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Croft Series X info

Hi all.
I´m newbie posting and I´m finding some help. I´m doing this same question in other forums.

Two weeks ago I received from a friend a Croft Series X amp.
I can´t find any info on internet about this amp, and want to ask if somebody can tell me something about the schematic, tubes, etc..
I tested it and the volume is very low, I don't know if the tubes are what they should be and if they are in the right place (have 2 12AU7, 2 12AX7). This use 6080 tubes in the power section.

I send it to a tech guy and he tell me exactly "sell that, it is a circuit that demands a lot from the valves, it is very hot and since it does not have transformers at the output, the speakers are at risk".
I see the amp in original condition, very neatly and I would like to investigate well before making a decision. I would rather keep it if everything was fine.

Many thanks
 
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Hi Carlos,

Welcome to DiyAudio!

From your description, it sounds like an "OTL" amplifier. My personal view is that I do not like them, and they are a poor match for a tube.

I understand where your tech friend is coming from and have often said the same thing. Things go like this. Some people think tubes sound better than solid state (they can both sounds pretty darned good). The hangup in tube amplifiers is considered to be the output transformer, plus they are expensive and heavy (for a good one). However, tubes are high impedance devices, high voltage and low current. So some bright eyed individual decided that they could drive a speaker directly from a tube. Sure, you can. However, tubes do not like high currents at all! Your speaker is a low impedance device meaning it wants to see a drive voltage with lots of current available. Far back in time, teams of brilliant engineers repeatedly found the best way to drive a loudspeaker is through a transformer. Tubes were the only active devices available, and even when transistors came on the scene they were delicate, low current things.

So from an engineering standpoint, a tube without a coupling transformer is exactly the wrong device to use for driving a speaker. It is simply not suited to the job. What that means is that this use will cause early failure possible, and when a tube fails it places an even higher voltage across the speaker than most transistor amps will. Historically OTL amplifiers have little to no speaker protection designed in.

How to deliver more current? Lot's of output tubes in parallel. The 6080 is a dual triode intended to act as a power supply pass tube. Generally 250 VDC maximum plate, I forget how much cathode current the rating is. It does represent a "high current" option in a tube, but not high enough except for low power with restricted output current.

I wouldn't own one. Now this is from my experience as an audio tech with over 45 years of experience. Having said that, some folks have them and like them, so go ahead and play with it. If you have to purchase it, then you really do need to have it serviced or at least examined by a good audio tech. There is cost associated with this, tube amps are not a low cost way to enjoy music, and I think OTL amps are on the higher end of running cost.

-Chris
 
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A few very important things to consider when using an OTL amplifier, is . . .

What is the impedance of your loudspeakers?
(not the manufacturer's nominal rated impedance; instead all the way from lowest frequencies to highest frequencies, across the whole audio band).

What is the efficiency of your loudspeakers?
Some manufacturers specify efficiency at 2.828Vrms at 1 Meter away.
2.828Vrms is 1 Watt into 8 Ohms.
But with an "8 Ohm rated" loudspeaker that dips to 4 Ohms(some of them do), 2.828Vrms is 2 Watts (so the manufacturer is Either trying to hide the truth . . . Or he is expecting you to use a solid state amplifier that can properly drive 4 Ohms.

Some OTL amplifiers work fine with 32, 16, or perhaps even 12 Ohms.
But most do not work well with 8, 6, and 4 Ohms.

Listening very quietly can sometimes help to reduce the problems listed above.

Just my opinions
 
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This amp may be intended for 16 ohm speakers, and/or very efficient speakers.
Try connecting both of your two speakers in series (and in phase) to just one channel, for an easier load.

If the gain is higher with the series connection of speakers, as is likely, the amp needs higher impedance speakers.
There is no loop feedback, and with the six 6080 output tubes per channel, the output impedance may be around 25 ohms.
 
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Some OTL amplifiers work fine with 32, 16, or perhaps even 12 Ohms.
But most do not work well with 8, 6, and 4 Ohms.
Just to be clear, 'most' OTLs are Atma-Sphere and can work quite well on 8 Ohms. The larger models can work well on 4 Ohms.

The Croft is a very low power unit and really is best used on a high efficiency speaker, but 16 Ohms should be fine.
 
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When I was in Thailand, I took a Taxi.
I told the driver where I wanted to go; we agreed on the price.
I discovered that he only had 3rd gear.
When we came to the giant traffic circle with many roads, it was a little scary.
The clutch grabbed and the car shook, and we managed to get up to speed in time for the rest of the traffic.

I view OTL as being somewhat like that wounded taxi. It had no 1st gear, and no 2nd gear.
OTL has no output transformer, and you are always starting out in 3rd gear.

Just my opinion.
 
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OTLs are a flawed concept. So are full-range drivers. And why use valves when good class D is available? But if it sounds nice, use it. Probably best if you can find loudspeakers nearer to 16 ohm than 8. Thirty years ago I heard Croft OTLs using PL509 and liked them.
 
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Hi atmasphere,
No, there are other brands of OTL amplifiers that preceded Atma-Sphere. All of which suffer from the same shortcomings. The Atma-Sphere works with 8 ohm speakers, but not really well. Not compared to tube amplifiers using output transformers, and especially not a decent solid state amplifier.

I agree that with selected loudspeakers, they perform okay. But they really are a misapplication of a technology. I am very familiar with these amplifier types.

-Chris
 
When I was in Thailand, I took a Taxi.
I told the driver where I wanted to go; we agreed on the price.
I discovered that he only had 3rd gear.
When we came to the giant traffic circle with many roads, it was a little scary.
The clutch grabbed and the car shook, and we managed to get up to speed in time for the rest of the traffic.

I view OTL as being somewhat like that wounded taxi. It had no 1st gear, and no 2nd gear.
OTL has no output transformer, and you are always starting out in 3rd gear.

Just my opinion.
Your analogy is inaccurate; your conclusion really isn't based on fact.

OTLs offer greater bandwidth and lower distortion than is possible with transformer coupled tube amplifiers. That translates directly to greater transparency and they have ruled the roost as the most transparent tube amplifiers made for decades.

Its not important that the smaller ones won't drive 4 Ohms well. If you want to get the most out of any tube amplifier, its really in the best interest of your amplifier investment dollar to use a speaker that is 8 or better yet 16 Ohms. The reason for this is the amplifier will have lower distortion and wider bandwidth all on account of the output transformer running more efficiently. This is easy to measure as well- all output transformers lose bottom octave bandwidth and can be as much as an octave from 8 Ohms as opposed to 4.

The downside of OTLs of course is a greater number of tubes and the obvious issue of being more picky about the loudspeaker used; but since 4 Ohms really isn't a good option for amps in general (solid state amps make more distortion driving 4 Ohms too) that's not much of a downside if sound quality is the goal. But if these issues are dealt with they can offer the most musical presentation of any of the tube amplifier types.

However, I've heard a number of class D amplifiers recently that challenge any tube amplifier made including the best OTLs, in that they are every bit as detailed and 3D (actually a bit better), with no hint of grain or harshness in the mids and highs. I've been playing OTLs for about 45 years but switched to class D about 2 year ago and not missed the tubes at all.
No, there are other brands of OTL amplifiers that preceded Atma-Sphere. All of which suffer from the same shortcomings. The Atma-Sphere works with 8 ohm speakers, but not really well. Not compared to tube amplifiers using output transformers, and especially not a decent solid state amplifier.
Of course there were predecessors ;)
But Atma-Sphere is the oldest OTL manufacturer in the world and has made more than all the other manufacturers put together. That is simply because we sorted out how to make OTLs reliable. If they don't blow up, you don't go out of business. The awards and reviews, most of which were done on 8 Ohm speakers, that our amplifiers have gotten over the years fly in the face of your comments.

Our biggest OTL, the MA-3, makes over 500 Watts into 3 Ohms and has no problem with 2 Ohm loads. Our MA-2 makes 220 Watts into 4 Ohms, so I'm taking your comments as inaccurate at best.

However, I'm of the opinion that all tube power amplifiers are on borrowed time at this point. Class D is (finally!) eclipsing them with the same liquidity and organic qualities in the mids and highs and that's really the only thing that's kept tubes in business. And you can get that with no heat and no need to replace hard-to-find exotic tubes. The war in Ukraine isn't helping. In 10 years the tube world is going to look very different, not because of home audio but because class D is working its way into the guitar amp world, which is the bread and butter of tube producers. As class D invades that market, tube producers will see their market shrink.
 
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Hi atmasphere,
All due respect.

If you throw enough money and energy at a problem you can develop something that works. Notice I said works. However if you approach the engineering problem from a rational viewpoint, you would use the most appropriate parts to solve a problem. An OTL amplifier is about the best example of ignoring engineering principles.

Let's look at the MA-3 for example. For each channel you are running:
42 - 6AS7G output tubes
12 - 6SN7
... and this buys you 500 watts at 0.5% THD. Almost 661 watts in heater power alone for the 6AS7 tubes, another 60 + watts for the 6SN7 - per channel! I'm not sure if you're using rectifier tubes or not. That's it just sitting there with zero plate current! What is the actual idling power used? The cost of a retube? OMG! Do those have to be matched? How many hours is the service life? I hate to ask, but is there any speaker protection in these amplifiers? Does it disconnect the load?

This amplifier looks like it uses a method similar to Carver to pre-regulate the AC. Nice (can be noisy electrically speaking).

I've been in service for 45+ years, working on some OTL amps, lots of tube amps (and designed a few) plus solid state amplifiers and designed some of those too. My direct experience has been that any decent solid state amplifier can drive a 4 ohm load without difficulty, and do it across the entire audio band at far lower distortion and better sonics than any tube amplifier is capable of. I'll include preamplifiers just from a signal standpoint. This is measured performance and also decades of opinions from individuals. We are going to exclude folks who like a particular sound. Children and women are the best, non-biased source of accurate opinion.

The worst piece of crap can get a good review! Lord knows enough have throughout audio history. I treat reviews as noise, and some reviewers I have met have terrible hearing and will wax on about stuff they simply are incapable of hearing! I've proved this more than once.

I have redesigned sections of amplifiers capable of delivering in excess of 500 watts for higher performance. I was warranty for Carver and personally serviced the Lightstars, and others like the TFM-75, Marantz 500 and down and countless other good amplifiers. You are talking to someone who actually does know quite a bit about audio design and practical products. This includes things like the CJ Premier One, some Hiwatt and other guitar amplifiers and lot's of MI product.

I was also warranty for Counterpoint. I redesigned the preamp power supplies to be reliable and higher performance, lower noise. The SA power amps had a novel approach I feel was better. They used solid state devices in place of an output transformer. Now, how they did this wasn't even close to well designed. I have redesigned that section as well as the voltage amp stage. But I feel that a hybrid design (tube front end, solid state output section) probably more closely approaches your design objectives without being stuck to tubes all the way through.

Thanks, but I'll stick to the excellent (far more efficient) solid state designs for myself. Tube amplifiers in the 20 to 80 watt per channel rage are fun, and I use them occasionally. Even 12 watt tube amplifiers can sound great. But, an OTL design? Hell no! Output tubes are used incorrectly in those, misapplication of a device.

-Chris
 
atmasphere,

In Defense of Both OTL amplifiers, And amplifiers with output transformers . . .
(are you surprised I am defending OTL amplifiers?)

I have never knowingly heard any loudspeaker that has the same extremely flat frequency response, extremely low harmonic distortion, extremely low intermodulation distortion, and extremely good looking square wave response as . . .
A multitude of very good vacuum tube amplifier models that Have an output transformer
(AND I will also include Your OTL Atmasphere amplifiers too, as being better than the loudspeaker).

I hope that settles what is "better", excellent loudspeakers, or excellent amplifiers.

But . . . There are lots of good synergistic combinations of loudspeakers, and Either OTL amplifiers, Or amplifiers with output transformers . . .
That reproduce music so well, they are "worth dying for".

Some listeners are never satisfied, no matter the sum of the parts of the sound reproducing system.
Pity those who can not enjoy listening!

Generalization:
There were two kinds of fighter jets:
Efficient US fighter jets.
There was the Russian fighter jet, 3 times the weight, 3 times the thrust, and 1/3 of the maximum flight Range.
And most airports have a limit of how much aviation fuel is on hand.
All engineering solutions have tradeoffs.

By the way, I never got to my destination in that Thai Taxi. I got out of the Taxi, paid the driver a decent fare to how far he travelled.
Then, I got into another Taxi, and finished the ride to the Hotel.
That was a tradeoff too.
 
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A lot of questions.
It regulates the AC power and this technique is very low noise.
You can test the tubes on the fly.
No matching required.
Bias and DC Offset is automatic.
Its perfectly safe with loudspeakers. If it weren't, you have to imagine we'd not have been able to stay in business over 45 years ;)
You can run it at 1/3rd or 2/3rd power by shutting off tube banks.
It runs cooler than our other amps (except for our class D) because the output section is so much more efficient owing to its lower impedance.
The service life of the power tubes easily exceeds 10,000 hours, partially owing to that efficiency I mentioned. They last much longer than power pentodes or tetrodes; triode power tubes are usually more reliable in that regard.
There are no tube rectifiers.
The amp has only one stage of gain and actually distortion is typically much lower than the 0.5%, which isn't a bad value considering that its a zero feedback design. Most of that is the 2nd and 3rd harmonic, owing to the fully differential nature of the circuit, the 3rd is dominant. The 2nd only shows up because cancellation isn't perfect.
The power tubes are low voltage tubes that handle high amounts of current. So the power supply voltage is only 140V (regulated). So the tubes are used properly and conservatively! They wouldn't survive the voltages seen in most tube amps.. We have a lot of them simply because they don't make a lot of power. When only the filaments are engaged, the tubes are cool enough that even if left on all day you can grab them and hang on without burning yourself. In spite of its size it is meant to be easy to live with and it is. Its also expensive, but I would expect in comparison to the amps you're talking about you'd be surprised to find out this one is more transparent. We get that a lot- the risetime in the output section is over 600V/uS. Most solid state amps are not that fast.
 
My Hernia surgeon does not allow me to pick up either powerful mono-block tube amplifiers of any design, or powerful stereo tube amplifiers of any design.

Most tweeters do not know what to do with 600V/us, and my ear does not either.
Music or square waves?
Which harmonic of a Piccolo requires a 6 Volt rise, in 1/100 of a microsecond? (10 nano seconds).

I have known listeners who need a special outlet to power their system (A stove outlet, or a clothes dryer outlet are good for their stereo systems).
 
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Hi 6A3sUMMER,
Completely agree with you. Also, excellent points on speaker response and bandwidth in general.

Hi atmasphere,
Okay, I understand AC pre regulation and have signed many NDAs, no need to be cryptic. I also understand how hot 16 watts per bulb gets, no you cannot comfortably grab it after it runs for even 15 minutes. Those suckers are HOT just on heater power. Try that with a 6L6GC, heaters only and they only draw 0.9 amperes (it's pretty warm), 6AS7 tubes draw 2.5 amperes. Both at 6.3 volts. Those are rated for 300 V plate to cathode, so conservatively run that way too.

It's good you are running low cathode current, extended service life and all. Those are low mu tubes, so easier to run open (no feedback). I cannot comment on your amplifier given I do not have a schematic to refer to.

I only see triodes and one gain stage consisting of several tubes may provide enough transconductance to have low distortion. Nested feedback loops by chance? I assume you are talking overall feedback.

Anyway, we are talking about the Croft OTL specifically, and the discussion expanded into OTL amps in general. We can't focus on the MA-3 and it is atypical as you've mentioned. Given this is a technical forum, sales comments are not helpful without some backup that is technical in nature. Meaning schematics, maybe missing specific component values but enough to see what you are doing.

I will say one thing. An Austrian technician that helped train me once said, and I feel this is very true, "there isn't anything you can design or do that can't be done by someone else - or has already been done". Details may differ, and the combination of circuit blocks may be unique, but there really are no secrets. I'm not demanding anything, but I will say one famous amplifier manufacturer that wouldn't release service information was incensed when I reverse engineered their product (correctly as it turned out) so I could repair it. It was repaired and a few months later they released the service information. I did not pass anything on, and this was before the internet. Their amplifier was an odd design, but not impossible to understand.

It seems every designer creates a unique and special design that can't be shared <yawn>. Once the details are known, there may be some inspired ideas, but the bulk is usually common or known well. In protecting the ip, the hurt their customers by creating products that cannot be efficiently serviced. Yet when you sell one, the secret is out if the right person runs into it. That includes potted subassemblies.

Yes, I know your brand is old. Not the only one, but good on you for that!

-Chris
 
My Hernia surgeon does not allow me to pick up either powerful mono-block tube amplifiers of any design, or powerful stereo tube amplifiers of any design.

Most tweeters do not know what to do with 600V/us, and my ear does not either.
Music or square waves?
Which harmonic of a Piccolo requires a 6 Volt rise, in 1/100 of a microsecond? (10 nano seconds).

I have known listeners who need a special outlet to power their system (A stove outlet, or a clothes dryer outlet are good for their stereo systems).
The amp section of the MA-3 weighs about 80 pounds. Not crazy for a 500 Watt class A amp. But its power supply weighs considerably more, which is why it has handles for two people. Your comment about 600V/uS suggests you don't understand what this value represents. It means that with a squarewave input, in 1 microsecond the output voltage would have reached 600V. Its not going to present 600Volts to a tweeter though :D It simply means that it has lots of bandwidth and ability to dump current really quickly.
It seems every designer creates a unique and special design that can't be shared <yawn>.
If that yawn is meant for me, I'm way ahead of you:

I put up a schematic of our M-60, which differs only in scale, a good number of years ago on this site. https://www.diyaudio.com/community/threads/what-tubes-for-a-otl-tube-amp.161112/
The MA-3 has a lot more paralleled tube sections in the voltage amplifier, driver and output section. But its the same idea and the voltages are similar. The AC regulation is simply an automated variac. There is a control transformer it feeds and the power transformers in the unit are designed for 95Volts so the control transformer is usually bucking the AC line down. In this way even if the AC line is drawn down to 100VAC it can still make full power. The output section employs 10 23,000uF 200V computer grade filter capacitors per channel. The driver power supply is fed by its own power transformer, also running on 95V. Its output is regulated and is a bipolar power supply of +/- 400V. I mention this only so you can get an idea of the difference in scale. But the concept otherwise is the same.

I've been doing some research on the Croft. As best I can make out, its a stereo amp of modified Futterman design (if not Futterman, definately a totem pole output), employing a 12AX7 as voltage amplifier and 12AT7 as driver, with two 6080 or similar power tubes per channel. Apparently it can be monostrapped.
 
atmasphere,

I tried to be realistic, and to be fair, so I used a remotely possible real world example:
Consider the Natural 5th harmonic of a piccolo playing a 4,000Hz fundamental note.
That 5th harmonic played on your OTL amplifier is very soft, it will not even be 6 Volts.
A rise-time of 600uV/microsecond = exactly 6V in 10nanoseconds.
Nobody needs that kind of rise-time to play Music.
Right?
(I am not talking about Woodstock # 1 at 10,000 watts or more (Where 6V is just noise), and I am not talking about Wendy Carlos' Moog synthesizer).

My surgeon says my maximum recommended lift is 25 pounds; not 80 pounds, Sorry.
I once wanted to purchase a Class D amplifier; but I no longer desire to own one.

In the very early 1970s, a lot of solid state amplifiers, and an OTL Futterman vacuum tube amplifier were listened to, and compared.
The test method was good, and it was a double blindfolded test.
Anybody that either wants to defend that test, and anybody that wants to deny the results of that test, is free to do so; go ahead.
But the fact remains that there was no statistical difference in the sound of music of all those amplifiers.
What did each solid state amplifier, and the Futterman amplifier have in common?
Three things:
Totem Pole Output stages
All were OTL
All had Lots of Global negative feedback.
Go think about all of that.