HELP: Scott 299 Bias
Yong-Chul Shin
Please let me know if someone has a manual or knows how to adjust
biases on H.H. Scott Type 299 Stereomaster.
Currently, the right channel output transformer is much
warmer than the other. The unit has no modification made and uses
original tubes (4 x 7189 and GZ34 as a rectifier).
Thanks.
Yong-Chul Shin
--------------
CEDAR, SUNY at Buffalo
John Byrns
In article <5k2o9d$a...@sadachbia.cedar.buffalo.edu>, Yong-Chul Shin
<ycs...@cedar.Buffalo.EDU> wrote:
> Please let me know if someone has a manual or knows how to adjust
> biases on H.H. Scott Type 299 Stereomaster.
>
> Currently, the right channel output transformer is much
> warmer than the other. The unit has no modification made and uses
> original tubes (4 x 7189 and GZ34 as a rectifier).
Hi,
I would refer you to my H.H.Scott web page, which covers this subject, but
my ISP has gone belly up, so the web page is out of commission until I
find a new ISP that supports web pages. My email won't work either, but
as soon as I get time to configure an email client for this ISP, I will
have email again, at a new address.
When you ask about the 299, I will assume you are talking about the
original H.H. Scott Type 299, rather than a 299B, 299C, or 299D. The 299,
299B, and their offspring, the LK-48, LK-48B, 222C, and 222D all use 7189
output tubes, while the 299C, and 299D use 7591 output tubes, and are a
different animal. I don't have the bias information on the original 299,
but assume it would be similar to the 299B. On the 299B H.H.Scott said to
set the bias controls for 55 mA of cathode current per pair of output
tubes. In the 299B and later amps, there is a resistor, normally shorted
by a link, in the cathode circuit of each pair of output tubes, to measure
the current. I don't know if the 299 has this feature or not. The DC
balance controls should be set for minimum hum in the output, with the
driver/phase inverter tubes removed. You can set the AC balance control
for minimum distortion, or for more even order distortion, to give that
nice warm single ended sound :)
On the later Scott amplifiers using 7189 output tubes, the LK-48, LK-48B,
and 222D, H.H.Scott recommended biasing each pair of output tubes for 44
mA of total cathode current at idle. Using this setting on your original
299 should help prolong tube and component life. The bias on the 222C was
not adjustable. If you have one of the higher power 7591 equipped
amplifiers, Scott recommended that they be set for 70 mA of cathode
current per pair of tubes. Again, less will help prolong the life of the
very expensive and hard to find 7591's.
Looking at your post again, I see that your amplifier uses 7189 output
tubes, and so is either a 299, or 299B. Also if it is not modified, you
should check or replace the coupling capacitors between the phase
inverter, and output tubes, and also check the bias/preamp heater
rectifier. The bias rectifier becomes weak with age, and the coupling
capacitors start to leak, both conditions can make it impossible to set
the correct bias.
Regards,
John Byrns
AMCMATADOR
Hi,
I own a Scott 299 and manual ( purchased from orig.owner) And can sell
coppies
Sinceerely,
Darren