andresound wrote:I found the problem ... VERY embarrassing
It always is!
Which configuration of master and level controls do you have now, and how is it working out?
I guess you already have a board and components for the FET stage?
FET board populated and biased as per your layout with 2 x 9V batteries in series. Ended up with 3k3 at Rs to read 9v to ground from top of 10k
Master is as per your drawing. Level next to that (both on front panel). Master works a treat!! Still need to tweak OD entrance and exit a bit more, otherwise well pleased.
Last edited by andresound on Sun Jan 17, 2016 4:44 pm, edited 1 time in total.
Had to order the FET jack socket (3-way Switchcraft). Will receive soon and then I can install the FET board and connect.
BTW, what would the the sonic effect of higher than normal plate voltages (212v I think) for v1 be? I am going to use the amp "as is" for a band practice tonight. But I could change the 2k2 dropping to something higher. I understand that the voltages for v1 will fall back in line to acceptable levels, once the FET board is connected to the B+5
You can install the FET board and connect it to the B+5 node and ground, and ground the input anywhere for now. It has the effect of drawing a few more mA through the whole dropping resistor string, so no one resistor change will do exactly that. An alternative is to put a 180k 2W to ground from B+5 as a FET simulator. The sonic effect of the lower voltages will be a bit earlier break up, but not a drastic change.
Check the voltage on the FET board after the 150k from B+5. This is another voltage divider where the output desired is ~20V. The lower part of the divider usually needs to be around 22-25k, and I think you have 8k2 there now. This step is described in my .pdf on biasing the FET. A great advantage of the battery bias method is that you can set the source (bias) resistor independently without the supply voltage moving around as the current change, which it will do if it is powered from the amp's B+.
Thank you Martin!
Put in a 22k. Measured 18v. Sounds about right. FET actually sounds "darker" than NORMAL. I prefer it.
I will now just learn to play the thing and let her burn in! Everything seems to operate properly. Tweaking can come later. Need to sort the cab and faceplate.
A BIG THANK YOU TO YOU MARTIN, COLOSSAL AND DAVID ROOT!!!
Without you guys AND all the encouraging words from all on this forum, this project would not exist.
Remember. I am in africa. I go to work horseback!!