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Alastair E 19th December 2010 01:29 PM

Autobias OTL....Any thoughts?
Ive breadboarded a simple Inverted Futterman OTL based on the Ciuffoli 'cheap and cheerful' design.

I noticed in the original design the neg/lower valve was apparently autobias, so Ive done the same with the top tube too....(100 ohm decoupled by 2200uF both tubes)

Its supply is only around 100V plus/minus for the O/P stage and I'm using 6C33 biassed at 175mA. so there'll be much lower power than the original.
--I dont need huge power, at a guess, Its making around 10-15W, not checked yet...

Its been run for a few days on all sorts of music and different sources. Ive made a few changes to the original design like the coupling-caps to the O/P pair are now 0.22 instead of 22uF and the grid-resistors are now 180K instead of 68K. (originals caused pumping/blocking)

A few more simple alterations, and I must say it sounds really quite good.

Any thoughts...?:)

poynton 20th December 2010 08:03 AM

Can you post a circuit, please?



SemperFi 20th December 2010 09:03 AM

That's very interesting! Makes for an even simpler ckt and no worries about thermal runaway.
But it does introduce large electrolytics in the signal path, and therefore eliminates some of what OTLs have going.
But I must try this myself;)

SemperFi 20th December 2010 09:14 AM

1 Attachment(s)
Something like this?

Alastair E 20th December 2010 09:45 AM


Originally Posted by poynton (
Can you post a circuit, please?



Hi Andy,
Andria's Schematic that I based this on is here-

The only fundamental change was the addition of the extra resistor and decoup cap in the top valve's cathode. The Speaker is connected at the anode of the lower valve, not at the cathode of the upper.

Ive changed a few values and tubes. I'm using 6SN7 for first and second stages and a 6J5G for the phase-split. The coupling-caps to O/P pair are now 0.22uF instead of the 22uF of Andria's design, and their associated grid-resistors are 180K instead of 68K (following the 6C33C guidance in the valve's spec. sheet, max recommended is 220K)
--I originally had 10uF here in the grid-drive, from P/S but found that under high O/P I was getting 'blocking/pumping' of the speaker-cone--Funnily enough, Only on one music track....:eek:

Offset volts with an 8 ohm load varies between 10mV and 50mV depending on what valves are used in the O/P stage, so I'm happy with that side of things.:)


Yes, your scheme is roughly the idea, (For the O/P pair) but the earth return of phase-splitter is returned to the speaker output point at the anode of bottom tube. (Futterman idea)
Guess there are Caps in the signal path, but I was looking to make a Very simple OTL with no worries of bias volts supplies or drift with ageing or different valves/mis-matched valves.

--Worst offset I get with two completely different 'gm' 6C33C valves is around 50mV, so the autobias seems to take care of most issues there...

It sounds very nice to my ear, comparable with my circlotron I made some years ago...:D

Brit01 20th December 2010 02:04 PM

nice idea.

R16 (16R) represents the speaker load?

Alastair E 20th December 2010 03:39 PM


Originally Posted by Brit01 (
nice idea.

R16 (16R) represents the speaker load?

Best to look at This diagram, its much more like what Ive built--The speaker connections are clearly shown,--no chance of error......:)

The above diagram posted by Semperfi doesnt have the right drive format, or the connection of the earthy end of phase-splitter to the speaker output connection--which makes a Futterman, a Futterman....:D

SemperFi 20th December 2010 04:33 PM

Yes indeed R12 (16ohm resistor) is the load. In this simulation using only one 6AS7 it is little interest in using a standard 8ohm load.
The 'drive format' is for simulation of the output stage only, although it can be done that way. And it's true I'd never do the Futterman way of doing it;D

Alastair E 21st December 2010 07:09 PM

Ive now built a second channel to see what its like in stereo...

Powered from the same PSU, there is very slight hum--which was expected, but overall I'm very impressed.

Great performance!

florindo 16th January 2012 09:02 AM


My name is Florindo.
I just set up a channel of this amp.

But i am facing some problems.At start up my fuses blows (1A)

All seems to work Ok without the 6c33 valves.Anodic is 160V witout tubes.

When i am inserting the tubes the voltage rise up only until 35V DC and when i switch on the full anodic tension the fuses blows.

Could you maybe provide me a list of the right values of tension in different parts of the circuit.
It could be a help

Thanks in advance

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