Hello everybody,

Next step this evening. I started to listen to the amp. after modifying
it following your advices.

Without GNFB : big hum, with GNFB hum (lot less of course). the gfb
loop seems to work.

I can hear a nice music with the hum (with a dummy speaker and a dummy
CD player.

Here is where we are now : http://cjoint.com/?emuwdkPsFE

I have questions for you :
- the transformer for filaments 6.3V has no center tap, so the
filaments are not grounded, is it a problem ?

- i have made the prototype on a piece of wood (a 50cm square), i have
tried to separe : heaters, ground, HT, signal. I don't think so much
hum can come from the path of the wires, can it ?

- if i raise the volume the hum increases with the music, strange ?

- when i took my measurements with my voltmeter (60Mohm internal
resistance), at the 60V point in the first 6sn7, the hum decrease.
what is the reason for that ?

- I cannot increase the 4mfd, because if i increase it the voltage is
too high. Maybe i can diminue the voltage by interting a power resistor
? (i have to buy it but i don't have idea of the value, power) And when
i tried 47mfd instead of 4mfd the hum was still there.

- i have burnt another 1.6A fuse... I have here 230V.
230*1.6 = more than 300w.
I happens when i plug it when the tubes are hot.
Maybe it is not a good fuse (slow fuse ?)
I now use 2.5A fuse, but it makes me think if i don't take risks.

Thank you,
Best regards,
Luc D.

said:

I have questions for you :
- the transformer for filaments 6.3V has no center tap, so the
filaments are not grounded, is it a problem ?


Yep. Connect 2 100 ohm/2 watt resistors in series over the 6.3 V
lines, and ground the junction.

--
Sander de Waal
" SOA of a KT88? Sufficient. "

"Sander deWaal" wrote in message
...
said:

I have questions for you :
- the transformer for filaments 6.3V has no center tap, so the
filaments are not grounded, is it a problem ?


Yep. Connect 2 100 ohm/2 watt resistors in series over the 6.3 V
lines, and ground the junction.

--
Sander de Waal
" SOA of a KT88? Sufficient. "

Do you mean in parallel across the 2 legs of the filament lines and gnd. the
junction of the 2 resistors?

west

Do you mean in parallel across the 2 legs of the filament lines and
gnd. the
junction of the 2 resistors?

west

I don't have 100ohm 2W, but i have 1000ohm 1/4w, is it ok ?

I smell troll .........

wrote in message
oups.com...


Do you mean in parallel across the 2 legs of the filament lines and
gnd. the
junction of the 2 resistors?

west

I don't have 100ohm 2W, but i have 1000ohm 1/4w, is it ok ?

a écrit dans le message de news:
...


Do you mean in parallel across the 2 legs of the filament lines and
gnd. the
junction of the 2 resistors?

west

I don't have 100ohm 2W, but i have 1000ohm 1/4w, is it ok ?

No, stay between 33 and 220 !
Temporary, just ground at least one side of the heather to ground, better
that nothing.

Yves

ok good, i will buy 100ohm 2W.
and at the moment i ground one side to see if it improves the hum
reduction.

Thank you.

Other topic :

I plan to but 50w 200 ohm reostat to reduce the HT (as i have too much
HT out of my main tranformer).
200ma*200 = 100V

So i can reduce the HT between 0 and 100v. (i plan to reduce by 60V)

Is there a reason to avoid doing this ?

Luc.

"west" said:

I have questions for you :
- the transformer for filaments 6.3V has no center tap, so the
filaments are not grounded, is it a problem ?

Yep. Connect 2 100 ohm/2 watt resistors in series over the 6.3 V
lines, and ground the junction.

Do you mean in parallel across the 2 legs of the filament lines and gnd. the
junction of the 2 resistors?


The resistors go in series, across the two heater lines with the
junction grounded.

Sloppy wording on my part, sorry.

--
Sander de Waal
" SOA of a KT88? Sufficient. "

said:

I plan to but 50w 200 ohm reostat to reduce the HT (as i have too much
HT out of my main tranformer).
200ma*200 = 100V

0.2 A X200V = 40 watts. A lot of heat to dissipate in a
resistor/rheostat.

You can use an additional transformer to reduce the AC HT voltage, per

http://www.ibiblio.org/kuphaldt/elec...s/AC/AC_9.html

Scroll down about halfway, look for the "bucking" config.

--
Sander de Waal
" SOA of a KT88? Sufficient. "