Krell KSA 50 PCB

neychi said:
Hi!

I finaly finished my Krell, but i have a problem. I can't adjust bias across emiter resistors. I have zero V and trimpot does not working on boat channels. DC offset was no problem to set, and I even listen to some music and sounds good, but bias adjustment doesnt working. I have Jan's boards with corrected traces, 2sc3955 for bias transistor. Any idea?

Thanks

Dean

There was some mistakes on the PCB, did you follow the instructions for correcting it?
 
neychi said:
Hi!

I finaly finished my Krell, but i have a problem. I can't adjust bias across emiter resistors. I have zero V and trimpot does not working on boat channels. DC offset was no problem to set, and I even listen to some music and sounds good, but bias adjustment doesnt working. I have Jan's boards with corrected traces, 2sc3955 for bias transistor. Any idea?

Thanks

Dean

There was some mistakes on the PCB, did you follow the instructions for correcting it?
 
Transistor Pin-out
 

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EDIT - I had Pinkmouse boards, so my response below may not apply...

I had a similar problem... read here

here

Turns out my output base resistors (R501 - R503 and R507 - R509 on PM board) were 2.2K ohms and not required 2.2 ohms.

Also make sure the signal ground is connected correctly - had an issue here too that made it difficult to set DC offset and Bias...
 
neychi said:
@Nico, yes I measured voltage on emitter resistor, between transistor emitter and speaker output (+)

@Andrew, 0.3V cca

You say that it plays music okay but you cannot adjust bias. What voltage are you measuring over the emitter resistor. I am thinking maybe that the Vbe multiplier is open circuit.

If this is the case then the amplifier will still operate but in class B or maybe a very small quiescent current, but you should still see a very small voltage change if you turn the pot.
 
Ok, now I saw that on Wiki:

"Slowly turn the bias pot to increase the output idle current, you can also measure this as the voltage across the 25ohm driver emitter resistors, nothing happens at first, basically until the board output voltage reaches about 1.2v across r127 & 128, then the outputs start to turn on and you will read a voltage across Re1...keep increasing until you have about 0.2 volts across Re1..."

I'll try once again tomorrow.
 
neychi said:
Thanks Nico

Maybe I didn't turn the bias pot enough. But I was thinking that after several turns some difference will appear.

I am happy for you that it is changing. Most multi-turn pots have 25 turns so it needs several turns to make a change, this is good. In the low bias state you can run the KRELL CLONE in class AB, it is quite happy and will operate cool under almost all conditions. For it to run in class AB mode adjust low bias for about 25 mV across Re of output transistors.


Good listening.

Nico
 
neychi said:
Heat sinks for output transistors and heat sinks for drivers are on the same temperature. Heatsinks for outputs first go hot around the transistors but after few minutes temperature is the same all around.

I can put my fingers on the transistors without problem.

WOW! That is a beautifully machined box, resembles the real KRELL stepped front panel and all.

Nico