Using series resistors in the secondary to affect Power Supply Dynamics

[SoulFetish]

I'd like to get some advise on simulating some power supply dynamics. When I designed for my transformer to be built, I opted for a larger transformer, designed for a higher load current. The power transformer has the toughest job in the amp, and I wanted it running cool. I scaled the secondary voltages accordingly, and was quoted a regulation percentage around %3 from the manufacturer (if I remember). From what I understand, this is pretty good for an EI core transformer.
On the other hand, I'm not necessarily going for the Lowest impedance power supply. I still like the idea of dynamics of power supply interaction under full signal drive. I would just rather have resistors take the heat for it. That's their job.
So, in a center tapped 2 phase rectifier in the secondary–I figure series resistors in each phase before the rectifier should be a close simulation to the normal series resistance in the primary and secondaries of the transformer. Right? Are there any good recommended resistor values? Also, during conditions of inrush current, what should I account for so that I can properly specify the power requirements for these resistors?
I'm not trying to get the sag of a tube rectifier, just some compensation for the natural loss of resistance I may find in a small transformer.

[mozz]

Just built a JTM45 clone out of a Fender bassman 135. Voltage was really high, i ended up putting 500 ohm resistor on the center tap to drop some voltage. Customer loves it, i asked if the sag was too much and he said no it was perfect. You can monitor plate voltage and watch it drop under signal, maybe you can pick a good value.

[Malcolm Irving]

I would work out the power dissipated in the resistor(s) at full load. Then approximately treble that power value (or maybe double at least) to specify the resistor. (Resistors dissipating their full rated power get very hot.) This will comfortably accommodate the inrush current, which only lasts for a very short time and is handled by the thermal inertia of the resistor.

[Enzo]

And don't worry too much about the transformer, it is just wire wrapped around the iron core. There are no sensitive silicon junctions in it. As long as it doesn;t get hot enough to compromise the enamel on the winding wire, it can get hot without causing problems.

I thought the power tubes had the toughest job, they are the parts that wear out all the time. The transformer is the transmission and the tubes are the tires, if you'll allow an automotive analogy.

[SoulFetish]

And don't worry too much about the transformer, it is just wire wrapped around the iron core. There are no sensitive silicon junctions in it. As long as it doesn;t get hot enough to compromise the enamel on the winding wire, it can get hot without causing problems
.

I can dig your point here Enzo. Core temperature and ambient temperature are important criteria trasformer manufacturers ask for so the can use the appropriate class material. Also, I believe Transformers can run extremely efficiently with a better regulation percentage, when operating pretty hot almost to the point of saturation. Heat wasn't the only reason i went with a larger transformer, but it was a factor.
As far as your point about tires and output tubes, I expect to replace my tires a few times over the life of my car. But If my transmition blows and needs to be replaced, that totally sucks. Output tubes do have a tough job. But we run 'em near the rails, and the give it a hell of a go while they can. Maybe it's fair to say that the power transformer has the most important, and most expensive job in your amp.

[Enzo]

yes, the PT supplies the power the whole deal runs on. it is important, I just didn't see it as being the most stressed part. I am picking at nits.

[SoulFetish]

I would work out the power dissipated in the resistor(s) at full load. Then approximately treble that power value (or maybe double at least) to specify the resistor. (Resistors dissipating their full rated power get very hot.) This will comfortably accommodate the inrush current, which only lasts for a very short time and is handled by the thermal inertia of the resistor.

Hmmm, I want to double check and recalculate the RMS current in each phase, for each resistor, at full drive. I haven't powered it up yet and taken any measurements, but the output tubes were designed to peak around 126mA, when operating in class B, plus screen current(with limiting resistors). Any suggestions for modest sag?

[trobbins]

You can use PSUD2 to simulate the B+ change (both static and dynamic using stepped current tap) without and with added resistor in secondary winding arms (ie. Diode plate circuit). That will also identify the resistor RMS current for dissipation assessment.

Perhaps start by choosing a power dissipation (eg. 3W for a 5W rating) and then use sim to identify resistor value, and to identify voltages, for say 3W diss. Then go to next resistor wattage rating and repeat, to get a feel for practical outcomes.

[SoulFetish]

You can use PSUD2 to simulate the B+ change (both static and dynamic using stepped current tap) without and with added resistor in secondary winding arms (ie. Diode plate circuit). That will also identify the resistor RMS current for dissipation assessment.

Perhaps start by choosing a power dissipation (eg. 3W for a 5W rating) and then use sim to identify resistor value, and to identify voltages, for say 3W diss. Then go to next resistor wattage rating and repeat, to get a feel for practical outcomes.

I've been having some issues trying to work in PSUD2 for some reason. I installed it using WINE (I use OS X), and I'm not sure how to update/edit the rectifier file.
I'm using MUR2100E and UF5408 diodes

[SoulFetish]

Wait, I actually figured a bunch of it out. Running sims now.

[Malcolm Irving]

Hmmm, I want to double check and recalculate the RMS current in each phase, for each resistor, at full drive. I haven't powered it up yet and taken any measurements, but the output tubes were designed to peak around 126mA, when operating in class B, plus screen current(with limiting resistors). Any suggestions for modest sag?

Might be worth doing a rough estimate, to compare with PSUD simulations.

Worst case load would be full output with a square wave signal. That would give a continuous 126mA into the output tube plates. Add (say) 20mA for screens and 5mA for pre-amp, gives about 150mA. Your transformer has 3% regulation. Let’s say the B+ is 400V, then 3% regulation is a voltage drop of 400 x 0.03 =12V. That would correspond to an effective resistance in the transformer of 12/0.15 = 80 ohms.
Adding artificial resistance of an extra 80 ohms would make the regulation 6% which is a more typical value. The voltage would then drop 24V in total at full overdrive.
80 ohms in the centre tap would dissipate 0.15 x 0.15 x 80 = 1.8 watts, so a 5W resistor would be good.
If you prefer two resistors in the plate connections, they should still be 80 ohms each but could be down rated to 2.5W resistors.

The above is only a rough and ready estimate and PSUD should give a more accurate idea.

Sorry for the delayed reply – I’ve been away for a couple of days.

[Old Tele man]

POINT TO REMEMBER: Voltage drop across a resistor is LINEAR, but across a vacuum-tube it's EXPONENTIAL.