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Cobra 29 T1 transformer specifications

350GA

350GA

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Oct 14, 2018
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Good evening everyone!

I'm working on a project Cobra 29, messing up around with direct injection, and so far it's working flawlessly! Super smooth audio with my rack gear!

I've been digging around and doing some research looking for a suitable upgrade for audio transformer T1. I found some Hammond audio transformers , but I'm not exactly sure on which one to select, and the ones I've price checked are around $70-$80 each, so not much room for buying several to try out. Lol https://www.hammfg.com/electronics/transformers/audio

I did find this old catalog that listed the specs for a TF-129-A with 20k primary and 600/250 ohms secondary impeadance and this seems like it might be correct, but would like to get one of the 20Hz-20KHz Hammond transformers to extend the frequency response range if any of theirs will work for this.

Any ideas on which one to go with or any other suggestions? Thanks!
 

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You can't simply "strap in a coil" - you're lower Line to Voice coils series is the ones you need to look at.

Because of the Line input - your TF-129 rating is in mA and maybe 1 amp draw - and needs the Line to Audio multi-tap impedance match approach.

The output of the TF-129 is ISOLATED speaker coil wind - so it's more of an Isolation type than a Line to Audio transformer tap.

The TF-129 works the way it does because of how the winds are "flipped" on the board - the output wind simply goes to the cap side of the Audio out, the Line winds are less - but carry the current - the last tap - the 3rd leg is where the audio or "auto-transformer" effect takes place.

Other radios use the Multi-tap approach to handle the current draw - but again suffer the fate of the DC to AC + Bias conversion and the inherited frequency response losses.
 
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Thanks guys! I really appreciate the responses!

Shadetree, if you happen to run across that thread, I'd definitely be interested in reading up on that. Thank you!
 
Shadetree Mechanic said:
ditching the transformer and using a Darlington pair to modulate with. Like what the 148 uses.
Click to expand...

You'll sacrifice over half the peak power you get from the transformer. It puts 24-Volt plus audio peaks onto the final's collector. The darlington/148 solution half that or less.

Would work, but wattmeter worshippers will hate it.

73
 
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Haha I hear ya Nomad. Nah I'm not one of those guys. I'd rather have that smooth sound and let the amplifiers do the work.

I currently have it setup with 3w carrier peaking 12.5w and 5.5w of forward average as measured on my Bird setup. So I've got my roughly 1:4 ratio from the radio, going into a pair of 1446's keying 50w, peaking 200w, then into a single 8877 keying roughly 500w peaking at 2kw. It all just seemed to flow perfectly with this setup! I'm planning on getting a 3CX5000 built in the spring.

I just really would like to get a little more bottom end through the radio. I did change C48 to a 1000uf electrolytic, and that made a very noticeable difference.
 
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the darlington mod you speak of was drawn up by Reverand Bow.
Good guy and knows his stuff.

here is a diagram of what he made:
bmedarlingtonmodulator1-1-jpg.39003


know that peak power will be pretty significantly reduced.
There are drawbacks to removing the transformer from the equation.
I've done it and it does work, but really unless you are doing direct injection there is no reason to do it.
sounds like you are, so it might be what you are looking for.
LC
 
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