Bipolar Cap Mult DC Heaters

[abbey road d enfer]

That article is obviously made for resistive loading only, and is correct in that context IMO.

The problem is that it does not define what constitutes the power rating of the xfmr.
Let's take a simple example, of a xfmr with two identical secondaries. The xfmr is rated at 100VA.
The article (and many others) considers that each winding is capable of 50VA. What happens when one secondary is unloaded and the other sees a load that draws 100VA? Unless the wire gauge has been made dangerously small, this loaded secondary will deliver a large percentage of the nominal VA rating. That is because flux circulates in both secondaries and flux is governed by the primary (and the core) alone.
In a center-tap configuration, each half secondary is instantly capable of delivering the whole power rating. In teh name of energy conservation, it can only do so half the time.
This situation may or may not be sustainable, depending on the safety factor the winder has taken. Note that this safety factor is not decided by the possibility of overload alone; actually it is the result of minimizing DCR.
I agree with the fact that the rms currents are smaller for the bridge configuration. However, paralleling two secondaries exposes to the risk of losses due to voltage unbalance.

 

[merlin]

And not satisfactory. Although secondary currents are different between bridge and CT, the actual efficiency is the same, so there's no "waste". Not 30% difference.

Just simulate the two circuits and you'll see the difference for youself.

 

[abbey road d enfer]

Just simulate the two circuits and you'll see the difference for youself.

I did, but I don't draw the same conclusion than you (and many others).
Read post #101.
I take a more holistic approach than all these articles about TUF (Transformer Utilization Factor).

 

[thor.zmt]

Just simulate the two circuits and you'll see the difference for youself.

Ok, let's do that.


DC out with identical DC current draw:


CT gives marginally more output voltage and power.

AC current into each set of transformers:



We see essentially the same peak current with negligible difference.

Observation, the CT Rectifier is more efficient as it is minus one diode drop relative to wiring parallel.

Second observation, even CT end up with a drop to ~6V at nominal mains voltage, which IMNSHO does not provide enough voltage headroom for regulators.



If we use the windings in series and assume a simple switcher LM2579 with a stepdown to 6V @ 3.4A, using recommended components should give over 85% Efficiency, meaning we will draw around 24VA from the heater winding.


This gives appx 16V DC before the switcher with modest ripple. In addition, we have soft start, adjustable current limit. etc.

So we have an impasse.

The OP resolutely refuses to consider switchers which is the only way I can see 6V heaters work well with the selected transformer and the Transformer is likely in the best case insufficient to provide reliable regulation and the OP resolutely refuses to consider alternative solutions.

Thor

 

[atavacron]

I just finished a quad mic pre using 12AY7 into 12AU7 as AF. 2 boards of 2 channels.
I used 2 Meanwell RS 15-12 (12v • 1,3A) for heating 2 tubes per board so 300mA current consomption per board (when hot cause the heaters deserve more current when cold > a simple 1,3A couldn't feed 4 tubes at starting and it went on standy)
And 1 Meanwell RS 35-48 (48v • 800mA) for phantom power and everything's doin' fine without any "noise"

Good on ya. I was looking at a 12AY7 front end but drilled down into CCS plate loads to make an all-6922 thing work. So you have stepped front end gain and a variable trim? And Are you adding any RC after either of those supplies?

 

[atavacron]

The OP resolutely refuses to consider switchers which is the only way I can see 6V heaters work well with the selected transformer and the Transformer is likely in the best case insufficient to provide reliable regulation and the OP resolutely refuses to consider alternative solutions.

Hey thanks for clearing all that up, Thor. The visual makes all the difference. In the 6V scenario, I might be able to get total draw down to 2.9A, which might help a little. It would involve a commitment to 6n23p, and assuming 325mA per. I had to figure out my actual relay requirements, which look like 270mA in one particular on state that is plausible, a far cry from the 500mA i was assuming, and i only got there after a significant redesign. If that doesn’t do it, i suppose it’s either series/parallel heaters, regulating series secondaries down to 6V, or a switching option. Oh and I figured out a way to avoid heater elevation entirely via a single triode output method.

 

[thor.zmt]

In the 6V scenario, I might be able to get total draw down to 2.9A, which might help a little. It would involve a commitment to 6n23p, and assuming 325mA per.

I suspect this will be still marginal.

I would instead recommend to base the design in reliable operation under all reasonable conditions regardless of the tubes customers chose.

Seriously, secondary windings in series, 16V DC and Stepdown simple switcher deliver this, complete with a cookbook that means you don't need to do anything except following the recipe.

And suddenly all problems go away.

As the Borg say "Resistance is futile".

Thor