Tube amp topologies

There are three basic output topologies for tube amps: Single Ended (SE), Push-Pull (PP), and "Output Transformer-Less" (OTL). OTL amps do not use an output transformer, are fairly exotic due to the number and size of tubes requires, and other factors. They are worthy of mentioning, but beyond the scope of a document aimed at a novice. .

Not sure I agree with this but of course I'm known as a proponent of OTLs. So maybe I can offer some information:

OTLs can be SE or P-P. They can be class A, A2, AB1, AB2 or B. They can be made with or without global negative feedback. They may occur in triode, tetrode or pentode embodiments (although I don't actually know of any tetrode examples right now).

OTLs can have adjustable bias, self bias or autobias.

So they can cover a similar range of topologies as transformer-coupled amps, only there is no transformer. To do this typically an OTL will have lower plate voltages (usually not over 175Volts) and higher currents. This means that the power tubes have to have capabilities of linearity at lower voltages and higher currents. There are 3 tubes currently (pardon the pun) in common use: the 6AS7, the 6C33 and the PL509 or variants thereof.

Quite often OTLs have a higher output impedance and so often need higher impedance speakers but this is not set in stone. The OTLs I use drive 8 ohms with ease, and I know of one OTL that makes its maximum power into 3 ohms. If enough feedback is applied, OTLs can behave like a Voltage Source, like any other amp. However, OTLs tend to have a lot of bandwidth so feedback has to be applied carefully. Too much and the amp might become unstable. There was enough of this in the old days that many people are convinced that OTLs are inherently unstable or unreliable, and neither is true- they are just as stable and reliable as any other tube amp if built correctly.

The advantage of OTLs is that without a transformer, they have more bandwidth and less distortion and so are more transparent. The OTLs I play have 20 Hz squarewave tilt that is unmeasurable. No other tube amp topology can make that claim. This means that OTLs can be considerably more transparent than conventional amps, something that is brought out in reviews on a very consistent basis. Contrary to popular myth, the larger number of power tubes does not seem to contribute to distortion, in fact it appears that as you add power tubes the distortion actually goes down. So then it becomes a matter of managing the tubes, but IME this is not particularly difficult- that amps I use don't even use matched tubes and have automatic bias.

Another often-cited concern is speaker safety. OTLs can be built to be as safe with speakers as any decent transistor amplifier. IOW they are not any more dangerous to a speaker than a transistor amplifier, assuming once again that it is built correctly, but you would have to assume that about a transistor amp too. The reason I chose to compare to a transistor amp is that OTLs use either an output coupling cap or they are direct-coupled. So it makes more sense to compare them to transistors in this regard.

In short, OTLs should be on table with all other tube amp types as an option for the newbe that is considering a tube amp. They are about the most tube you can get :)
 
Referring to Wigwam's earlier post: Here are a few more conceptual thoughts for you and anyone else who might benefit.

One possible analogy is that you might wish to think of a power amp in terms of an actual engine and transmission in most any typical car. If there was a single cylinder, like a small lawnmower-type motor, you could see how its single piston does all of the work 'alone'. As an aside here, ironically enough, the (European) English term for most any tube is a "valve", which is a fairly accurate description of what most tubes perform electronically. You might even consider the small, front-end input tubes to act somewhat like the valves in an engine's cylinder. Going with this analogy, the output tubes might act as the pistons, since they do the bulk of the 'work', with the required 'help' of the valves, of course and the engine's transmission would be the output transformer.

You might easily see where a two cylinder lawnmower motor might have more power with two pistons working together in most any form, whether thru a V configuration or Posche/early VW 'pancake' style, and these two or more will produce more power than two single-cylinder motors separately combined. That should serve as a reasonable analogy of the efficiency gains realized by PP amps in relation to SE amps; semantics aside as to what ultimately sounds..."best".

Moving to the OPT (output transformer); it acts as a transmission of sorts and is technically more of a torque converter for the output tubes. A car's engine is hardly practical without a suitable transmission and the same would be true for most any tube amp that requires the high voltage, low current output tubes' downconversion to lower voltage, higher current that the OPT enables for the benefit of the spkrs. The output tubes 'like' the particular (high impedance) load that the OPT presents on the primary side. The low impedance secondary windings inductively respond to the signal swings running thru the primary, yet since there are far less windings on the secondary, they present a reduced version of the primary's high voltage signal swings which are now converted and suitable for the low impedance loads that most spkrs present.

Although nothing is 100% efficient, there is a basic proportional formula for transformers and transmissions, so the reduction ratios equate closely to those you might find in a car's engine RPM reductions thru its transmission with the same types of results: same power at a lower speed generally equates to much more torque and vice versa. So there you have it; maybe not a perfect analogy, but enough loosely-based similarities to be worth casual consideration.

Ok with this said,more tubes the better, just how many output tubes can be tied together in a push pull parallel. I use 6-6v6 tied together for 30 watts.Magnavox designed it. Can you do this for all types of output tubes.
 
Ok with this said,more tubes the better, just how many output tubes can be tied together in a push pull parallel. I use 6-6v6 tied together for 30 watts.Magnavox designed it. Can you do this for all types of output tubes.

In theory, yes. Any number of tubes can be set up in parallel in any of the configurations, however there are always practical imitations in the about of current drawn by the output stage, the heat produced, the power output, the tube type and many more that others could speak to with much more authority then I. I would point out however that some tubes want and need loads of several thousand ohms. To use a tube that wants a 2000 ohm output stage in push pull down to 200 ohms you would need to use 10 tubes. If it was push pull or totem output that would be 10 pairs.

Shelly_D
 
Great thread and I am glad I read it first. I use a Tube Pre and a Hybrid amp but have been looking into alternatives for when I have to downsize in the future. Some folks have been raving about Flea Watt SET amps and single driver speakers so I auditioned a setup last week.

They did not have a flea watt so I had them setup a JAS Bravo 2.3 15wpc integrated and a pair of Gemme folded horns. I was not impressed with the speakers but the Bravo was very nice. They also setup a pair of Sq-10s which are Steve Monte's (Quest For Sound) own speaker with a horn and 10" driver and very good efficiency. I also was not overwhelmed by this setup. The $800 KEF bookshelf speakers actually sounded best. None of it sounded better than the setup in my signature.

I want to go back and listen to some Raysonic Push Pull units with other speakers.

It was briefly touched on in the original post but I was wondering if there is a simple description of each topology to it's expected sound.

Such as:

SET usually sounds like X
Push Pull has characteristics of Y
OTL will sound like Z

Can anyone touch on that and other that power (wpc) why do you want one over the other?

Thanks for the education!
 
Providing those types of comparisons is really hard because the idea of "sounds like" is purely subjective. Given different tastes of listeners, you will inevitably end up with good and bad adjectives being applied to the same characteristics depending upon the tastes of who is posting.

Shelly_D
 
Quick question: in a stereo 300B SET design, are you able to use one power transformer for both channels, or must you use one for each?

Also (and this may be related to my first question), in the version of a WE91A pictured here, what is the function of the two toroidal transformers? Are they for the heaters?
 
Oh, wait, never mind. I see that he used the toroids to handle the 230V/50Hz euro mains. So, if I understand that particular design correctly, he is using one power transformer and two OPTs, correct?

If that's so, I would like help understanding what function the transformers are performing in the WE91A shown here. It looks like T2 is its own separate transformer providing heater voltage, and T3 is...what? It looks like it's going to the cathode on the 300B.

Any responses are much appreciated, and since I'm an obvious noob, please feel free to give any background info that you think might help fill in the gaps.
 
Tube amp powered with regulated switching power supply would be the best. IMO
It is difficult to properly filtering the secondary High Voltage side just on the Anode.
Much easier to have a two staged power supply with primary side puffer & energy storage. I mean much less low frequency ripple on the secondary side!
 
Tube amp powered with regulated switching power supply would be the best. IMO
It is difficult to properly filtering the secondary High Voltage side just on the Anode.
Much easier to have a two staged power supply with primary side puffer & energy storage. I mean much less low frequency ripple on the secondary side!

Have not played around with switching power supplies so can't say anything with certainty. They do use higher frequencies to transformer the voltage.

Advantages include smaller, lighter, less expensive transformers for the power required.

Possible disadvantages: Possible high frequency inductance into the circuit, possible noise on the DC out, Possible objectionable ripple frequency in the output.

Properly designed for audio equipment any power supply will be simply an appropriately quiet source of ripple free DC. Getting to that point is the trick.

Shelly_D
 
Thanks! I used to build tube stuff,when I was a kid,back in the early 70's;between your post,and a book called Radio Tube Fundamentals,by someone named Christ,I'm slowly remembering things I've forgotten...is there a good book on soldering/chassis layout/assembly?
 
Ok guys followed concepts. at the end of the day can anybody tell me why nobody makes vacum tubes today that sound like old tubes?
 
Love the info! Making the dive into tubes in a few weeks and need all the info I can get! Awesome post!
 
Ok guys followed concepts. at the end of the day can anybody tell me why nobody makes vacum tubes today that sound like old tubes?
Economy of scale.

Tubes are obviously not currently produced in the quantities they were when they were a common electrical component.

This has a couple major ramifications:
1) Smaller pool of requisitely skilled technicians to draw from for QC and maintenance of tooling. :dunno:
2) Undocumented methods/accrued expertise/industry secrets of the past have since retired and died with the workforce. :angel:

Reconcile that with the fact that:
A) Most of the technology we currently rely on and consume was developed by the defense and aerospace industry, which accomplished most of this with funding from the American taxpayer. During the build up to WWII through the beginning of the Cold War, a significant amount of these subsidies were shoring up the manufacturing base of vacuum tubes via government contract. :deal:
B) Current industry standards are dictated by the largest market share, which is tube instrument amplifier manufacturers, not HI-FI amp manufacturers. :guitar:
C) Scarcity of minerals has driven up the cost of metallurgical purity. :sigh:
 
Descriptions look really good to me. I would bring up 2 possible improvements.

1. If you can - small diagrams of a typical push pull arrangement and typical single ended arrangement.....

if anyone ever finds or posts such diagrams (suggested above by Shelly_d) please please let me know it is exactly what I need to take me to the next step of learning, that'd diagram would be priceless. Especially if it was like a picture of the inside of an actual simple tube amp with arrows pointing to each part in a sort of diagram format.

How awesome would that be for newbs like me
 
I'm writing this for a few friends, it's meant to be an entry level primer on tube amp topologies. I'd like to keep things very general, so that a newbie to the hobby can gain some vocabulary for the various tube types and output topologies. There are exceptions to probably everything I will say, but I wanted to share what I could in as general language as possible. There are other threads for hard-core techies, this one is for the novice.

Triodes have one and only one "control grid". Triodes can be used in Single Ended Triode (SET) circuits, and in push-pull circuits. If a true triode is used in any circuit as an output tube, it can not utilize "pentode" or "ultra-linear" mode.

Pentode tubes have three control grids, and the addition of the extra grids allows the pentode to be run in various different output modes: triode, pentode, and ultra-linear. Triode mode mimics a true triode tube, is often of a lower power than the others, but is popular because of it's sonics. Pentode mode usually uses a tube to more of it's maximum output potential, and often gets the highest power ratings out of a tube. It's generally fallen out of favor, but making a comeback in certain DIY circles. Ultra-linear mode is sort of a compromise between the two, with power ratings between triode and pentode. Ultra-linear mode is only possible if the output transformer has a special tap that supports this use.

There are three basic output topologies for tube amps: Single Ended (SE), Push-Pull (PP), and "Output Transformer-Less" (OTL). OTL amps do not use an output transformer, are fairly exotic due to the number and size of tubes requires, and other factors. They are worthy of mentioning, but beyond the scope of a document aimed at a novice. As both SE and PP amps require an output transformer (OPT), the characteristics of that transformer will dictate many factors about the amp. Generally speaking, an output transformer will be either a single ended OPT, or a push-pull OPT. Either PP or SE OPTs can have an ultra-liner (UL) tap. If your output has it, you can optionally use it. One could argue that the output transformer is the most critical part in a tube amplifier.

A Single Ended amp uses a single output tube. If the tube is a native triode (300B, 2A3, etc.) this is a SET amp. You can run a pentode in Single Ended Pentode (SEP) mode as well, but it's not a SET amp. Using a pentode, you can have a Single Ended Pentode (SEP), and if your transformer supports it a Single Ended Ultra-Linear (SEUL) mode as well. It is possible to switch between these modes.

Push Pull uses a pair of tubes to power the speaker. Triode, Pentode, and UL modes are all available here, too, depending on the choice of tube and transformer.

Parallelism is also possible, usually for more power. A Parallel Single Ended (PSE) amp will use more than one tube run in single ended mode. A Parallel Push Pull (PPP) amp will use two or more pairs of outputs. While it's usually easy to spot a Parallel Push Pull amp because of the number of output tubes (4, 6, 8, etc.), the most common Parallel Single Ended amps can resemble a Push-pull amp, as each have 2 output tubes. Generally speaking, parallel amps are not as common as non-parallel. Vintage parallel push pull amps were often PA type amps. Parallel Single Ended amps seem to be more common in modern times, as people want more power while trying to retain some single ended sonic characteristics.

Single ended amps are generally prised for their sonics, but put out less power than push-pull amps. They are generally simpler and have less parts than a push-pull amp. They tend to be fussier about hum and noise, so particular attention must be paid to the quality of the power supply, amplifier layout, careful heater wiring, and other factors that induce noise. In some ways, parts selection becomes more critical because there are fewer of them to begin with. This can work to a DIYers economic advantage too, as sometimes it's possible to use a budget for fewer but higher quality parts. While circuit choice is still very important, due to the simplicity of the design, most SE circuits resemble each other to a great degree.

Push-pull amps generally have more power than SE amps for any given tube. Generally speaking, they tend to be more complex then SE amps, with a higher parts count and more tubes. They have some noise-rejection qualities inherent in their topology, and sometimes this can present less of a problem to get noise free. Choice of circuit can be of particular importance in a push-pull amp, some can really sound better than others. There is also a greater number of circuits to choose from, as different designs handle the more complicated push-pull circuitry in different ways. Parts selection is still important, but perhaps circuit choice is even more critical in a push pull amp.
thank you so much for the write up,this is exactly what sort of information that newbs(like me ) to the tube amp game really appreciate,thanks!
 
Transconductance is a measure of the change in plate current to a change in grid voltage, with plate voltage held constant.
 
Back
Top Bottom