OTL Amp

[ba3jar]

Hi

does anyone know where to find books and information on how to build OTL amp and calculate the output impedance of it?


thanks

[imaradiostar]

You might try some of the DIY hi fi audio forums. There are a few OTL designs that are somewhat common. You won't likely find many here because musical instrument amps tend to use an output transformer.

jamie

[J M Fahey]

The *first* parameter you must consider is peak current into the load.
For, say, 15W RMS into 8 ohms, you´ll need 2A peak.
Considering that a 6L6 can provide, say, 200 ma, you´ll need 20 x 6L6.
10 each side of the Push Pull.
Obviously not a good choice, so you'll need some exotic tube which can supply at least 1 Ampere, and with relatively low voltages so it's not *so* unefficient.
Lately some people were using a weird Russian tube designed for flight controls in MIG fighters or something like that.
Or design for 800 ohm speakers as Philips did in the old days.

Basically this is it.

[Jazz P Bass]

OTL Amplifier Design: the Broskie OTL
Google 'OTL amplifier'

[ba3jar]

thanks for the info , most of the otl amps use the 6as7 tube , what about using preamp tubes as output such as 12at7 and 12au7 as option for otl output amp?

[sgelectric]

Bruce Rozenblit has a book out called "Audio Reality" that has an entire section dedicated to an 80 watt OTL design.Check it out.


http://www.transcendentsound.com/Tra...o_Reality.html

SG

[km6xz]

Maybe a better starting point is describing what you are trying to get the amplifier to interface with, and operational constraints. Sure 12AT7s would work in very low power applications, milliwatt range, at very low current, some meter and instrumentation amplifiers use that sort of power level, to drive a meter movement. OTL circuits of any useful power are really complex for operating point stability or just plain flaky stability-wise. Circuits like the Futterman were notoriously unstable but sounded good. They relied on EL509 in later models but hard to get sweep tubes originally(6LF6's, a bunch of them) for about 8-12% efficiency. That means a gigantic power supply for just a little output. It cooked the sweep tubes pretty often because they are pulse tubes and can't handle sustained high current that OTL requires. Only the old original EL509's work now because the newer tubes marked 509 from New Sensor and JJ are no where near the ratings of the original tubes.
High current regulator tubes could work but those have low gain as a rule so drive becomes more of an issue.
What characteristic of your application needs the traits of a OTL? I am sure there are more effective solutions available.

[ba3jar]

about the characteristic of the application is actually a project i want to work on a tube amp for guitar , i do like the 12at7 characteristic sound in the preamp , so i'm looking for a way to use the 12at7 as output section as well using about 8 12at7 in parallel a single ended output , instead of using a transformer i want to make it as otl one, the reason using 8 12at7 because in the data of this tube i was looking for the voltage and current basic to give me about 2watt on the plate , that total on one tube may have about 4 watts and about 30watt total in parallel 8 12at7 , i'm not sure if i'm in the right direction , the question here how does that work in otl design

[J M Fahey]

Dear baejar, the critical parameter you asked about is, as I mentioned before, current capacity.
Permissible plate dissipation is reasonable, sort of, you could draw a few watts ... into a high impedance which will require an impossibly high impedance speaker ... or the "normal" solution: an output transformer, just what you are trying to avoid.
You set for yourself conflicting specifications.

[imaradiostar]

Just for kicks, why not try a 12at7 with a fender reverb transformer and see if you like it?

jamie

[J M Fahey]

^^^^^^^^^^
Possible spammer

very well my friend

[teemuk]

I know it's heresy but why don't you just take the logical route and buffer a 12AU7 voltage amp with a solid-state push-pull follower?


OTL tube amps are interesting and challenging - but also totally pointless - projects. They practically make sense only for the sake of building a tube power amp that doesn't need an output transformer and uses a very different kind of architecture than usual. But what you really gain at best with that route is a very "solid-state-ish" circuit that introduces massive power losses and pestering stability issues.

[Chuck H]

+1
If the point is to avoid using an output transformer then a tube preamp and SS power would seem like the way to go. To hit any reasonable guitar speaker load (say 32 ohms, two 16 ohm speakers in series) for a low power amp (under fifty watts more than two speakers seems silly) I think you would need to parallel over two hundred 12at7 tubes?!?

[ba3jar]

i wouldnt compare otl output with solid state output , both sounds different , i'm talking about pure tube distortion without the transformer distortion , plus with the tube you could make more functional tone controls to control the tube frequency which each tube has its own tone and character much realistic in sounding rather than a solid state you need to hear more sounds to understand and compare , about the watts as i said i'm talking about 8 12at7 tubes in parallel it means 16 triodes each one has about 2 watts the total is about 30 watt , that would be a single ended otl , about the output ohms i dont know how much will i get but if i get it at least to 64 ohm , i could then make a 4 speakers in series with 16 ohms each

[J M Fahey]

i wouldnt compare otl output with solid state output , both sounds different

How do you know?
Have you ever listened to an OTL amplifier?
It´s not the typical transformer loaded common emitter pentode we all associate with "tube sound" but triode cathode followers: unity gain , low output impedance and low distorting

i'm talking about pure tube distortion without the transformer distortion

not much tube distortion there.
In fact it´s the cleanest flattest circuit you may build with tubes.

plus with the tube you could make more functional tone controls to control the tube frequency which each tube has its own tone and character much realistic in sounding rather than a solid state

what is "tube frequency"?
A cathode follower is the flattest tube circuit you can build.

you need to hear more sounds to understand and compare

fine, maybe, have you listened to OTLs and compared them to the suggested solutions?

about the watts as i said i'm talking about 8 12at7 tubes in parallel it means 16 triodes each one has about 2 watts the total is about 30 watt

Well, that might be their plate dissipation, don't understand how that parameter might relate to power output across a speaker

that would be a single ended otl , about the output ohms i dont know how much will i get but if i get it at least to 64 ohm , i could then make a 4 speakers in series with 16 ohms each

A single 12AT7, saturating, has almost 4mA plate current at a saturation voltage of 50V
I´m thinking of an OTL stage with 300V B+, with 12AT7 in series resting at 150V each, and swinging up and down to 250V and 50V.
8 x 12AT7 means 32mA into the load.
The peak power would 0.032A squared x 64 ohms= 65mW
Now 300x12AT7, as suggested elsewhere, would add up to 300x.004A=1,2A
1.2 squared into 64 ohms=92 peak watts, or 46W RMS.