12A/71A SET project

[Rocky]

Finally, holidays are here, and I can spend some time finishing my 12A/71A SET.

The setup is quite simple, amorphous core volume trannies into UX-112A loaded by Pimm BBMCCCS, coupled by 0.2uF silver mica capacitors to the UX-171A output tubes, loaded by Tango XE-20 S output transformer configured as 5K:16R. Sensitivity of the setup is about 4.5Vrms for full 700mW output. Here's a pic taken yesterday right after drilling and mounting the first bulk of components..

It is originally intended as a horn amplifier for use above 300Hz, but I think it'll have to serve preamp duty also, utilizing the TVC inputs and BBMCCCS loaded 12A's as preamp stage for bass amps...

[Rocky]

Not much inside yet..

[Giaime]

Originally posted by Rocky
Sensitivity of the setup is about 4.5Vrms for full 700mW output.

I must admit that, when I first read your post, I misreaded those values and thought: "pretty good amp! 4.5Wrms with 700mV input... I've got to try those tubes!"...

then I read more carefully...

Great project btw! Some details about your horn system?

[Rocky]

Originally posted by Giaime

then I read more carefully...

Hard to get sensitivity on a SE DHT amp without using 3 stages or those elusive KC3, RS242, RS241, etc... I went with cheaper and more available tubes so I could afford to bank 3 globe pairs of each.. Replacements can come in handy when using electronics from the 20's and 30's..

Originally posted by Giaime

Great project btw! Some details about your horn system?

Thanks

I only use horns for mid-highs, they are TAD4001 drivers mounted on round mahogany tractrix horns milled by a local woodworker. They sound so darn good I am absolutely positive they will stay in my system for decades The lows are another story.. Relatively small onken cabinets employing 12'' altec drivers. Nice speakers really (by sound not look) but I can hardly integrate them with the mid horns. What I want is goto or ale bass compression drivers and horns (80Hz horns or so), but that won't happen the next 5 years I'm afraid

I have nothing on top of the TADs. They do fall off around 15kHz (they beam in this band), so I might try a bullet horn tweeter or something from 5kHz but it is not a priority. I will have to try before I buy... the HF rollof doesn't bother me so..

[Rocky]

What a nice sight; hooking up the SLA batteries to the filaments for the 1st time

[Rocky]

Some more wiring in place, still alot to come. The new CCS boards aren't soldered yet either..

[Rocky]

only the BBMCCCS circuits left now, but I need some stuff from the lab and it's easter-closed this week Perhaps I'll just mount them on proto boards from the local shack.. don't think I can wait a week for them to power up I'll figure out what to do in the morning..

[Sheldon]

Originally posted by Rocky
I have nothing on top of the TADs. They do fall off around 15kHz (they beam in this band), so I might try a bullet horn tweeter or something from 5kHz but it is not a priority. I will have to try before I buy... the HF rollof doesn't bother me so..

Purty!

Earl Geddes claims that musical content above about 10 kHz is almost nil. I wouldn't really know as my hearing falls off pretty fast above about 12k anyway.

Sheldon

[Rocky]

Sheldon;

Geddes may be very right on that.. ofcourse it all depends on the horn Fo, but mine are big, so they really don't perform too well in the highs.. thing is, with tractrix flare and round horns, in a small listening room like mine, the sweetspot is very small anyways, so it is not very noticable to me at what frequency they start beaming.. Also, I'm not a sucker for high frequencies even though I could hear 21kHz last time my hearing was checked. What I pay the most attention to in music is the band I built the horns for; voices and classical instruments, plus percussion - wich sound even better on the horns than it does in reality

That being said, I haven't had high frequencies in my setup for quite some time... I might not realize what I am missing out on?? but who cares.. someday I'll find out... perhaps sometime soon

[Rocky]

The build has taken a little longer than planned as I decided to put chokes in place of the CCSs.. they're in the mail somewhere.. most other stuff is done though, a switch is wired up for selecting RCA or XLR (only two inputs), and another switch paralells the primary windings of the volume trannies for selectable 0dB or +6dB gain. Even though this is a small integrated amplifier, I designed it more like a preamp, using RCA jacks for all outputs; a passive output, a CF output from the 12A (impedance in the 5K5 range), and 16ohm + 32ohm outputs from the 71A and output transformers for driving headphones and my horns... Inside looks like a bombed ratsnest already...