Power Transformer/Power Supply "Rules of Thumb"?

[SRT]

Hi all,

I've been looking for a quick 'rule of thumb' for choosing a power transformer that I intend to use for a tinkering platform project (essentially a desk-sized breadboard that I can easily swap parts in).

I realize that the heater secondary needs to be calculated by summing all of the Ih specs on the data sheets. Which of the other specs do I need to sum to come up with the HT/B+ requirements? The plate max voltage and plate current?

Hammond makes a 700V C.T.@201mA,5V@3A,6.3V C.T. @ 5A unit (the 273BX) that looks like it will suit my needs. I'd prefer it to be reasonably future-proof as my knowledge grows (along with my ambitions/delusions of grandeur) and I expand to larger output tubes and more complex configurations.

Further to all this, do I need to worry about the max voltage of the filter caps being able to accept the entire 700V output of the PT or just 350?

I've ordered Merlin Blencowe's books and they'll be here any day. While I think they'll be immensely illuminating in the long run, I'd like to build the test platform while I've got a bit of "play" money.

On a complete tangent, has anyone out there ever modeled a Fender amp in MultiSim or LTSpice and is willing to share their models? Some "virtual tinkering" would help me learn the basics without burning out too many components

Many thanks in advance, oh wise tube gurus!

--SRT

[greekie]

Such a PT would normally get half-wave rectified - that is 700/2 = 350VAC x 1,4 = about 490V DC unloaded.

Think about what kinds of circuits you would like to experiment with. I have a setup like the one you ar describing for trying out different preamp ideas and so on, as well as lower power (<50W) power amps. Anything larger, I hook up to what I already have built.

I have a node on the power supply that is variable, like power scaling. This is very helpful when you want to try out, say, a 6V6 amp or an AC30-style build, where the 490V would be a bit over the edge.

YM2C

[ThermionicScott]

700CT is another way of writing 350-0-350, so you could fullwave rectify it like normal. The rest of the math still works.

[J M Fahey]

If you want to experiment with different power amps, from Champs to anything sporting, say, 2x EL34/6L6, I think you should explore the availability of some transformer with a tapped secondary, say from 250 to 350V AC, in, say, 20V steps and enough current for a 50/60W amp plus, say, 20%.
If bridge rectified it would supply a *wide* range of useful B+ voltages, simply switched in seconds with a 5 pole switch.
That plus a generous 6.3V winding and an extra 50V , low current for biasing, would be a true universal bench-supply transformer.
If not commercially available, its usefulness might still having it specially wound worth the somewhat higher price they would ask.
You gave me ideas, maybe I'll wind one myself for personal use.
In the not_so_long run, it will be cheaper and more convenient than having a bunch of single trick ponies.
PS: you *needn't* have a 2 diode "full wave rectified" transformer (which would complicate switching); of course you would lose some rectifier sag, if applicable, but you can always put a 50 ohms 10W resistor in series with the diode bridge .

[Gingertube]

This is the Hammond Transformer Catalogue.
http://www.hammondmfg.com/pdf/5C08.pdf
In the introductory section (page 11) there is a summary sheet of AC voltages and currents required to give particular DC Voltages and Currents for most of the common types of rectifiers.
I find it VERY handy as a quick reference.
Cheers,
Ian

[d95err]

Such a PT would normally get half-wave rectified - that is 700/2 = 350VAC x 1,4 = about 490V DC unloaded.

It's not half-wave. Center tapped PT with two diodes is full wave rectification.

[trobbins]

Full-wave on the output DC rail. Half wave on the secondary half windings. ;-)

SRT - you don't seem to elaborate on your vision for an output transformer, or a valve type - they usually dictate the HT rail and current/power level requirements. Hoping to make a generic HT supply is fraught with tradeoffs.

Ciao, Tim

[Enzo]

Well, in fairness, what he is wanting to do is build a bench supply, so he can knock out various breadboard tube designs without having to build in a new power supply for each one. Obviously, whatever the maximum current ratings of the supply are will limit the range of experiments.

[trobbins]

The hassle is to change the HT voltage level for an output stage. A variac is one means, but requires a separate filiment transformer. A capacitor input, or a choke input, or a doubler provide three substantially different levels. A large VVR would be another means. Otherwise large power dissipation droppers I guess.

[J M Fahey]

The hassle is to change the HT voltage level for an output stage.

I would have somebody wind me a custom transformer with a generously rated 6.3V winding (or two), and a single tapped HV winding, with, say, 20V steps, from, say, 210VAC to 350VAC and, of course, bridge rectification.
A switchable built in series resistor would reasonably mimic tube rectifier loss and corresponding effect on sag, I so interested.
Such transformer should cover *a lot* of experiments, from Champs to 120W firebreathing monsters.
jm2c

[tubeswell]

You have a 5V3A winding so you can use various rectifier tubes. A crude R.o.t. for rectifier tube voltages is:

SS = HT x 1.4142
5AR4 = HT x 1.31
5U4G = HT x 1.21
5Y3GT = HT x 1.1

But this depends on a whole lot of things. Personally I'd prefer to use Merlin's 2nd book to work it out now, but FWIW there's a cheat sheet for various tubes attached that one of the other guys did over at the Hoffman forum but IIRC there's a couple of flaws (e.g.: The EL34 heater draw is wrong - should be 1.5A)

[R.G.]

Well, in fairness, what he is wanting to do is build a bench supply, so he can knock out various breadboard tube designs without having to build in a new power supply for each one. Obviously, whatever the maximum current ratings of the supply are will limit the range of experiments.

In my pile O' stuff, I found an HP tube amp bench supply, complete with variable HV, screen, and heater outputs. I was amazed to find such a useful box, and I'm at least a little ashamed that I haven't found time to rebuild/refit it. It was in a quarter-ton gift from a now-deceased ham radio friend.

[Enzo]

When I was a kid designing hifi amps, oh some 50 plus years ago, I made a bench supply for B+. Ut was only 300v, but that suited my needs. My dad came along and asked what I was doing. I told him I was building a 300 volt power supply. His immediate reaction - as expected - was, "Well why don't you build it for less volts so it won;t be as dangerous?"

I had a variety of filament transformers around ready to clip in.

I have a collection of electrophoresis power supplies. No heater supply, but they are adjustable up to about 500 volts DC, with volt and current meters on the front. The majority of them use a small variac inside to run a high voltage transformer. That is what gets adjusted.

[J M Fahey]

I told him I was building a 300 volt power supply. His immediate reaction - as expected - was, "Well why don't you build it for less volts so it won;t be as dangerous?"

You make me remember a similar one:
My brother used to be a commercial pilot.
A very common joke among them was:
"This pilot guy tells his wife he's going to work .
She answers: "OK, but I'm worried about you, please fly low and slow""
The joke lies in that "low and slow" is a sure fire recipe to crash and get killed.

[SRT]

Hey, all! I've been on holidays for the last while.

My idea is essentially to build a Fender Pro Reverb-based circuit but run 4x6V6 instead of the 2x6L6 power stage. Because I also want to experiment with cathode biasing as well as a few replacing the Normal Channel (Tweed Bassman or Hiwatt, perhaps?), I feel that messing about in a "test-bench" environment is the best way to prove my ideas before ordering up some ClassicTones or Heyboers to throw in the chassis.

Now that all of my books have arrived (and I delved into them while sitting on the dock at the lake), I think I have enough of a grasp - along with the info you all have pitched in - to go ahead.

Many, many thanks!