Adding Bias Adjustment Pot

[martin manning]

I'm guessing that adding a 10k linear pot between the two resistors (as per MP Manning's helpful diagram), and reducing the value of the series resistor by 25% or so, should get me in the ballpark, no?

I assume that point "E" in the schematic is the bias supply you want to make adjustable. If you want to use a 10k pot (wired as a variable resistor), I'd reduce the 12k to 6k8. That will allow you to vary that leg of the divider by about +/-40%.

Moreover, is there any implication to the power supply if I add a single bias adjustment pot for each pair of output tubes? I don't trust that my 7189As are actually a matched quad, and frankly it doesn't seem to matter much if they are or not, as there are two discrete amps working here.

Any insights you all have, I'm all ears!

But, let me see if I understand... you have two separate power amps, and each of them has a p-p pair of 7189A's? And, is point E in your schematic supplying bias voltage to both p-p pairs?

[slor]

You got it exactly. The way this amp is laid out, the power supply is in a chassis at the bottom of the cabinet, and the bias feed resistors are up top with the tubes and the rest of the circuitry. It's a bit crowded, but thre's room to install a pot between each pair of output tubes. I'm wondering if the two bias adjustments will interact with each other, given that they're fed from the same supply (point "E").

[martin manning]

I think the best thing to do for separate bias supplies is duplicate the whole network: the 22k, 10k pot, and 6k8, and the 25uF (or 22uF) cap. You'll be drawing twice the current from that branch, an additional 1.4 mA, unless you were to double the resistor and pot values. You'd have to get some 25k pots and recalculate the fixed resistor values. 1.4 mA isn't much though, and it will probably be ok. Have you replaced the bias supply electrolytics, including that 100uF in the power supply chassis? Probably a good idea to do that.

[slor]

Interesting...implementing this circuit would cause an increase in current draw? Can you explain? The theory is still a bit beyond me at this point.

And yes, every electrolytic in the amp (and the early model diodes, 1N1697) are being replaced.

[martin manning]

Simple. If you put two parallel voltage dividers in there then the resistance to ground from the recifier/100uF cap junction in the lower unit through that branch is halved, and the current will double.

[slor]

Duh. Thanks!