Q: I've read references to modifying PP OTs, adding a mysterious "gap" (mysterious to me at least) for SE use. What does that mean? 

A:Probably this: process of adding a gap means carefully unstacking an interleaved PP transformer, restacking the E's and I's into non-interleaved chunks and reinserting the bobbin with a spacer. This gets you a transformer that has a lower primary inductance, but that won't saturate as easily. It's probably not what you would originally have designed for any given SE circuit, as the primary inductance is now lower, but it might work. And it REALLY appeals to the home-handyman tube hacker crafstman instinct. 

Q: What are "gaps" in output transformers, and why are they good? 

A: The "gap" in single ended transformers is just that - a space where there is no iron. The way this is done with E-I cores is to stack all the e's together and insert them into the bobbin in one chunk, then put a paper or fiber spacer across the end of the "E"'s, and then the I's are laid on in a chunk. The point of this is that there is now a gap where there is no iron bridging it that makes the magnetic field jump across. This linearizes the magnetic properties of the structure as a whole, as the properties of the gap are so different from iron that they dominate, and the gap cannot saturate like the iron can. 

The overall primary inductance is much lower now than if you had interleaved the laminations, alternating the direction of the E's in the center of the transformer, though, so you must use much more iron and copper to get as much primary inductance and low frequency response as you would have had if you'd interleaved the laminations, so the transformer gets big expensive, and can support much less output power for its size than it might otherwise. Ten watt gapped single ended OT's for instance may be four to six times as heavy as Class AB transformers designed for fifty watts out. 

Q: Can I use a push-pull OT in my single ended amp? How about if I re-stackit to have a gap? 

A: The use of PP OPTs for SE is cautionary. The problem is that you must run the bias current through the primary as DC, and this offsets the core magnetically. A transformer which uses interleaved laminations (almost all PP OPT's) has a much higher primary inductance, but is easy to offset and make the iron saturate. Introducing a gap makes the primary inductance lower, but makes the transformer much more resistant to saturation, good for SE OPT's. Using an interleaved PP OPT in SE use necessarily limits the amount of DC bias that can be used without saturation and the corresponding distortion. It works if you keep the DC bias current low enough to not saturate the core, but that limits the amount of power you can use. So in general it probably works if you use a BIG PP OPT for a much smaller SE amp. As a tube hacker, I can understand the urge to just hook up whatever you have that might be a good output, but as a former transformer designer, there are going to be cases where it will not work or won't work well. Experiment if you like, but be prepared to toss the transformer and/or output tubes if they fry, or tear it all down if it just doesn't sound good. 

Q: What is an "ultralinear" output transformer? 
A: It's a transformer with a tap at about halfway between the B+ connection and the plate connection to which you can attach the output tube's screen grid. This connection provides some feedback to the screen grid as well as a bias voltage and has been found to act like a connection halfway between pure pentode modes and pure triode mode, with lower distortion than either. It's almost a requirement for tube hifi ampliers. Bass amplifiers use it to get large amounts of clean power. It usually sounds too "sterile" or clean for guitar players' tastes. 

Q: Can I convert an amplifier with an ultralinear connected OT to normal use? 

A: Yes. Just disconnect the ultralinear taps and make sure they don't short to something, then connect the screen grids to a screen voltage supply. This is a common mod to UL connected amplifiers for guitar use. 

Q: Can I substitute two single ended transformers for a single plate-to-plate transformer? 

A: If you have two single ended transformers, these are running effectively in parallel. It makes no difference whether you are driving them from a phase inverter or not, because the phase on the two (independent) output windings can be changed either way by reversing the leads. This is not going to give you the same operation of either the transformers or the tubes as a true plate to plate transformer. The real plate to plate transformer actually combines the tube power in the flux in the iron, which can never happen in two separate transformers. 

Hooking two transformers together to combine the output power is always tricky, series or parallel, because if you combine them wrong, there can be large circulating currents, which can kill the transformers. How did you determine the proper phasing on the outputs when you connected them up? It is coneptually possible to series two outputs and have everything work, but in practice, it's hard to do this well. 

For a series connection, you MUST get the secondary voltages to add. If you get the secondary voltages offsetting each other, the series connection is effectively open circuited, no matter what load is on the series combination no current flows because the secondary voltages cancel out. In this connectio, you probably kill the primaries by arcing over the internal insulation, possibly followed by burning the winding open when a turn or two shorted from the puncture in the insulation and the B+ caps dumped through that short. 

The safe way to find out the phasing is to use two resistor-loads, one per secondary, and then to connect one end of each secondary together; drive the amp with a signal generator and measure the AC voltage across the free ends. If the voltage is 2X the voltage on either secondary, you're phased correctly, and you can leave the center connection where it is, remove the resistor loads, and put one speaker across the free ends. 

If the voltage across the free ends is smaller than the voltage across either secondary, they're phased wrong, and you need to swap the ends of one secondary. Combining the powers of two SE transformers is best done acoustically. Drive a second speaker. 

Q: I've seen circuits that use reverse biased diodes connected from ground to the plates of output tubes as "transient spike preventers". How does this work? 

A:The 1N4007's serve mainly as an amulets against the voltage gods in this case. An inductive flyback pulse will go to literally ANY voltage until it finds a discharge path. Ideally, transients that would cause very high positive voltages on one push-pull plate would cause high negative voltages on the other plate, and the diodes on the negative going plate would clamp the voltages on the positive going plate through the output transformer. This does indeed happen for small, extremely-tightly coupled transformers. However, any leakage inductance between the two primaries prevents the tight coupling that would let the negative going diodes protect, and worse yet, it's the leakage inductances that cause the spikes on transients anyways. 

What really happens is that the first few flyback pulses that occur will break over the 1N4007's rather than than arcing the plates on the positive side, so there really is some protection, it's not just where it looks like it is. If you're lucky, the 1N4007's break over before the transformer insulation punches through, and all is well until the 1N4007's go leaky or short. Probably better than nothing, but not a whole lot of additional protection, either. Heck, amulets are not harmful, I guess. 

Q: Can I use an ordinary pushpull output transformer as a single ended ultralinear output transformer? 

A: Maybe. It's cheaper than getting one originally designed for that use, but you do have to consider it as experimental - it may or may not return good results, and you may have irretrievably damaged a working OPT, which may or may not be a tragedy, depending which transformer it was and whether you paid real dollars for it and how many. You can try it, but consider the transformer expendable. 

Q: What does the "impedance" of my output transformer mean? 

A: Transformers don't have impedances, they have impedance RATIOs. This is an important distinction. 

Transformers transform impedances as a pure ratio. That is, a 4400 PP to 8 ohm transformer makes any load on its secondary look like it's 550 times bigger to a tube at the primary. An 8 ohm secondary load then looks like a 4400 ohm load at the primary. It also makes a 16 ohm load look like an 8800 ohm load if you hook 16 to it, 2200 if you hook a 4 ohm load to it, and similarly for all values in between. Power tubes have a power output that depends on matching - that is, they have sweet spot load that they do best on, most power out, and other loads will get less power because the tube itself limits how much power it will transfer out. [Actually there are two sweet spots, one for highest power, one for lowest distortion; the two spots are not the same for any known tube. From zero ohms loading up to some ill-defined number of ohms higher than the optimum power load, power tubes do not destroy themselves, they merely change how much they transfer to the load. So - if you have a tube amp with a tap for 8 ohms, you will get the nominal power of the amp only with a "matched" 8 ohm load. If you hook 16 ohms there, the power tubes "see" a proportionately higher ohmage on their plates, and can only put out about half the nominal power. If you hook up a 4 ohm load to the 8 ohm tap, the power tubes "see" a load about half of the matched one, and again will put out only about half of the nominal power. This "half the nominal" power is not fixed because of the 2:1 change in load, but varies from amp to amp and tube to tube, and may not be exactly 2:1. In addition, speakers are NOT single impedance loads. It is convenient to think of "8-ohm" speakers, but the plain fact is that the speaker's impedance varies with frequency and also with the acoustic loading (cabinet and other things) that the speaker sees. That impedance meter is not going to be a huge help, because you have to specify the frequency being tested as well as the impedance to have something meaningful. 

Q: Why do I have to match speakers to the output impedance of the amp? 
A: You'll get the most power out of the amp if the load is matched. 

Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load? 
Simple A: Within reason, no. 
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up? 

For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load. 

For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways. 

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance. 

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. 

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important. 

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. 

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer. 

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. 

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once. 

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load. 

Q: Can I use two single ended output transformers in a push-pull circuit and then parallel the outputs? 

A: What you're making really IS two class A output stages run in parallel. With no magnetic coupling between the two half-primaries, there is no interaction on the secondaries, either. You have to run them Class A to keep from having distortion because they really are separate amplifiers. It's not clear what happens if/when you try to use feedback from the secondary into the (presumably)common driver stage. 

On the secondaries, you have two 8 ohm outputs that you can connect in series to drive either two 8 ohm loads separately or one 16 ohm by placing the secondaries in series; the resulting power capability, given that you get the rest of the circuit right, is the sum from each transformer, or about 2X the power of each Class A amp by itself. Note that this is far less than you'd get by using a proper push-pull OT and driving it in Class AB, probably ¼ the power. 

If you try to parallel the two, you can get some interesting and possibly disturbing results. If the transformers really are IDENTICAL, then for equal primary drive, you get equal secondary voltages, and you could parallel them OK to drive a single load. If there is a difference between primaries, secondaries or drive voltages, then the secondaries try to make different voltages, and fight it out. The differences are reflected into the primaries as a kind of push-back voltage on the output tube plates. Tubes being the forgiving things they are, this will probably not kill anything, but it will at least act like a different loading than you're expecting on a per-tube basis. I'm not certain exactly what effect this will have on linearity or life. If you were driving the primaries from a low impedance source, something would burn. 

Q: What are the things about output transformer that cause the differences in tone? How do differences in output transformer construction combine with tubes to give differences in tone? How do I design/modify an OT for a tone I like? How do I duplicate the tone of a OT I already like? 

A: What you have asked, translated into transformer-geek language, is "How do I completely describe the equivalent circuit of a transformer and the circuit it resides in?" 

To be truthful, there isn't all that much mystery about transformers, but it's not like the rest of your everyday electronic parts. Transformers are susceptible to electronic modelling, and once you get the model correct, you can twiddle the values until you get the "tone" you want, including nonlinear effects. The later versions of SPICE include nonlinear transformer models for exactly this use. 

You won't like the answers, primarily because of size. To understand a transformer's effect on tone, you have to be able to model the whole power amp/tube/OPT/speaker chain and account for the effect of changes in the OPT model, then synthesize back to real hardware once you get the response you like. You've asked for a couple of semesters equivalent worth of information on transformer modelling and design linked to a course on the design of the output stage of a tube audio amp. 

I suggest that if you really want to know this stuff, you find a copy of Nathan R. Grossner's "Transformers for Electronic Circuits", which is out of print, but available at many technical libraries. I put this reference in the Tube Amp FAQ to answer this kind of question. 

You can model any transformer as a shunt primary capacitance across the primary winding, a series leakage inductance to the primary winding, a series resistor equal to the winding resistance, a nonlinear inductance representing the primary inductance, with a nonlinear resistor in parallel with the primary inductance to represent core losses, primarily from eddy currents. Then an ideal "perfect transformer" to convert the voltages and currents correctly, a series secondary winding resistance, a series secondary leakage inductance, and a shunt capacitance across the secondary. A shunt capacitor from primary to secondary completes the model. 

Get those component values correct, and you can accurately model everything about any transformer. There are no mysteries hiding in there. The component values are all measurable, and to a certain extent predictable from the start. Any transformer can be copied, Fischer and his ilk to the contrary. 

So - tone effect of a OPT? first - what does the base transformation ratio do to the reflected loading on the tubes as a function of frequency, including speaker loading. This is fairly independent of the transformer model, depending only on that "ideal transformer" in the middle, but has a big effect on how the tubes put out power. 

Next - What are the values of the model components? That is, how much leakage inductance, shunt capacitance, and core loss is there? At what points in the excitation does the core start going into saturation, and from the composition of the iron, what is the irreducable energy loss per cycle to magnetizing losses, which shows up as pure third harmonic distortion. Core saturation sounds like any soft limit on a signal; its effect on tone also depends on the symmetry of the limiting. You get primarily third, but smaller amounts of fifth and seventh harmonics on pure tones. Combine with the tone of the tubes? I have a problem with that, and I'm not just being difficult. First, define "tone" unambiguously... 

The power response of the tubes will be affected a lot by the degree to which the reflected loading on the plates matches the "power transfer sweet spot" for the tube, and this is a function of frequency, depending obviously on the speaker impedance curve and the other parasitics in the model. 

The size of the core and the number of turns have a direct effect on the low frequency response, but they affect it by changing how much the primary inductance loads the tubes at the lowest frequency of interest. Good designs make this NOT be a consideration in most cases. Poor designs make it a critical factor, and you hear the poor design as either core distortion or low frequency restriction. The winding inductances are entirely subsumed into primary and secondary inductances and have no effect on tone whatsoever - except to the extent that the physical location and sectionalization of the windings contribute to the leakage inductance and shunt capacitances. The effect of the loading on the plates IS a major contributor. Each tube type has a power response curve, power out at a given impedance. There is also a curve of distortion versus loading. In general, the sweet spot of max power is not the sweet spot for lowest distortion, so changes in loading cause the amount of power out to change as the amount of distortion changes, too. Changes in plate loading will cause big changes in tone - and speakers all by themselves have impedance versus frequency curves that vary by four or more to one. To get a good grip, first get some good background. There is not enough room in this FAQ to type in what you've asked. Get a book, preferably Grossner, but any other that describes the basics of transformer modelling; then I can point you to some books on transformer making that will give you an idea on how to change the things you do in making one that can change those parasitics. A final thought. If the totality of what a transformer does to tone can be modelled by the ideal transformer and some non-ideal components, could you take a transformer with very small parasitics, close to ideal, and add in external "parasitic" components and make it look like any one of a number of less ideal transformers? Yep. You can add inductors and caps to OPTs to make them look more like some transformer you like better, as long as you're not haveing to add negative inductance and/or capacitance. The iron alloy also has an effect, and it's tied up in that business about the BH curve and nonlinearities. If you drive a transformer from a voltage source, 0 ohms impedance, then there is no distortion of the secondary voltage as a result of the BH nonlinearities, as the source can provide any current to keep the voltage correct. If however you use a source with a real impedance, like the plate impedance of a pentode, then the nonlinearities demand current, and the plate impedance then limits the current available, so the voltage waveform is distorted on both primary and secondary. Unfortunately, we need the transfomer BECAUSE the tube has internal impedance, so we can't just wish that away. As a sidelight, this is one of the classical arguments for triode output tubes over pentodes or beam power tubes during the golden age hifi years - triodes have a much lower internal impedance and hence lower the distortion of the transformer. 

What you CAN do is to do some fairly simple tests to map the BH curves of the iron you have, 

[sidelight: if you find a "magic" transformer, much more "tone" (whatever that means to you) than any other, you can do the work to measure the BH curve nondistructively on the core properties that it has, and then go duplicate them.] and then either get different iron or introduce air gaps to change the effective BH curve of the iron to make a core nonlinearity that matches whatever sounded good. You may not be able to match the iron perfectly, but it's the core properites, not just the iron, that you're looking for, and there are things you can do there. 

There are a number of grades of transformer iron, but I doubt that there are larger laminations made especially for audio these days, as there is effectively no money to be made; the money is all in laminations for power transformers. There are several grades of good, linear, high permeability silicon iron made for power transformers, and I suspect that these are what ALL new manufacture OPT's come from. 

Note that this may (and probably will) result in a core that is bigger than the one you're copying, you may have to rework the windings to get the necessary primary inductance, shunt capacitancs, leakage, etc, etc. to duplicate the response of an iron core you can't get. 

Also - there are other ways to introduce the specific nonlinearities that make for a good sound if you can ever define what "good" is well enough. 

Q: How can I tell if my output transformer is live or dead?
A:There are some simple tests you can run to quickly determine if a transformer is grossly bad. This is much simpler than determining if it will work well and sound "good" for you. The tests of relative "goodness" are also possible, but require a lot of equipment and experience to do correctly. For the quick and dirty tests described here, you'll need a means of measuring AC voltage and current simultaneously, such as a pair of VOMs or DMMs, and a 110/120 to 6.3VCT filament transformer, and either a variac (variable transformer) or a light bulb socket in series with the primary of the filament transformer to limit the power you put into the transformer under test.