6L6GC AB2 Amp

[chrish]

In this thread Amp design suggestions Eli was suggesting a 6L6 AB2 design based on Mullard 5-20 topology with 12AT7 front end and ECC99 LTP and Mosfet source followers. Respecting the original thread, Eli asked to start this new one.

I am interested in progressing with this design, looking for any assistance

Cheers,

Chris

[DougL]

I am Assuming that AB2 implies triode operation.
AB implies fixed bias.

IIRC, the recommended operating point was 10 ma and 450V

My next step would be to pick the primary impedance for the output transformer.

After choosing an output Transformer, Next step is to determine the input voltage swing on the 6l6 for full output.

This nails down most of what we need to know about the output stage.

TUBECAD PP calculator may be very handy for both choices.

HTH

Doug

[SY]

AB2 can be triode or pentode. 10mA seems awfully light. I'd go with the tube datasheet recommendations for AB2 operating points as a start.

[Eli Duttman]

Chris has proposed Tamura F684 O/P trafos. If somebody has a link to an English data sheet, please provide it.

[N1ESE]

Originally posted by Eli Duttman
Chris has proposed Tamura F684 O/P trafos. If somebody has a link to an English data sheet, please provide it.

Not a full data sheet but this link has a lot of specs with dimensions:

http://www2.big.or.jp/~sunaudio/tamura/out1.html

[HollowState]

Originally posted by chrish
I am interested in progressing with this design, looking for any assistance

Hi Chris,
If you really want the efficiency of P-P class AB2, you will need a very good output transformer. One that is well designed and has very tight coupling between windings because crossover distortion could well become a problem as you leave AB1 and come closer to class B. AB2 is somewhere between these two. You'll be able to see it on a sine wave with an oscilloscope across the output of the amp as you adjust the bias more negative.

McIntosh ran their amps in AB2 but avoided crossover distortion through the use of a very complex bifiliar wound transformer with lots of feedback in the output stage, by way of the cathode windings, as well as to the driver stage and also global. Without a super-duper output transformer and plenty of feedback you may come to find that the sound is not so good compaired to AB1 which most transformers are rated for. It is always said that the O-T is the most important part of a tube amplifier. Well, that's even more so with AB2.

Victor

[N1ESE]

English and Japanese transformer catalog showing a little more info with some curves:

http://quarkconcept.free.fr/Tamura-C-1005-9.pdf

[chrish]

Thanks for the input so far...

I am not "set" on AB2, it is just that Eli suggested this as an interesting possible design on another thread.

I am just about at the end of my academic year with my university work (mature age student doing university degree and full time job) and will have a little time to work on audio projects when my final exams for the year are finished in a few days.

I have collected so far a pair of Tamura F-684 output transformers, so these will be the basis of the design. Specs are 6.6K primary, 0-4-8-16 Ohm secondary, 150mA x 2 max DC current, freq response 30-30K, 3rd 16 Ohm winding for NFB. Here is a link to the PDF Tamura output transformer data sheet

Eli linked here Eli suggestions to 'Mosfet follies' and Tubelab's 'Power Drive' circuits (sorry, in India with work and the crappy hotel internet connection here wont link to those sites) as interesting suggestions. Tubelab's experiments with power drive look promising. I am no engineer, but from my reading it looks like he gets good results, with lower distortion at lower power levels and the advantage of the ability to absorb high transient music loads without clipping (have I got this right?).

The output iron is a given, it has already been purchased, so the design will have to start from here. I have also:
Russian 6SN7
Russian 6SL7
6N1P-EV
6N2P-EV
6N6P-I (ECC99 equiv)
2 x Mullard built ECC81
4 x JAN (GE) 12AT7WC
8 x Russian 6P3S
8 x Russian 6P3S-E

2 x Aluminium chassis

Happy to buy other valves if required, just listing what I have if this helps. Have yet to purchase power transformers. Waiting to find/design a topology before this purchase.

BTW, I am not an engineer. Can solder and do chassis work, but will need gentle guidance with any design.

Thanks again, hope there are others interested in a project like this. I am looking forward to getting in to it!

Cheers,

Chris

[weinstro]

Originally posted by HollowState

If you really want the efficiency of P-P class AB2, you will need a very good output transformer. One that is well designed and has very tight coupling between windings because crossover distortion could well become a problem as you leave AB1 and come closer to class B. AB2 is somewhere between these two. You'll be able to see it on a sine wave with an oscilloscope across the output of the amp as you adjust the bias more negative.

McIntosh ran their amps in AB2 but avoided crossover distortion through the use of a very complex bifiliar wound transformer with lots of feedback in the output stage, by way of the cathode windings, as well as to the driver stage and also global. Without a super-duper output transformer and plenty of feedback you may come to find that the sound is not so good compaired to AB1 which most transformers are rated for. It is always said that the O-T is the most important part of a tube amplifier. Well, that's even more so with AB2.

What about something like a Crowhurst twin-coupled configuration? That design proposes simple output transformers, as feedback is taken from the primary side of the OPT.

In my infinite spare time, I've been given thought to this topology, but with something like tubelab's PowerDrive to support AB2. I don't have an approach yet on how to adapt the design to PP, however.

Also, a question on the post above. I thought AB2 was when the tube's grid is driven positive. I had read that when this happens, the grid impedance suddenly drops, and unless the driving stage is designed with adequate capabilities, gird current starvation can result. Is there something else about AB2 mode that results in worse crossover distortion than plain old AB?

Thanks!

[N1ESE]

Originally posted by weinstro
Also, a question on the post above. I thought AB2 was when the tube's grid is driven positive. I had read that when this happens, the grid impedance suddenly drops, and unless the driving stage is designed with adequate capabilities, gird current starvation can result. Is there something else about AB2 mode that results in worse crossover distortion than plain old AB?

The '2' means that the grids ARE driven positive and grid current will flow. An output stage in which there is grid current such as AB2 or B must be transformer coupled instead of R-C coupled. If R-C coupling IS used the grid behaves like the plate of a diode and rectifies some of the audio signal which makes the grids more negative. Instead of class AB2 you would have a kind of moving class AB1 and a lot more distortion than you had counted on. If you were to attempt class B with R-C coupling, the extra grid bias could actually bias the tubes beyond cutoff and lead to heavy distortion. The rise in average, DC, plate current may be anywhere from 50 % to 200 %.