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 Post subject: Re: 12B4 Preamp
PostPosted: March 14th, 2021, 7:20 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
ratbagp wrote:
Thanks for the info, especially the shunt regulator.

I actually built a SE amp years ago with 5 or 6 12B4A per channel. It sounded pretty decent but I moved on to other things and disassembled it. I was influenced by some commercial line stage that used multiple 9 pin tubes per channel and supposedly produced more detailed sound. I've still got the tubes kicking around.

ray


My understanding is Futterman designed OTL amps with 12B4s as the output tube back in the day...

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 14th, 2021, 9:59 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 448
https://www.pearl-hifi.com/06_Lit_Archive/07_Misc_Downloads/Futterman_12B4_OTL.pdf

I looked up the 6BA8 on ebay and found that a bloke in a town half an hour away has two for sale so I will try to buy them and build the OTL, just for the fun of it. Hopefully the bloke will turn out to be a tube enthusiast.

ray


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 Post subject: Re: 12B4 Preamp
PostPosted: March 14th, 2021, 10:30 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Ray,

Saw the same article. 10-12B4 per amp! I hope you have quite a supply! I think I will keep with 2 at a time. I have about 20 in my collection, but for preamp use need to be picky. Though I seem to luck out with the first pair of JAN12B4A tubes I picked from the pile. When you are talking OTL tubes, looking for tubes that have even lower Rp helps keep the number of tubes down. Regulator tubes like 6AS7 (6080), or Russian 6C19p have been candidates, but the latter is getting scarce and expensive. For OTL, you want cheap since you need alot of them.

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 14th, 2021, 10:40 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 448
I also have a stash of 6C19p. Future projects!

ray


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 Post subject: Re: 12B4 Preamp
PostPosted: March 14th, 2021, 11:56 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 306
I was going to say the same as Roscoe--microphonic as heck. But if you found quiet ones, hurrah!


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 Post subject: Re: 12B4 Preamp
PostPosted: March 15th, 2021, 6:14 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Roscoe, Grover,

Well, if I want to be honest, if I tap gently on the tube with a pencil, I do hear a "ping", but it is pretty faint. More out of one tube than the other. Now it is either luck of the draw on tube selection, or it is because my mounting is lossy. Since I have the tube sockets on standoffs from the lower chassis, there may be some lossyness with that style of mounting. Second, I have silicone rubber O-rings on the tubes at the micas that help dampen the glass and structure. Finally, using amps with lower sensitivity inputs (6V or more recommended), means that the amplifier gain does not amplify the noise in the preamp tube as much. Also, I am not using horns, but am using high efficiency monitors (Aria 5R) in my near field system. Once I move this up to my main rig, it may be a different story.

I will have to try with a different set of tubes, I have around 20 or so to try.

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 15th, 2021, 9:23 am 
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Joined: July 24th, 2015, 4:17 pm
Posts: 1374
Location: Parkville, Maryland
David McGown wrote:
Roscoe, Grover,

Well, if I want to be honest, if I tap gently on the tube with a pencil, I do hear a "ping", but it is pretty faint. More out of one tube than the other. Now it is either luck of the draw on tube selection, or it is because my mounting is lossy. Since I have the tube sockets on standoffs from the lower chassis, there may be some lossyness with that style of mounting. Second, I have silicone rubber O-rings on the tubes at the micas that help dampen the glass and structure. Finally, using amps with lower sensitivity inputs (6V or more recommended), means that the amplifier gain does not amplify the noise in the preamp tube as much. Also, I am not using horns, but am using high efficiency monitors (Aria 5R) in my near field system. Once I move this up to my main rig, it may be a different story.

I will have to try with a different set of tubes, I have around 20 or so to try.

David

Welcome to the "microphonic club." All tubes are microphonic to some level and calming things down with tube rings can certainly help. I moved away from the silicone tube rings a long time ago and found that the Teflon rings do a better job although not cheap. Next up are Pearl Tube Coolers. They not only damp the tube but also provide cooling and a metal cover that can control secondary emissions. Nice!

https://www.ebay.com/itm/Duende-Criatur ... Sw31xfrk3V

https://www.ebay.com/itm/PEARL-TUBE-COO ... Sw6HJZxRv-

BTW -- your workmanship is excellent and some manufacturers could learn from you. :clap: Good workmanship doesn't cost anymore that sloppy work.

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Walt


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 Post subject: Re: 12B4 Preamp
PostPosted: March 15th, 2021, 10:20 am 
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Joined: March 5th, 2013, 9:35 am
Posts: 232
Location: Highland, MD
Say, that dual-DN2540 plate-load CCS PCB looks familiar. ;)

I like the enclosure's appearance as well, and the compact layout. I also like the large control knobs!

Neat circuit: thanks for sharing!

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- Guy


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 Post subject: Re: 12B4 Preamp
PostPosted: March 15th, 2021, 10:26 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Guy wrote:
Say, that dual-DN2540 plate-load CCS PCB looks familiar. ;)



Guy, Thanks for the boards, they have been really handy.

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 15th, 2021, 10:29 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
SoundMods wrote:
Welcome to the "microphonic club." All tubes are microphonic to some level and calming things down with tube rings can certainly help. I moved away from the silicone tube rings a long time ago and found that the Teflon rings do a better job although not cheap. Next up are Pearl Tube Coolers. They not only damp the tube but also provide cooling and a metal cover that can control secondary emissions. Nice!

https://www.ebay.com/itm/Duende-Criatur ... Sw31xfrk3V

https://www.ebay.com/itm/PEARL-TUBE-COO ... Sw6HJZxRv-



Walt,

Thanks for the links on the tube dampers. I may pick up some of the teflon rings.

David


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