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leadbelly 13th February 2006 04:46 PM

Why no single triode OTL power stages?
In my searches of OTL schematics, I have yet to come across any examples of single triode cathode follower output stages. They are all multi triode push-pull cathode follower, White cathode follower, etc.
This site indicates that it can be done with a single triode section:
I understand that the power output would be so small for such a stage as to make hardly it useable, say only as either a headphone amp or driving some 100+ dB/W/m speaker or tweeter. Other than that, is there anything bad about something like a single 6C33C cathode follower OTL headphone amp?

tubetvr 13th February 2006 05:17 PM


The main problems with a single tube OTL is to get low enough output impedance and useable output power. A 6C33C which is one of the most suitable tubes available have a ra of ~80 ohm and a mu of ~2.5 this give an output impedance of ~22ohm as a cathode follower, this can be lowered by parallelling tubes even though 22 ohm should be good enough for a headphone amp.

A cathode follower OTL will always be inefficient and you will always have the problem providing a good cathode load be it a inductor or some kind of CCS.

Most people building OTL amps want to have a reasonable output power and the easiest way to get that is by using 2 tubes in some kind of push-pull circuit. The inverted Futterman circuit always provide the lowest output impedance for the same tubes, thereby eliminating the need for parallell connceting a large number of tubes.

Why this interest of single tube OTL? what are you trying to achieve?

Regards Hans

leadbelly 13th February 2006 05:33 PM


Originally posted by tubetvr
Why this interest of single tube OTL? what are you trying to achieve?
Thanks for your detailed reply. Basically, I want to build as inexpensive an OTL as I can for a learning experience. A single tube cathode follower appeals to me since it can be powered from a single rail, the same rail as the input stage. I have looked at OTL schematics and they are far from inexpensive, since the cost of the OPT is more than offset by the extra cost of multiple rails, extra filtering to fight bad PSRR, etc.

tubetvr 13th February 2006 05:37 PM

The first OTL I built was a SEPP without phase splitter using 2 pentodes, it was similar to the one described as "Philips original OTL" here it has the disadvantage of needing a output capacitor and it also give quite a lot of 2nd order distortion because of the lack of symmetry but the one I built sounded so good that I after that always wanted to build a real OTL.

I can recommend this as a first project and it should work quite well as headphone amp.

Regards Hans

leadbelly 13th February 2006 05:48 PM

Is that the same as this one: ?
Wasn't that built for 600 ohm speakers? Will it work as a headphone amp into 32 ohm headphones?

markanica 13th February 2006 06:18 PM

Single output tube otl
2 Attachment(s)
It is possible!!

I made a one output tube otl using one 6AS7 for each channel
The resulting output is about 60 Volts into an 800 Ohms speaker
Output impedance is about 200 Ohms
Freq. response -1dB is 40 Hz to 200+ kHz

I use Philips 9710AM speakers. Also sold under the name Norelco iirc.
Lately i found some nice 800 Ohms speakers in a Philicorda organ, haven't tested them yet though

Sound is sweet and fast, unlike any transformer coupled design i have heard so far.
I build it with parts out of the bin, to keep costs low.
See att. schematic

tubetvr 13th February 2006 06:33 PM


Is that the same as this one: ?
Yes that is correct but I didn't use EL86 but some other tubes I had at the time. The ouput impedance is a bit high but for a headphone amp it works quite well even though it would give very low efficiency and low output power when loaded with say a 8 ohm speaker.

Regards Hans

andrei 13th February 2006 10:49 PM

The trouble with various OTL schemes is not getting output impedance low (as people tend to think), but limited current drive capability. If you are running two sections of 6AS7 in parallel as a Class A cathode follower at 75mA idle per section, the maximum current you will get into the speaker is 150mA, which is just 0.1W into 8ohm load before clipping sets in. Not very efficient. (One can get 5W easily from the same tube in conventional transformer-coupled common-cathode output stage.) To get around this problem, OTL are usually made in push-pull configuration, and run deep into Class B (which is ugly if you ask me).

leadbelly 14th February 2006 01:32 AM

I do like the looks of that Philips amp more and more. Are there improvements you would make to the circuit, e.g. larger electrolytics since they are cheaper nowadays? Also, what kind of current requirement for the PS is needed?

tubetvr 14th February 2006 05:25 AM


To get around this problem, OTL are usually made in push-pull configuration, and run deep into Class B (which is ugly if you ask me).
There is no reason to run an OTL in class B, AB1 is sufficient to get both high power and reasonable efficiency. My own OTL have an idle current of 200mA and the tubes deliver 2.5A in the peaks without drawing grid current.

Output impedance is important in order to get reasonable efficiency, an OTL with very high output impedance like for instance the original EL86 based Philips design need very high anode voltage to get any kind of output power when loaded with low impedance e.g 8 ohm. Output impedance can of course be lowered by feedback but efficiency will still be the same.

Regards Hans

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