London Power ad

Search the Forum

(Advanced Search)

Setting bias with multiple pots and tube
Hi Guys

You see many amps that are fixed-biased built with a single bias pot for the output stage regardless of how many tubes are present. This is definitely poor design.

In TUT3 and TUT5, we see 2-tube output stages that have two bias pots, one for the 'push' side and one for the 'pull' side. In the 4-tube output stages of the Plexi, 800, Swede, Standard, Custom Special, and the 6-tube SVT, there are individual bias pots for each tube. It is easy to understand how to set the single pot, but how do we adjust multiple pots?

First, when setting bias in an amp, turn the Master Volume or Volume to zero so the readings will be just DC and not noise or signals. In a Power Scaled amp, also set Power Scale for maximum voltage. We set all the bias pots to minimum so that each tube ideally is 'off'.

For a 2-tube output stage with separate bias pots for 'push' and 'pull, which is essentially the same as individual bias pots per tube in this situation, we set the first pot by meter so we know the tube is biased within a safe range, then set the second pot by ear for lowest hum. If you measure the currents they will be slightly different. This is exactly what we expect considering how nearly all output transformers are wound. There are typically more turns of wire on one side of the primary centre-tap than the other but the manufacturer is trying to have the same inductance per side. Even with interleaving and other techniques, most OT primaries are "unbalanced" on a DC resistance basis.

With more tubes, which is generally equal numbers per circuit half, we have to alternate using a metered setting, then an ear setting. Say the first tube is on the 'push' side'. We set that tube by meter to a safe idle current. We go to the first tube on the 'pull' side and adjust it for minimum hum. At this point, only these two tubes are influencing how the OT behaves, whether there is hum or not.

Now we go to the second tube on the 'push' side and set it by meter. We may move the negative meter lead to do a "zero" between it and the first tube we set on this side. Then we set the second tube on the 'pull' side for minimum hum. If there are more tube pairs, we continue along on this procedure until all the tubes are adjusted.

What if there are an odd number of output tubes?

We have to make sure that both sides can be set to about the same net value of current. The side with only a single tube would be set first by meter. The first tube on the other side would also be set by meter to some value that allows the difference to be safely carried by the third tube. The third tube is set by ear for minimum hum.

How about dissimilar tubes?

For either side that has a small-bottle tube, set the small-bottle first to a safe value for itself. If the other side has a small-bottle tube, set that one by ear to cancel the small-bottle hum from the first side. Proceed with the other tubes. If it is just 2-tubes then set the small-bottle tube first by meter and the large-bottle tube by ear for minimum hum.

Forum Jump:

Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
Please remember these three principles: respect, sharing, community.
Not familiar with The Ultimate Tone book series? See discussion topics, or click here to visit London Power/Power Press Publishing.

Tube Amp Forum Hosted by London Power
London Power logo