technical reason tube amps need a load?

[dbw]

My dad is an electrical engineer and he's helping me repair a solid-state 80's Carvin bass amp I found.  I asked him if it was safe to run a solid state amp without a load, and he said "of course, why wouldn't it be?"  I said I knew it's not safe to run a tube amp without a load and he said that sounded like "bunk".

What's the technical reason this is bad for the amp?  I said "the amp is designed to be run with a load and it's bad for the transformer" but he started talking about step up transformers and voltages and my eyes glazed over and I couldn't provide the level of detail he wanted.  Maybe one of you guys can...

 

[chuck7]

I'm going to go with "Because CB says so"?

 

[GoDrex]

I have no real idea about amps, but it seems to me that anything that puts out power has to have something to deliver that power into. Not sure if it really the same but I know that a radio transmitter should only be run if it's hooked to an antenna or a dummy load, otherwise the power feeds back and will damage the transmitter's components. The antenna also has to match the output of the transmitter frequency (sort of like speaker ohms I guess) or you'll get power feedback into the transmitter. There's always a small amount of inefficiency, but you try to get it as low as you can.

 

[jackthehack]

If you do not have a load on a tube amp the power amp stage can overheat and self destruct.

 

[dbw]

If you do not have a load on a tube amp the power amp stage can overheat and self destruct.

Sure, that's the conventional wisdom, and I believe it... but why exactly?  Pretend you are talking to an electrical engineer with 35 years of (non-amp related) experience.  CB is probably the only one who can answer this...

 

[jackthehack]

When there is no load connected to the speaker jack, the output of the transformer is shorted to ground. Now while this is far from an ideal condition, it will, for a short duration, prevent damage from occurring in the output transformer. It will, however, cause damage to most transformers if this condition is sustained for any extended period of time because too much current is being drawn from the transformer.

 

[-CB-]

Let me quote where jack failed to... (no offense, just for clarification)

(Quote http://www.musicianshotline.com/archive/monthly/ask_budda/08_05.htm )

I tried plugging my guitar into a Peavey tube head vtm 60 (no speakers, no dummy load) I then connected the Peavey via the effects send to the input jack of my Fender Blues DeVille. The Peavey sounds great as a preamp. From what I've been reading this is BAD. Is it?
Thanks, Jeff Schwartz

Jeff,
Yes BAD is a good word to describe what you're doing. You see, a tube amp, unlike a solid state amp, needs to have a load on its output. The AC current and voltage on the primary and secondary sides of the output transformer are directly affected by the load on the secondary side. The optimum conditions exist when the secondary (or output) side of the transformer is properly loaded. Some tube amps use shorting jacks for the speaker outputs. When there is no load connected to the speaker jack, the output of the transformer is shorted to ground. Now while this is far from an ideal condition, it will, for a short duration, prevent damage from occurring in the output transformer. It will, however, cause damage to most transformers if this condition is sustained for any extended period of time because too much current is being drawn from the transformer. Since I believe your Peavey uses open jacks for the speaker outputs instead of shorting jacks, an opposite but potentially more dangerous condition exists. When there is NO load on an output transformer, it attempts to develop more voltage on the secondary side. This can cause the internal insulation of the transformer to break down and high voltage arcing can occur causing shorted turns or open windings.

There may be, however, a couple scenarios that may have spared your output transformer. First, is if the effects loop is a simple series loop, and the "send" jack actually breaks the signal path. This is not probable, as most amps use the "return" jack to break the signal, but I have seen a few designs that use the send, so it is at least possible. The next is if the effects loop in the amp is "pre-master", and you had the master volume turned off. This would prevent any signal from driving the output stage, so the tubes and transformer would not be attempting to drive a speaker load.

The best advice I can give you is to hook the amp up correctly and see if it still functions properly. If it does, and you still plan to use the amp in this manner, run right out (do not pass go, do not collect two hundred dollars) and purchase a proper load for the amp. THD makes a nice unit called the Hot Plate, and there are others made by Weber, Dr. Z, Tube Amp Doctor and Marshall. I believe most of these even have a line out on them that you can use instead of the effects send of the Peavey. That would give you the total sound of the amp including the output stage, where some nice things happen if you drive it hard enough. The tone and response of the output stage is a very integral part of the tone of our Budda Amps, which we, of course, recommend never be run unloaded.

Remember, a loaded amp is a healthy amp! Wait a minute, Let me re-phrase that. Ah, nevermind.

Jeff Bober
www.budda.com


(end quote)

Sort of.... its actually a safe condition if the transformer was sized and specified properly to begin with.  When things are marginalized, not telling.  FWIW - Marshall did, and still does, marginalize their transformers.  Its part of their signature tone, having to do with the saturation of the transformer, or put another way, when its magnetic properties do not permit any further "transformation" from one coil to another (in laymans terms).

You father should know that ANY transformer that is run continuously with no load is going to run hot.  Worse, transformers are made to transform AC, but output transformers on guitar amps have a large DC current which is always present.  What happens on the primary side (tube side) depends on what is presented on secondary side.  If there is an open secondary, the primary is basically a DC electromagnet, since no "transformation" takes place, and it quickly overheats.  The shorted secondary, while not ideal... should actually last indefinitely (I differ with the Budda guy) so long as the transformer itself is not under spec'd to begin with.  There are _those guys_ that will use a 3w rated transformer on a 5w or 8w output, because they like the saturated sound.  Performance has it price, nuff said. 

Hope that clears it up a bit.

 

[dbw]

Not at all!  But I'll show it to my dad and I'll relay his comments back.

By the way, when you say the transformer will run hot.... my transformer always gets real hot!  I thought this was to be expected?

 

[-CB-]

Yes, to be expected that they run hot.  But for instance, I work with 28.8 v transformers all the time.  You have no load... they get SMOKIN hot and run cooler with a load.  However, they cannot be run into a shorted secondary - because the primary impedance is effectively "zero" (well, a few ohms... not many).  On a tube amp output transformer, the impedance on the primary (with tubes in place) is rather high, so shorting is ok - its self limiting, within the ability of the transformer to withstand it.

The no-offense part was for Jack.  I didn't mean my complete quote to be a one-up on his partial quote.  Its just that it seemed awfully familiar, and I was able to quickly locate the source page for it, and give the whole quote in context, as it seems rather helpful, but perhaps a wee bit over cautious.

 

[dbw]

Okay, here's his response:

Thanks - I read it and kind of understand what they are talking about, but still it does not make sense.

Ok, there are two components to consider:  AC and DC.

Lets take AC.  The secondary output is tied back to the power stage's pre-amp via a feedback network.  This feedback will measure the higher than normal output voltage, due to 'no load'.  the result of high feedback will be to lower the signal (AC) drive to the output stage to lower the output voltage.  This loop (feedback) will, instantenously, lower the signal, until everything comes to an equilibrium.

There is no DC transfer in a transformer.  Transformer can only transfer AC.  If you can invent a DC to DC transformer - you may have something.  There is DC flowing in the primary.  This is called the bias current.  this bias current is then modulated with the AC signal (of interest).  If there was no bias current, the transformer would 'run dry' on quite passages and this would clearly cause major distortion.  So the bias current is set such that the drive circuit can reduce it and increase it as required by the incoming signal and this change is what gets coupled across the transformer.  This is a simple Class A setup.  There are more complex schemes, all designed to lower the amount of the bias current. The bias current is where all the heat and power in the output stage is going into.  But, the bias current is in no way related to the secondary.

But, maybe the guitar amps are different.  maybe they do not use feedback from the secondary.  In audio, the output transformers have multiple taps 94, 8, 16 Ohms) for ideal matching of the secondary and the load.  When the two impedances are matched, the power transfer is maximized.

However, in the audio world, people will tell you to connect your speakers to the tap that has the feedback connected to it.  this is usually the 16 Ohm tap.  If you connect the 16 Ohm tap to a 4 Ohm speaker, the power output will be reduced, but the sound quality is optimized as the amp is controlling the speaker load more accurately.
But, that is another story.


Good luck with the solid state amp.

Best,
Dad

 

[-CB-]

Lets take AC.  The secondary output is tied back to the power stage's pre-amp via a feedback network.
The output voltage will not be higher when there is no load.  It seems like there would be.. but there isnt.  When the output is open, the transformer quits transforming.  But even if the voltage were higher, the feedback network only admits a very very small out of phase signal to (usually) the phase inverter.  It cannot really perform the task you imagine as its rather limited in that regard.  Its there to just barely tame the supersonic high frequency, and also to provide a means of damping the output a bit more.  Amps run pretty much the same with the NFL disconnected, and thats a common mod... for a bit of extra high end grit.  Also, consider the case of speaker mismatch to the output.  If you run an eight ohm output into sixteen ohms do you expect the voltage to double?  And therefor the NFL to be twice as strong?  And therefor the output to be much lower?  It doesn't.do that.


The bias current is where all the heat and power in the output stage is going into.  But, the bias current is in no way related to the secondary.
Correct.  The primary just goes pure DC and turns into an electro magnet and self distructs.

But, maybe the guitar amps are different.  maybe they do not use feedback from the secondary.
They do

In audio, the output transformers have multiple taps 94, 8, 16 Ohms) for ideal matching of the secondary and the load.  When the two impedances are matched, the power transfer is maximized.
Some amps have multiple taps, for 2, 4, 8, 16 even 32 ohms.  Usually its three successive impedances on that scale, such as 8, 16, 32 or 4,8,16.  Most amps are just straight secondary windings on the OT, and the impedance is what it is.  And for those, they usually run them a bit more robust, such that you can add a matched load to that normally seen , as an "ext. speaker", and not tear things up.  Some amps - sort of split the difference between favorable impedances.  That is, they run the output to be best matched with say... a 5.6 ohm load, a happy medium, and not terrible for the 8 ohm internal speaker load, or the combined 4 ohm load if used with an external speaker.


Good luck with the solid state amp.
Never touch that bastards... they make me sick!

 

[dbw]

... interesting.  Clearly some people are really into this.  I think I would still like to be convinced more, but, maybe, I'll just go home and get some sleep.

See ya and do not forget to connect your speakers.

Dad

Thanks, CB... I think you and my dad would get along