Bias adjust HH Scott 299 amp
wi...@gnn.com
I have an HH Scott 299C and 299D which I think are super quality
amps. I would like to know the procedure for biasing the output
tubes. There are two RCA jacks labeled "bias points", a bias
adjust switch and bias adjust trimmers (2 on the 299C and 4 on the
299D).
Thanks
Mark
John Byrns
Mark,
I don't have the Service Bulletins for the 299C or the 299D, although I do
have the H.H. Scott Service Bulletins for the 208 power amplifier, and the
LK-72B kit. I believe all 4 amplifiers have virtually identical power
output stages, rated at 40 wpc. The 208 and LK-72B use the same power, and
output transformers, and I assume the 299C, and 299D also use these same
components. The main difference is that the 208 and the 299C have non
adjustable fixed bias supplies, while the LK-72B and the 299D have bias
adjust pots, and current sensing resistors in the cathode circuits of each
pair of 7591 output tubes. The following discussion is based on the LK-72B
Service Bulletin.
For both the 299C and 299D, the first step is to check that the rectifier
for the negative bias/heater supply is in good condition. These tend to
become weak with age, resulting in under biasing, and the slow death of the
7591 output tubes. You should expect something like -56 Volts at the first
filter cap on the output of the heater/bias rectifier. If the voltage is
low replace the rectifier stack with a silicon diode bridge from Radio
Shack.
Once you have the proper output from the negative bias/heater supply, you
will need to adjust the bias pots on the 299D. The 299C bias is fixed by
the resistors in the bias divider. The bias pots on the 299D should be
adjusted so that each pair of 7591's draws a total of 70 mA. The current
can be measured by removing the bottom cover, and measuring the Voltage
across the current sensing resistors in the cathode circuits of each pair
of 7591's. In the LK-72B these resistors are 3.3 Ohms, so there should be
0.23 Volts across each resistor. You will want to check to see what value
resistors the 299D uses, and if it is different, calculate the expected
Voltage drop for a current of 70 Ma. In any case you should measure the
actual value of the resistors, to see that they haven't drifted with age.
Before adjusting the bias, you should remove the phase inverter tubes,
6BL8/6U8. The two RCA jacks labeled "bias points", and the bias adjust
switch, are a hooky scheme that was designed so that kit builders without
even minimal test equipment, could adjust the bias. I would suggest
removing the existing circuitry associated with the switch and RCA jacks,
and connecting the wires coming from the two 7591 cathode current sense
resistors to these jacks. This will allow you to measure the current of
the 7591's without removing the bottom cover of the amplifier.
Once the bias is set to 70 mA for each pair of 7591's, you can adjust the
DC balance controls. This applies to both the 299C and 299D. To adjust
the DC balance, first remove the 6BL8/6U8 phase inverter tubes, then
connect a 16 Ohm resistor and a sensitive AC-VTVM or scope across the 16
Ohm output of the channel being adjusted. Adjust the DC balance control
for minimum 120 Hz hum on the output. The DC balance and bias adjustment
on the 299D may interact to some extent, so recheck the bias, and the DC
balance a couple of times. Replace the 6BL8/6U8 phase inverter tubs and
the job is done.
The bottom line is, the bias should be set so each pair of 7591's draws 70
mA.
The DC balance is adjusted for minimum 120Hz hum at the output, with the
phase inverter tubes removed.
Regards,
John Byrns