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Little 12BY7 S.E.P. Amp
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Author:  Geek [ 04 Mar 2012, 06:33 ]
Post subject:  Little 12BY7 S.E.P. Amp

Scrounging through my stock, I came across a nice NOS pair od 12BY7A tubes. These are video output pentodes, hence very linear. They are also forgiving, being used by many ham radio manufactuers to drive a pair of 6146's in their SSB transmitters.

Like most video pentodes, they are also fairly reasonable for transconductance - 11,000uMhos.

Out of curiosity, I did a set of loadlines for this at 230-250V... said it would be happy with 7K-9K anode load and reasonably linear within that. Super! What boatanchor geek doesn't have a little bin of, vintage 6AQ/BQ5 SE outputs with 8K primaries?

I needed another little amp. The wife conscripted my beloved "Baby Amp" (6FD7 S.E.T.) and the only other tube amp I have is my 90W, EL34 PP DynaMutt test bed. Using the 90W amp on multimedia speakers is silly to say the least.

There's more than enough gain to make a spud, but I wanted to be different.

Since we had gain to burn, lets use the OPT secondary as cathode feedback. The arrangement shown with my transformers translated into about 10dB of CFB. I have learned from experts that such CFB has a 1.5x enhancement on damping factor over gNFB - so we get the equivalent of 15dB of NFB for the price of 10.

Also, this forms a local FB loop, so we have serious tightening of the bass, without dulling the sound like (too much) gNFB has a terrible habit of doing.

At this point, it was taking about 6.5VRMS to drive it to full output of 1.98W. Let's add a rather transparent tube to the front and do more local FB.

I added a 6N1P, for no other reasons than: 1) I like their sound of neutral with a slight brightening, 2) The gain was low enough for the project and 3) I have quite a few of them.

Since the 6N1P had more gain than I needed and I was running it at reasonably high current, I chose to do another local loop - anode follower. I get 300mVRMS sensitivity now. Perfect!

Oddly enough, the anode follower resistor (150K shown here) has a significant impact on frequency response. With 150K, it's 21Hz to 21KHz. With 100K, it's 18Hz to 18KHz. With 82K, it's 17Hz to 17KHz.... it just shifts the whole amp's F3 up or down Shocked Any idea why? With it removed, it's 27Hz to 30KHz (21Hz if you make the cathode bypass on the 12BY7 1000uF from 220uF - my 220uF caps were just a zillion times the quality of the larger ones).

I should note that I have drawn DC through the secondary of the OPT. Given the phase it has to be in, the DC de-saturates the core a little, extending the frequency range somewhat as well as increase the power handing capability.

You could of course use only AC coupling of the CFB. That's accomplished by moving the leg of the 82R resistor in the 12BY7 cathode that's on the negative side of the cap, to ground and let the AC only come through the cap.

So, how does it sound? DOOD! If perfect mids, piano and female vocals are your dream, this is the amp for you.

What bass 2W peak can give, is pretty authoritive. I have 82dB speakers on my computer that drop like rocks below 90Hz. Playing Patricia Barber's Avante Garde Jazz genre, "Regular Pleasures", it ran out of steam pretty quick, but was clean enough to be enjoyed. Her stand-up bass heavy song "Use Me" was a treat, as the bass was very three dimensional and her vocals clearly jumped out into the room.

What made me decide to schedule this for build rather than just a breadboard project, was Diana Krall's "Jive Jack" - the details were there that I only heard on my larger speakers at much higher volumes. Anything that makes my Nanaimo Songbird sound that good, gets built!

Schematic: http://geek.scorpiorising.ca/contrib/Ge ... Amp_v1.png

How she looks (looked - it got sold in a week to a client that visited):

Image

Image

Image

Cheers!

Author:  gofar99 [ 04 Mar 2012, 17:34 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

Hi, I like clever projects. I ran into a similar situation when I designed my phono preamp. Adjusting the value of the one resistor shifted the whole range of audio. I finally determined that the first tube's anode resistance when combined with the follow on resistor essentially summed and the combination saw the next capacitor (the 100nf in your case) and made a rather wide band pass filter. Very small adjustments in the resistor (5-10%) would shift the whole response envelope up or down. Drove me crazy ( a short trip BTW ) until I figured out what was going on. The solution is to try other values for the capacitor. Try small changes first.

Good listening
Bruce

Author:  Geek [ 04 Mar 2012, 19:14 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

Thanks!

OMG, phono pre's I let the computer get me in the general area... otherwise my phono projects would be measured in cases of wine, rather than bottles, LOL! Image

Cheers!

Author:  Isophon [ 05 Aug 2013, 11:20 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

Hi Gregg,

Cool design, my favorite thing is to use odd tubes for audio.

How does this new amp compare to your Baby SET with 6FD7 (currently hooked-up in my basement, check my signature you will recognized some names :D )

Would 5K OPT be OK ??

With Schade partial feedback I`m sure it sounds good.

Any advice on the 12BY7 brand I should buy ?

Thanks,
Iso

Author:  mwhouston [ 05 Aug 2013, 12:46 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

Geek: cute looking amp. I like the timber collar around the power tranni. In the past I have found input RCAs at the front have never worked for me I would have them at the back. The timber sides are a real nice colour. Your OPTs must be compact to fit in the chassis. I have found the small cheap open frame 10W Edors perfect for these types of builds. My Paris Duo (6EM7 amp) with OPTs in a compact cast Al chassis used those real small Edcors. Paris Duo sounds amazing for its size the Edcors don't let it down at all.

I'm yet to do a spud but I have a few ideas.

Author:  Geek [ 05 Aug 2013, 17:43 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

Hi,

Isophon wrote:
How does this new amp compare to your Baby SET with 6FD7


Sounds more like a 45-ish to me.

Quote:
Would 5K OPT be OK ??


You'll need to draw the loadlines and rebias.

Quote:
Any advice on the 12BY7 brand I should buy ?


All the ones I rolled seemed to work.

Quote:
In the past I have found input RCAs at the front have never worked for me I would have them at the back.


Would not work for this design. AC pickup would have been intolerable (wires and parts are totally "Japanesed" in that chassis), so I chose the shortest path to grid.

Cheers!

Author:  mwhouston [ 05 Aug 2013, 19:17 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

For a number of tube amps I have built lately instead of having the driver tube upfront I'm setting it back into the amp and placing the RCA just behind it. I don't use shielded cable in my amps but using this method means the wires from RCA to grid are only a few inches long. I use heavily twisted wire warp wire to connect RCA to grid. Haven't had a problem yet.

The idea of having the driver tube upfront of a power amp seems wrong to me. By having the driver tube at the back of the amp (when RCA are mounted rear-ward, more often than not) it would make sense to have the driver tube back there. Wires from RCA to driver are therefore shortest. In most cases the driver tube is some small unexciting 6N1P or 12AX7. Even a 50 year treasure (black glass and all) with ceramic base as I have used a few of lately can take back row to say a Shuguang 300B-Z.

With driver tube upfront and RCAs on the back you will you have the worst situation for long wires, shielded or not.

Author:  Geek [ 05 Aug 2013, 19:44 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

If you so wish :D

Not all amps have to be built the same way and the client is happy:
http://www.glowinthedarkaudio.com/se12by7a.html

Author:  Isophon [ 05 Aug 2013, 19:45 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

To avoid AC pickup from RCA input connector from the back of the amp to all the way in the front I was considering powering the unit via a small DC power supply (laptop brick style) and adding a DC/DC converter to increase the DC input (ex 16Vdc) to a voltage of about 200Vdc. One converter per channel.

I have seen this on YouTube with a 6V6 guitar amp, thought the idea was great for low power amp.

See here : http://www.tayloredge.com/storefront/Sm ... index.html

Rgds,
Iso

Author:  Isophon [ 05 Aug 2013, 20:00 ]
Post subject:  Re: Little 12BY7 S.E.P. Amp

mwhouston wrote:
For a number of tube amps I have built lately instead of having the driver tube upfront I'm setting it back into the amp and placing the RCA just behind it. I don't use shielded cable in my amps but using this method means the wires from RCA to grid are only a few inches long. I use heavily twisted wire warp wire to connect RCA to grid. Haven't had a problem yet.

The idea of having the driver tube upfront of a power amp seems wrong to me. By having the driver tube at the back of the amp (when RCA are mounted rear-ward, more often than not) it would make sense to have the driver tube back there. Wires from RCA to driver are therefore shortest. In most cases the driver tube is some small unexciting 6N1P or 12AX7. Even a 50 year treasure (black glass and all) with ceramic base as I have used a few of lately can take back row to say a Shuguang 300B-Z.

With driver tube upfront and RCAs on the back you will you have the worst situation for long wires, shielded or not.



Hi Mark,

I never have hum problem with RCA in the back and driver in the front, please check your layout.

The 6N1P-EV is an excellent driver when bias properly (9-10mA), Vp 200V it sings :D

Rgds,
Iso

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