Ok, so clean power and rectified heaters and some other stuff were covered in the previous requirements post.
Tube Biasing – 1st Stage
As I understand it from my readings, the standard way of biasing, self biasing, is a complex reaction. Essentially as the power drawn from a tube increases, the bias goes more negative, somewhat thwarting the increase in power. This could be a very good thing. To my mind, this sounds like a plausible explanation for why compression happens. Now, as you all know, my understanding of what’s going on in these amps is mild, at best. My thoughts for biasing are actually three fold. Here is the first stage of the GA-5 :
The Fender Champs (5E1 and 5F1) and original Skylarks biased at 1.5V. Interestingly, the schematics from Crestline Skylarks (GA-5 and GA-5T) are annotated with voltages and despite the fact that these are exactly the same circuits up through the second stage, the ’5 shows a bias voltage of 1V while the ’5T shows 1.5V
I want to use a potentiometer here, to play with various values. Note that setting the bias is not really an issue for 12AX7 / 6EU7s in this case. The circuit is self biasing. I just want to see if fiddling here would change the tone.
Additionally, I want to be able to switch the bypass cap on and off. The 5E1 Champ had it but the 5F1 champ did not. Finally.. and here’s the kicker, I am considering a straight bias option. Sending a fixed voltage to the cathode. This is how tubes used to be biased, with a battery or other source directly supplying the right voltage. So this would be a complex little bit of business, just so I could play around with biasing options. Knowing me I’ll find a setting I like and never change it. Dave Hunter’s Two Stroke amp has a three way switch with two different caps and a no cap selection.
But still, that’s not complex enough! Next up an LED/diode combo to tell me when the grid voltage is higher than the bias voltage (Actually when Vg + Vb >= 0). No point in buying a lot of extra tubes if I can give myself a warning before I cathode strip them.
More Coming…
are you thinking of using an LED across the grid and cathode as sort of “check valve”? i can’t find an LED that has an operational volt range of more that 2-3 volts…, unlike a 1N400* diode.
Not as a valve specifically but just to light up when the Grid goes positive.
I’ll probably have to think about that a little more though.