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Acrosound family reunion: S1001/UL120

Brice

Brice

AK Subscriber
Subscriber
Hello guys,

I did some work on the only Acrosound pre-amplifier ever produced, with the idea of making it work together with it's mate, the UL120. Thanks to Diamondsouled for finding me this one.

First I thought it would be an easy caps swap and go. As usual, it did not happen this way.

First I started to notice some oddities with the boards versus the schematic. After some time, I realized that the model I had did not follow the schematic as it has 2x7199 per board and not a 7199 in low input followed with a 7247.
This is odd but reality: the version I have is not the "regular" model, but maybe an early version.

After a change of all electrolytic caps (1 HV cap was shorted), and set the correct tubes, it came to life.

But It had a high level of hiss that I tried to chase.

Long story short, I ended up changing all the caps and CC resistor with new CC resistor and orange drop mylar caps, tube sockets, added some shielded cable in strategic places. And also the tone control old resistor/caps which did help a lot. And I went DC on all tubes heaters.

I still have some hiss when I go very close to my speaker, but I am guessing this is the nature of this design.

I only use the medium amplification AUX, so only one tube is used. I tracked that the noise is generated by the first pentode after the tone control, and not the second stage triode. Even with the screen pin 7 to ground, the hiss is still there. I don't get it.

Anyway you have to get your hear very close to the tweeter, else you can't hear it.

I am now listening to the great Acrosound duo: the sound produced by these products is quite astonishing for such old equipment. And I like the look :) and the preamp controls are funny to use.

Here is pictures of the set, and the schematic of the second amplification stage on my model.

Thanks for reading,

Brice.

IMG_4228.JPG

IMG_4229.JPG S1001-Zoom-7199-2.jpg 20170629_194615.jpg
 
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Love it!!!
Fwiw acrosound made the exact amp and preamp for radio shack and branded them realistic.ive always wanted to try a acro amp and preamp but none ever showed up when i had the cash to buy.enjoy !
 
Well thats certainly different, a 7199 doing something other than phase inverting. Cathode follower output too, nice touch.

Didn't even know this piece of gear existed. Nice looking, and it goes well with that UL 120.
 
Thanks guys,

Radioative, cool posters.

Kinda Gadget. The PT is different as there is no tube rectifier in the UL120 but SS.

There is difference in component values but the schematics are identical.

Also the ULII has a TO600 while the UL120 has a TO600A: difference unknown to me.

In terms of sound they are very similar.
 
Brice -- In my opinion, any remaining 7199 left in the world is just crumbs being passed around time and again when one is found out to have the noise and other quirks it has. In a unit I just had for a client, the problem was bad enough that the section that tube was used in was redesigned to use a more conventional, readily available tube.

Pin #7 is the control grid for the pentode section of the 7199. Grounding that pin simply grounds out the audio signal applied there, so you would still hear any hum and noise developed within the particular tubes used in that location.

A very sharp looking piece! Congrats!!

Dave
 
Hey Brice,

That looks spectacular! Can't wait to hear it!

I've got a bunch of 7199s and I'll screen them for noise. I'll bring the best with me for our next beer. Hopefully we can improve the noise problem.
 
Thank you all. That is indeed a nice piece, and it sounds superb.

I've also tried NOS 6GH8A with an adaptor but the noise level is similar. I suspect that the other model with the 7247 has the same hiss. Anybody has one ?

One last question: the bass and volume pots have 4 connectors. I am unfamiliar with this.
Is there a way to replace them with regular 3 points potentiometer? Did they use those to simulate a logarithmic pot?

Here is a picture of the pot.20170704_133151.jpg


And here a quick view of the inside.


20170707_161319.jpg
 
I don't know what an effort it might be. I was just wondering how hard it's be to breadboard the other version to see if its quieter or better in some way. Its likely pretty involved. Also you're not likely to use it that way in the end so probably a waste of time.
 
That is a sharp looking piece. I like that gold/black color scheme, with the slightly techy, but not overly so, front panel
 
I have the 7199/7247 version that is self powered and mine is hiss free but it suffers from a low level hum, possibly from the aforementioned power supply (tube rectifier). I have also modified it with a separate DC supply for the filaments. Aside from the hum it is one of the most detailed preamps I have heard.
 
Thanks for your input Ramjet. That's what I thought. Using a 7199 at that spot is probably not a good idea.

Funny, mine is hum free but hiss, and your's is the opposite.

The hum is certainly reducible by some measures.

One obvious, the wires going to the OUT of the preamp are not shielded and run close to the AC HV and heater supplies.
Increase the supply caps on the board. If you have the original caps, they are probably at the end of their life.
I went DC heater for all tubes with a simple modern bridge and a big 10000uF cap.

I agree, the details and sound stage is really good. I was expecting a tubby sound: not at all. It has the Acrosound signature.
 
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