Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss

Pre-amplifier

0 views
Skip to first unread message

LMMP

unread,
Jun 8, 2002, 5:28:27 AM6/8/02
to
Hi,

Yesterday I fired up for the first time my home-made pre-amp. It is an EF86
based design, similar to early Quad's and Leak's, with feedback in the first
tube.

The PSU is regulated. It is tube rectified (GZ34) and the pass tube was
first a 6L6GC, with a 12AX7 as error amplifier. But it did not work in this
configuration because the output voltage was only about 60 V on the cathode
of the 6L6. The anode was 330V and in the grid over 300V too, but when I
turned it on, the 6L6 started to become blue just under the electrodes and
then there was a lightening inside it. I took it off and replaced it with a
EL34 and the PSU worked just fine. Any ideas of what can be wrong with the
6L6GC?

I also have a lot of hum from the pre amp circuit. In fact I could only play
it at low volume because of the hum.I think all earth connection are right.
I played around the input signal wire and could hear some hum variation, but
could not get rid of it. On the other hand, the EF86 datasheet says that
this tube has a noise of a couple of microvolts when properly installed.
What can I do here?

Best regards

Miguel Pedroso

Paul D. Spiegel

unread,
Jun 11, 2002, 2:46:11 AM6/11/02
to
LMMP <mig...@eq.uc.pt> wrote:

> I also have a lot of hum from the pre amp circuit. In fact I could only play
> it at low volume because of the hum.I think all earth connection are right.
> I played around the input signal wire and could hear some hum variation, but
> could not get rid of it. On the other hand, the EF86 datasheet says that
> this tube has a noise of a couple of microvolts when properly installed.
> What can I do here?

If your heater supply is AC, converting to DC may help. You could
also shield the input wire right up to the tube pin.

LMMP

unread,
Jun 11, 2002, 5:28:41 AM6/11/02
to
The heater supply is DC.
I saw that if I disconnect it from the source (CD player or turntable) the
hum is almost gone. So I think that the proble may be some discrepancy with
the earth connection of the pre amplifier and of the sources. What can be
done?

Best regards

Miguel Pedroso


"Paul D. Spiegel" <p...@ktb.net> escreveu na mensagem
news:ae46bj$oun$2...@dns.ktb.net...

Doug Schultz

unread,
Jun 11, 2002, 11:46:58 AM6/11/02
to
Try Running a ground wire from one chassis to the other so that your ground
isnt made through the audio cables.

"LMMP" <mig...@eq.uc.pt> wrote in message
news:ae4frm$qj8$1...@rena.mat.uc.pt...

0 new messages