Biasing

[Grubey Doo]

First off I bias using a cathode interrupter. That being said...
Lets say I am installing a quad of tubes. Two are matched and the other 2 are matched but the 2 pairs are not matched to each other. How would you all recommend biasing a quad like this?

[Rob Mercure]

GD,

Well, first I really like individual bias controls and current sense resistors for each and every output tube - but somehow I suspect that this may more work than you're ready for. Since you want to achieve a DC balance in the output transformer primary you'll want to put one tube from each matched pair on either side of the primary winding - that is, if the four sockets are lined up put one matched pair in the two "outside" sockets and the other pair in the two inside sockets. This will at least achieve a DC balance.

But if each tube pair has wildly different control grid needs to achieve the same idle current then you're going to have to come up with some method of individually biasing the tubes.

Rob

[Grubey Doo]

I hear you that would be ideal if I put in the indy bias controls. How do I do that? Doing the work is no problem I just never put seperate bias controls in an amp.

[Rob Mercure]

Rather than provide a generic reply - what amp do you have and which output tubes?

Rob

[Grubey Doo]

Hi It's a 1974 Marshall 100 watter

[Mars Amp Repair]

technically, you really should have each tube match gain & bias point. Matching the bias with all 4 tubes is only half of the issue solved. If the gain is not the same then the tubes, tho having the same dc bias, will not all be funcitoning together for the maximum power...like a 4 cylinder engine with different compressions & or different strokes, not all tubes will be contributing to the max power...tho I won't argue that the resultant sound might actually be desirable with the mismatch...just not the most efficient.
I go for matching both & one bias adjustment. g

[Enzo]

AS was said above, split your pairs ABBA or BAAB. You will probably find one set of tubes will draw a little more current than the other. Measure each and see how far apart they are. Pick a socket, and bias one tube the way yuo like. Now plug each other tube into the same socket and do not alter the bias setting. Note what each draws.

People have their 70% rule and who knows what other rules for bias, but it really is not a critical adjustment. If your target current is 38ma, how much difference will it really make if the tube is 36ma or 40ma? Not much. If the tubes are WAY different from each other, than maybe they are not the best to pair up in the first place. If they are within the ballpark of each other, then average them. Let us say one set of tubes did draw 36ma, and the other did draw 40ma. That is 76ma per side, and we can call it 38 - close enough.

Whatever they do, and whatever you target, you should be able to find a middle position for an average reading.

next time order a quad.

[Grubey Doo]

But how do I install the individual bias controls?

[R.G.]

But how do I install the individual bias controls?

Find a place on the chassis to put four potentiometers, and install same. Connect the pots to ground on one end and the -Vbias on the other. Take the grid bias resistor for each tube to one pot. It probably helps if the physical positioning of the pots reflects the physical positioning of the tubes from right to left. Pot value probably should not be less than 250K. Be sure that with four pots connected to your -Vbias supply that the pots do not pull the bias voltage down to below maybe -65V to ensure that there's enough voltage there to turn a tube off. Put a high value resistor, perhaps 470K, from the -Vbias end of each pot to its wiper. This last is a fail-safe resistor to force the tube fully biased cold if the bias pot wiper goes intermittent.