http://www.montagar.com/~patj/pc_speaker.htm

Here is a page with a schematic for a one tube PC speaker amp. It uses
a 6.3VAC fillament tranny as the output tranny! I wish i could just cut
and paste the bullet list, but it's a .gif instead of text. The gist of
the gif is this: The turns ratio is 17.5:1; square that and you get
305:1 which gives you the desired 2000 ohms at the plate with 6.6 ohms
on the speaker side of the tranny.

The tube used is a 70L7 wired for 1.8 watts.

"Karl" a écrit dans le message de news:
...
http://www.montagar.com/~patj/pc_speaker.htm

Here is a page with a schematic for a one tube PC speaker amp. It uses
a 6.3VAC fillament tranny as the output tranny! I wish i could just cut
and paste the bullet list, but it's a .gif instead of text. The gist of
the gif is this: The turns ratio is 17.5:1; square that and you get
305:1 which gives you the desired 2000 ohms at the plate with 6.6 ohms
on the speaker side of the tranny.

The tube used is a 70L7 wired for 1.8 watts.

Not too bad with such low B+.
Try to obtain a 230 / 12.6 tranny, so you will have the same turn and Z
ratio and stay far away from core saturation.
Anyway adding a gap looks uncontournable !
Voids using of a toroïd here.

If, as suggested in text, you plan to go in AB2, you should use Zener
biasing for the 70L7.

A simple week end project. Have fun.

Yves.

Yves wrote:

"Karl" a =E9crit dans le message de news:
...
http://www.montagar.com/~patj/pc_speaker.htm

Here is a page with a schematic for a one tube PC speaker amp. It use=
s
a 6.3VAC fillament tranny as the output tranny! I wish i could just c=
ut
and paste the bullet list, but it's a .gif instead of text. The gist =
of
the gif is this: The turns ratio is 17.5:1; square that and you get
305:1 which gives you the desired 2000 ohms at the plate with 6.6 ohm=
s
on the speaker side of the tranny.

The tube used is a 70L7 wired for 1.8 watts.

Not too bad with such low B+.
Try to obtain a 230 / 12.6 tranny, so you will have the same turn and Z=

ratio and stay far away from core saturation.
Anyway adding a gap looks uncontournable !
Voids using of a toro=EFd here.

If, as suggested in text, you plan to go in AB2, you should use Zener
biasing for the 70L7.

A simple week end project. Have fun.

Yves.

Since an input transformer is used, some NFB could be taken from the 70L7=
GT
plate to the bottom end of the input transformer to improve the audio
performance if you like. But you might need more filtering of the B+ supp=
ly
since that kind of NFB will make the amplifier more susceptable to PS hum=
=2E A
small choke would do the extra filtering task well.

Cheers, John Stewart

John Stewart wrote:

Yves wrote:

"Karl" a =E9crit dans le message de news:
...
http://www.montagar.com/~patj/pc_speaker.htm

Here is a page with a schematic for a one tube PC speaker amp. It u=
ses
a 6.3VAC fillament tranny as the output tranny! I wish i could just=
cut
and paste the bullet list, but it's a .gif instead of text. The gis=
t of
the gif is this: The turns ratio is 17.5:1; square that and you get=

305:1 which gives you the desired 2000 ohms at the plate with 6.6 o=
hms
on the speaker side of the tranny.

The tube used is a 70L7 wired for 1.8 watts.

Not too bad with such low B+.
Try to obtain a 230 / 12.6 tranny, so you will have the same turn and=
Z
ratio and stay far away from core saturation.
Anyway adding a gap looks uncontournable !
Voids using of a toro=EFd here.

If, as suggested in text, you plan to go in AB2, you should use Zener=

biasing for the 70L7.

A simple week end project. Have fun.

Yves.

Since an input transformer is used, some NFB could be taken from the 70=
L7GT
plate to the bottom end of the input transformer to improve the audio
performance if you like. But you might need more filtering of the B+ su=
pply
since that kind of NFB will make the amplifier more susceptable to PS h=
um. A
small choke would do the extra filtering task well.

Cheers, John Stewart

Another thought here. If you don't already have a 70L7GT a good alternati=
ve
would be one of the similar 117 volt tubes such as the 117P7GT. That way =
you can
avoid the heater series dropping resistor. Cost for any of the 117 volt t=
ubes is
about the same as the 70L7GT.

Real Class A2 operation is not possible with the standard pentode SE hook=
up as
we know it. Reason is, that with normal bias, the +ve going voltage excur=
sion on
the plate loadline could not be made as long as the -ve voltage excursion=
since
the tube runs way into cutoff at larger signal levels. You need to also a=
djust
to a steeper loadline for Class A2 operation, otherwise there is no incre=
ase in
power output. A steeper loadline means a lower impedance reflected to the=
power
amp plate.

However, Class A2 in possible with PP pentode & Beam tubes. Some are desi=
gned
from the ground up for that mode of operation. Typical is the 2 volt batt=
ery
type 1E7G double pentode. In the correct bias condition the 6F6/6F6GT cou=
ld also
be run in PP Class A2.

Cheers, John Stewart