Bias capacitor hum

[alange5]

A couple years ago I built an amp based on the normal channel of the 6G6-B blonde Bassman, with the addition of bias tremolo and a "type 2" master volume (dual pot replacing the power tube grid leak resistors).  It sounded good, but had a persistent hum independent of volume control.  Removing the PI tube removed the hum, as did lowering the master volume.  Now that I have some time on my hands, I decided to revisit the amp and track down the hum.


I more or less tore down and rebuilt the amp, experimenting with transformer placement, different grounding schemes, filtering, filament wiring, etc. But the hum remained.


As a last-ditch effort, I replaced the 25uf/50v bias filter cap with a 100uf/100v.  The hum completely disappeared, and my 6L6 plate voltage dropped about 10v.


My voltage is still within spec and the amp sounds great, so I'm not concerned, but I'm wondering if someone could explain to me the relationship between that cap, the hum I was experiencing, and the subsequent drop in plate voltage.


Thanks!

[tubeswell]

The bias supply cap is a filter cap (like the other filter caps, but for the bias supply). In bias supply circuits where there is only one cap, it acts as both a reservoir cap and ripple filter cap, and filters the power supply ripple out of the bias circuit. In bias circuits where there are 2 caps, the 1st cap will be the reservoir cap and the 2nd cap will be the ripple smoothing cap. But for the purposes of most single cap bias supply circuits, which only carry a tiny amount of current, 1 cap does the job without needing a ripple smoothing cap. Either way, the bias supply circuit is connected to the output tube grids. If the bias cap is bad, the amp will hum.

So you should replace it when you replace the other filter caps.


If the bias supply voltage is insufficient due to a bad bias supply cap, the output tubes won't be properly biased, so they will run hot and (because the output tubes carry far more current than all the other tubes combined, and therefore have the most influence on where the B+ sits, because they present the biggest component of electrical load), it will drag down all the B+ voltages in the main power supply rail

[alange5]

Makes sense.  Thanks for the info.


I never suspected the cap because my negative bias voltage was spot-on, and I was able to bias several pairs of 6L6's with minimal adjustment.

[tubeswell]

Makes sense.  Thanks for the info.


I never suspected the cap because my negative bias voltage was spot-on, and I was able to bias several pairs of 6L6's with minimal adjustment.

Bias voltage is not critical. Its output tube current that's critical. (Bias supply voltage is just a means to regulate output tube current). And yes, if all the output tubes are running hot, all loading down the B+ voltage together, then it will seem like you are biasing the output tubes with minimal adjustment, if you are just going by the bias voltage