Power supply question

[Redboy]

Power supplies are a bit of a mystery to me. My next project involves rebuilding the power amp section of a vintage integrated onto a new chassis. The existing PS is a simple CRCRCRC... design, feeding four different voltages to the various sections of the amp.

When I rebuild, I want to dispense with the phono pre and the preamp section. Can I simply "lop off" the portions of the PS that I don't need (using just the first CRC) or do I need to accomodate for the change otherwise? Will I have excess power with nowhere to go, or will the power transformer just run a whole lot cooler?

[Tom Bavis]

Well, you can add a bleeder resistor to draw the current that the preamp and phono amp would have used. This will maintain the same voltages. You can also raise the value of the R in your supply to get the same voltage. Less heat that way. Your heater voltage might be a little high too, with fewer tubes.

[cademan]

The amperage (current) may be a little high, but the voltage will remain the same!

[jaymanaa]

Nate, loping off the unused stuff shouldn't cause any problems, and should indeed make the TX run cooler, but try some different arrangements as well, remembering that inductance is aways better than resistance in power supplies (read choke). Have you ever used the the Duncan PS simulator? It's free and will get you fairly close on vdc. I rigged up a load simulator a few yeras back that uses several light bulbs in series. I calculate what the load should be (near abouts), and install the correct wattage bulbs. Then I can try all sorts of arrangements and see what my vdc ends up at "before" mounting stuff to the amp chassis. I just mounted 6 closet type bulb sockets to a board and wired them in series. For most stereo amps we fool with, 5, 15 watt bulbs work good (just stick a screw in type Edison base fuse in unused spots). As simple and goofy as the thing is, I use it on every build/mod, and couldn't imagine trying to do power supply stuff without it.:thmbsp:

[jaymanaa]

Here's a visual for ya (top right by the scopes). :D

[Redboy]

Now that's cool, Jay! :yes:

I did download PSUD the other day but I think I'm in over my head with it. I don't know enough to know what questions to ask. My electronics background doesn't exist, and all of what I know about tubes and tube amps came from this forum.

I understand what you're saying about using a simulated load, but how do I caluculate what the load should be from staring at my schematic? I'm working on a Heathkit A-151 here, building from the 6AN8A tubes back.

[bricktop]

Use "constant current" loads in psud, and usually in datasheets they give an idle current load (in milliamps) for specific circuits, which you would plug in as the load. If you are taking a circuit from an old amp, usually they mark some voltages (and sometimes currents), you can use the voltages and resistances to figure out how much current they were drawing.


Nice scope Jay! :yes:

[electroking]

The amperage (current) may be a little high, but the voltage will remain the same!

This is a funny statement that should be explained. If you disconnect
some loads on a B+ bus, you should expect the voltage to get somewhat
higher, which may in turn lead to somewhat higher currents in the remaining
loads, but most probably less current than before overall.

[shelly_d]

Now that's cool, Jay! :yes:

I did download PSUD the other day but I think I'm in over my head with it. I don't know enough to know what questions to ask. My electronics background doesn't exist, and all of what I know about tubes and tube amps came from this forum.

I understand what you're saying about using a simulated load, but how do I caluculate what the load should be from staring at my schematic? I'm working on a Heathkit A-151 here, building from the 6AN8A tubes back.

Usually they will give you the voltages between the different sections of the power supply. Take the difference between two adjacent sections. That's the voltage drop caused byt the current through the resister seperating those two sections. To get the current draw through that resistor use Ohm's law, which in this case is the voltage difference devided by the resistor value. All voltages need to be in volts, all resistances in ohms and all currents will be in amps. Convert to milliamps if you must AFTER you do the calculation.

Hope this is clear.

Shelly_D

[cademan]

This is a funny statement that should be explained. If you disconnect
some loads on a B+ bus, you should expect the voltage to get somewhat
higher, which may in turn lead to somewhat higher currents in the remaining
loads, but most probably less current than before overall.

Probably a plausible explanation. :yes: What I meant was if you remove a tube or two, 6.3 volts off of the transformer will still be 6.3 volts. If you have a mono amplifier with 4 6v6 output tubes and remove two of them, then the remaining two will red plate from excess current. :thmbsp: