Tube amp topologies

Here is a link to some youtube videos on basic tube amps and how they work. I found them to be great. I hope they will help someone else also.



Hello,
Thank you, just fantastic job finding this information and sharing it.

Could you please find videos explaining how to identify unknown Power & Output Transformers?

I have 2 - sets of Baldwin Transformers from Hammond Organ 4 - 6L6 Power Tubes. I would like to know how to identify the wiring and be able to test them and know what their capabilities are!!!

Thanks again!!!
 
Hello,
Thank you, just fantastic job finding this information and sharing it.

Could you please find videos explaining how to identify unknown Power & Output Transformers?

I have 2 - sets of Baldwin Transformers from Hammond Organ 4 - 6L6 Power Tubes. I would like to know how to identify the wiring and be able to test them and know what their capabilities are!!!

Thanks again!!!

Here is one on the output transformer by the same person as the other videos

 
Hello,
Thank you, just fantastic job finding this information and sharing it.

Could you please find videos explaining how to identify unknown Power & Output Transformers?

I have 2 - sets of Baldwin Transformers from Hammond Organ 4 - 6L6 Power Tubes. I would like to know how to identify the wiring and be able to test them and know what their capabilities are!!!

Thanks again!!!


And power transformers

 
Thanks for all that info! I have a question... What type of tube amp would power a turntable w/o a preamp? I am looking to try to switch from a solid state receiver to tube power. Would I need a preamp in addition to the vacuum tube amp? Also, are these new 'combo' SS) tube hybrids useful for anything other than Digital DVD players, ipods, etc? Thanks a lot!

An "Integrated Amplifier" means it has a pre-amp built-in. There are plenty of tube integrated amps. If it does not have a pre-amp, you will need an external pre-amp for most turntables.

Note that the pre-amp section is most important from a noise point of view. So, IMHO, a tube pre-amp connected to a solid state amp makes (some) sense, (although you will not get the full "tube sound"), but connecting a solid state preamp to a tube power amp makes no sense to me.

One thing (and this part is more a question for discussion, not an answer) is that combining a modern tube pre-amp with a vintage tube power amp does make sense to me.

There seem to be lots of cool tube pre-amps available at low cost, but the vintage power amp hardware can be found at a good price, even if you allow some budget for repairs/modernization.

Some of the modern tube pre-amps use modern solid-state power supplies, but stay pure tube in the signal path.
 
I didn't read through all the pages so pardon me if this is already stated. First I should say that I'm kind of a newbie to tubes but not totally. I can also be very dumb when it comes to understanding some stuff. I think your first sentence should read to the fact that there are three basic style of tubes that you typically run into when dealing with the audio equipment. Obviously the triode and the pentode, but you did not list the beam powered tetrode.

Where are you mention amplifiers that don't have output transformers I think it would be wise to include information about amps that use field coil speakers and the danger of the high voltage that runs in the speaker wires feeding such speakers.
 
it's difficult to write such a primer that is totally accurate as there are so many exceptions! So most of the statements you make really need a little qualification! Things are not so simple in practice. But thanks for trying.

OTL amps also are not just a variation, but instead can be single-ended or push pull, in class A, B,C, or D. The main difference is that the output stage is designed to put out full speaker amps, a lot more amps than your typical output stage.
 
What are some of the issues with running output tubes in push-pull parallel? I have a little Cary amp from years ago that uses 4 el-84 (can also use 7189 type) per channel. The output is operating in Ultra-Linear mode.
I use the russian 6p14 tubes, and while the sound is fine, I am wonder if output tubes in parallel present any special concerns with tube matching.
As an aside, I only use this amplifier in cool weather months. She’s a heater!
 
What are some of the issues with running output tubes in push-pull parallel? I have a little Cary amp from years ago that uses 4 el-84 (can also use 7189 type) per channel. The output is operating in Ultra-Linear mode.
I use the russian 6p14 tubes, and while the sound is fine, I am wonder if output tubes in parallel present any special concerns with tube matching.
As an aside, I only use this amplifier in cool weather months. She’s a heater!

It would be best to match all 4 tubes, in theory. But since they are operating ultra-linear, that bit of negative feedback makes matching less of an issue.
 
After quickly glancing through this topic, is it true that OTL adds the least amount of "tube color" to sound? My SS setup sounds a bit sterile and I'm interested in trying a tube buffer or a pre to see if it helps to bring in some life. Do buffers or preamps even come in non-OTL flavors? ..............

Good questions. Tube designs and parts can cause a host of different quality sonics, compared to true accuracy. Locals here have worked towards perfect accuracy (outputs sound just like the inputs via multiple listening testing procedures). I designed SETs, then went OTL with unbelievable results sonic wise. It was really really good.

For those into specs,

20 tubes/channel,
triode connected,
200 watts rms output into 8 ohms with
0,5% THD.
FR sub 5hz to ~1.2mhz.

Never had a problem with oscillations nor from external interference. The one problem was the inefficiency; 450 watts AC power just sitting. Good for winter, bad for summer.

Then worked on PP designing and found, with exception of power output, that PP can be virtually perfect in sonics. The caveat is the reactance of the speaker load vs frequency. This was solved by a combination of speaker crossover and speaker wire adjustments.

To give an idea of the sensitivity of the system, altering one of the parallel 6 foot long
18 gauge speaker wires from 10 to 9 conductors was clearly perceived. A correlation
was also perceived by rotating the speakers 0,5mm, or a third correlation, adjusting
a low ohmage resistor by 1 part in 1,000,000 (matching the full range driver to the woofer).
Power supply decoupling capacitors were all poly caps except the main high voltage to output tubes.

Such sensitivity demonstrates the possibility that you might be able to reposition your speakers just slightly by either height, depth from the wall, separation between speakers, from the side walls, slight rotation or combo. It might take some time, but tape around the speaker base so you can return to the original position if a movement is not fruitful.

keep on truckin

joe
 
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Many people are afraid by the direct coupling of the speaker to the tubes, but you also find this on most of solid-state amps, and nobody fears though !

It is correct that most (I would say all contemporary) solid state amps don't have output transformers, but generally speaking, the voltages involved in solid state amps are low, and there is no reasonable failure of a SS amp that is going to put lethal voltages on the speaker terminals (or any place else that a user may come in contact with).

Even very high power SS amps have rail voltages at most around half of the line voltage, and these usually contain speaker protection circuitry that protect not only speakers -- but also listeners -- should there be a failure.

Even without speaker protection circuitry (or assuming multiple failures), comparing a 1000W professional stage amplifier to a 10W amp (or pre-amp) is not really fair. In a fair comparison, there is no SS 10W amp that is going to present significant voltage on the output terminals under any reasonable/imaginable failure modes.

That said, There are safe and unsafe Tube OTL designs.

OTL Tube amps that essentially just substitute a capacitor for the OPT (speaker connection on the plate side, conventional power configuration), are unsafe. Capacitors very often (I'll even say usually) fail shorted (or very leaky). Classic OTL designs seldom employed speaker protection. This means that a single and common failure puts lethal voltages on the speaker terminals/speaker wires, etc. This is by definition (not by what I say) -- unsafe.

There are safe (or safer) OTL designs.

--- If the speakers (outputs) are connected to the cathode side (again, other things being of a conventional design), a single failure that would put High Voltage on the speaker terminal is unlikely. Most failures would either short out the power supply, blow the fuse, or just make the tube stop conducting (open).

-- If the speakers are connected to the anode, but the cathode is run at a large negative voltage. Failure modes are similar to above. Given a safe PS design, these can be relatively safe.

-- Dual rail designs are a combination of the above, and work similarly to a modern SS amp. The speaker terminals are kept near 0 volts without an output coupling capacitor, and there is no single point failure that could allow high voltages on the speaker terminals (for more than an instant). These are again -- pretty much as safe to the user as a SS amp. I believe the amps shown in the previous post work this way.

--- There are also low-voltage tube designs that would not employ high voltages at all. These would be pre-amps, headphone amps, or very low power designs. (Tube car radios? and yes, there were tube hearing aids.)

In case anyone is wondering: Conventional tube amps (with Output Transformers) also have only a single component between the Plate Supply voltage and the speaker terminals -- this component, of course, being the Output transformer itself. So what's different than amps that do the same thing with a capacitor? After all, transformers can and do fail shorted.

If a short develops in a transformer, most failures will also result (one way or another) in a short to ground. With the secondary (negative speaker connection) referenced to ground, and the transformer core/mounting firmly attached to the chassis, a failure that would present HV on the secondary without something else happening that shuts down the HV is very unlikely.

is it true that OTL adds the least amount of "tube color" to sound?

The Output transformer is just as critical to the sound of a Tube Amplifier as the specific tubes used or the rest of the design. An OTL amp is going to sound different than a conventional (with OPT) tube amplifier. An OTL design will still have softer clipping than a SS amp, and perhaps the "pleasant distortion" qualities of a tube amp, as opposed to "harsh distortion" of a SS amp.
 
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Yes, ione can prevent max current from reaching the speakers.

1. Capacitor output coupling will definitely protect unless 40 years old. Make sure the output capacitor has voltage rating above the B+ DC supplied voltage.

2. I have never had an output transformer (opt) short pri to secondary. A simple protect is to ground one side of the secondary.
The secondary dc resistance is very very low, so virtually no dc tickles the speaker. Besides that the fuse should blow promptly.

I also have seen many solid state amps, with included "protection", fail and take a speaker down. The rail voltages can also easily kill, clamping the heart, being DC in nature.

A push pull amp with opt can sound more accurate than an otl amp, and certainly better than a solid state amplifier with its own forms of distortions.

keep on truckin

joe
 
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Yes. I'll even go further : the best and the worse can be encountered both in OTL, OPT and Solid State amps circuits !

And yes indeed, Tubes doesn't imply Good, nor SS imply Bad : this is a matter of design, among other points...

T

I agree, almost all tube and solid state components are not optimally designed. The quality
of both vary considerably with either.

There is another area, that of inherent attributes of each kind of device, both inside the
device and without (required in the overall design), that will limit or allow for
its highest potential, and accomplished the worlds most accurate.

keep on truckin

joe
 
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How about transformers?
A SE transformer requires better design than a PP transformer, because it works with a DC magnetic component. That means that a SE transformer can enter saturation abruptly, at the "knee". That's mitigated with an air-gap in the magnetic circuit (to soften that knee transition). This requires careful design and core installation.
The working point has to be selected exactly in the middle of the linear portion and excursion of power limited.

A PP works around this by canceling the two DC magnetic components. It can use more than double of the magnetic excursion when compared to SE. Is efficient because it doesn't need an air-gap.
The small non-linearity in the PP class AB, around the zero transition, is minimized by the Global Negative Feedback, that incorporates the transformer core in the loop (from secondary side).

Power_Transformer_Basic_-_The_Magnetic_Circuit_Fig3.JPG

Power Transformer Basics: The Magnetic Circuit - Technical Articles (eepower.com)
 
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.
4/12/23 Thank you so much for taking your personal time to put this together. I had to print it out so I can go over it with my hi-lighter. I will indeed have questions , u can count on it. Thanks again.
 
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