Leslie Combo Preamp Power Transformer gone missing

[jrcreative]

I got a great deal on a Leslie combo preamp and then realized why... the power transformer was missing. I'm looking for a power transformer with 34VAC output and it's apparently not a thing people have around anymore. I reached out to Jameco to see if they had something that would work in its place and they sent me here. The manual (page attached) says it's a TI Power Transformer 020446 which gives me nothing helpful in a google search. Any thoughts on what could replace it? I don't care so much about keeping it stock, so if the circuit can handle a different output voltage that's more commonly available, I'm happy to use that. If it requires custom winding a transformer I'm not sure it's worth it.

 

[dmp]

It is filtered to 48v DC then down to 40v for the preamp, I believe (posting the schematic would have been helpful). You could get this to work with other vac transformers.
The physical constraint on it might be more tricky.

 

[jrcreative]

yeah, here's the circuit, and you're correct. It's filtered to 48vDC, and then 40vDC.

Any recommendation on a replacement would be much appreciated. I think there's enough room in the chasis to accommodate a larger transformer depending on how much larger. Would it be able to generate enough volume to power the Leslie Amp if I were to replace it with a 24VAC transformer? Those are easy to find.

 

[dmp]

You could probably adapt it to use a 24vac transformer if you make it a full wave rectifier. (Add a diode bridge. )

 

[Brian Roth]

Combo preamp,,,,a blast from the past 50+ years ago in my garage band and music store service dept. days! In decades past, a Hammond Organ dealer and/or a music store selling Leslies would be a likely source, but alas.....

I found a couple of Youtube videos showing the guts. One was for a 230VAC version, and the part number stamped on it matches the service docs. It is an open frame package. . The other video shows a 120VAC version, but that transformer has metal end bells and I can't seem to read the part number.

I see some 34ish Volt possibilities in the Hammond Mfg. (NOT Hammond Organ) transformer catalog....assuming they will fit. The first thing is measure the hole spacing for the two mounting holes.

Can't link directly. Search Youtube:

Leslie Combo Preamp Inside for 147 230V Version Circuit Breaker Overload Protector Switch​

Other link:



Bri

 

[nashkato]

2x18V are readily available ?

 

[sahib]

2x18V are readily available ?

Indeed.

Visit Mouser web site. They have fixed 36V output too, and the rest.

 

[Spiritworks]

Do you have the Leslie speaker? It should be noted that these pre-amps were usually speaker model-specific.

 

[MidnightArrakis]

Do you have the Leslie speaker? It should be noted that these pre-amps were usually speaker model-specific.

[these pre-amps were usually speaker model-specific] -- How and/or why would that be? Isn't 8-Ohms (or, whatever.....) 8-Ohms???

Besides, a "pre-amp" is before the power-amp that is driving the speaker. So, isn't the power-amplifier also acting as a "buffer" between the two? Or.....does the pre-amp also have anything to do with the Leslie-speaker's rotation?

/

 

[MidnightArrakis]

yeah, here's the circuit, and you're correct. It's filtered to 48vDC, and then 40vDC.

Any recommendation on a replacement would be much appreciated. I think there's enough room in the chasis to accommodate a larger transformer depending on how much larger. Would it be able to generate enough volume to power the Leslie Amp if I were to replace it with a 24VAC transformer? Those are easy to find.

[Any recommendation on a replacement would be much appreciated] -- Is it possible for you to come up with some relative space/volume dimensions (HxWxD) of the amount of space that this transformer has to fit within? I know of some companies that manufacturer small power-transformers.

[here's the circuit] -- Do the transistors on the PCB show any actual Part Numbers on them (i.e., 2Nxxxx, BCxxx, etc.)? If so, could you provide a list of what the schematic transistor REF DES (i.e., Q1, Q2, Q3, etc.) are in relation to what
an actual Part Number is?

/

 

[jrcreative]

[Any recommendation on a replacement would be much appreciated] -- Is it possible for you to come up with some relative space/volume dimensions (HxWxD) of the amount of space that this transformer has to fit within? I know of some companies that manufacturer small power-transformers.

I don't have exact measurements handy, but I've attached some images to give some relative size. There's also room in the chassis to mount it elsewhere if necessary.

[here's the circuit] -- Do the transistors on the PCB show any actual Part Numbers on them (i.e., 2Nxxxx, BCxxx, etc.)? If so, could you provide a list of what the schematic transistor REF DES (i.e., Q1, Q2, Q3, etc.) are in relation to what
an actual Part Number is?

Yes here are some of the transistor part numbers
Q1 - MSPS 4382
Q2 - MSPS 4382
Q3 - 2N 3859A
Q4 - 2N 3859A
Q5 - I'm not sure, It's not in the documentation that I have and the device it at a buddy's house where we're working on it. I'll get that info when I'm over there next. It is in the power supply, though, so it's probably the one most relevant to this discussion.

Another idea I'm playing with is bypassing the power supply and replacing it with this 36vDC switching power supply like this:

https://a.co/d/iDTRztB

Seems like an inexpensive solution and would probably provide cleaner power to the preamp. is this a viable option? Am I missing anything?

And THANK YOU everyone for your input on this. What an amazing community!

 

[jrcreative]

[these pre-amps were usually speaker model-specific] -- How and/or why would that be? Isn't 8-Ohms (or, whatever.....) 8-Ohms???

Besides, a "pre-amp" is before the power-amp that is driving the speaker. So, isn't the power-amplifier also acting as a "buffer" between the two? Or.....does the pre-amp also have anything to do with the Leslie-speaker's rotation?

/

The preamp includes a switch to control the motor rotation, and the version I have is matched with the Leslie Speaker that I have, a type 47. It has a 6-pin cable that sends power to the amp, switching to the relay that controls the motors, and and unbalanced signal which is, of course, boosted by the preamp to a line level. The Leslie is expecting a line level from an organ so this is meant to simulate that so guitars and keyboards can be plugged in directly.

In the case of the type 47, there is only one motor speed, Fast or Off. The 2-speed Leslies (the 147 and 122) have Fast or Slow, and there are others with Fast, Slow and Brake. They have different cables with more conductors (9-pin and 11-pin) to accomodate for the additional switching, and balanced signals. There are different Leslie Combo Preamps to accommodate the different models, but you're correct that the preamp proper just boosts the signal to line level as you'd expect, and is not concerned with the speaker impedance.

 

[MidnightArrakis]

I got a great deal on a Leslie combo preamp

[I got a great deal on a Leslie combo preamp] -- I also meant to ask you..... -- WHICH -- "Leslie Combo" do you have? There are several different types available.

/

 

[jrcreative]

[I got a great deal on a Leslie combo preamp] -- I also meant to ask you..... -- WHICH -- "Leslie Combo" do you have? There are several different types available.

/

It's the original model, with the single switch and 6-pin cable output.