Submitted for your (dis) approval, OTL, Autobias Inv. Futterman

[Alastair E]

Ive been meaning to try good old-fashioned Autobias for an OTL for a couple of years.
I finally settled on a pretty basic simple Inv. Futterman having two gain stages and a concertina phase-splitter.

I was looking to make it reasonably simple, and not have the usual problems of bias drift and runaway sometimes associated with OTL using the 6C33C

I built this before Christmas, a Stereo 'Breadboard', and its been used every day.

Sounds wonderful and has been utterly reliable. At a guess, its done around 400 hours.
(Yup, I like to TEST my breadboards!)

I was surprised when I built it, that the 'worst DC offset' with my most mis-matched 6C33 was only 0.25 volt with an 8 ohm load...

With a reasonably matched pair this is on average 20mV!

It has two 6C33 per channel, and all heaters series wired and running off the O/P +B transformer AC side with an additional 'underwind' to take the 55V to 49-50....

I'm only running the O/P stage with plus/minus 100 V supplies as I'm using a common or garden 500VA 0-55 0-55 toroid with a couple of extra turns added to the existing windings.

Ive not checked the actual O/P power, but reckon on around 15W ish. Not a lot, but I have a pretty small listening room....

[banat]

Congratulations Man
I`m not the big fun of Futterman/Inverted Futterman Amp topology,anyway......
You done Nice variation of `Inverted Futterman` OTL Amp topology.
Bet you that this your small ~15 w OTL Amps sounds 1000 times better than any~ 15w WOPT or SS based Amps

With only ~18w idling per each 6c33c plate ,this output tubes will last for really long time.
With your Autobias configuration for each 6c33c is `sacrificed` maximum available output peak power of Amps performance but on the other hand actually rises the Amps reliability for sure.
Just wonder, since you are not show the PSU schematic:did the transformer CT secondary of B+and B-(+- 100V)goes to the ground or is `floating`?

Best Regards

[Alastair E]

I'm pretty sure the same idea could be used for any OTL topology, certainly a circlotron could use it...

Yes, it may have the drawback of a lower power O/P but the benefits IMO far outweigh the loss...

After all, How often do you use full power?

The plate-diss currently is 14W, --100V Minus whats dropped over the cathode-resistor (at the moment, 18V) so that makes around 14W ish...

It definately runs cooler than other designs Ive made.

The PSU for the O/P stage ground is connected to main ground of the amp. Its not floating. Just a pretty simple plus/minus grounded supply..

I would like ideally to raise the supplies to the O/P stage to around 140V and trim the cathode-resistors to give 180-190 mA keeping the P-diss down to around 20-25W
This will give more headroom, although when playing as it is, there's no apparent issues even when played loud.
--It just quietly and gracefully goes into clipping at the limits of its power.

--Looks like I'll have to add some more turns to the Toroid!

Initially when I built it, I used quite large coupling-caps to the G1 of 6C33, (10uF) but this gave me blocking/pumping effects, when payed at full output --on certain tracks only though!

I tried 0.22 and eliminated that effect, with little if no loss in LF performance in listening tests.

[banat]

Since 6c33c tube generaly is not suited well for `deep` A2/AB2 OTL operation(like 6h13c) this 6c33c tube autobias is very good for lower eficiency A1/AB1 Amp operation.It is posible to use some sort of dual mode bias-(autobias + fixedbias) with DC coupled CF drivers to,for improved OTL output stage power efeciency.
And Yes,pure autobias like you did cood be good for 6c33c OTL Circlotron to.Of course than that topology includes grids coupling capacitors,with all sonics(coupling capacitors) drawback of that design aproach.
Agreed with you: OTL Amps usualy dont run for extreme output power but for Best of the Best Amps sonics !
30 W(150V/0,2 A) for 6c33c plate steady state bias is Apsolute Maximum for optimum tube performance and longest tube live(My expirience),since your Amps run far from this plate Maximum
dont wory,they well live amazing long,just check the tube heater pins & sockets pins contacts for burn out (corosion) sometime.

Best Regards

[Alastair E]

Banat,--

You have some very valid points. I initially did think of a combined 'dual-bias', using a combination of auto and fixed, but I was looking to make a Really simple, no nonsense amp,--Maybe the sort of thing a less experienced DIY'er could try if he's interested in OTL, Something nice and basic that works well and a pretty simple PSU...

I think there's more to be gained from the basic design as it is, with a somewhat higher +B supplies to the 6C33's but still retaining the 'reliability' of the autobias format
--I really should try this and do some proper power measurements, Getting the time these days is the problem!

Currently, this is in bread-board format, and needs work on the PSU, currently, I have just a little hum if you listen right at the speakers.--a 'fault' of the topology, and too little smoothing.

Over the years, Ive esperienced the issues of poor heater-pin contact with certain sockets, in fact I have two different types on this current amp.

One Russian white square ceramic type, --which has had a very hard life in other projects in the past, and I'm using three of the brown phenolic type sockets.
These brown ones grip the pins Very firmly, its Much harder to remove valve from this type socket than the ceramic type....
--The top of the socket in contact with the valve has gone black, but the pin contact is still very firm

Ive had to tighten the ceramic type a few times over the years (in other projects), and one of its heater connections had gone very loose on the rivet...

Heat and time will tell how the phenolic type last!

I should have added, that the 6C33C-B I'm using are well burned in and pretty well seasoned, they probably had about 100 hours on them at the time I made this amp.
I believe Burning in of these valves is Mega important....

[banat]

You are 100% right about this square ceramic Russian 6c33c tube socket, with fork type contacts.I seen everybody use them as original,(Profesionals & DIY) for 6c33c OTL aplication,but this ceramic sockets is not just reliable as Original 6c33c brown type (phenolic),but ceramic one is just OK for 6c41c tube. Example:
If Amps expirience some accidental mechanical shock,lets say during the Amps move from one room to other, 6c33c tubes will fall down from their sockets and broke

With this brown type Russian socket this is simple imposible,+ elecritcal & thermal contact with tube pins is superior !
About your autobias OTL amps again :Some electrical measurments have to be done(in one of future day),until than Enjoy
Finaly Only what is count on the end is the final judgement of Your Ears,nothing else !
Best Regards
Ps: Mr Karsten(Atmasphere) explained somewhere on this DIY site one very good method for 6c33c tube precondition,which is prety important for future normal OTL Amp operation.

[DouglasSelf]

You might be interested to know that I have just opened a Wireless World archive page on my website, and the first item is a 1957 article on transformerless valve amplifiers.

The Wireless World Archive

I hope that's useful.

[banat]

Thanks Mr Douglas,it is very old OTL related article but is very interesting article to.

[banat]

About 6c33c-b tube socket again,there is picture of four Russian 6c33c-b socket type.
Type 1,2 and 3 is just OK, type 4 is with fork type contacts,not bad but previous three types is much better and reliable.

[banat]

Type 4