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Old 27th May 2009, 02:57 AM   #1
Pano is offline Pano  United States
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Question Need small SET transformer (12B4)

Hi guys - I've been looking at the few 12B4 SET designs here and on the interweb. Can't really find the info I need.

What's a good, small OPT for a rig driven by a little 12B4 triode? All the projects I've seen use OPTs pulled from old consloes. I thought it might be fun to put my 12B4As to good use, if the right OPT is found.

It does not need much power (natch), nor does it need bass. All I want to do is run 16ohm compression drivers on big Altec horns. Minimum x-over would be 320Hz, more likely 500Hz. Active crossover, so no bass worries.

Where can one find such an animal? Any ideas for something off the shelf? Or a common vintage OPT? Thanks for any ideas.
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Old 27th May 2009, 03:34 AM   #2
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Hmmm... well. I may have answered my own question. If I had been looking in transformer catalogs instead of circuits, I'd have seen that both Hammond and Edcor make iron that might work just fine.

Other suggestions welcome.
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Old 27th May 2009, 03:56 AM   #3
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I'd do Edcor over Hammond, but I'd also email Electra-Print and see what Jack can do.
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Old 27th May 2009, 03:57 AM   #4
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What impedance are you looking for?

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Old 27th May 2009, 04:00 AM   #5
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rp's about 1K, so 4K to 5K is probably a good target.
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Old 27th May 2009, 04:00 AM   #6
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I'm thinking a small line transformer run parafeed could work fine. Parts express has them cheap. I personally used old 5k:8 ohm console pulls for my 12B4 SE amp last year.

In hindsight, consider a direct coupled design, .. I ran into blocking distortion with a (low) B+ of about 220V in my RC coupled design, even running into 97db/w/m full range speakers
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Old 27th May 2009, 04:05 AM   #7
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OK, I've used Edcor stuff and it's not bad at all. Can't beat it for the price!

I dunno Dave. I see SET circuits for this at 5K to 20K.
Tho there are not a lot of circuits floating around - not a common use for this tube. 5K seems kinda low to me.
Sure don't need much power. Suppose I should run the numbers....

EDIT: Zobsky posted while I did. 5K worked for you?
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Old 27th May 2009, 04:05 AM   #8
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The ones i have are 10K:4 OHM IIRC. Real nice German ones.

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Old 27th May 2009, 07:41 AM   #9
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Default Re: Need small SET transformer (12B4)

Quote:
Originally posted by panomaniac
I see SET circuits for this at 5K to 20K.
Tho there are not a lot of circuits floating around - not a common use for this tube. 5K seems kinda low to me.
Sure don't need much power. Suppose I should run the numbers....
It really depends on the amount of power you need. The higher the primary impedance, the lower the output power. A load of 4 to 5x rp is pretty standard as a good balance between power, distortion, and damping.

Edcor is cheap enough to buy different values and see what works


Quote:
Originally posted by panomaniac
All I want to do is run 16ohm compression drivers on big Altec horns. Minimum x-over would be 320Hz, more likely 500Hz. Active crossover, so no bass worries.
Quote:
Originally posted by planet10
The ones i have are 10K:4 OHM IIRC. Real nice German ones.
With a 16 ohm load, those will run at 40K. They surely don't have the inductance to do that. Now, since he doesn't need any bass, that might not be an issue, but the power will still be really low.
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Old 27th May 2009, 11:59 AM   #10
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Quote:
Originally posted by dsavitsk
rp's about 1K, so 4K to 5K is probably a good target.
Got it. OK. Too bad I can't use Dave's nice vintage German iron....

I'll be on the lookout for a small transfo that's about 5K:16R It may end up as Edcor.

Thanks guys!
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