DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently October 31st, 2024, 7:48 pm

All times are UTC - 5 hours [ DST ]


Switch to mobile style


Post new topic Reply to topic  [ 17 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Wanted: Lundahl OPT Pair
PostPosted: September 29th, 2021, 12:54 am 
Offline

Joined: April 22nd, 2013, 12:58 pm
Posts: 293
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


Top
 Profile  
 
PostPosted: September 29th, 2021, 10:47 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 497
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?


Top
 Profile  
 
PostPosted: September 30th, 2021, 2:20 am 
Offline

Joined: April 22nd, 2013, 12:58 pm
Posts: 293
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.


Top
 Profile  
 
PostPosted: September 30th, 2021, 7:01 am 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 497
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.


Top
 Profile  
 
PostPosted: September 30th, 2021, 10:39 am 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 497
Oh, and I also have a pair of James OPTs, 5K p-p with 16, 8 and 4 ohm outputs. Those aren't bad.


Top
 Profile  
 
PostPosted: September 30th, 2021, 5:05 pm 
Offline

Joined: April 22nd, 2013, 12:58 pm
Posts: 293
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 9565 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.


Top
 Profile  
 
PostPosted: September 30th, 2021, 5:22 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 497
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.


Top
 Profile  
 
PostPosted: September 30th, 2021, 5:50 pm 
Offline

Joined: April 22nd, 2013, 12:58 pm
Posts: 293
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


Top
 Profile  
 
PostPosted: September 30th, 2021, 9:09 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 497
Oh, great! I'll pack them up Saturday and send them off. You can send me $20 via PayPal.

grovergardner at charter dot net.


Top
 Profile  
 
PostPosted: September 30th, 2021, 9:45 pm 
Offline

Joined: April 22nd, 2013, 12:58 pm
Posts: 293
Done...
I included a small tip.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 17 posts ]  Go to page 1, 2  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 27 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group