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Author Topic: Switching tube amp between two cabinets  (Read 3390 times)


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Switching tube amp between two cabinets
« on: February 18, 2017, 08:57:35 PM »
I want to see if I can settle a debate I'm I've had with multiple people (or perhaps start a new debate). If your tube amp is on, and you have no signal on the input (shorted to ground) can you switch the loads without causing damage? I get mixed responses, but one source has his PhD and has been building tube amps for 30 years...he says that as long as you can guarantee there isn't any signal it shouldn't matter at all. Then there are guys on the internet that say you shouldn't do this under any circumstance.  I'm inclined to believe that the few milliseconds of no load plus the time that it takes for switch bounce to subside shouldn't make any difference. What do you guys think?

Assume that make before break switches don't exist.
« Last Edit: February 18, 2017, 08:59:13 PM by tra »


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Re: Switching tube amp between two cabinets
« Reply #1 on: February 18, 2017, 11:30:41 PM »
I'm not qualified to answer your question directly....

Anecdotal evidence that it *may* not matter...

I once "repaired" a guitarist's tube amp by replacing the speaker cable.
(Head and speaker cabinet)

He was complaining that it had been randomly cutting out for some time, and then finally, it cut out completely.
I was there at the time it finally cut out completely (rehearsal place, mid song) and I told him to turn it off.
Made a quick check and the speaker cable (was) intermittent and finally broke one of the wires.

Replaced the speaker cable and *his* amp appeared non the worse for it.

I don't know if there was any long term negative effect from the amp running for "some time" with an intermittent speaker connection,
and then for several minutes with NO speaker connection.

But it appeared ok then, and for some 6 months of regular use afterwards.
He moved away after that....

Your amp may be different....

I am aware of the danger of running a tube amp without a load connected. Tx damage.

I can't see how the milliseconds of switching loads would matter.

It is probably dependent on the quality of the iron.
A cheap output Tx might be easily damaged from only a few seconds of no load.
If it ain’t broke I’ll fix it until it is.


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Re: Switching tube amp between two cabinets
« Reply #2 on: February 18, 2017, 11:40:46 PM »
After a quick Google of this I found this post....


Note the last paragraph.
Maybe it will help?

If the amp was built with a switching jack where it's contacts are closed and grounded when there is no plug inserted, then there is usually no problem even if you bang a handfull of power cords trying to find out what is wrong.
In that scenario, the output transformer will "sense" the speaker load to be what ever the actual DC resistance of the OT secondary is... VERY low but still some kind of load.
The problem with unloaded OTs is when there is no load, like an open connection.
Then the OT senses the load to be very very high impedance... like infinity.

Remember that the OT "reflects" the speaker load impedance to the power tubes and multiplies that impedance to match what the power tubes need to operate correctly.
If a pair of tubes need 4000 ohms and the speaker is 8 ohms then the ratio of secondary to primary is 500:1.
If there was a speaker load of 16 ohms connected to the 8 ohm tap then it would be, 500 x 16 = 8000 ohms.
A miswired cabinet at 32 ohms would reflect back to the power tubes at 16,000 ohms! etc.
An open circuit, as with no speaker load, would be so high that the power tubes would see an infinite high reflected load or something stupidly high in the many many tens of thousands of ohms impedance.
The power tubes will try to put current and high AC (audio) voltage into that outrageously high impedance load.
When that happens, AC voltage in the OT will rise to a very high level.
The abnormally high AC voltage in the OT could jump across the very windings of the OT primary and blow up the OT or, jump across the tube socket lugs trying to find a path to ground. This draws mongo current form the power supply and will probably blow the fuse but wrecks the tube socket and sometimes the tube too.
Here's were a very well made OT can save you for a little while and a cheap one blows up.

Here's an old trick....
connect a 5watt 270 to 470 ohm resistor to ground across the highest secondary tap of the OT.
Use a 270 ohm for (4 to 8 ohms speakers) and 470 ohms for (8 to 32 ohm speakers).
Now no matter what happens, there is always some kind of "load" on the OT secondary that will help protect it from destruction if the cable comes unplugged, the jack fails or the speaker blows open.
The resistor is so high in DC resistance that most of the audio still goes into the speakers so it is invisible.
Hope you found that useful.....
« Last Edit: February 19, 2017, 12:27:58 AM by galaxiex »
If it ain’t broke I’ll fix it until it is.


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Re: Switching tube amp between two cabinets
« Reply #3 on: February 19, 2017, 12:45:08 AM »
Yes the extra load resistor was common and does exactly what *galaxiex* noted in the last paragraph of that link. Lots of older valve schematics have the safety resistor across the speaker output socket. Most modern Amps don't bother with it as it adds cost to the build,, or they is just to lazy to bother. :trouble :grr

I agree it's really dependent on the amplifier design and some designs push the boundaries of what Valves can do and may be more prone to open circuit failure compared to designs with more conservative designs.


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Re: Switching tube amp between two cabinets
« Reply #4 on: February 19, 2017, 02:48:17 AM »
This is right in there with seat belts.  You should ALWAYS wear a seat belt when driving a car.  OK, how about if I just want to back it out of the garage onto my driveway?  No traffic, no other cars.  I might belt up just because it is automatic for me at this point, but I hardly think it necessary.  But someone would argue that always means always.

The danger to your amp is if a loud spike goes through the OT with no load, a huge primary spike might develop.  Ground the input?  OK, but that doesn't stop pops in the system.  SO they are possible.  Maybe not likely, but possible.

A lot of amps these days have protection diodes on the OT primary.  These clamp the ends of the primary to prevent damaging spikes in many cases.

If the amp has a standby, use it, or just flip the damned power off for a second or two.  Why look for reasons to justify iffy behavior?  But to answer the question:  in the scenario described, a grounded input amp running, will it hurt anything to pull one speaker cord out and plug another in?  Probably not.

The millisecond you describe?  That is plenty of time for a spark, electricity is fast.  And switch bounce?  Opening and closing of the circuit is exactly what causes the problem in the first place.  But in the context, those are red herrings.  The question is this:  will you get away with it?  In all likelihood, probably yes.  Does getting away with it make something a good idea?  No.


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Re: Switching tube amp between two cabinets
« Reply #5 on: February 19, 2017, 09:24:21 PM »
Agree with above that it is not good practice.
As far as hypotheticals, for debate, not only must you guarantee there is no input signal, but also that there is no incidental noise.  You could have the input jack grounded and no signal, but you could still have a microphonic preamp tube squealing away, or an inaudible ultra-sonic oscillation.  Either could cause damage if the OT was unloaded.


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Re: Switching tube amp between two cabinets
« Reply #6 on: March 28, 2017, 06:26:32 PM »
It is correct, as noted above, that an output transformer may be easily destroyed without a load.

In somewhat more detail, the reason is that the core flux couples the primary to the secondary, with the number of turns determining the ratio between input and output.  The AC current at the primary induces an AC current at the secondary, with the relationship of voltage and current being determined by the winding ratio.  Current through the secondary is then dissipated as heat by the load (speaker).

The problem arises when no load is present at the secondary.  As the flux tries to collapse there is no place for it to go; it cannot be converted into current because the secondary lacks a complete circuit for current to flow.  So the voltage begins to climb until current may flow.  At some point that flow occurs via insulation breakdown, aka an arc.  This arcing problem happens with transformers, it happens with solenoids, motors, etc.  The voltage can be considerable: an output transformer with an input voltage of a few hundred volts may arc at several thousand volts.  That arc may go through the transformers insulation, destroying it, or it may go through tubes or adjacent terminals at the tube base, destroying them.

The standard technique is a reasonably high value resistor across the secondary terminals, in parallel with the normal load (speaker) to always provide a load of some sort.  As per Ohm's Law, E = I x R, if R is a few hundred Ω (say 470 Ω) it passes relatively little current at a terminal voltage of, say, 40 V (about 9 mA) compared to the speaker, say 8 Ω, which passes 5 A at the same 40 V.  This parallel resistor does not much affect the overall load impedance (470||8 = 7.9 Ω).

But there is an even better solution to be added in addition to the parallel resistor: the flyback diode.  These go by other names depending on whether they are used with transformers, motors, or solenoids.  In alphabetical order: catch diode, clamp diode, commutating diode, freewheeling diode, snubber diode, and suppression diode or suppressor diode.  If you've ever seen a relay circuit with a diode across the coil, that's the purpose: arc suppression.

When the inductive kick produces the reverse voltage the diode will conduct, and harmlessly dissipate the arc by completing the circuit.  It's like a switch that only triggers on an arc.

One thing that must be remembered is that the arc voltage is many times higher than the continuous voltage, so the PIV rating for the diode must be very high.  A few hundred V of B+ can result in a diode seeing 4 kV.  High voltage diodes are available and are not very expensive (few dollars) but one must be aware that the voltage ratings of common diodes at 1 kV (or lower) are inadequate for this purpose.

Placing a MOV (Metal Oxide Varistor) across the terminals is less idea.  These devices have a finite lifespan as each over-voltage condition which causes breakdown damages the device.  So a single cycle of operation without a load may be sufficient to destroy the device and then destroy the amplifier.  This is an inherent problem in the MOV construction, which in socket strips is known to fail as a short and then burst into flame.  One is technically supposed to regularly measure MOVs to detect a 10% difference in characteristics and then replace faulty devices.  (Yeah, right.  I don't make the specifications, I just report them.)

Anyway, flyback diodes and parallel resistors are inexpensive and highly effective.  No good reason exists to not protect hundreds of dollars worth of output transformer and tubes using a few dollars worth of components.

Oh, and since someone will ask, the reason manufacturers do not include protection devices is that it increases the cost which ripples into the retail channel at many multiples.  If consumers as an aggregate do not understand the difference and ask for better circuits, they will not receive them.


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Re: Switching tube amp between two cabinets
« Reply #7 on: March 29, 2017, 11:09:57 PM »
the reason manufacturers do not include protection devices is that it increases the cost which ripples into the retail channel at many multiples. 
Until the point where the cost of warranty claims pressure them to include protection.  Then they will make an adjustment to achieve cost equilibrium.
The big time manufacturers have exact percentages to achieve as far as warranty claim costs.  And they keep track of the specific nature of field failures so they can make adjustments to achieve those numbers.
  Of course, the fact that most warranty work now requires the end user to pay the shipping costs has shifted this balance in favour of the manufacturer (an end user is less likely to return a defective product due to shipping cost).
So a bit less % units returned under warranty means the product seems (falsely) more reliable as far as the manufacturer is concerned and the bean counters tell them to cut costs to get back to that previous equilibrium.  ::)