Gentlemen,

because of personal reasons I have been away
from RAT for some months.

Meanwhile, my first tubed PP amp DIY project
made good progress. Here are some links:

Amp schematic: http://www.ndh.net/home/schlangen/ro...p_amp_full.gif
PSU schematic: http://www.ndh.net/home/schlangen/ro...p_psu_full.gif

My RAT's nest :-) on the breadboard:

http://www.ndh.net/home/schlangen/ro...eadboard03.jpg
http://www.ndh.net/home/schlangen/ro...eadboard04.jpg

Most problems I experienced are solved by know (thanks
to lots of helpful RATs!) and this thing already sounds
rather promising to my ears at not that easy to drive
speakers (Isophon Avalon; D'Appolito with bandpass woofers).

With 20dB of global NFB, the sensitivity is 0,3 Vrms
for full output, which is ca. 12 watts/ch. Yes I know
that there are less complicated ways of getting two
dozen watts from some tubes ;-) but this project serves
me very well in exploring PP technology.

Please feel free to make suggestions how I could further
optimize the circuit shown, but please keep in mind that
I will not make big changes to the topology anymore,
like using other tube types, other PI principles, and so on.

Thanks alot for your attention.

Last but not least Merry Xmas & Great New Year!

Tom

--
fnord (now you see it, now you don't)
- R.A. Wilson / FZ

"Tom Schlangen" wrote in message
...
Gentlemen,

Please feel free to make suggestions how I could further
optimize the circuit shown, but please keep in mind that
I will not make big changes to the topology anymore,
like using other tube types, other PI principles, and so on.

Thanks alot for your attention.

Last but not least Merry Xmas & Great New Year!

Tom

--
fnord (now you see it, now you don't)
- R.A. Wilson / FZ

Looks good, how about adjustable AC balance for the driver tubes by
replacing R105 and 107 with a potentiometer. This way you can balance the
drive to the output tubes and minimise the amount of feedback you need. You
can scope the "open loop" output and adjust the balance to least
distortion...

Also those caps to the output tubes look quite small, what is their rolloff
freq?

Hi Henry,

Looks good, how about adjustable AC balance
for the driver tubes by replacing R105 and
107 with a potentiometer.

First I built the LTP without a CCS tail,
using a simple resistor instead. Different
plate resistors (or a trimmer, as you suggest)
to match the different gains were mandatory
with this setup, indeed.

Still, this first setup didn't deliver enough
undistorted voltage swing from the AC-grounded
section.

Adding the CCS fixed all these problems, more
than enough undistorted voltage swing and
excellent symmetry (per section and between them)
now.

As I understand, with the CCS tail, the gains of
the sections of the diff amp are set only by
the precision of the plate resistors, or am I
wrong?

Also those caps to the output tubes look quite
small, what is their rolloff freq?

Time constant is ~0,02 seconds, which is a
little bit limiting on the low frequencies,
indeed, but I consider it okay with a "little"
amp. But I will follow your suggestion and
change to 0,05 seconds and remeasure things.

Tnx2u for your input!

Tom

--
When in doubt, use brute force.
- Ken Thompson