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The good, the bad, and the ugly OTL amplifiers

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Quote from the Transcendent audio site for the purpose of looking at the feedback aspect of the OTL amplifier.


Let’s look at the Transcendent T8 as an example. It uses 8, EL509 tubes in push-pull. Each 509 has a plate impedance of about 150 ohms. 150 ÷8 = 18.75 ohms. This is unacceptable. Negative feedback is employed to achieve proper performance. The amplifier uses 33 dB of negative feedback which reduces the output impedance to 0.4 ohm thereby achieving outstanding woofer control and the ability to drive 4 ohm speakers. This specification was verified by Stereophile when they reviewed the amplifier.

Note: the 6AS7 has a plate impedance of 280 ohms so if my math is correct 280/20 sections is 14 ohms which is further reduced by the negative feedback that is applied. The feedback is 23db.

So, in your opinion is this amplifier capable of driving a 4ohm load that I would encounter with my speakers in reproducing low frequency?
 
Quote from the Transcendent audio site for the purpose of looking at the feedback aspect of the OTL amplifier.


Let’s look at the Transcendent T8 as an example. It uses 8, EL509 tubes in push-pull. Each 509 has a plate impedance of about 150 ohms. 150 ÷8 = 18.75 ohms. This is unacceptable. Negative feedback is employed to achieve proper performance. The amplifier uses 33 dB of negative feedback which reduces the output impedance to 0.4 ohm thereby achieving outstanding woofer control and the ability to drive 4 ohm speakers. This specification was verified by Stereophile when they reviewed the amplifier.



Different tubes will provide similar results. The configuration of the OTL output stage does not matter whether it is series connected push-pull or balanced. The controlling factors here are the number of output devices and the plate impedance of the devices. A 6AS7G is a dual triode with a plate impedance of 280 ohms. An amplifier that uses 8 such tubes will have an output impedance of 280 ÷ 16 = 17.5 ohms. A fourteen tube amplifier will have an output impedance of 10 ohms.

The amplifier I am considering has 10 6AS7GA's in it per channel thus reducing the output impedance to 14 ohms. This coupled with 23db of negative feedback should allow the amplifier to drive low frequency without a problem in my opinion.
 
Just an opinion,
I would speculate that most of the sonic advantages of a OTL amp can be achieved by transformer coupling a few 6AS7's into a quality toroidal output transformer. Run the 6AS7's at high current and low voltage. This type of setup gets around the inherent limitations of an output transformer, because high current low voltage has a much lower stepping ratio than a normal output tube which drastically improves the frequency response. Of course you will be running PP.

This is a very rarely tried configuration (because suitable OT are rare and 6AS7's are not liked by any but the OTL crowd) so most people cannot express an honest opinion of its sonic merits.
Running an OTL amp just doesn't make sense on so many levels, when there are good alternatives out there.

Shoog
 
Shoog said:
Just an opinion,
I would speculate that most of the sonic advantages of a OTL amp can be achieved by transformer coupling a few 6AS7's into a quality toroidal output transformer. Run the 6AS7's at high current and low voltage. This type of setup gets around the inherent limitations of an output transformer, because high current low voltage has a much lower stepping ratio than a normal output tube which drastically improves the frequency response. Of course you will be running PP.

This is a very rarely tried configuration (because suitable OT are rare and 6AS7's are not liked by any but the OTL crowd) so most people cannot express an honest opinion of its sonic merits.
Running an OTL amp just doesn't make sense on so many levels, when there are good alternatives out there.

Shoog


I disagree. I'm a big fan of OTL amps, and own several.

(Of course, they're all solid-state.)

I would wonder if inexpensive tubes (availible by the truckload on eBay) could be substituted. With 32 tubes/channel, you'd need a huge power transformer, but think of the output power!
 
burnedfingers said:


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?

My name is Giaime, and I was referring to original Klipsch Cornwall speakers. And I usually don't talk about things I don't know or heard.
 
Ive been trying to find the source I read about it in the past been, its been frustrating.

Some of what I read boils down to that the tube is alot less dependable at 60watts dissipation than 45watts. Also that just because you can get away with quite impressive far beyond datasheet 2A peaks, doesnt mean that youl have great MBTF doing so. The parameters may shift abit with burnin and/or hard use.
 
Hi,

Yes, it seems to be more or less common knowledge that 6C33C will have very long life and be stable if used at 2/3 of max dissipation or there about. Viktor Khomenko at BAT who uses large quantities of this tube has described that parameters can start to drift if the tube is used at max dissipation continously, but he has also said that life is very long if run at or less than about 40W.

As I descríbed before I think it is important to design an OTL amp in a sensible way, I don't think this really matters on which tubes you use but due to the specs of 6C33C it has the advantage of being able to deliver useful output power from only a few tubes.

In my amp with 2 6C33C tubes I use 160V B+ and 200mA idle current so the idle dissipation is only 32W. With max output power which is 25W in 8 ohm the peak current is 2.5A but the average current that is determining the thermal load of the tube is then only 0.8A so the max dissipation will be ~115W which is still way over the max of 60W.

However you can argue that an audio amplifier will only spend a very short time on peak power and it is reasonable to assume that even if you play at clipping level the average power is at least 10dB lower than max.

10dB below max gives in my case an average current of 0.28A and an average dissipation of only 45W so the end result is that you get very long tube life which is also what I have experienced, in my own amp I am still using the original tubes after more than 6 years of use.

Regards Hans
 
burnedfingers said:
Note: the 6AS7 has a plate impedance of 280 ohms so if my math is correct 280/20 sections is 14 ohms ...

It depends on the circuit. If the load is in the plate circuit of the 6AS7's (as is the case in a typical amp with an output transformer) then your math is correct. However, if the circlotron circuit is used (as is the case with Atmosphere) then a single 6AS7 will have an output impedance of something in the neighborhood of 80 ohms. That's both sections of one tube in a push-pull circlotron circuit. So 20 tubes in parallel will have an output impedance of about 4 ohms before NFB.

The White cathode follower and Futterman circuits also provide an output impedance that is lower than the plate resistance, but I don't know off hand what it is. You could probably find out with a little googling.

-- Dave
 
I think the 6C33C-B got pad press because of the cheap sockets that were available for it a few years back. The metal was so flimsy that it made poor contact and fried. Newer sockets available now have taken care of that problem. Also, some people used them at higher than acceptable dissipation and/or voltage. Most designers prefer less than 150V. I always pre-age mine for a few hours on a seperate rig to stabilize the parameters and they work fine.
 
The White cathode follower and Futterman circuits also provide an output impedance that is lower than the plate resistance, but I don't know off hand what it is.

See here: http://members.aol.com/aria3/otlpaper/otlhist.htm

Lowest output impedance you will get with an inverted Futterman Rp/2*(1+u) which for 6AS7 is 280/2*(1+2) = 47ohm The Circlotron is worse and give Rp/2+u = 70ohm. Figures for 6C33C are 11 and 17ohm respectively.

Regards Hans
 
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I designed a disasterously unsuccessful OTL amplifier based on 6C33 about 10 yrs ago. Take a look at my website for the dire saga. ;)

The 6C33 has the potential to be an excellent tube for OTL, PP and SE amplifiers if certain simple rules are adhered to, and will be reliable if you get a good tube. (Not all tubes available are good. :xeye: )

As pointed out elsewhere in this thread a good socket is a must.

:hot: Delay B+ until cathodes are fully warmed up - this will avoid cathode stripping and more importantly the arcing that will kill your load. :hot:

:hot: Don't run them beyond 45W dissipation, they become much less reliable and drift strongly at higher dissipations. :hot:

Make sure the section to section match is good.

Make allowances in the driver design for the considerable miller capacitance of several of these in parallel, and provide for some limited grid current drive capability as well. (Cathode follower or White CF driver stage?)

My understanding is that you can run the filaments about 5% below nominal and limit inrush current as well for a substantial improvement in life.

Otherwise this might be the best choice for an otl of currently available tubes, a pair will get you about 20W into 8 ohms - try that with a small quantity of 6AS7G.. :D
 
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