What's your OTL tube amp experience and suggestion?


Are OTL amps in general much less reliable due to their nature, or due to the implementation quality, or both?

Perhaps this has been discussed a zillion times in the past.   Perhaps, however, makers have now improve on past experience?  So it could be worth re-visiting.

My past encounters with OTL tube amps are among the most negative: Wonderful (but never great) sound during the brief period that they work.  Otherwise, major fire hazard.  Overheating, red hot plates, sparks, consistently toasted fuses, burning smell, you name it.  My past OTL amps are like crying babies on an changing table - don't you ever walk away from an arm's distance.   The used market seem to reflect such as well --- way more 'as-is for parts' or 'totally refurbished' units than reliable 'used' units that rarely need service.

Beg your pardon if it's just my poor judgement that keep getting the lemons.   What's your experience, and tips to keep OTL amps up and running happily?


bsimpson

The baby on the changing table analogy is a good one. I called my Joule Electra amp a labour of love, perhaps because I had to treat it like a baby. I really liked the sound, it was wonderful within my system, but I battled buzzing and humming which would occasionally arise. That said, I never experienced any of the problems that you listed, other than feeling like I was sitting in a sauna.

These were my issues:

Grounding – For me, the IC’s had to be inserted “just right” to stop a slight buzzing. This was hard to get right. I suppose I could have got a technician to check this out, but I was always able to resolve the issue myself with patience. I never found out if it was a design issue, or just a problem with a loose ground at the rca connector. The latter was a good possibility, as I used to have heavy, unwieldy rca cables that may have loosened something inside the connector.

When a tube went bad, it was more complex to diagnose because of the several output and driver tubes. In my experience, 6c33c output tubes going bad created a hum through the speakers. There were 8 output tubes. When only one was bad, it was easy to figure out. But when 2 or more output tubes went bad at the same time, it was difficult to troubleshoot. My original tubes lasted 5-6 years. It was only the last few years that I owned the amp that I had problems. I ended up just buying a whole new set of output tubes. Same thing with my driver tubes. When they were going bad, I went nuts trying to figure out which one was causing the problem. I knew something was wrong because one of the dc offset meters built into the amp got stuck at  “-1” and I could not set the correct value (I should have been able to set it to 120). An entire set of new driver tubes (10 of them) fixed that issue. It would have been much cheaper if I could have bought only the tubes I needed, but I couldn’t figure out which one or ones were bad. I suppose a tube tester such as a Hickock tester would have been handy for the driver tubes, but as far as I know, there was not an easy way to test the big output tubes.

Also, my OTL amp was susceptible to noise from dc on the electrical line. For example, when sliding a dimmer switch to the low position, it would create an audible buzz through the speakers. It took awhile to figure this one out, as the dimmer switch was located in a hallway far from my music room and it didn’t dawn on me at first that a switch outside the music room could be causing the problem. I changed the switch to a new one, problem solved. But there are other amplifier designs that also have problems with dc, so I wouldn’t say OTL’s are inherently worse than other types of amps when it comes to dc.

Tips: Make sure grounding is good, not just with the amp but throughout your system. Keep good, spare tubes around for testing. Minimize dc as much as you can (maybe too much to ask?). And most of all, give your amp room to breathe.

Bottom line, I wouldn’t say the OTL topology is inherently problematic. And bear in mind there are different ways to design an OTL amp, with some designs better than other designs, just as with any amplifier topology. I have owned and heard other high end amps in my system and at dealerships that made more noise than my Joule ever did (Vac 70/70 in my system, for example). I have only owned one amp that ever went “poof” and stopped working, and it wasn’t the Joule. The Joule was a workhorse that never failed me over the course of 8 years, other than the fixable, relatively minor issues that I wrote about above.

Well, wait a minute, there was the one time when the fuse holder for the variac melted and there was that chemical-like burning smell … but that was because the manufacturer installed a 7.5 amp value fuse holder when it should have been 10 amps (according to the technician who fixed it). So then again, you might be onto something!


Hi Thaluza,
You certainly have demostrated considerable tolerance/ patience. I would imagine that your current simpler designed amplifier has simplified things for you 😊😊
Charles
My advice for keeping OTL amps running smooth and trouble free:

Buy AtmaSphere.
" What's your OTL tube amp experience"

Try not to get to close when you turn it on. 
No issues with past Atmasphere amps. Very safe and trouble free.
The only OTL other than an Atma-Sphere I would own would be the Counterpoint SA-4, designed by Roger Modjeski of Music Reference. One advantage of the SA-4 is it's lower output impedance, providing more predictable frequency response (read John Atkinson's tube amp test results and his comments on the subject---a major tube-amp factor ignored by many owners).
Not all OTL are the same in tube topology some are very reliable than others
Look into David Berning's ZOTL designs built by Linear Tube Audio.
Not too sure about the SA-4, I was always of the understanding that they were one of the problematic designs. Agree that Atmasphere is certainly dependable and at least in my experience the Transcendent T-8 I owned without a hiccup. The Berning ZOTL designs are very dependable as well but the design is not a purist OTL without risk of opening up that debate again. 
@tubegroover - agreed, no more debates needed since it's certainly not a traditional OTL.  I suggest only having a listen. My MZ2-S pre and ZOTL40 combo powering my Janszen electrostats is the finest sound I've experienced.
bsimpson- I've owned amps by the 3 major players in the OTL game at one time or another.  Here are my thoughts:
Joule-Electra 80 wpc stereoblock- Big, somewhat warm sound, build quality of basic chassis (no idea about music block) not as robust as it could be.  I suspect that thaluza's problem w the hum/buzz was due to bad joint at the input jacks.  Tremendous amount of heat.  Not recommended for those w inquisitive children.  The most "fiddley" of the 3.  Somewhat cubersome power-up procedure.
Berning ZH-270- One of the Berning built amps, not the newer LTAs.  Not fiddley at all.  Auto-biasing and auto-dc offset nulling.  Tube life should be very long.  I found the sound not all that engaging, but then again, I used it w/o a pre-amp and I have found that the pre-amp is a critical part of a system.  Lightest weight and lowest heat output of the 3.
Atma-sphere- I have owned the M-60s and now the S-30.  Manual bias and dc offset.  No reliability issues at all so far.  More neutral sounding and "faster" than the Joule.  Best customer support imaginable.  UPgrade path available, which extends warranty.  One of my 2 "go to amps" (the other is a VAC Ren 30/30). 

bsimpson, in case you may still be following your thread.

I've owned a pair of AtmaSphere monoblocs and spent time with a few different Joule Electras.  From that experience I can offer two considerations.

First is the appropriate match with speakers.  That is a question of both power and load impedance.  Proper mating with lower than 8 ohms can be problematic.

Second is the issue with heat.  More output tubes may be required per channel to drive lower loads with OTLs.  My AtmaSpheres had 10 outputs per channel to produce 100 watts which I attempted with 4 ohm speakers.  They put out more heat than similarly rated transformer coupled tube amps.  But with less demanding (i.e. complex) music at reasonable levels they were magical.

That was all some years ago and now I look forward to auditioning the ZOTL 40s that lancelock recommends. 

my OTL experience is now more than 10 years ago, but the OTL magic is still a guiding light in my sonic compass.

in 2001 i was moving away from Wilson WP6’s and top level Mark Levinson #32 and #33 mono blocks. i looking for a more involving and musical presentation and purchased Kharma Exquisite Reference 1D speakers and starting searching for an amp to go with it. tried a few and then had the opportunity to demo the Atma=sphere MA2 Mk2.2’s in my room and really liked it. i was ready to pull the trigger but then had another opportunity to try the Tenor 75 watt OTL mono blocks. ok; here was a modestly powerful amplifier but this one had a fire in the mid range and was linear top and bottom. it seemed like a direct connection to the music. so i purchased the Tenors and had a 3 year love affair.

like some other love affairs, it was painful too. the darn Tenor would explode on a regular basis and take out drivers in the Kharma’s. that direct sound and lack of transformers did have it’s dark side. so I ended my love affair.....but looked around for a something else that could be similar but was reliable.

i ended up with darTZeel NHB-108 solid state which combined that fire in the mid range and transparency of the Tenors, but also had the solid state attributes of lower noise, more authority, and reliability.

back to the Tenor OTL’s; eventually Tenor went bankrupt; but the good news is that another group bought the assets and raised it from the dead, and the 75 watt Tenor OTL’s got their design cleaned up and did become reliable......and i would strongly recommend a set if you can find one (and make sure it was built after 2005 or upgraded.

it’s a magical amplifier. or get a darTZeel solid state and never look back. i did.

@mikelavigne ,
back to the Tenor OTL’s; eventually Tenor went bankrupt; but the good news is that another group bought the assets and raised it from the dead, and the 75 watt Tenor OTL’s got their design cleaned up and did become reliable......and i would strongly recommend a set if you can find one (and make sure it was built after 2005 or upgraded.
This statement seems to be incorrect. To the best of my knowledge they only make solid state amps; I'm sure we would have heard about it if they still made OTLs.

FWIW the MA-2 Mk2.2 dates from about 17 years ago...  and in the intervening time we have managed to make some progress :)  The current incarnation of the MA-2 is Mk3.3. People have described the improvement as a 'transformation'.
Hello All,

I have the Zotl 40 paired with the MZ2S preamp and after 4 days of use,only playing my worst sounding recordings as a test I find they all sound better than I've ever heard them.I'm using Mullard XF2's from 1966 and haven't used the gold lion Kt77' yet.

More evaluation time needed to truly sort this out,but I do believe I have found a pairing that I will be happy with for a long time.

Best,
Kenny.

This statement seems to be incorrect. To the best of my knowledge they only make solid state amps; I’m sure we would have heard about it if they still made OTLs.

FWIW the MA-2 Mk2.2 dates from about 17 years ago... and in the intervening time we have managed to make some progress :) The current incarnation of the MA-2 is Mk3.3. People have described the improvement as a ’transformation’.

Ralph,

Tenor makes hybrid tube amps (an update to their Hybrid 300 mono blocks I owned briefly in 2004) now. not sure about any other products. however; back in 2005 a group of investors did purchase the assets of Tenor and did redesign/update the tubed OTL amps and sell them as a current product for some years. there are still a few out there new for sale but not sure it’s a current product anymore.

regarding my personal Atma=sphere MA2 experience; if you read my post it says this happened in 2001, 16 years ago. i’ve had many positive things to say about you and your products over the years and just because i preferred the 75 watt Tenor OTL’s back then, does not change the fact you make great products and take great care of your customers. i’ve heard your more recent products many times at shows and they are as you describe.

@mikelavigne , I remember when that happened, but even back then the story I heard was they were only doing the transistor amp. In all that time, I've yet to hear about a Tenor OTL that was made after the 1st company went under. I suspect what you are seeing for sale are older amps that have been refurbished.


Ralph,

http://tenoraudio.com/press/press-releases/product-press-releases/classic-soundpieces.html

re-manufactured.

here ya go.....

http://tenoraudio.com/high-end-products/high-end-audio/350m-ultimate-reference/

the current model tube hybrid mono blocks....

Yes- apparently we're on the same page here, if you'll pardon the expression :)
Simple fact is Most OTL on the market are not built Like Atma-Sphere. These companies that try to work on OTL cant seem to figure out a Stable build, Atma did this decades ago.

The Simple answer to OP issues is, If you want OTL Stick to Atma-Sphere and you will have Years of trouble free operation.

Sorry but DartZeel is not on the same league as the Atma-Sphere Amplifiers. The Atma is a lot more transparent, faster and throws a larger sound stage and does not sound like a tube nor a solid state. Should I even mention the Atma not only has better transparency it is a lot more emotional to listen too. It draws you into the music and does not sound mechanical.

DartZeel make good solid state amp but I'm sorry it cant be compared to a Atma-Sphere.
Tenor has a bad reputation in Asia. They also gave OTL in general a bad reputation. Most buyers don’t understand their were tons of companies coming out with OTL and only to go out of business as none of them could figure out how to make a Stable OTL Design.

Everytime i speak of OTL to someone, Tenor would come up into the conversation as something that used to blow up like a dynamite!!!

Given how dynamic and lively a good OTL can sound, the world would be full of models being offered, if they didn't have their problems.  Most of the ones I've encountered were not fully reliable.  The notable exception are the Atmasphere models which don't seem to die or destroy speakers.  But, I have also heard a number of setups with Atmasphere OTLs and high efficiency speakers where a hum or buzz could not be ameliorated.  

My personal preference in OTL sound, from commercially available amps, was the big Joule OTL (warmer sound than the Atmasphere or Berning), but, the model I heard had problems with tube sockets going bad which caused it to intermittently put out a HUGE pop; a friend with 105 db/w speakers said the pop was as loud as discharging a shotgun indoors.  I heard that pop on far less efficient speakers and almost had a heart attack.

I did not understand the appeal of the Tenor at all.  In the systems I heard the Tenor in, it did not deliver the incredible dynamics and liveliness that is the primary reason one goes for OTLs.

I also liked the sound of the SAP OTL, but, that amp too, turned out to be unreliable.  

The only practical alternatives out there that I know of are Atmasphere and Berning models.  Both are certainly worth an audition.  I am a tube fan, but, I don't like most high powered tube amps.  If I had speakers that demanded more than 40 watts, I would certainly be looking into OTL amps.
I found that the Atma-Sphere Amplifiers sound warmer if you change out all the 6SN7 Tubes provided by Ralph.

The one Ralph gives with the amplifier are Chinese made tubes which to me sound a little bright in the upper regions. Once I swapped these out for good pair of NOS tubes everything changed.

The Noise is most likely from the Tube not the Amplifier. I had the same issue with one channel constantly buzzing, These tubes I used in other pre-amps were dead quiet but in Atma-Sphere they seems very noisy. Once i changed them out for other NOS tubes the noise was gone.

The noise is not from the Pre-amp or the amplifier but the Tubes them selfs, Some of these tubes never show signs of noise in my other pre-amps but for some reason they do in Atma-Sphere.

So the goal is to feed these babies good quality low noise tubes. Once you got the tube noise down and the system dialled in with the tonal balance the system can have a nice warm balancing sound.

Just an advise for those who want a warmer sounding OTL on Atma-Sphere. Use some of the RCA, Sylvinia, Ken-rads. The Sophia Electra used on the pre-amp softened up the system too much to my liking.

dragon_vibe,

i did not make the connection between darTZeel and Atma=sphere......you did. i said that in back to back listening sessions ’16 years ago’ i preferred the Tenor OTL’s to the Atma-sphere MA2’s.....which was my opinion back then. and at that time, many agreed with me and did acquire Tenors and loved them. and i absolutely agree that back then those Tenor OTL were unstable and did blow up tubes, which i mentioned. later that design flaw of the Tenor OTL was cleaned up and I provided a link for that information.

but we are just going to have to disagree on how we view darTZeel and the current Atma-sphere amplifiers.....which i did not bring up. but, of course, i’m referring to my current darTZeel’s, the 458 mono blocks.

we all have a right to our opinion.

that said; i’m a fan of both Ralph and Atma-sphere and do strongly recommend them and have recommended them.....and agree it’s a solidly built product well supported.....and on my short list of favorite tube amplifiers.

Sorry mike I misread your post.
no worries dragon_vibe.
Also don't forget using an auto former like the Speltz to pair with a wider variety of speaker impedances.  

If anyone wants to hear the Atma-sphere gear in NYC give me a shout out.  TriodePictureSound. 
Thaluza
I have a Joule-Electra VZN 80 and I have never had any of the issues you speak of .
Did you bias the tubes including the driver tubes. It was easy to tell if an output tube was going bad it would not hold Bias. The joule OTL are the best sounding of any I have owned or demoed .
Atma-Sphere M-60 3.3 with the upgrades Ralph offers.
Plus I use dual Zero’s with my 8 ohm 90+db Hi-Eff speakers. I can play the music as loud as I once did with my Pass Labs X350-5.
And... the music is so smooth and perfect... No turning back
I am also using Russian output tubes and Sophia input tubes.
ozzy


Ozzy, have you experimented with the RCA Output Tubes?


To answer the original questions, there can be reliably and potentially safety issues with OTL amplifiers, especially early designs.  The best implementation I've seen are the Berning ZOTL amps.  They are safe, and reliable.  Most won't require retubing for years, or decades. They have autobias, and protection circuits, plus very low output impedance,  making them very versatile in matching them to a wide variety of speakers.

As well as the models available on his website, or licensed by LTA, David has a collection of small, lower power models available he doesn't promote on the website that includes  300b SET and PP, 6b4 SET and PP, 845 SET, etc. All the models I've heard (and owned) sound outstanding. 
@rusty_jefferson 

Our (Atma-Sphere) claim to fame is we made the first reliable OTLs. That was in 1976. I don't know of any OTLs that were safety issues, including early designs. I've heard this said before, but as far as I know its some sort of myth.

FWIW, the Berning amplifier is not an OTL. It is a uniquely different design and does employ an output transformer (unlike an OTL; 'OTL' stands for Output TransformerLess).
Who made the amp?Do you know the tube topology?Looks like homebrew from inexperienced TUBE MASTER.
@atmasphere

Just to be clear, I'm not Trolling you, or your company, just answering the question in the thread title, "What's your OTL tube amp experience and suggestion".

By stating you made the first reliable OTLs does indicate that some or all of your predecessors were in fact, not reliable. And by safe, I didn't just mean not catching on fire, but rather safe and reliable in our systems.  The Berning ZOTL amps have no chance of passing DC, no popping noises, and extremely long tube life, again, on the order of many years to decades of daily use. If your amplifiers are capable of similar safety and reliability, I apologize for not knowing so.

And if some think ZOTLs aren't really OTL amplifiers because they don't meet some engineering criteria that fits their needs, well, I don't know what to say about that. I don't think anybody has ever sued Berning for false advertising.  Here's a nice discussion from the Berning website, including pros and cons of different OTL implementations, for those interested. http://davidberning.com/products/qz
It is accurate to point out that the Berning amplifiers utilize a "nontradional" output transformer that operates at very high radio frequency levels. This is a different type of transformer as compared to the common or traditional output transformer. This is different from the Atma-Sphere which does not utilize an output transformer at all. Both companies have earned reputations for excellent sound quality. Obviously each has their own unique character.
Charles 
@rusty_jefferson 

Just to be clear, I'm not Trolling you, or your company, just answering the question in the thread title, "What's your OTL tube amp experience and suggestion".
No worries- I didn't think you were. But most people don't realize the ZOTL amplifiers have not only an output transformer, but also semiconductors in the output section. It is quite unique!

By stating you made the first reliable OTLs does indicate that some or all of your predecessors were in fact, not reliable. And by safe, I didn't just mean not catching on fire, but rather safe and reliable in our systems. The Berning ZOTL amps have no chance of passing DC, no popping noises, and extremely long tube life, again, on the order of many years to decades of daily use. If your amplifiers are capable of similar safety and reliability, I apologize for not knowing so.
Sooner or later, if a tube fails the Bering will make a popping sound. But it unlikely to threaten a speaker.

You might think about this for a moment: If our amps were damaging speakers, we would not be able to stay in business for very long, instead of the 41 years we've been around. I'd be quite nervous if I thought for a second that one of our amps could threaten my speakers- the drivers in them are quite expensive. That sort of thing has a way of coming home to roost!

We protect the speaker with a simple mechanism which is highly effective. But that is not the only reason our amps are safe- we are the first OTL manufacturer to make an amplifier wherein the power tubes are in fact properly under control (which is really important; think about what that means for distortion too) and also, the first OTL that is unconditionally stable (which is to say: stable regardless of input condition or output load).

We too get long tube life (we warrant all the tubes in our products for a year) often on the order of many years to decades... so apology accepted- no hard feelings  :)

And if some think ZOTLs aren't really OTL amplifiers because they don't meet some engineering criteria that fits their needs, well, I don't know what to say about that. I don't think anybody has ever sued Berning for false advertising. Here's a nice discussion from the Berning website, including pros and cons of different OTL implementations, for those interested.
When the amp first appeared I read the patent, as it is patented. This is from the abstract of the patent:

"A linear audio amplifier includes a push-pull pair of vacuum tubes operating in a linear amplification mode coupled through a pair of dc-dc switching power converters to an external load impedance. Each power converter includes a **transformer** with one or more secondary windings that drive rectifier circuits, and the resultant dc voltage sources are loaded by their respective tubes."  (emphasis added)

Although connected in a quite unconventional manner (hence the patent), it is the transformers that effect the transformation from the high impedance of the power tube to the low impedance of the speaker, which otherwise is a rather conventional way of describing the operation of an output transformer.

In a letter to the editor of Positive Feedback back in 1997, David mentioned that he was originally going to call the amplifier a Zero Hysteresis amplifier but he found that to be difficult to explain. When he told people it was similar to an OTL then the glazed expression went away. It didn't help that Harvey Rosenberg suggested the ZOTL acronym, which Harvey told me specifically didn't mean anything as he knew full well the amp was not an OTL.
Had the Tenor 75 and Atma s30

The Tenor gave me no problems and had some work done on it  prior  to me buying them. Crazy great sound and  awesome looking but created too much heat in my Florida listening room. The equal to Atma

Atma s30 why did I sell it?? Stupid not to have gone to the m60 and just deal with the heat. Now in the market to buy a pair. Support is impeccable and reliability is off the charts great.  Bulletproof in my experience with the S 30.