Replacing power transformer - is it enough? -cloning an old Leslie sim-

[Kevin Mitchell]

Typically when replacing power transformers I choose one of equal or greater VA rating and all is well, no questions asked. In this case I'm considering using a 12VA transformer in place of a 15VA transformer. While I believe the 3VA difference is likely negligible in this circuit I'd like to know rather than "believe".

Here's more info on what I'm working with. Original PT, wired for 34V-CT, shows 15VA on the brick. Note: It has a 160mA primary fuse for 220v, suggested to use 315mA for 120v. It also two 315mA fuses on the end of the two secondary.


I'm thinking of using an LP-34-340 which is 34V-CT 12VA. Secondaries are rating at 320mA for series, 640mA in parallel.
(LP-34-340)

Now, knowing that the secondaries have 315mA fuses on each end (with CT grounded) is the LP-34-340 just enough to work here without cooking?

What's a good rule-of-thumb for selecting power transformers?
Of course knowing the current draw/load is the biggest factor. I'm guessing the secondary fuses in this circuit I'm replicating is the giveaway - unless I'm misunderstanding something which is very likely.

[antonis]

While I believe the 3VA difference is likely negligible in this circuit I'd like to know rather than "believe".

And for which circuit we're talking about..??

P.S.
Transformer fusing is a very sad story..
(it has nothing to do with their nominal working current..)

[Kevin Mitchell]

And for which circuit we're talking about..??

What, I can't keep my secrets? 

I'm wrapping up a CLS-22 clone. Just working out the last bit of the power supply so I can prototype it hopefully by next weekend.
Here's a schematic of the CLS-222 as it's easier to read. Not much different than the 22, uses the same PT but utilizes the extra secondaries to power a relay which is not in the 22 model.

[Kevin Mitchell]

P.S.
Transformer fusing is a very sad story..
(it has nothing to do with their nominal working current..)

Curious to know the story. This is actually the first example of a circuit with secondary fuses I've come across - besides an older circuit with fuses post single-phase rectifier.

With the secondary fuses being there anyways, it's safe to suggest the circuit is not pulling more than 315mA on each secondary. So I suppose I shouldn't be worried about burning up the substitute I'm looking at - assuming 320mA in series means 320mA rating for each secondary winding.
I suppose it's the translation of the specs that I'm not too keen on. Well, besides everything else I'm also not too keen on 

[PRR]

This is too complicated, too many unknowns.

I guess the proposed transformer is in your hand? And fuses? Then just try it. A commercial 5VA-25VA transformer is not going to blow-up quick. If overloaded, it will sag bad and run hot. Gross sag is your cue to get bigger iron. (BTW, weight is often the best guide.)

[Kevin Mitchell]

Thanks, PRR. It was my understanding that sag and heat would be the sign before anything blows. I haven't been through this implication before so it's reassuring to hear it from ya.
If the 12VA model isn't up to par I suppose I can get creative and snub in the 24VA model for the first batch of PCBs. I was trying to side step it because the project is aimed to fit in a 1U rack.

I had posted a preview over at diyaudio last night (where I came across the idea of cloning the CLS-22).

[amptramp]

If you have two secondary windings as you have shown in the schematic, you can always use two transformers to replace the one.  t is usually easier to find a transformer with a single output winding than two of the correct outputs.

[Kevin Mitchell]

If you have two secondary windings as you have shown in the schematic, you can always use two transformers to replace the one.  t is usually easier to find a transformer with a single output winding than two of the correct outputs.

That's not a bad idea! Got me thinking further, running two completely in parallel should allow it to handle twice the current (which is an effective equivalent to the 24VA model). Will keep this in mind as potential workarounds for the proto if needed.

All this just to save 0.25" vertical space in a 1u rack

[anotherjim]

Is it not an option to use a normal doughnut toroid traffy in that or is horizontal space short too? I got the impression you don't need an extra secondary for the relay version. Mind you, in 1U, you still need to allow for the toroid clamp bolt and make sure the top can't never-ever short to the lid.

[Kevin Mitchell]

Sorry for the confusion. The schematic and my replacement use dual secondaries wired in series for center-tap operation - which is fed through a fully wave rectifier for bipolar DC. The original transformer had extra voltage taps which was used in the later model to power a relay - which is something I do not need.

I haven't used a toroidal yet but was under the impression finding one that fits my required specs was out of range (being power requirements and/or form-factor) - as the story usually goes when I explore that route.

There's plenty of horizontal space. In fact the rack I'll be using has more depth than the original.
A 1u rack has ~1.5" of height space to work with. I don't think there's a toroidal within the required specs that'll fit.

EDIT: BUT! I could totally use two small 7VAC transformers - provided they can handle ~350ma on each secondary. That's probably what amptramp meant.

[anotherjim]

The 15VA toroids I can get * are 1.4" high (36mm). I've fitted one in 1U. Primaries are 115v+115v and 18v+18v secondaries.
Tight for height but doable. Vigortronix brand, which may not be available over there.

*stocks are always low or zero when I want one!

[Kevin Mitchell]

The 15VA toroids I can get * are 1.4" high (36mm). I've fitted one in 1U. Primaries are 115v+115v and 18v+18v secondaries.
Tight for height but doable. Vigortronix brand, which may not be available over there.

*stocks are always low or zero when I want one!

Ah I see the MCTA015/18 (multicomp brand) available through newark. 1.2" height! Just one of those would fit the bill for sure.
I'm going to give my first choice a shot - it's PCB mounted and fits my vision. If that fails I know I now have a couple of options   

Thanks, anotherjim!

[Kevin Mitchell]

All seems to be well!
Didn't feel like making a new thread for this so anyone curious enough will see what I had come up with.
Best listened to with headphone or decent speakers.

[amptramp]

Don't keep us in suspense - what transformer option did you go with?

[Kevin Mitchell]

Don't keep us in suspense - what transformer option did you go with?

It's in the photos & video!
I went with my original choice of the LP-34-340. No sag, no trouble. Current draw is within it's limits.