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Need small SET transformer (12B4)

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Pano

Pano

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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.
 
Pano

Pano

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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. :)
 
zobsky

zobsky

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Joined 2002
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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
 
Pano

Pano

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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?
 
dsavitsk

dsavitsk

diyAudio Member
Joined 2005
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panomaniac said:
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....
Click to expand...

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 ;)


panomaniac said:
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.
Click to expand...

planet10 said:
The ones i have are 10K:4 OHM IIRC. Real nice German ones.
Click to expand...

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.
 
zobsky

zobsky

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Joined 2002
zobsky.blogspot.com
panomaniac said:
.

EDIT: Zobsky posted while I did. 5K worked for you?
Click to expand...

Yes, .. note my caveats about the low output power though. I would re-build it as direct coupled if I didn't have so many other non working / semi working pieces that demanded my attention.

That apart, great amp , .. really musical. I used a 12AT7 as driver. Sounds a bit like a 2A3, . creamy
 
S

Sheldon

Member
Joined 2003
panomaniac said:
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.
Click to expand...

I've got a couple extra I wound for this project: http://www.diyaudio.com/forums/showthread.php?s=&threadid=125488&highlight="801+loftin+white"

Sheldon
 
G

G

Member
Joined 2002
zobsky said:
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
Click to expand...

Pardon my ignorance but what is blocking distortion? I am about to start building a 12B4 SET amp and I would hate to think I wasted a lot of money on the parts.
 
G

G

Member
Joined 2002
dsavitsk said:


http://www.aikenamps.com/BlockingDistortion.html
Click to expand...

Thanks for the link dsavitsk. I ran across that when I googled the term after my last post. I had planned on using 1K carbon comp resistors as grid stoppers on the 12B4s in my circuit. Hopefully it will work well. At this point I have all of the parts and therefore I'm committed to go ahead and build it. The speaker I will be using are around 95dB 1w/1m and I don't anticipate trying to drive them too hard with 1.25 watts. We shall see.
 
zobsky

zobsky

Member
Joined 2002
zobsky.blogspot.com
G said:


Thanks for the link dsavitsk. I ran across that when I googled the term after my last post. I had planned on using 1K carbon comp resistors as grid stoppers on the 12B4s in my circuit. Hopefully it will work well. At this point I have all of the parts and therefore I'm committed to go ahead and build it. The speaker I will be using are around 95dB 1w/1m and I don't anticipate trying to drive them too hard with 1.25 watts. We shall see.
Click to expand...


.. and if you search on my username / 12b4 , theres a thread on this forum with the relevant o-scope traces and potential fixes, if that's your cup of tea
 
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