays, anything with keysand some strings, but knows very little about amps or pedals. When I got back Inoticed things moved and put away differently, I asked what was up and he toldme he couldn't get my new Stang Ray, head and cab, or pedalboard to work so hegave up after some trying. I ran him through the proper setup procedure and hetells me he missed a cable and didn't plug the cab to the head, PUTZ! I nearlybusted a nut, I hadn't yet turned anything on and thought everything was friedbut it seemed fine and worked. Anyways, I thought I'd ask, thanks for the reassurancethat it should all be ok.
Vadim


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## dcole (Oct 8, 2008)

jimt11 said:


> I was considering installing a permanent resistor across the output of an amp I am "planning". I was thinking something like 160 Ohms, but I have no idea if this would be a good plan, or how to calculate the appropriate resistance. What do you do?
> 
> Jim


What I recall seeing is a 100 ohm load on the 8 ohm tap. Some speakers impedance goes that high at resonance anyways. The 160 would seem like a good number to use on a 16 ohm tap.


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## WCGill (Mar 27, 2009)

Why so low? You'll probably need a 25 watt minimum chassis-mount resistor at these resistances. I use 680R 10-watters, much easier to source and mount.


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## jimt11 (Jun 6, 2012)

WCGill said:


> Why so low? You'll probably need a 25 watt minimum chassis-mount resistor at these resistances. I use 680R 10-watters, much easier to source and mount.


Good point. I was only thinking of the trade off between protection and loss of power to the speaker. I do not know how low I have to go to protect the output transformer so I just dropped the resistance as far as I thought I could without affecting the sound. As I was thinking about a 5 watt practice amp at the time I did not think about the dissipation in the resistor. I'm glad I asked here. Knowing that your 680R resistor protects your amps helps quite a bit. I will plan on the larger resistor. It will keep the heat down and that's always a good thing.

Jim


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## dcole (Oct 8, 2008)

WCGill said:


> Why so low? You'll probably need a 25 watt minimum chassis-mount resistor at these resistances. I use 680R 10-watters, much easier to source and mount.


Can you show me that math as to why you would need a 25 watt resistor?


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## greco (Jul 15, 2007)

IIRC, I read somewhere that that the wattage rating for the power resistor (in a dummy load) should be somewhere around 2 to 3 times (minimum) the maximum power rating of the amp.

Cheers

Dave


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## WCGill (Mar 27, 2009)

Since current flow is inversely proportional to resistance, obviously less watts are dissipated in higher value resistors. I use 3 25R, 25w resistors in parallel as an 8R, 75w dummy load and they get so hot you can't touch them, not surprising as your garden variety 40-50 watt amp will put out 75 watts or more clipping.


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## dcole (Oct 8, 2008)

dcole said:


> Can you show me that math as to why you would need a 25 watt resistor?


WCGill,

I guess I should clarify my question. If a 100 ohm load is used to prevent the primary OT voltage from flyback, why does the 100 ohm load need to be rated at 25 watts?

Other than loading the output, in my research I found two other methods. Flyback diodes on the OT primary and a shorting jack to short the output when not loaded with a cab.


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## greco (Jul 15, 2007)

dcole said:


> Other than loading the output, in my research I found two other methods. Flyback diodes on the OT primary and a shorting jack to short the output when not loaded with a cab.


Any chance you could post links to descriptions of these 2 other methods. 
I'd like to learn/read more.

Thanks

Cheers

Dave


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## WCGill (Mar 27, 2009)

Lower value resistors need to be rated higher so that the full output of the amp (unlikely but still possible if someone cranks the amp with no load) doesn't cause the resistor to go open. Higher ohm values won't dissipate as much power. I never liked diodes on the outputs as I believed I could hear them. The shorting jack is foolproof but obviously won't work with OPT's with multiple secondaries. My last 2 builds I put no resistors in at all, trusting that the clients, who are both experienced musicians with many road miles, won't hammer on the amps without speakers. Hopefully.


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## Wild Bill (May 3, 2006)

WCGill said:


> Why so low? You'll probably need a 25 watt minimum chassis-mount resistor at these resistances. I use 680R 10-watters, much easier to source and mount.


Quite right, WCGill! You don't want the protection resistor to be as low as a typical speaker. It will then suck up a good portion of the output power and will have to be BIG and EXPENSIVE to dissipate that power as heat!

On the other hand, too high a value means that you are not keeping the output load low enough with no speaker plugged in to prevent those high voltage flyback signals.

There is no easy way to calculate the best value but over the years designers have made some good guesses. Pete Traynor often used a 250 ohm, 10 watt wired inside some versions of the YBA-3 Custom Special bass head. It must have worked 'cuz I have never seen one with a blown OT from not having a speaker load plugged in.

Wild Bill/Busen Amps


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## WCGill (Mar 27, 2009)

Maybe Pete just got lucky. 2 EL34's running hard across a 250 ohm resistor will generate enough voltage to dissipate 16 watts in said resistor. How much would that monster Custom Special generate? More than that poor 10 watt resistor could handle I'll guarantee. The whole safety resistor idea is one of those things that works really well if you don't need it. If the value is too high it's unhealthy for your amp. If the wattage is too low and the resistor opens it's unhealthy for your amp. Indeed, running a tweed Bassman into a 16 ohm load, or a Super Reverb, could cause grief. If you design for worst-case scenario, ie the amp is extremely loud and you have an open circuit, then your resistor has to have a huge power rating so it won't open and a low enough resistance so it's not too much of a mismatch. I don't think this is attainable, but worst-case is an extremely rare event. No, I think the resistor is a good safety measure for the majority of times when you've just turned the amp on and no sound, so you find out you forgot to plug in the speaker. No damage done as volume levels are usually low. I do the same thing quite often as I mentioned before and don't have safety resistors in place. Of course I don't keep turning it up because I can't hear it.


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## Wild Bill (May 3, 2006)

Ah but MCGill, that 250 ohm resistor will show a FAR higher load to the plates of the tubes! This load will be far outside the power curve for those tubes, making it unlikely they would be trying to pump out anywhere near the same power.

Besides, that 250 ohm resistor was never intended to be a dummy load, continuously soaking up power. Music power at that, which again is NOT continuous power! It was supposed to keep voltage spikes down, that's all.

One would expect that when you thumped the strings of your guitar you would start to check things out when you didn't hear anything! Only a nimrod would keep playing on 11 with no sound coming out, until something finally fried!

That resistor simply provided some safety for voltage spikes in the trannie, for a couple of minutes at most until the owner finally figured out he should plug in his speaker! And it was CHEAP! Real protection would have been much more expensive. Why should everyone else pay more for to save an amp owned by a nob?

Wild Bill/Busen Amps


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## WCGill (Mar 27, 2009)

Yes it's very hard to make things foolproof because as they say, fools are ingenious.


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