It's an Ampeg v2 actually. I'd like a v4 though, I've heard good things.
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Running a 16 ohm and 8 ohm cabinet at 5.33 ohms
- Thread starter Mr.Sharkfist
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JeezyMcNuggles
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- Feb 23, 2018
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I used to spend some time on the Marshall forums, and much like we have our designers, techs and tinkerers here so do they. I do remember once getting into a back and forth about mis-matched impedance. According to many posters there, "Santiago" I think is his name, is/was one of Marshall's designers and has (more than once) stated that mismatches aren't as big of a deal with modern Marshall amps as it used to be.
I bowed out. Ok. Who am I to question their big dog?
OP I would take this query to the Marshall forum. There may be a devil in the details specific to your situation since it is a LEM and not just a modern reissue JCM. I cant remember which tap he/they suggested for this mismatch, but I do remember this has been covered, at least in regards to 8==16 loads paralleled on their tube heads.
I bowed out. Ok. Who am I to question their big dog?
OP I would take this query to the Marshall forum. There may be a devil in the details specific to your situation since it is a LEM and not just a modern reissue JCM. I cant remember which tap he/they suggested for this mismatch, but I do remember this has been covered, at least in regards to 8==16 loads paralleled on their tube heads.
I do agree that there is is a consistent amount of mismatch either way (33.33%). The amp will not necessarily see it this way.
There is sort of general rule that tubes amps are safer with a low mismatch. The assumption is it's better to run 5.33 ohms on the 8 ohm output instead of the 4 ohm output. Unfortunately this rule is not accurate for all tube amps.
For example, Mesa consistently recommends a high mismatch with their tube amps. The say 8 ohms on a 4 ohm output is fine, but don't recommend 4 ohms on an 8 ohm output.
I believe which is best depends on how the output section of the amp is set up. So it will vary from one brand and model to the next. What's going on? The output transformer matches the low input impedance of the speakers to the high output impedance of the tubes. But the impedance match is not exact. Also, some designers may choose to set up the tubes in a low loading situation, while others may choose to set up the tubes in a high loading situation. The loading changes some aspects of how the amp sounds and performs, and has consequences regarding whether the amp is safer with a low impedance mismatch or a high impedance mismatch.
Also I believe with some amps you can run 8 on the 4 ohm tap, and 16 on the 8 ohm tap, and the amp will still see the expected load. I am not saying this is safe with any of the amps we have mentioned. AFAIK whether it is safe or not depends on how the output transformer is wound. I believe it relates to whether the transformer has one winding tapped at multiple locations or separate secondary windings for each impedances. Although I don't necessarily understand why it makes a difference, apparently it does.
The impedance ratio is wound into the transformer.
But the transformer wire gauge has to be designed correctly for the current needed.
In solid state you usually keep adding more output devices to handle the current load.
With a tube transformer. To handle more current the wire gauge has to be increased.
So when the dont recommend 4 ohms on a 8 ohm tap.
It has to do with the amount of current a 4 ohm load pulls on
the wire gauge used for 8 ohms.
16 or 8 ohm taps can use thinner wire than a 4 ohm.
4 ohm taps and especially 2 ohm taps use much heavier gauge wire
for the needed current.
In the OP case a 5 ohm load on a 4 ohm tap is not a problem
since 5 ohms requires less current.
Far as impedance mismatch 2:1 is accepted.
That is the fine art of designing transformers.
Heavier gauge wire, and heavier gauge insulation
can handle more current. And tolerate higher voltage with insulation strength.
Problem is with HiFi heavy insulation and heavy wire can cause frequency loses.
So if you want heavy gauge wire for lower impedance, but good
frequency response. Usually you need to start doing more interleaving windings
to gain back frequency response with heavy wire.
With musical instrument transformer, frequency response in the highend
isnt a real priority so you dont usually see much more than 3 layers of interleaving.
Hifi can interleave up to 5 or 6 layers for high current taps.
And often 4 ohm taps are actually 2 windings in parallel
so its 2 windings with 5 to 6 layers. So cost goes up dramatically
Heavier gauge wire, and heavier gauge insulation
can handle more current. And tolerate higher voltage with insulation strength.
Problem is with HiFi heavy insulation and heavy wire can cause frequency loses.
So if you want heavy gauge wire for lower impedance, but good
frequency response. Usually you need to start doing more interleaving windings
to gain back frequency response with heavy wire.
With musical instrument transformer, frequency response in the highend
isnt a real priority so you dont usually see much more than 3 layers of interleaving.
Hifi can interleave up to 5 or 6 layers for high current taps.
And often 4 ohm taps are actually 2 windings in parallel
so its 2 windings with 5 to 6 layers. So cost goes up dramatically
Rather than going by a general rule, I think it better to follow the advice of whoever designed the amp.
My understanding is when the impedance of the speaker is too high, it restricts current flow in the transformer secondary. As a result, the energy is not efficiently transferred out the transformer and into the speaker. So a bit of excess energy remains in the transformer.
When the signal starts flip polarity, the transformer continues trying to transfer the excess energy into the speaker instead of tracking with the output tubes. At some point the transformer can no longer sustain the current flow because the voltage source has disappeared. At this point, the electro magnetic field around the secondary collapses rapidly.
The rapidly collapsing magnetic field then induces a flyback voltage into the primary winding that is much higher than normal. If the insulation is not thick enough, the voltage will arc across the windings and damage the transformer. Essentially if a high enough flyback develops it punches through the insulation.
So it's not just about excess current when the load is too low. It's also about having adequate insulation to handle the resultant flyback when the load is too high.
Also, AFAIK, it's not just the transformer turns ratio that establishes the expected load. Changing the effective plate voltage affects the output impedance of the tubes. Also I believe the amount of bias current passing through the tubes has an impact on their output impedance. Furthermore, the impedance of speakers is not fixed; rather it changes with frequency.
Designers don't necessarily select a transformer turns ratio and operating characteristics so that the amp expects exactly 8 ohms. The expected impedance may be a bit above 8 ohms or a bit below. If the amp expects the impedance to be a bit high, going up to 16 ohms will be less of an impedance mismatch than going down to 4 ohms. In other words if you go up, the mismatch is <1:2, and if you go down the mismatch is greater >2:1. If the amp has a robust transformer with heavy wires and thick insulation, either may still be fine. If the transformer is has light wires and/or thin insulation, neither may be fine.
My advice is run the expected load unless you have clear advice from a legitimate expert that the amp can handle the desired impedance mismatch. Don't assume the amp is safe with a 2:1 or 1:2 impedance mismatch. It may be safe with one or the other, both, or neither. If you go against this advice and damage the amp, expect an expensive repair.
I'd agree if the output transformer shall power an electrical tooling machine at 100% duty cycle.
Audio and music instrument signals do rarely demand 100% duty cycle at full output power of an amplifier so it all depends on the amount of duty cycle the amp was designed for "full power".
edit,
in real practice its very common standard in the industry to overpower transformers up to about 70% (or even more) above the manufacturers rated nominal power IF the expected duty cycle of strain provides the possibility to do it within safe operation margins.
It helps to save costs and weight (and materials) IF its foresseable there is no need for 100% duty cycle.
Even Lemmy strained his amps with (likely) way less than 50% duty cycle at full power of the amp.
Unfortunatelly there are still "engineers" out there which try hard to understand that average rms is the way to go with.
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This szenario was only possible and true for class A tube amplifiers where the iron core of the OT (often) has got a smallish air gap which stores magnetic energy.
All of the class A/B transformers don't have a air-gap in the core and these transformers don't store magnetic energy inside the core.
With these kinds of transformers all of the electrical power is "directly" transformed from the primary to the secondary side of the OT and there is nowhere power stored inside the transformer core which could induce overvoltage by a collapsed magnetic field.
edit,
even class A/B transformers store a little amount of magnetic energy in the stray field coupling between primary and secondary windings but these transformers don't store by far NOT the amount of magnetic energy such as it was true for class A transformers which store nearly the total amount of transformed power inside the air gap of the core.
And if the air gap in the magnetic core of an ignition coil was filled with some iron material then the engine of the car would refuse to run.
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That's interesting. Then what are the diodes in the red oval for?
Everything I have read says these diodes are for flyback protection. Supposedly MOVs (metal oxide varistors) work even better, I believe because they are bidirectional.
The schematic is from a Trace Elliot V-Type (V4). AFAIK, It's a class AB output section. I have seen this arrangement on other instrument amps as well.
I experienced this very amp make a very loud sound, like an explosion accompanied by a bright flash of light. I believe it was because one of the tube sockets was a bit loose, and the amp produced a huge flyback because of the intermittent connection. I tightened up the pins on the output tubes and the amp has run reliably since.
protects against Inductive kickback
Inductive kickback can be a very short burst.
Its a strange phenomenon
Even basic 24 volt relays can have 300 volt bursts without a protection diode.
Far as " airgap" its actually more a shim between the E/I laminations.
More often seen in single ended topology. Because DC bias is not cancelled like push pull. So the constant DC on the transformer will cause it to saturate faster and increase inductive leakage.
Main concern is saturation, which reduces frequency response and overall power handling is considered lower since the transformer saturates faster.
So with anything there is tradeoffs, or you toss more cost and money at the transformer to get performance. Such as thinner core laminations to decrease eddy current, larger core than needed to reduce saturation at proposed power level. DC gap or shims between the cores which can also lower frequency response but reduce saturation etc etc
Inductive kickback can be a very short burst.
Its a strange phenomenon
Even basic 24 volt relays can have 300 volt bursts without a protection diode.
Far as " airgap" its actually more a shim between the E/I laminations.
More often seen in single ended topology. Because DC bias is not cancelled like push pull. So the constant DC on the transformer will cause it to saturate faster and increase inductive leakage.
Main concern is saturation, which reduces frequency response and overall power handling is considered lower since the transformer saturates faster.
So with anything there is tradeoffs, or you toss more cost and money at the transformer to get performance. Such as thinner core laminations to decrease eddy current, larger core than needed to reduce saturation at proposed power level. DC gap or shims between the cores which can also lower frequency response but reduce saturation etc etc
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For the most part.
A lot of the " magic" with musical instrument tube amps, especially guitar amps.
Is using relatively cheap transformers which are allowed to saturate quicker.
Frequency loss can be relatively high with cheap small cores getting saturated, specially with minimal laminations, and larger lamination thickness.
So the transformer ends up being somewhat a filter as well under high distortion. Reduces high frequency
And actually creates more distortion when saturated.
Main issue is still wire gauge for higher powered amps for lower impedance taps.
50 to 100 watt amps its easier to get away with cheap transformers.
Once you start pushing higher current with 4 ohm and 2 ohm taps with 200 or 300 watt amps.
The transformer needs to be designed correctly. Regardless its already accepted the core will be cheap
and saturated. But when pushing high current and voltages. You cant skimp out on wire gauge and insulation.
Ask marshall about the major. They tried to get away with typical cheap practices used with 50 and 100 watt amps. Didn't play out well with higher voltage 200 watt amp.
in first instance the flyback diodes help to protect the tubes rather than the OT itself.
Even an class A/B OT stores some magnetic power but this stored power/energy has got nothing to do with the power which is transformed from the primary to the secondary side of the OT.
For example if the OT transformes 100 Watt (full power) from primary to the secondary side and deliveres 100 Watt to the cab than these 100 Watt are nowhere stored inside the transformer.
The little amount of magnetic power which is actually stored inside the transfomer equals (at any time) the power that was stored if there was no load (open load) presented to the secondary side of the OT.
If there happens an issue at the primary side such as a sudden broken wire or contact issues with the tubes sockets then (of course) the smallish amount of magnetic power stored will cause some overvoltage but, as the stored amount of power is quite small the "stored energy" was not sufficiant enough to "melt" the insulation of the wire windings of the OT.
But the "overvoltage" induced by the "smallish" collapsed magnetic field (the primary side) may be sufficiant enough to harm the tubes by transient overvoltage. There is only very little power necessary to kill a tube if the anode voltage becomes too high.
Similar like Fet inputs which are very "touchy" vers highish electrical static fields. Fets can be "destroyed" nearly powerless this way.
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To clarify, I didn't suggest damage from high current. I suggested damage from voltage in the transformer getting so high that it punches through the insulation. Whether it's true or not, a lot of techs claim the diodes/MOVs across the primary protect the transformer to some degree, and cite a lower transformer failure rate when this sort of protection is used. I do understand that the devices are also protecting the tubes and tube sockets.
I found multiple explanations that the flyback is actually generated by the speaker, and this is more of a problem when the load provided by the speaker is higher than the amp expects. I assume we would more accurately refer to this as back EMF.
A quote from the 3rd post on this page. This part of the post discusses what happens when the impedance is higher than expected: Clarification on impedance mismatches? | MarshallForum.com
.A speaker is a current operated device in that it responds to the current through it to generate a magnetic field that works against the magnetic field of the speaker magnet to make the cone move in and out. Thinking in very short amounts of time, when the output charges up the voice coil with current, then the signal goes away or gets reduced, the cone system moves the voice coil back to its home or resting position. As it is moving back, it generates a voltage that is fed back up the line into the transformer and appears in the output circuit of the amp. This generated voltage is often referred to as flyback voltage, because we are charging up an inductor, then when we disconnect or stop charging the inductor, the magnetic field in the inductor collapses and induces this big voltage into itself. This big voltage then ‘flies back’ to the source of the charging current. There is a mathematical formula to determine how big the voltage is and it is related to the inductance of the voice coil, the amount of time it was fed current, and how much current it was charged with. The bottom line is that the voltage fed back to the output circuit is oftentimes much higher than the voltage that was used to drive or charge up the voice coil initially. This voltage gets transformed up by the turns ratio of the output transformer, and in many cases can be over 1,000 volts. What happens then is that arcing can occur between the pins on the output tube socket. Once this has occured, a carbon path forms on the tube socket between the pins. The carbon path allows a steady current to flow between the pins and eventually burns up the socket due to the heat that is generated. For example, it wouldn’t be too uncommon to see a transformer turns ratio of 30:1. If we had a voltage fed back from the voice coil that was around 50 volts, 30 times 50 would be a 1,500 volt spike at the plate of the output tube. This is why you often see designers connect diodes in a string between the output tube plates and ground. They are trying to suppress these spikes and dissipate the energy in the diodes rather than allowing an arc to occur at the tube socket. So, when you use a higher impedance load on a lower impedance tap, the turns ratio is higher and resulting fed-back (flyback) voltage gets multiplied up higher than what it would have been with the correct impedance load.
I think most of us know well about this nasty feeling that sometimes happens when you step out of a car and then touch the electrostatic high voltage loaded car chassis with the fingers.
Normally this high voltgage would kill people nearly at once IF there was right enough electrical energy stored in the "source".
As the power stored in the "source" is quite small the high voltage does nothing but "ouch".
Its possible to repead this "nasty thing" many times but after all (as a summery) it won't kill
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Sometimes folks try to overthink essential basics.
Even SS amplifiers "try" to produce a higher voltage swing at the output terminals once the load was going up to higher numbers.
The voltage swing will reach its possible maximum for open load.
If a tube amplifier powers a 8 Ohm load on the 4 Ohm tap then of course (similar like SS amplifiers) the voltage swing was increased at some content while the output power goes down a little bit.
If the 100 Watt Marshall can deliver full power 100 Watt into 8 Ohm load on the 8 Ohm tap but only 70 Watt into 8 Ohm at the 4 Ohm tap:
100 Watt for 8 Ohm load - 8 Ohm tap,
transformer output voltages:
8 Ohm tap ~28 Vrms
4 Ohm tap ~20 Vrms
70 Watt for 8 Ohm load - 4 Ohm tap,
transformer output voltages:
8 Ohm tap ~34 Vrms
4 Ohm tap ~24 Vrms
It can be easily seen that the voltage swing throughout secondary (and also primary windings) would (in this case) raise by about +20% once a 8 Ohm load was connected to the 4 Ohm tap.
There may be some tube amplifiers out there which may become "very touchy" in any regards of stability once the output load changes too much and due to "mismatch" start to oscillate which then can damage the amplifier. But (anyway) this was not the question asked and (honestly) I have never seen a musical amplifier that was not robust enough desigend to handle some Ohm mismatch on the output tap
Every momvement of a cone störes mechanical power as well as the near field of the loudspeaker stores reactice acoustical power that runs back into the cone and via the cone movement that acts as "electrical" motor then flies back to the amplifier.
That's one of the reasons why it is so important to design ported cabs "properly" just to get best sound performances out of a cab.
Tschbyscheff alignments are more "critical" versus Butterworth alignments or LR-alignment. Sealed cabs provide benefits vers ported cabs.
The negative global feedback loop (if present) of the amplifier helps to keep these "unwanted" movements of a cone as small as possible.
This also means that the "fly back" energy (which is induced by unwanted cone movement) is terminated by the amplifier at the secondary side by pushing regulative power (voltage) at the primary of the OT.
Even SS amplifiers "try" to produce a higher voltage swing at the output terminals once the load was going up to higher numbers.
The voltage swing will reach its possible maximum for open load.
If a tube amplifier powers a 8 Ohm load on the 4 Ohm tap then of course (similar like SS amplifiers) the voltage swing was increased at some content while the output power goes down a little bit.
If the 100 Watt Marshall can deliver full power 100 Watt into 8 Ohm load on the 8 Ohm tap but only 70 Watt into 8 Ohm at the 4 Ohm tap:
100 Watt for 8 Ohm load - 8 Ohm tap,
transformer output voltages:
8 Ohm tap ~28 Vrms
4 Ohm tap ~20 Vrms
70 Watt for 8 Ohm load - 4 Ohm tap,
transformer output voltages:
8 Ohm tap ~34 Vrms
4 Ohm tap ~24 Vrms
It can be easily seen that the voltage swing throughout secondary (and also primary windings) would (in this case) raise by about +20% once a 8 Ohm load was connected to the 4 Ohm tap.
There may be some tube amplifiers out there which may become "very touchy" in any regards of stability once the output load changes too much and due to "mismatch" start to oscillate which then can damage the amplifier. But (anyway) this was not the question asked and (honestly) I have never seen a musical amplifier that was not robust enough desigend to handle some Ohm mismatch on the output tap
Every momvement of a cone störes mechanical power as well as the near field of the loudspeaker stores reactice acoustical power that runs back into the cone and via the cone movement that acts as "electrical" motor then flies back to the amplifier.
That's one of the reasons why it is so important to design ported cabs "properly" just to get best sound performances out of a cab.
Tschbyscheff alignments are more "critical" versus Butterworth alignments or LR-alignment. Sealed cabs provide benefits vers ported cabs.
The negative global feedback loop (if present) of the amplifier helps to keep these "unwanted" movements of a cone as small as possible.
This also means that the "fly back" energy (which is induced by unwanted cone movement) is terminated by the amplifier at the secondary side by pushing regulative power (voltage) at the primary of the OT.
Personally I think this guy tries to discuss arc welding transformers rather than audio output transformers.
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I recall many years ago I myself mismatched crossover distortion artifacts between bank A and bank B at full power with saturation of the OT.
I had been totally wrong!
There are a couple of guys here on TB who know one or two things about OTs and these guys showed clearly that OT saturation is (likely) the most unwanted artifact with saturated tube amplifiers.
In short words a saturated OT might sound anything but by no means may this artifact sound musical for the most part of musicians.
Which kind of current are you talking about respectively which number for the Peak to RMS ratio for current (and voltage) do you think was most common and reasonable "expactable" in real practice with the bass guitar?
I have got one of the so called "oversized" legendary vintage TE amplifiers and there are some people out there who claim for sure the transformer of this amplifier must be oversized.
But to the contrary the transformer was designed for ~50/50 duty cycle.
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