Testing power supply before completion

[dpb703]

These are likely noob questions, so I apologize in advance.

I’m in the middle of my 2nd build, a Marshall 45 watt plexi, and just finished the power section and really want to make sure that everything is working as it should before I continue with the build. I have checked continuity, but was wondering what other steps I can take as I have the B+ wired and it would be comforting to know that the voltages are within range.

Is there any other ways to test the power filter section before I move on?

As always, Thank you all for your wisdom

Dan

[xtian]

Without a load (tubes), the B+ supply and following nodes (B+1, B+2, etc) does not drop to expected voltages, so there's no way to test what the final voltages are going to be. However, you can test to see if you've wired it correctly by powering it up (preferably with a bulb limiter, to prevent burning things up if you've made a mistake) and making sure you've got B+ at all the nodes, and nothing drawing unexpected current.

[martin manning]

Can you link or post the schematic you are following and indicate how much of it you have wired up?

[dpb703]

This is where I'm at. I've attached the schematic showing the power section. Additionally I have the filaments wired for all the tubes and have the power tubes completely wired.

So grateful that I have such experts from this site willing to help.

Schematic cropped.jpg

[martin manning]

You can power that much of it up and get the unloaded voltage. I think you probably only have up to the "D" node, since "E" is on the main board. Make sure that all of the filters can take the full unloaded B+. Do you have the OT wired in?

[didit]

Hello --

Testing is doable and recommended. A suitable dummy-load power resistor is needed. As an example, for testing mid-level Wattage amps that have a single conventional tube rectifier socket I cobbled an octal plug with a string of small power resistors, as shown here in the first post of :: tale of two BFDRs.

A similar dummy-load using a small board to hold the resistors along with some carefully placed alligator leads works for silicon rectification or to test DC power after rectification. One does need to work through arithmetic of suitable load resistance and dissipation requirements for each situation. There are caveats as filament loads may have a measurable impact, particularly on transformers right at the available power threshold. I'll do this on new experimental designs to confirm a power supply has met target expectations. I've also used one to assess and diagnose amps with questionably misbehaving power supplies.

Best ..

[TUBEDUDE]

And be sure the bias voltage is at the pins before installing the power tubes.

[dpb703]

The OT is wired. I have yet to wire the main board which includes the bias, PI and the Preamp. So long as I'm not going to blow it up by plugging it in, it would be nice to see to check it out. Once the main board goes in it is extremely crowded, so much so that I have to modify the main board just to make it fit in the chassis. Oh, how I wish I had spent some time with the layout and schematic before buying this kit. On the flip side, I'm learning more than I would have ever learned otherwise.

Is it the Transformer that requires the dummy load or is it the tubes which require the dummy load?

Thanks to all,

Dan

[martin manning]

The load resistor didit talks about is for loading the power supply. You don't need that if all you want to do is check for correct wiring up to where you are (no shorts) and unloaded voltage. Anytime you put tubes in, and go off standby, you need a load on the OT secondary, a speaker or a dummy load, to protect the OT. You can check the OT phase at this point, like this: https://ampgarage.com/forum/viewtopic.p ... 94#p463994

[sluckey]

I usually check the power supply operation before installing the board. That includes plugging in all tubes to be sure all filaments light up.

Don't forget to drain those filter caps! Even better, permanently install a 220K 3W resistor across the first filter cap to always drain whenever the amp is off.

[R.G.]

Don't forget to drain those filter caps! Even better, permanently install a 220K 3W resistor across the first filter cap to always drain whenever the amp is off.

+ 1 on the drain-down resistor. The life you save could be your own.
I like to put an LED in series with the 220k to have a glowing indicator of "this is still live".

[didit]

helo --

Thank you clarifying my posting Martin. I do this only occasionally, and still perhaps more often than really necessary. Since a few usefully sized power supply dummy-loads are there in my toolbox, it takes just a moment to clip in and test voltages and more before moving on.

If the schematic is followed there's the 440kOhm series pair for balance, which will fairly rapidly provide the drain at node A.

Best .. Ian

[imo1]

I’d like to jump on this topic if ok, and ask ask for a mild clarification:

If building and testing power supply, where would you stop your build for a first test of rectified voltage?

In my case, building a single channel deluxe with solid state rectification, I’d like to build my full power supply and test it. I know voltages will be higher without tube loads.

-currently heater winding is only connected to lamp with 2 100r resistors to ground.

-bias circuit is going through the 10k pot and 10k resistor to ground.

-hv has a choke, and 2 10k power resistors

Id like to test to this point. I know I’d leave the OT unconnected and the HV will be higher.

Im assuming that I would be able to test this way with my 2a slowblow fuse and be fine. Does this sound correct?

[martin manning]

Sounds fine to me. Here is a case where I've tested the unloaded voltage on a current project with the PT and choke tack-soldered to the board, Heater circuit is not connected, of course. https://ampgarage.com/forum/viewtopic.p ... 85#p464385 No fuse, I just brought it up slowly with a Variac.

[imo1]

Glad I checked it. The primary was faulty.
I had a different pt from another old amp. Using a little higher HV. No bias voltage on this one, so tapping of HV