Help Needed with BIAS & BALANCE Adjustments on a SUNN 300T

[dc10bass]

I would like to get some input from anyone that has done this before...

[The Following is copied from the 300T manual]

Page 10:
300T Output Tube Bias Indicators and Controls

Output Tube Bias Adjustment Instructions
1. Turn on your 300T and let it warm up for at least two
minutes with the STANDBY switch in the ON position. Make
sure the amplifier is properly connected to a speaker.

2. Remove the bias controls cover box.

3. With a digital voltmeter set to its most sensitive DC
voltage scale, measure the voltage between the BIAS
100mV test points (test points 1&2) and adjust the BIAS
control for a reading of .1 VDC (100 mVDC).

4. Next, connect the digital voltmeter to the BALANCE 0V
test points (test points 2&3) and adjust the BALANCE control
for a reading of 0 VDC.

5. Replace the bias controls cover box.

Notes
1. Make bias and balance adjustments ONLY when
necessary. If you are unsure about setting bias and
balance, take your amp to the nearest authorized Fender
Service Center.
2. For optimum sonic performance, set bias to 100mVDC
then set balance.
3. If the output tubes can not be biased to at least
80mVDC, replace the 6550C output tubes.
4. If the output tubes can not be balanced, check the bias
setting and/or replace the 6550C output tubes.
5. The power tube trouble LEDs are NOT indicators of worn
tubes. If your tubes are worn and sound weak the LEDs
may still glow green. The trouble LEDs turn RED only after
tube failure has already occurred. DO NOT WAIT FOR
TUBE FAILURE TO REPLACE TUBES. (See Tube
Replacement Section, page 11).


....Now here is my issues;
I was able to set the BIAS at the requested .1 VDC (100mVDC)
...But when I was trying to set the BALANCE, I could not get it down to 0 VDC.
The lowest I could get the BALANCE to was (-57.0mVDC)

...Am I using the right setting to do this on my Multimeter?
I have 5 settings I can use under VDC;
600
200
20
2000m
200m

I have it set to 200m. (is this correct?)

Thanks you for your help!!!
Curt

[mckinnon audio]

  Hi there,yes your meter is on the right range,200mv.If the tubes won't balance,refer to Note #4,check bias and/or replace tubes.There's a similar set up on SVT's,older ones for sure.Good luck,Mel.

[loudthud]

200mV is the correct range on your DVM.

I suspect you have a bad tube or some other fault in the amp that prevents one tube from conducting. The amp is setup so the "bias control" biases one side and the "balance control" biases the other side. To measure the bias on the second side connect the DVM from J9 to J11. Can't tell from your description if you should read 157mV or 63mV. If everything was working the "balance control" should be able to set it to 100mV just like the bias control sets the voltage from J9 to J10.

Tubes V5, V7 and V9 are on one side and the even numbers are on the other. Swap the tubes around, try to re-bias, and you should be able to move the fault to the "bias control" side if the problem is a bad tube. Otherwise the problem is inside the amp and should be refered to a tech for service.