Fixed Bias Question

[Jennings]

I've been renewing a circa 1970 Fairview Music Stage Two amp...never heard of them before, and it looks like a sort of competent prototype that someone built.  My question is around the fixed bias circuit, which I've traced out and hastily draw in blue in the attached picture.  It feeds a pair of KT88 which have 600VDC measured at the plates (plate to cathode pins, no output valves in the sockets at the time).  I'm aiming to bias the new set of tubes to circa 40mA current on my bias meter, but at lowest adjustment I can't get much less than 60mA.  I tried increasing the pulldown resistor (100k, then 150k) and that doesn't seem to get much more out of it.  I even tried coupling the -68V directly to the pair of 220k, and that didn't get any real reduction in current on the probes.  What would folks suggest next?  I'm happy to rework the bias circuit...I already see that adjustment pot as a risk in its current format.  Screen resistors are 470k FYI.

[sluckey]

I doubt your screen resistors are 470K. Maybe 470Ω. 

-68VDC is the most you can get from that 48VAC source. You need a higher voltage source. I suggest rewiring the bias circuit like this...

     http://sluckeyamps.com/phoenix/phoenix.pdf

Then select a proper bias range resistor as follows... Set the bias pot to mid range. Then adjust the bias range resistor value up/down to give the desired negative voltage at pin 5 of the KT88s (no tubes plugged in at this time). For example, let's say you want -72vdc on pin 5 for your desired bias. Simply experimentally choose a bias range resistor value to give you that voltage. That's all! Now you will be able to adjust the bias hotter or cooler than your desired operation bias point.

Oh yeah, if you have a FWB rectifier supplying the B+, my bias circuit will not work.

[Jennings]

Yep, you're right...470r screen resistors.  my typo! 


Thanks for helping confirm I'm going going to get anything more out of that 48VAC tap, and the suggestion for an alternative bias circuit.  The main B+ to this amp is fed from a 400VAC PT tap via a SS FWB rectifier.  The PT is a big lump of iron, but only has those two secondary VAC supply taps.

[sluckey]

The main B+ to this amp is fed from a 400VAC PT tap via a SS FWB rectifier.

In that case, I suggest a simple voltage doubler instead of your half wave diode.

[Jennings]

Thanks Steve...I'll sketch one out.

[Jennings]

All sorted…thanks for the expert guidance 😃👍 Voltage doubler worked a treat and gives plenty of sweep. Here’s what I implemented off the 48V tap.