small and cheap OTL schematic needed

[neazoi]

hello,
I was wondering if there are any OTL amplifiers using small tubes. I have found one which I attatch but I wonder if there are any other out there.
thanks

[TheGimp]

Grid and plate are swapped in the output of that schematic.

You also have a strong possibility of dc bias thorugh the speaker.

The only other OTL i've seen discussed lately was based on 6AS7, which while a dual triode does not qualify as a small tube.

Actually, there are several errors in the schematic. bias reisstors for one tube not ten in the output stage. 700V supply for triodes designed for 250V max, -213 V on the grid (currently shown as plate) of output with -13 on the cathode ....

Hopefully someone else can point you to a schematic of an amp that meets your requirements, and works.

[cnpope]

The circuit looks quite strange, but actually it is what is intended. The design, by Steve Bench, runs the output tubes in "inverted triode" mode. More details can be found at Inverted Tube Operation, and other articles by Steve Bench at Steve's Tube Pages. I think this is a mirror of his original, now defunct, website. He seems to like exploring slightly unusual circuit designs!

Chris

[Andrewbee]

12B4 Amps12B4 Amps

[Sch3mat1c]

Simple answer: either use a metric sh**load of tubes in parallel, or forget it. OTL is a fundamentally flawed concept around vacuum tubes. The impedances just aren't right.

If you insist, I recall there are some Russian types with very low plate resistance, analogous to 1/2 6AS7 in a 9-pin bottle. I don't remember the number.

Tim

[danzup]

Originally Posted by Sch3mat1c

Simple answer: either use a metric sh**load of tubes in parallel, or forget it. OTL is a fundamentally flawed concept around vacuum tubes. The impedances just aren't right.

If you insist, I recall there are some Russian types with very low plate resistance, analogous to 1/2 6AS7 in a 9-pin bottle. I don't remember the number.

Tim

6H13C ( with europeen letter is 6N13S ) but this is in 8 pins socket
direct equivalent of 6AS7G
of course , but you need a power plant to lite up 10 pieces of that tube for a decent low output impedance !

[danzup]

Originally Posted by TheGimp

Grid and plate are swapped in the output of that schematic.

You also have a strong possibility of dc bias thorugh the speaker.

The only other OTL i've seen discussed lately was based on 6AS7, which while a dual triode does not qualify as a small tube.

Actually, there are several errors in the schematic. bias reisstors for one tube not ten in the output stage. 700V supply for triodes designed for 250V max, -213 V on the grid (currently shown as plate) of output with -13 on the cathode ....

Hopefully someone else can point you to a schematic of an amp that meets your requirements, and works.

As posted by fellow user that schematics really work !
But no need to say that are lot of other OTL schematics better that that :
- circlotron OTL
- futterman OTL
- technics OTL variant of futterman
- ciufolli OTL variant with 6C33C
- atmasphere variant (Nice AT M60 schematics will you find on this forum !) with 6H13C
- and of course old Philips OTL with capacitor on output (like an SRPP but with pentode )

[Andrewbee]

if you insist, i recall there are some russian types with very low plate resistance, analogous to 1/2 6as7 in a 9-pin bottle. I don't remember the number.

6c19

[danzup]

Originally Posted by Andrewbee

6c19

Yes that is in 9pin glass bottle .

[cnpope]

An example of a more conventional simple OTL is the design by Tim Mellow, which appeared in the February issue of Audio Express. It's 25W per channel. Admittedly the output tubes (a pair of 6C33C's per channel) could hardly be called "small" (unless you compare them to transmitting tubes or something like that!), but the design is about as simple as they get. I have built one, and it sounds great. Another nice design, which I've also built, is the one by Hans Beijner, again using 6C33C's. Both of these are essentially modified Futterman designs. I've built a circlotron too, using a design by Alan Kimmel, using EL509S tubes in the output. This one is a bit more complicated, because of the need for two separate floating output power supplies per channel.

Interestingly enough, it is really rather easy to get a low output impedance with an OTL, particularly with the use of negative feedback. A fraction of an ohm output impedance is quite routine for such amps.

Chris