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The good, the bad, and the ugly OTL amplifiers

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Ok, I'm looking to buy a OTL amplifier at a good price. I need information in order to formulate my decision of to buy or not to buy. I need your opinions, facts, judgements and so forth. Please convince me one way or the other here.

I have been told they are dirty little buggers that cannot be trusted. Some tell me they go thru tubes like they are going out of style and yet others tell me they can be trusted to deliver good and trouble free service. Some tell me they have no bass response and yet other people rave about the low end.

Please help seperate fact from fiction here.
 
Number one: what speakers are you using? OTLs tend to be VERY finicky about their loads. If you answer "Quad ESL," then you're a candidate.

They are horribly inefficient, tend to be unreliable, and eat output tubes. I've owned some original Futtermans (Futtermen?) and a home-made monster using 16 6528. Interesting experiences, but I'll never do that again.
 
Yes, 16 per channel. The heat load just from the output stage heaters was nearly a kilowatt. It was a very inefficient, inelegant design, but a friend had been given 64 6528 out of a NASA shake table and wanted to do something with them...

What does the impedance curve of those Cornwalls look like?
 
6C33Cs have the potential for thermal runaway if care is not taken

I have never heard or experienced that, can you tell me more? I have used 6C33C in OTL amps for many years but never had a serious problem. As a reliability test I was running one amp at full power for 24 hours without anything bad happening except that everything got very warm, I seriously doubt that there could be problems with thermal runaway and why should it happen with 6C33C?

On the subject of OTL amps I think it is important to choose a amp that is designed well, meaning that idle dissipation must be much lower than allowed maximum, that during normal operating conditions average cathode current should not exceed max allowed cathode current, that if parallell tubes are used current is derated, and that ventilation is good enough to remove the heat. However it seems that very few commercial amps fulfil these very basic requirements.

Regards Hans
 
Anything with 16+ tubes is going to have a failuire in at least one tube on a regular basis. Make sure to get a tube tester and a few backups while you're at it!

As a side note, you might want to consider trying the SEWA solid-state amplifier found in the Pass Labs section. It's supposed to be a pretty good 7 watt amp, and if you already have an appropriate transformer/PSU components, the cost is very low. (It's also supposed to work very well with a tube preamp - it has no gain!)
 
Hi Burnedfingers , for sure you'll realize your dream ;) OTLs are for me the cream top of amplifiers . I did my own ones as i didn't want to use 6c33 (many failure) , 6528 (not good in OTLs) , nor 7542 (hard to find) ... king of the OTL is 6080/6as7 . With 10 per channel , you should drive 5 ohms loads without problems . Some tweaks : as previously said , make them run UNDER max rate, have a tube tester to match them , and ... :att'n: heat the NEW ones 24 or 48 hours long without HV to evaporate residual dusts : 10 years long without a failure :rolleyes:
 
You mention the dampening factor is bad with OTL. I haven't seen a tube amplifier with high dampening factor. As a matter of fact there was a thread here a while back that discussed dampening factors in tube amplifiers and if higher dampening factors were better than lower ones. If my memory serves me correctly I believe that lower dampening factors were favored in tube amplifiers and very high dampening factors are favored with solid state amplifiers. The higher the dampening factor in solid state amplifiers the better the amplifier. I can respect your opinion here and would just like some facts and figures to go along with your opinions because I'm still on the fence here.

What is the dampening factor of a OTL amplifier, a push pull amplifier and a SE amplifier if I may ask?

With respect to feedback...Some here surely don't want any while others feel that a small amount is needed. Naturally any amplifier will sound somewhat better with a small amount of feedback.
The solid state amplifiers absolutely need it to sound listenable and for stability. Where do we draw the line here anyway?
 
I am presently using a Transcendent amp that uses 2 6C33C-B's per channel with quite a bit of NFB. My speakers are homebuilt with good efficiency but low impedance - around 97db and 3 ohm minimums. It is one of the best sounding amps I've had. In my top few favorites of all times.
In my opinion the NFB may not be as harmful in an OTL since it is not being used in a loop that includes a less than perfect output transformer. Just my thought as to why it does not seem to hurt the sound much.
Sometimes the best way to find out if your system will sound good with different gear is to just try it out. Not everything can be figured out with measurements, statistics, and opinions.
 
I made a Transcendent T8 several years ago. It is not a finicky amp at all, has plenty of bass, will easily drive 4 ohms, sounds really great & all that. Downside, it throws off some serious heat & the price of all those power tubes. Owning an OTL was my dream for many years & after I got one, I'd just as well listen to my little Loftin-White.
 
Never heard an OTL amp going too good with the Cornwalls.

Gamie,

Have you ever heard Cornwalls? Anyway they are Cornwall in size and that is about it. They are a highly developed design by DJK that totally kicks the Klipsch to the curb. They test almost ruler flat also. I refer to them as a Cornwall clone because they look almost the same. In terms of performance they are so much smoother and higher efficiency.

I'm interested in your opinion don't get me wrong. Have you heard any OTL amplifiers in which you could lend a opinion based on this information?

Quote:

McIntosh + Klipsch is something that one must hear in his life.

Well, heard them and wasn't impressed. Mac is ok but there are far better amps out there to be had.

Getting back to the OTL amps

So far we have established that they run hot. This is because of a large number of tubes producing mega heat. Note: Good for winter heat in home. There is considerable cost to replace the output tubes. They may or may not go up in a puff of smoke. They do require some feedback to sound halfway good. Note: Feedback
is 23db for the design I am looking at.

Safety: In checking into various designs it should be noted that some do not use transformers in their power design. No design like this should be considered or purchased.:hot: This notice is for SY and the diyaudio board and is meant to protect us so we don't fry our butts.

It shall be noted that some consider the OTL to sound quite good.
We should consider the type of speaker driven and the impedance curve of same. OTL's don't like to drive low impedance loads unless they have a massive amount of output tubes. Some claim long tube life if broken in correctly. I should mention that the 6C33 has a tube socket problem as it cannot withstand the massive heater current and this leads to problems. The 6C33 is a great tube however.

It would be of great interest to see some of the OTL amplifier designs that some of you have made in the past or are currently using.
 
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