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 Post subject: Wanted: Lundahl OPT Pair
PostPosted: September 29th, 2021, 12:54 am 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 253
Hi all,

I'm looking for a pair of Lundahl output transformers and wondered if anyone in the group is willing to part with a set that are surplus to requirements.
I'm building a tube power amp and I need a split primary, which almost all Lundahls have.
I'm open to a variety of power and impedance combinations for this project, so I'm kinda fishing with a magnet at this point....

Thanks, Bruce


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PostPosted: September 29th, 2021, 10:47 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 396
FerdinandII wrote:
Hi all,

I'm looking for a pair of Lundahl output transformers and wondered if anyone in the group is willing to part with a set that are surplus to requirements.
I'm building a tube power amp and I need a split primary, which almost all Lundahls have.
I'm open to a variety of power and impedance combinations for this project, so I'm kinda fishing with a magnet at this point....

Thanks, Bruce


What are you looking to build? I have quite a few OPTs with split primaries. What plate impedance are you looking for?


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PostPosted: September 30th, 2021, 2:20 am 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 253
I'm planning to use triodes in push-pull, so anything 5k Plate-to-Plate and above would work.
4 and 8 ohm taps on the secondary would be ideal.


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PostPosted: September 30th, 2021, 7:01 am 
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Joined: June 4th, 2013, 2:39 pm
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FerdinandII wrote:
I'm planning to use triodes in push-pull, so anything 5k Plate-to-Plate and above would work.
4 and 8 ohm taps on the secondary would be ideal.


Do you plan to use true triodes or triode-strapped pentodes/beam power tubes? Feedback or no feedback? Is there a schematic you have in mind, or are you going to try something from scratch?

I ask because I have a number of options here I won't get around to using, but it depends on what you plan to build. I've got a pair of Eico HF-22/35 OPTs, 7500 ohms p-p with 43% UL taps, with 4, 8 and 16 ohm outputs. They can do 25 watts. They'd probably afford the best bandwidth for a no-feedback amp. They're big, they're potted, they need repainting and the leads will need to be extended, but they're in good condition (or were the last time I checked).

I've got a pair of Heathkit W4 OPTs, 10K p-p with 4, 8 and 16 ohms taps, good for 20 watts. These are small and potted, very nice little trannies, but I've never measured them without feedback. They'd probably do okay without it. I have an accompanying schematic for a swell 10wpc triode-strapped EL34 amp that is very zippy. You can see the project here:

https://audiokarma.org/forums/index.php ... ld.795322/

And then I've got a pair of Sophia Electric OPTs from a Sophia stereo p-p EL34 amp that someone trashed with extensive (and unreversible) modifications. I rebuilt it as a PP stereo 300B amp but it wasn't impressive enough to dwell on, and I'm not interested in buying 300Bs anymore. ;-) If they're what I think they are, they're 5.5K p-p with 4 and 8 ohm taps, and Sophia still sells them for $700/pair. No split primary, and plate taps only. They're potted. These might actually be the best thing for your project if you plan to use true triodes and no feedback. I could throw them on my "hacker" amps and see what the bandwidth looks like with no feedback. You're welcome to these, I have no use for them, they're just sitting here.


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PostPosted: September 30th, 2021, 10:39 am 
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Joined: June 4th, 2013, 2:39 pm
Posts: 396
Oh, and I also have a pair of James OPTs, 5K p-p with 16, 8 and 4 ohm outputs. Those aren't bad.


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PostPosted: September 30th, 2021, 5:05 pm 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 253
Hi Grover,

I'm going to try adapting this output stage from a preamp. As you can see, the two separate windings in the primary provide a good place to insert the DC blocking capacitor. I suppose the cap could be inserted at either end of the primary winding, but I like the symmetry of the arrangement with the cap in series at the 'inner junction' of the two windings. Maybe I should try that arrangement with some of the OPTs I already have, before I add any more OPTs to my stockpile. ;)
Attachment:
Output Stage.png
Output Stage.png [ 62.36 KiB | Viewed 917 times ]

Did I use the right term when I said I was looking for a "Split Primary"? Some of the ones you mentioned don't look like they have two separate primary windings.


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PostPosted: September 30th, 2021, 5:22 pm 
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Joined: June 4th, 2013, 2:39 pm
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Oh, I see. No, I knew what you meant. ;-) The Eico HF-22 OPTs and the Heathkit W4's have separate primaries, as do the Acro TO-300's I have. If you decide you want to try any of them I'm happy to send them out. It's only $20-40 shipping via priority mail.


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PostPosted: September 30th, 2021, 5:50 pm 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 253
Thanks Grover..... :obscene-drinkingcheers:
The Heathkit W4's would be a great starting point, I think.
What are your preferred payment options to cover the shipping costs?

Shipping address is:
Bruce Normann
1126 Park St. NE
Washington DC 20002


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PostPosted: September 30th, 2021, 9:09 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 396
Oh, great! I'll pack them up Saturday and send them off. You can send me $20 via PayPal.

grovergardner at charter dot net.


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PostPosted: September 30th, 2021, 9:45 pm 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 253
Done...
I included a small tip.


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