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

David

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

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

I also have a stash of 6C19p. Future projects!

ray

I was going to say the same as Roscoe--microphonic as heck. But if you found quiet ones, hurrah!

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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Walt

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

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

David

Walt,

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

David