Why output transformers are necessary on a tube amp ?

Bob Latino wrote:Someone Emailed me and asked me "Why output transformers are needed on a tube amp?" I don't proclaim to be a tube amp expert but I thought that the answer I gave might be appropriate for the forum here ..

"Consider that the actual output from the plates (pin #3) of each output tube is a very high DC voltage (400 - 500 VDC depending on the amp). Riding on this high DC voltage is the audio signal which is a small AC voltage. You don't want that high DC voltage to go directly to the speakers because the high DC voltage would destroy the speakers. What you do is run the high DC voltage and the small AC audio signal from each output tube into your output transformer. The output transformer has two windings - a primary winding where the voltages come in from the output tubes and a secondary winding that connects to your speakers. Since transformers only transfer AC, they are very effective at blocking DC voltages. What goes in on the primary side of the transformer is your high DC voltage with the AC audio signal riding with it. What comes out on the secondary side is just the AC audio signal. This is all done by electromagnetic INDUCTION. There is no electrical connection between the primary and secondary sides of an output transformer. The wires (primary and secondary) are just placed close together and the AC audio signal is "induced" or carried over to the secondary side."

Side note - There are some "OTL" (Output transformerless) tube amp designs out there that use either capacitors or other methods to block DC from getting to your speakers without the use of a transformer. So in reality, the output transformer is not absolutely "necessary" as long as you use some other method to block DC from reaching the speakers.

Bob

if I may add to this with regards to OTL tube amps.
Some people swear by capacitors used in the output. Using a capacitor in the output stage tend to generally give better quality sound, ie less interfered with, than a transformer, however, well designed OTL amps tend to be very expensive as the output circuits tend to be more complicated and are forced to use top quality capacitors.
Like a lot of amp designs, trying to keep costs down and circuits relatively simple is always a compromise.
A well designed and manufactured output transformer, as used in the new generation ST70, ST120 and the M125, will go a long way to not interfere too much with that AC signal, and the output circuit is a much simpler and well proven design.

Do various configurations of output transformers then also determine the output in watts, impedance and other factors? I have an old Silvertone 1483 guitar amp and the OT was out...many replacements out there, including "original" ones from Mercury Magnetics that were like $130 to $150 just for the tranny. My tech put something in that "worked with 6L6's" at much lower cost and it is a monster. Seems to have more watts, more balls....just a screamer gigging amp. Not sure if the new OT had anything to do with it but holy crap, clean, powerful....a great amp would put it up against Marshalls or the high end boutique amps.

A couple of other things. Mostly what you described is a single ended style OT which requires an air gap because of the high DC magnetic field. In a push pull OT as all of the Dynaco style amps the DC windings in the primary are 2 and wound opposite of each other which cancels almost all of the DC magnetic field in the primary. Hence smaller transformers than required for single ended amps. Also the transformer does lower output impedance and improves efficiency of the amp. A OTL single ended does require a capacitor.  A OTL amp in push pull requires no output transformer or capacitor. But it does require a strange tube output stage compared to a transformer style output stage. It is also very inefficient and requires a lot of output tubes to produce wattage. I know because I have owned all 4 types of amps. OTL push pull and single ended and transformer push pull and single ended. They all have there advantages but bang for buck the Transformer push pull will always win.

the output transformer (OPT) also provides a "load" for the output tubes, and the impedance of the transformer has to match the required load for the output tubes.
For example, the smaller EL84 tubes in a 15w push-pull (PP) amp usually use an 8K to 10K primary. Tubes like EL34s (and also 6L6 and KT88) used in an ST70
need a load of around 4K-6K. For higher powered amps running PPP (parallel push-pull) such as Bob's M125 monoblocks, they need a load of around 1900 ohms.
The load the speaker presents also has a bearing on how much power is transferred efficiently. The transformer has a fixed ratio of primary to secondary windings,
and though many OPTs have 4-8-16 ohm output windings, if the speaker you use is hooked up to the wrong tap, or the transformer doesn't have different output windings,
then for example a 4 ohm speaker load would present a different load to the output tubes than an 8 ohm speaker does. Most speakers have a widely variable impedance, although they may be rated at 8 ohms (nominal) they may have a dip to 4 ohms and possibly a peak of 25-30 ohms. That's part of speaker and crossover design,
but it does affect how the amplifier itself performs, as the load will be mismatched at certain frequencies. Fortunately, tubes have a wide range of operating parameters,
and are designed to work best in the middle of the design range.
Another interesting tidbit for you is the windings ratio, lets say a 5000 ohm primary and an 8 ohm secondary. That's a ratio of 625.
We are converting a high voltage signal with little current (relatively speaking) into a low voltage signal with a lot of current.
BTW, if you short out your speaker terminals, you'll burn out the transformer, and most likely burn out the output tubes, which will in turn burn out the rectifier,
which will in turn burn out the power transformer, which will at some point possibly blow out the fuse! Too bad it doesn't work the other way around!

I am pretty new to the world of tubes but my understanding was that since tubes are a voltage amplification device, the main reason for output transformer was to convert the high impedance, high voltage, low current signal of the output tube to a low impedance, low voltage, high current signal required by the loud speaker...and with OTL amps, you would need multiple output tubes to provide the required current to drive the loudspeaker.

Yea. OTL amps must produce there own current without conversion and also must produce a low impedance for damping factor. In a SE configuration the output tubes must be in a cathode follower configuration, which means far less than 1 amplification factor but the exchange is lower impedance and full amperage. You also have to choose your style of tube very carefully. You need A tube that produces high amperage at low voltage. None of the normal push pull tubes fall into this category. The KT88 group are high voltage low amperage tubes.