Simplest 6080 or 6AS7 OTL Project?

[Michael Rothacher]

Tube Gurus:

What's a good and simple OTL using 6080's or 6AS7's that would be suitable for a first timer?

[fdegrove]

Hi,

If you don't need more than 25W into 8 Ohm i can recommend this design by Bruce Rozenblit:

6AS7G OTL

Cheers,

[ThorstenL]

Konnichiwa,

Originally posted by mrothacher
What's a good and simple OTL using 6080's or 6AS7's that would be suitable for a first timer?

I would not think a OTL is AT ALL suitable for a first timer. What you could try is to build a reasonable WOT (With Output Transformer) Amp, using 6AS7 or not (that's another issue).

Now, how much power are you looking for?

Sayonara

[Michael Rothacher]

Thank you both! I'd be satisfied with 5 - 30 Watts. I'm very experienced with solid state, and I've built tube kits, but this would be my first scratch-built tube project.

Your input is much appreciated.

Mike

P.S. I already have a fair stock of 6080's I'd like to put to use, but since it's my first scratch built amp, I've been looking for something as simple as possible.

[ThorstenL]

Konnichiwa,

Originally posted by mrothacher
Thank you both! I'd be satisfied with 5 - 30 Watts. I'm very experienced with solid state, and I've built tube kits, but this would be my first scratch-built tube project.

A simple and inexpensive solution would be to build Menno Van der Veens "Maurits".

http://www.plitron.com/PDF/PB/Article/Atcl_13.pdf

The advantage is that due to it's fairly low internal impedance you can actually get away in the case of the 6AS7/6080 with using a normal torroidal Mains Transformer (115V+115V : 6V+6V with the 6V windings parallel will give a 8KCT:6R Transformer) witout running into saturation or other troubles.

Torriods by inheret design have rather low leakage inductances, which helps the high frequency response, the low impedance of the Output Valves helps with the low end, you cannot misuse EI mains transformers that way, BTW.

The powersupply could be improvised from a 115V+115V : 115V+115V mains insulation transformer with a few added turns for the heaters and also a few for the HT to be added. A nice 330VA or bigger mains insulation torroid would fit the bill. You can usually find neat looking diecast metal enclosures to enclose your Tooridal Output and mains transformer, giving the Amp a clean and simple appearance.

If you slightly re-adjust the output stage operation (based on the Svetlana Datasheet) you need around 370V +B (can be gotten with a little extra work from a 230V AC voltage and a few turns added by hand), make the common cathode resistor around 750 Ohm, use a 50VA or larger 115V+115V : 9V+9V as 5K-CT : 8 Output Transformer and you will have around 6 - 10W RMS available from this modifed "Cheapo Maurits".

This should give you a simple and inexpensive start to schratch building. After that you can consider how you feel about a more complex Project, with many 6AS7 Output valves (you need a lot to get the output impedance in OTL operation to anything sensible) and you will be better able to judge if you actually want to do that kind of thing or not.

Anyway, this would be my take on a simple "beginners amp", that could be build for very little money.

Sayonara

[Michael Rothacher]

Thanks! That looks like a good place to start.

[alexg]

Originally posted by mrothacher
Thanks! That looks like a good place to start.

Did you ever built this amp?

How does it sound?

Following the success of my parafeed 6080 amp using toroidal mains transformer for OPT, I want to do a PP version.

Thanks.

[Steve Cresswell]

Hi Alex

A 6AS7 or 6080 pp is a superb sounding amp. I've attached a schematic of my own take on the 6AS7 pp idea.

I use low current drive in my amp and love the sound it makes. Others might prefer higher current drivers but either way the 6AS7 can be made into a stunning sounding push-pull amp.

Mine uses differential amps with constant current sinks in the tails of both the driver and power stages. The driver uses a simple LM317 as a sink with an STC 12E14 regulator tetrode as the sink for the output stage.

It is a work-in-progress and I'm thinking about reducing the number of stages to 2 by putting a 6SL7 or similar as a driver and removing the first stage. By doing this I'm trying to take full advantage of the low Miller capacitance of the 6AS7 by using a high gain low current driver. That's for the new year.

As it is though the amp sounds simply beautiful.

On another thread Shoog has done the opposite to me and used high currents and interstage transformer coupling. Also, his constant current sinking arrangements are a bit more sophisticated than mine.

Both approaches are equally valid and will produce an amp that sounds superb. I can guarantee you won't be disappointed whichever option you choose.

Steve.

[Jeb-D.]

An OTL from scratch would be very difficult for a first time tube builder.
You may want to consider an OTL headphone amp. Or a kit from http://www.transcendentsound.com/ The prices are very reasonable, it would honestly probably cost you more to build one of equal quality from scratch using a schematic for reference. I know it's not 6as7 based, but the T-16 stereo kit looks to fit your requirements.

[Jaime]

Steve Cresswell
I believe you can go for an 600:10k+10k input trans and reducing the number of stages to 2.
It is posible?
Best regards Jaime