Negative DC on hum balance pot

[rf7]

The Ampeg B15S ties the negative bias voltage to the center of the hum balance pot. I've never seen this before. Is that a good idea? What are the drawbacks?

B15S schematic

[tubeswell]

Its actually just another way of elevating the heaters (i.e.; the heater is elevated to the bias voltage, which just happens to be negative). The bias voltage is set by the bias supply - not the heater.

[cbarrow7625]

I don't know how they are "elevating" the heaters with negative voltage. Usually heaters are elevated (with positive DC voltage) to reduce Heater to cathode voltage to within acceptable tube limits (like in the case of a cathode follower with 100+ volts on the cathode). If the Heater to cathode voltage is too high it can lead to premature failure of the preamp tube.

In this case the heaters are actually being made more negative WRT the cathode. I too am not sure why they would want to do this. What is the advantage?

[Enzo]

The whole point of elevating the heaters is to make them more positive than the cathodes they heat. That way any loose electrons boiling off the AC driven heater won;t be attracted to the more positive cathodes and thus introduce hum. it would seem to me making the heaters more negative would make the problem potentially worse.

[cbarrow7625]

Yeah, exactly Enzo. I hope someone can make sense of that and tell us what the deal is. It doesn't make sense unless there is some other advantage that we are completely missing.

[jrfrond]

A heater center-tap (as well as cable shields) can be floated at 0V (ground), negative or positive. In any case, this is usually arrived at empirically by the design engineers during the debugging phase. If this is what worked for that design, than so be it. My attitude toward this is, if it ain't broke, don't fix it. Also, let's remember that Ampeg had seriously crack engineers back in the day, with guys like Oliver Jesperson (Jess Oliver) and Eugene Andre cranking out designs that STILL withstand the test of time and engineering level.

[tubeswell]

Yeah exactly John. I knew somebody would make the same sense of it that I would (so that we could tell everybody what the deal is etc...).

[cbarrow7625]

OK, cool....so you don't know either. Of course they had great designs. Those guys were geniuses. Part of the purpose of this forum is to enable us all to learn something we didn't know before. We all do a lot of things empirically in designing. I would like to learn what problem they were solving. Maybe I've had the same problem and didn't know that this was a solution for it.

Mock if you want, but I'd still like to know if anyone actually understands the reasons for putting a negative voltage on the heaters. Why would a designer think they need to do this? My assumption is that it helps reduce filament induced noise in some way. I'd just like to know if that is correct.

[jrfrond]

Who's mocking? I am serious! I have done amp design work and debugging, as well as other electronic gear. Sometimes things just work, and as long as you are not violating specifications of the components involved, it is OK. Even design engineers don't have all of the explanations, so they will try things, aka experimentation. If it goes up in flames, it's back to the drawing board, but if it works and it's safe, it's called a solution. Circuit debugging, especially when it comes to grounding is part science, part black art. The design work is actually the EASY part. Debugging can be brutal.

My guess is that the thought process went something like:

"OK, we have heater hum with the center-tap grounded".

"Have you tried floating the heaters positive?"

"Yes, but we still have unacceptable hum levels"

"OK, well try floating it negative, and see if THAT works"

[tubeswell]

Yep - well sometimes I don't know, and other times I'm just plain wrong (as many of my posts will testify), but not this time methinks. Others get to be right too sometimes (No hard feelings - just a small claim of the winner's spoils once in a while . Sorry to rub it in Is that too cheeky?)

[jrfrond]

There probably IS a very valid scientific explanation. If someone comes up with it and can back it with data, I'd like to know too. Until then, I have to solve OTHER problems for customers, some of which I have no answers for.

It's OK not to know everything, because no one does.

[cbarrow7625]

Tubeswell, please don't misunderstand, I'm not saying you were wrong. However your explanation just covers the "what" and not the "why". It is fairly obvious that the reference level of the heaters is being changed, I believe that is all you pointed out so I'm not sure what exactly you are "rubbing in".

This thread is just getting weird now....

[tubeswell]

Yeah fair enough point. (I was just indulging in gloating )

Anyhow, placing the heaters at a more negative overall potential compared to the ground return in the amp will increase the heater-to-cathode voltage, and I guess care will need to be taken w.r.t. any cathode-biased tube that the max heater-to-cathode voltage rating isn't exceeded, esp where PI or CF cathode biasing are involved.

But as far as the science goes, aren't you just placing the entire heater VAC swing at a lower potential than ground, thereby avoiding any induced AC disturbance in the ground potential (arising from the 'pull' of the varying heater voltage on the ground potential) from causing hum back through the signal path? You could just as easily put the entire heater VAC signal clear above ground potential to achieve the same effect. That's about the limit of my understanding.

[Enzo]

That isn't where the hum comes from. With all due respect to the success of this particular design, in general, the hum is created thus. The heaters are running on AC, and the hot filament emits some electrons. The electrons are attracted to the positive cathode nearby. This flow of electrons is a current, an AC current. That current can add itself to the signal currents and thus add hum. COnventionally, you elevate the heaters to a positive voltage above that of the cathodes. Now any free electrons are not attracted to the less positive cathode and no such AC current flows.

Normally I would expect a negative voltage on the heaters to make it worse. Just why it worked here, I couldn;t tell you.

I have no problem accepting John's explanation.