Real-life transformer simulation

[Luthierwnc]

Hi Guys;

I have accumulated some mystery power transformers and want to know what they can do. PSUD helps up to a certain point but when you don't know some of the load specs, it is hard to know what kind of voltage they really drop. Has anyone rigged-up a breadboard to just plug in and check the DC output under load? I was thinking along the lines of a cap input with a large (200w-300w) ceramic variable resistor.

Thanks, Skip

[mchauck]

Hey Luthierwnc....I don't have any idea of how to do what you asked about, but I did want to say my parents lived in Brevard and how pretty your city is!!!!

Brian

[Phil_S]

I think this link should work. Look for my post ont his topic.

https://tubeamparchive.com/viewtopic.ph ... highlight=

[Luthierwnc]

Thanks Brian, it is pretty.

Phil; I was thinking of butting a breadboard together with a choice of rectifiers, a hefty cap and a variable power resistor -- one of those 200 watt ceramic jobs with a sliding mount to dial in different loads.

The impetus for this is I have a project on the board to do a low-voltage, high current, class-A job along the lines of a Black N Tan. As near as I can figure, it is a choke input arrangement with untralinear taps. Mine will use dropping resistors on the screens but you get the idea. The transformer being considered is from a Hammond 100E organ. It has two HV taps designed to run simultaneously. One put out 450VDC through a 5U4 into a pair of 7591's. The other ran a P-P reverb driver with two ECL86's. This shows 273-0-273, no load. Massive unit, Z-mounted. It goes 4"X3.25"X3" high.

My preliminary noodlings on PSUD give me around 234VDC with a full-wave diode string (if I am doing it right). Surprisingly, a bridge gives me about the same. That is a little low so 300-0-300 might be better - the Toneslut seems ideal. Still, there is something to be said about a trannie I already own.

Any ideas are welcome,

Thanks, Skip

[Phil_S]

I think you've about summed it up. If you build that thing, you can get a real good idea of what that tranny is able to do for you. Pay attention to temperature. Remember, once you let the smoke out, you can't put it back.

I like used iron. The price tends to make it very attractive. You should see the nice iron I've got in some of my amps. If you can come up with used iron + decent used chassis, it cuts the price of the build by at least 30%, sometimes more. Some of those old hi-fi OT's are terrific.

[skyboltone]

Thanks Brian, it is pretty.

Phil; I was thinking of butting a breadboard together with a choice of rectifiers, a hefty cap and a variable power resistor -- one of those 200 watt ceramic jobs with a sliding mount to dial in different loads.

The impetus for this is I have a project on the board to do a low-voltage, high current, class-A job along the lines of a Black N Tan. As near as I can figure, it is a choke input arrangement with untralinear taps. Mine will use dropping resistors on the screens but you get the idea. The transformer being considered is from a Hammond 100E organ. It has two HV taps designed to run simultaneously. One put out 450VDC through a 5U4 into a pair of 7591's. The other ran a P-P reverb driver with two ECL86's. This shows 273-0-273, no load. Massive unit, Z-mounted. It goes 4"X3.25"X3" high.

My preliminary noodlings on PSUD give me around 234VDC with a full-wave diode string (if I am doing it right). Surprisingly, a bridge gives me about the same. That is a little low so 300-0-300 might be better - the Toneslut seems ideal. Still, there is something to be said about a trannie I already own.

Any ideas are welcome,

Thanks, Skip

Hey skip. I use a piece of garolite with bout 20 1K 15 watt resistors tied end to end. If I had it to do over I'd use 30 watts. this way you just clip in between the pair with the right current match and your'e set. As I mentioned in Phils's thread perform all tests at dc. otherswise you'll get false expectations of capacity.

dan

[Luthierwnc]

Thanks guys,

Here's what I'm thinking of:

[Phil_S]

Why are you bothering with a tube rectifier for a test rig? What I'd want to find is a good approximation of the mA rating of the HT secondary. With that and the voltage, what happens with a tube rectifier is rather easy to predict. I'd lose the tube rectifier. Also, be very careful not to touch the load, as it will get quite hot. Although, it is not a problem per se, you might think about heat sinking it just to be on the safe side.

In addition, you can perform similar tests on the filament windings, but you need to be careful not to let the smoke out of those either.

In the end, it is important to keep in mind that the VA rating of a PT is based on the capacity of the primary winding. The primary has a limit beyond which it won't go. So, even if the sum of the capacity of two filament windings plus a HT winding is a particular number, it is possible for it to exceed the primary VA limit. So, as with most work, it is appropriate to derate the PT in the application, so as not to push it too hard. This is the reason for the temperature measurement.

[Phil_S]

I did see you put a switch in there for tube or ss. I just think it's more complicated than it needs to be. Forgot to mention this in my other post. YMMV.

[Luthierwnc]

The redux is simpler - no tube. I shouldn't need to test the filaments either. Most of these trannies ran a lot of preamp tubes in addition to the outputs. Decade boxes are pretty cheap on EBay but shipping them is as much as the purchase price. I'll probably use a minimal assortment of cigar-style resistors to mix and match combinations.

Uncharted territory is the draw on this specific application. At 250VDC and 90 ma per tube (idle) plus a couple preamps, that's in the neighborhood of 1250 ohms. Any resistor is going to get a little warm.

sh