Help with unknown OTs

[Mr Sanderson]

Hi folks - this is my first post here after lurking for some time.

For some background, this is my fourth scratch amp build and my first step into push-pull circuits. I like to source my own parts rather than buying kits and I recycle anything I can from old radio/phono units that are getting tossed locally.

I’m in the process of building a 5E3-based head with iron I pulled from a 1951 Electrohome radio/phono unit. I need some help with the output transformers. Someone was mucking around in the power amp at some point so I’m not sure what’s original and what isn’t.

The power transformer is a huge Hammond 272BX25 (25 for the old 25hz power up here by Niagara Falls). This one is labelled and it’s testing fine. Excited about this one. There’s also a Hammond filter choke, which I was also able to identify.

There appear to be two output transformers. The smaller one is labelled Hammond on top with no other identifiers. It’s got three black wires on the primary and two (1 red, 1 yellow) on the secondary. No shorts from primary to secondary or to the casing.

I tested by putting 1.06V ac on the secondary and measuring the primary. Testing primary wires 1+3 together gave me 0.745v and any combination of wires 1 or 3 with the centre wire gives about half that (.373). I’m assuming that centre wire is the centre tap.

The turns ratio I get is 351:500. This is weird right? Am I looking at a pooched transformer or something else?

The bigger output transformer has “LK 421” stamped on top and “338” on the bottom. I’m getting around 50:1 as the turns ratio, which gives me around a 10k input impedance with a 4ohm speaker load. I’m thinking that’s probably not a great match for the 5E3 circuit if my math is on.

I’m mostly wondering what my options are with what I have here. I’d prefer to use what I have but not at the cost of a good sounding amp. Since somebody was in here doing a bit of homebrewing I wanted to get your opinions on what’s happening  - is the smaller transformer doing something I’m missing?

The tubes in the amp when I got it were 2x6v6 in push pull with a 6J5 in the PI spot and a 5U4G rectifier. Have to assume the 5U4G is not original (black plate though - pretty!).

Any help or recommendations would be much appreciated. Please let me know if taking any more readings would be helpful on my end.

[kagliostro]

There appear to be two output transformers. The smaller one is labelled Hammond on top with no other identifiers. It’s got three black wires on the primary and two (1 red, 1 yellow) on the secondary.

May be the winding you call secondary is the primary winding and what you have isn't a small OT but an interstage Transformer used to drive the PP Power Section

Franco

p.s.: Which is the exact model of the radio you dismanted ?

[Mr Sanderson]

Doyoy - should have included the model number. It’s a PCCF102-438Z.

I was able to find a schematic and it shows a filter choke and an OT with the right coloured wires to match the bigger of the two unknowns. I don’t see an interstage transformer anywhere but I could be missing something? That’s an interesting prospect I hadn’t considered.

Here’s the schematic:

https://pacifictv.ca/schematics/electchipp3fmdata.pdf

[Mr Sanderson]

Also want to note that I’m only using the power amp portion of the amp for this project. The RF chassis is separate and doesn’t appear to have been modified.

The big papa Hammond PT seems to be a replacement since the chassis is marked for 60hz and the PT is for 25hz.

[kagliostro]

The interstage was only an idea, I suppose you dismanted the transformer from the chassis, don't you remember where it was placed and to what it was connected ?

K

[Mr Sanderson]

The smaller OT was placed on top of the power amp chassis. The larger OT and the choke were mounted *inside* the chassis. The wires for the smaller OT entered through crude holes drilled in the chassis.

I’m working on deciphering where it was connected with the pictures I took before I dismantled everything. Lots of dust. Definitely doesn’t match the schematic.

I think the red wire goes to pin 2 of the “inter chassis connector socket” and the yellow wire goes to pin 3 of one of the 6V6s. Which should be where the original OT is connected on the schematic but it’s been rerouted. Trying to figure out where…

Edit:
One red/brown wire of the larger OT primary is connected to pin 3 of the other 6V6, the other goes to pin 5 of the 6J5. I can’t tell where the third (black/blue) primary wire is going. The secondary is connected to the speaker socket just like the schematic.

[acheld]

Just from the schematic, it looks like that's an "Ultra-linear" OT.  Red is the UL tap, blue and brown are the primaries.   

It can be tough to tell colors on these old parts . . .

Edit:  my bad. 

[Mr Sanderson]

Ahh thank you!

I would have expected more wires on the primary for UL but now that I’m looking closer I see that the red is tied to the grid of both power tubes. I’ll save it for a headphone amp or something then.

I’m going to order a Hammond 1750E and just go with that for the OT on this build. I got more than my money’s worth with the two steel chassis, the glorious PT, and a ready-made cabinet for the head (the drawer fits the power amp chassis perfectly) for the twenty bucks I paid for it.

Still trying to figure out what that other little guy is for but at least I’ve made a choice on the build now.

[sluckey]

Just from the schematic, it looks like that's an "Ultra-linear" OT.  Red is the UL tap, blue and brown are the primaries.   

Say what??? Red is the center tap and is connected to B+. The screens are connected to the same B+ node. This is very common in small amps. Nothing ultra about this OT.

[Mr Sanderson]

Thanks for clarifying. Outside of reading about UL OTs I don’t have any experience with using or identifying them.