Gallien Krueger RB400 Bass Amp Adjusting the Bias Trim Pot?

[TAC57]

I've replaced the power transistors on a Gallien Krueger RB400 Bass Amp and was looking for pointers on adjusting the bias trim pot.

 

[JMFahey]

In principle, do not mess with it.

Measure voltage drop across emitter ballast resistors, post it here, also resistor value, so we can caculate current.

I guess 20-30 mA per transistor would be fine, adjustment could be needed if , say, <2-3 mA (including zero) or higher than, say, 70-100mA

 

[llwhtt]

There should be a 2-pin header on the board, those are the bias test points, connect your meter to them and adjust for 5mv.

Craig

 

[TAC57]

@JMFahey I read 986 mV DC across R26 and 1.028 mV DC across R27. With the 0.33 ohm resistors that would give me 2.99 and 3.11 mA.

I assume messing with R14 would allow me to increase these values.

FYI this amp was dead and I traced it down to Q12 being bad was which was a MJ15002. I did replace the two MJ15002s (Q11 & Q12) with MJ15004s, and both MJ15001 (Q8 & Q9) transistors with MJ15003s.

Feeding a 500 mV sine wave into the input the amps seems to work fine. Is there any components I should deal with before returning the amp to it's owner. I've read I should change a bunch of transistors. I do have some MPSA06 and MPSA56 transistors, but MJ15003 and MJ15004 transistors are on back order.

What are your thoughts on pulling the 8 power supply caps and replacing them with two 2,200uF / 63V caps?

 

[llwhtt]

That's an older version with TO-3s, disregard the 2 pin header test points. I'll have to find my GK Service Manual CD and let you know. Have you tried contacting GK, they're usually pretty helpful.

Craig

 

[TAC57]

Here is a picture of the PC board for my amp.

One thing I find interesting is if I plug my speaker into the 'direct out' the sound is very clean with no 'pop' when I power on the unit. If I plug into the 'SPEAKER' output it pops when I turn on the unit and there some buzz in the back ground. In both cases I'm feeding the amp with a 200 Hz 500 mV sine wave

 

[TAC57]

Can anyone tell the vintage of this amp from the picture?

S/N is 68470, and below the s/n 3 - 88,

 

[TAC57]

@JMFahey I adjusted R14 to get the voltages across the bias 0.33 ohm bias resistors (R27 & R27) to 5 mV DC which is 15mA.

The owner of this amp is picking it up tomorrow so I hope this is OK. That's what it said to set it to in the service manual.