Matching the leads in a Thordarson OT to a Ceriatone 36W RP amp

[lacrebob]

I have a 36 watt RP 4xel84 Certiatone amp that works fine. I also have an old NOS Thordarson T-17S13 Output Transformer rated at 34 watts that I’d like to try in place of the existing transformer.

I could use a little help matching the old transformer data sheet info on each lead wire to the amp.

The Data Sheet reads:

Primary
Brown - Plate (Start of Winding)
Red - B+
Blue - Plate

Secondary
Black - Secondary
Yellow - 4 ohms
Blue - 8 ohms
Red - 15 ohms
Green - 250 ohms
White - 500 ohms

Tertiary Winding (located on the same side as the three Primary wires)
Brown-White - 10% Inverse Feedback Winding
Red-White - 10% Inverse Feedback Winding
Blue-White - 10% Inverse Feedback Winding
* Red-White is Center Tap

Note: The Brown Primary wire labeled Plate (Start of Winding) is a larger guage than every other wire

Q: Is the Black wire on the Secondary side labeled “Secondary” the same as a ground wire? There’s no other choice, so it would seem so.

Q: Which wire is the “Center Tap?” Is it the Red wire from the Primary side labeled B+, or the Red-White wire? My guess is the Red B+ wire is the one i want, and that the Red-WIre is the center tap specifically for the feedback which I won’t be using.

[maxkracht]

Yep, black wire would be negative, not necessarily ground, but would connect to ground in your amp. Primary Red is your B+/center tap. I'm guessing the tertiary wingdings are for an electrodynamic speaker?

[martin manning]

The primary leads are typical for push-pull output stages (Red CT, Blue and Brown plates), but be prepared to swap Blue and Brown if you guess wrong on the phasing.
The secondary is straightforward, black is common.
The tertiary winding is identified as a feedback voltage source. If you build according to the layout you posted, taking the feedback from the 16 Ω tap, you can tape the leads off and ignore it.

With ~400V on the plates, 6k6 primary impedance is quite high for 4x EL84. I would consider miss-matching (connect 15 Ω secondary lead to 8 Ω) to get 3k3. If you do that, you'll want to adjust the feedback network too.

[lacrebob]

Thanks for helping out.

In another post in this forum, I read that 6K6 would be okay to use so I thought I'd give a try. I'm not bound to it though, and I had also thought to use 3K3 as suggested since that is closer to the 4K most often for a 4 x el84 amp.

I just rechecked some old email messages. Nik Azam at Certiatone said I have an early version and he's not exactly sure which OT was used (there are no markings). Nik said the existing OT would either have an impedance of 5K2 (Trainwreck) or 3K4 (50 watt Plexi), but it looks like I have to disassemble first to find out. If the existing is 3k4, then I wouldn't have to change the feedback section.

[Phil_S]

You don't need to disassemble anything. Just test the OT for the turns ratio with low AC voltage into the secondary and see what comes out of the primary. I use the filament winding to supply the low voltage. The impedance ratio is the square of the turns ratio. That math is good enough for what you are asking. Search the forum for turns ratio. This has been covered previously and more than one time.

I was going to raise the mismatch question, but I see it has been covered.

BTW, the 1942 catalog says that OT cost $7.20. The inflation calculator says that's $115 today. However, given the market for boutique transformers, I think there may be considerable intangible value making it worth even more. I'm not sure. Anyway, I think it was pretty high end back in the day.