SOLVED M800 Biasing Issue

[Guitarslinger79]

Hey there! I've just finished and M800 and I'm trying to complete the bias stage.
I've managed to get all the transistors to bias to 4.5v except Q2. With the trim pot all the way down, I can only get it to go to 5.3 volts.

I've checked all my resistors twice. I've tried everything I can think of short of pulling the transistor and Trim Pot and replacing it with a new one.

Anyone have any ideas what I'm doing wrong??

I mean. It doesn't sound bad like this. Bit muddy unless I dial the Q4 way to the left, but the notes don't have regular sustain. It kind of drops off in steps. Any thoughts?

Thanks!

 

[temol]

It is possible that you need to increase the trimmer value for Q2, e.g. to 200k. 100k may not be enough with a fairly high value of the source resistor (10k). What kind of transistors did you use? Where did you get them?

 

[Guitarslinger79]

That's an interesting idea.
I did use J201s as directed. I'm new at this so I'm not going off the path too much. I got them from GuitarPCB.com
All the others Biased up well. If that particular Transistor were faulty, would the whole pedal fail to function?

 

[temol]

I assume that transistor is not faulty. Available range of the voltage adjustment depends on the parameters of the transistor and trimmer value. But value of the source resistor also affects drain voltage. So you may be out of the adjustment range here. 100k may not be enough to set 4.5V (because of the high Resource value). Look at the schematic - other source resistors are lower in value so no problem with setting 4.5V.
The other story is - what's the point of such high value of the RS and biasing Q2 at 1/2 of the supply voltage... In a tube JCM800, 10k is for a reason. There is no trimmer that allows you to set the anode voltage. And for some reason this voltage is not 1/2 the supply voltage.

 

[Guitarslinger79]

Wow thanks for this. I'm pretty new. I can read the circuit diagrams, but I don't really know enough about what they mean to modify them on my own. What I think you're suggesting is take R5 and boost it up to a slightly higher value like 15 or 20K? Seems simple enough.
It's strange that nobody else who's built one has come across this. That's why I thought I must have a faulty component somewhere.

 

[temol]

I explained why in my opinion you can't set 4.5V and R5 is involved. I suggested increasing the trimmer value to 200k. But you can lower R5 to 5k6 for a test. You should not increase but decrease the value of R5.

 

[Guitarslinger79]

Thank you for your help. I really appreciate it.

I swapped the trim pot out last night and it sounds even MORE amazing now!