I got it all wired up. I do not have the -2 to -50 v range on my bias. I am only getting -31.4 to -42.8. I used the 124 layout values of 10k pot 27k bias resistor and 3k3 resistor. How can I adjust this to get a better range for both 6L6 and EL34 tubes. I have pins 1 and 8 grounded and a 1.5k off pin 5 straight to 220k resistors. I have a 5watt 470 resistor across pin 4 and 6. I put some 6L6 tubes in just to see the range of bias and the lowest I could get was 47 ma. Right at 70% but I'd like to bias them colder if needed. Didn't try the EL34s yet.
Here are my voltages. Are they OK (will research them after I post this)?
pin1 pin6
v1 219 221
v2 221 221
v3 294 304
pin3 pin4 pin6
v4,5,6,7 445 444 445
+ AC
Rec 445 both 325
Will post some pics soon. Tks in advance for any advice!
edit: just checked the schem...it has 190v on v1 200v on v2 and 280v/290 on v3. Also found out you need some tubes in the preamp to measure their voltages
As Tone Merc suggested try some different tubes before adjusting the string.
Tubes draw different amounts of current, which will affect the voltage they sit at.
Sometimes as much as a 10v difference can be observed between two different 12ax7's.
You don't say what you're using in V1, V2, V3. I have found that old Amperex Made in Holland (Heerlen factory) seem to draw more current than most. Old GEs can be good too.
I took those readings with EI 12AX7s in V1-3. I use them as test tubes as they are just too microphonic...although they sounded great when they did work. Those are higher gain if I remember right. Does high gain have anything to do with higher voltages? Thanks all for the advice so far. There is alot more you can tweak with these amps than I am used to. I've built a bunch of BF amps and Plexis and didn't ever find the need to tweak except to mess with the bright caps. I haven't even touched the trim pots on the board yet. I just set them in the middle. I changed the safety tail to 22k. Its now at -29 at the low end of bias range and I was able to bias the EL34s at 37 ma. At least its a little lower than the 70% of 39ma. I am really enjoying the sound. I think I need to roll all the tubes I have and take measurements and learn a little more.
