Lowering the bias range on 20W plexi

[pjd3]

Hello,

This is likely a worn-out topic but, in case anyone is willing to bite.

So finally powered up one side of the stereo amp (two amps in one chassis, really). Most of the voltages appeared to be proper around the amp BUT, the bias only gave me a range of about -49 to -60. What I did since there was a 55vac bias tap on the transformer was lower the input resistor from 220k (as in a Marshall tapped of the HT outer coil leg) down to a 470 ohm resistor (like a Fender might do). I figured that this would be a good place to start.

Now, while I certainly can play with resistors to shift the bias into a more usable range for the 6V6's, I want to make sure I don't compromise the stiffness of the bias dc voltage. I can either play with the bias input resistance or, play with the 47K resistor that is in series with the bias adjust pot.

OR, since this is the more elaborate bias circuit with the two 25uF caps and intermediate 15K resistor, would it just be best to forgoe the -55vac tap and re-install the 220K resistor and do it the original Marshall way to avoid complications? I may be mixing incompatible approaches that are causing issues, but I wouldn't know that at this point.

I was considering looking at the dc bias at the grid with a scope set to AC and see if the bias maintained a good stiffness as I reduced the 47K resistor until a useful bias range occurred for 6V6s which, I imagined might be centered around -40 volts.

Thanks for hanging with this, after all of this work, surely don't want to compromise the integrity of a good working bias system!

Best,
Phil D (pjd3)

[mhuss]

The 'stiffness' of a supply is controlled by the size of the filter caps, and to a lesser extent, the impedance of the AC source. This is probably why the Marshall style uses a Pi filter (C/R/C) to compensate for the high source impedance. If you have a dedicated bias winding that can supply enough voltage, that's always a better choice. You can just use Ohm's law to figure out the divider values.

[pjd3]

Hi Mark,

Thank you for commenting on this. By the way, this project is two of Sluckey's 20W plexis in one chassis.

So, you don't see any issues with using the Pi circuit with the -55vac tap then?

When you mention the divider, where in the circuit are you referring to, the input resistor or the 47K resistor in series with the adjustment pot to ground?

I figured I'd try to find out if one is any less or more compromising than the other.

Thanks very much Mark, it will be good to get this as right as possible.
Best,
Phil D

[sluckey]

Use the bias tap and replace the 470Ω with 1K. Re-evaluate.

[pjd3]

Thanks Sluckey,

I will do just that tonight.

Best,

Phil D

[pjd3]

Hey, thanks for the tip.

What I did, since I realized I'm dealing with lower voltages and not much current was pull out the 470 ohm resistor and clip in a potentiometer preset for around 1K ohm. The range was still high so, tweaked the pot until -40vdc was almost in the middle of the pot rotation. The pot measured 11K something or other so I soldered in a 10K resistor.

Perfecto!

Thanks for helping me hone in on this. Now to do the same to the "other amp".

Best,

Phil D