A bricktop SE amplifier

bricktop · May 27, 2008

I while ago I got the urge to try single ended, mostly because I have 96 dB/1w/1m open-baffle speakers that are pretty suited to a low-power amp. I didn't really want to spend a whole lot of money on this, so I used all the parts I already had as much as possible. I picked up some Edcor XSE25-2.5K-8 transformers for $30/each and went to work. I used an old chassis that already had a bunch of holes in it. I already had a Stancor PC-8412 and C-1401 for a choke-input power supply, so that was mostly taken care of, but I didn't want to run anything at 330VDC, so I did a little badass regulation scheme to get my lower-volts, and plus the extra tubes look cool.

This will also be an easy way to get a real low ripple supply - which is necessary for zero feedback designs. I ended up using only two 100uF caps for B+ for the whole thing. I decided to use the 6JM6 because I have a ton of them, and other real triodes are crazy expensive. I tried the 12AT7 as a driver first and got great distortion specs. A more linear driver tube here actually increases the total distortion because the 12AT7 + 6JM6 tube has some distortion canceling effects. (See the 6LU8 amp thread in DIY) I get about 5 watts out at 5% THD, not too shabby. The amp sounds pretty fantastic on my open baffle speakers. Well, enough with the talking...here are the goodies...

jon_s · May 27, 2008

I just want to note that the distortion specs on this are better than most 300B amps I see tested in Stereophile

The squares look mighty-nice too. Kudos to bricktop and Edcor. And to the power of sweep tubes.

gadget73 · May 27, 2008

Interesting choice of tubes, don't see too many TV tubes in audio use, though some should take well to audio use. Whats that other tube in there behind the 6JM6 and in front of the choke? I don't see it on the schematic unless I'm missing something.

Pretty darn cool in any case

currituckco · May 28, 2008

I think that's the 6080, along with the 12AV7 for his regulator....that thing's a beauty. I'll have to study it for a while to figure out what all is happening in there...

GordonW · May 28, 2008

A note on this design: This should be a very cool design, as there are LOTS of old TV sweep tubes that can probably be directly-substituted.

A short list, of those that I have found to have nearly identical parameters:

6JM6, 6JN6, 6JB6, 6JU6, 6KM6, 6JF6

Also, ANY other voltage options of these tubes (ie, 12JN6, 17JB6, etc) would be just as usable, with the caveat that you would have to find a way to power the heaters separately (ie, a separate small transformer for them)...

These tubes are, almost literally, EVERYWHERE. In fact, I managed to round up NINETEEN 6JB6s, spending hardly anything, for a project I'm working on (I wanted to make sure I had plenty of spares for assembly error recovery

, upgrade or retubing- I will be using either 4 or 8 at the time). As opposed to the higher-wattage tubes (6LF6, 6JE6, etc- 30 to 40 watts plate dissipation) that have been snapped up and made expensive by ham radio and CB linear enthusiasts, these smaller tubes (17-20 watts plate dissipation) were more common in the first place (more TVs used this size range than any other), and were less-commonly used in RF designs... which means there's lots of supply, and little demand... at least as of yet...

I also like the use of the 12AT7- it's a very good tube, and it's also the least-hoarded version of that type (compared to 12AU7 and 12AX7, primarily). Also, there's other tubes that would easily substitute (with same parameters- just different heaters and pin-outs)... the 6DT8 and/or 12DT8 should give the exact same performance.

In short- I like designs (like this) that use readily available parts. Just good design practice, IMHO.

Regards,
Gordon.

dnewma04 · May 28, 2008

Can you give any more specific details about what comprised the 5% THD?

It looks great and I love the choice of tubes.

dnewma04 · May 28, 2008

gadget73 said:
Interesting choice of tubes, don't see too many TV tubes in audio use, though some should take well to audio use. Whats that other tube in there behind the 6JM6 and in front of the choke? I don't see it on the schematic unless I'm missing something.

Pretty darn cool in any case

It's on the PSU schematic and is a 6080 tube used in the regulated supply along with a 12AV7. If I understand it correctly, sweep tubes like the one used like to have a regulated G2 supply.

EDIT: curritucko beat me to it and I missed it.

EDIT #2: it doesn't look like a regulated G2, it looks like it's just a regulated power supply.

EDIT #3: I'm a moron, don't pay attention to me.

soundmotor · May 28, 2008

On your power stage schematic you have D+ 2X.

Shouldn't the 6JM6 plate supply be marked B+ or did I miss something really obvious?

I like the design very much too.

Squidward · May 28, 2008

The 6JM6 plate supply goes through one half of a 6080 for each channel. In the power supply schematic, you can see that B+ is accounted for at the start of the regulation scheme, and you get D+ out (twice).

Nice work Bricktop! I've got a bunch of 6080 tubes, I might copy your regulation scheme at some point! Mind if I ask some noobish questions later?

bricktop · May 28, 2008

dnewma04 said:
Can you give any more specific details about what comprised the 5% THD?

At 5%, there was a little clipping on the tops and bottoms of the waves, due to g1 current draw and cutoff, respectively. You can see in the 5% pic where the top is chopped off abruptly -> g1 current draw which comes on fast as 0V g1 is approached. This may or may not be the driver stage - I'll check later - because I know it is biased at only a volt negative or two. The bottom is more smooth, where cutoff is approached as the output tube nears zero current draw at the bottom of the loadline. 5% is pretty well into clipping so I attached a 1% distortion pic too, which is at about 2.8 watts output - much smoother. In all pics the top is the distortion residual (not to any scale - adjusted automatically by the analyzer) and the bottom is the signal at the load.

I may do a little more experimenting later to find the exact best operating points for the driver and output stage that give the lowest distortions, but I think I'll just listen to it now for a while

Squidward said:
Nice work Bricktop! I've got a bunch of 6080 tubes, I might copy your regulation scheme at some point! Mind if I ask some noobish questions later?

bug away :thmbsp: I'll be helping out with the 6LU8 amp as best I can without actually building it. I'll probably be doing some more 6AV5 experiments for Kegger pretty quick.

currituckco · May 28, 2008

Squidward said:
...I've got a bunch of 6080 tubes, I might copy your regulation scheme at some point! Mind if I ask some noobish questions later?

Make sure to ask them here, so we all get the tutorial! I have some 6080s as well.

zenith2134 · May 28, 2008

Nice measurements....I also like the unique tube choices. Can I build one??

spaceman · May 28, 2008

Very cool! :thmbsp: I don't understand enough yet to really appreciate your efforts, but I love those tubes. :smoke:

jaymanaa · May 28, 2008

Beautiful workmanship as usual Micah, but I can't decide which I like more; the amp or that killer 4 channel Tek scope.:drool: I've been wanting to do a "boneyard" build too, and not buy ANYTHING. I doubt that I could make it look that nice though. Top Notch! :thmbsp:

Urizen · May 28, 2008

Super nice work, Micah.:thmbsp:

Pics of the speakers?

bricktop · May 28, 2008

jaymanaa said:
Beautiful workmanship as usual Micah, but I can't decide which I like more; the amp or that killer 4 channel Tek scope.:drool: I've been wanting to do a "boneyard" build too, and not buy ANYTHING. I doubt that I could make it look that nice though. Top Notch! :thmbsp:

Your recent maggies came to mind as I was thinking about wood trim for this. Your too modest Jay

I got the scope off ebay for $125, but it wasn't working. I fixed it up and cal'd it, one of the better buys I've made.

Urizen said:
Pics of the speakers?

The speakers are the ones I built last Thanksgiving with Hawthorne silver iris coaxials. I think they were at AK fest this year with the sterling version :scratch2:

Urizen · May 28, 2008

bricktop said:
The speakers are the ones I built last Thanksgiving with Hawthorne silver iris coaxials. I think they were at AK fest this year with the sterling version :scratch2:

I heard a version of those at the Fest. I was very impressed with their sound.:thmbsp:

Kegger · May 28, 2008

Very nice Bricktop!! Way cool!

And great choice in speakers to, as that's what I plan on running the 6LU8 on as well!

soundmotor · May 29, 2008

Squidward said:
The 6JM6 plate supply goes through one half of a 6080 for each channel. In the power supply schematic, you can see that B+ is accounted for at the start of the regulation scheme, and you get D+ out (twice).

Thanks Squidward, I completely missed that.

bricktop · May 29, 2008

Kegger said:
Very nice Bricktop!! Way cool!

And great choice in speakers to, as that's what I plan on running the 6LU8 on as well!

Great! I think you'll be pleasantly surprised. Have you built/bought the speakers yet? They're pretty incredible at the price point. The amp drives them to pretty loud levels, no shortage of watts here...

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