Help Me Identify This Preamp: Audio Radio Corporation - ARC 15

[SirCVH]

Hi PRR and mjrippe

Thanks so much for your help! I have attached a more complete schematic with resistor values (I learned to read
resistor bands after putting it off for too long). I will clean and finalize soon, but
I believe this to be pretty accurate. I am excited that "D" is a feedback circuit (and not riaa correction or something else), I am familiar with feedback circuits sourced from output transformers
being brought back into the circuit in Fender amps, but didn't realize this was a concept within preamps, very cool to learn.

PRR, if the added resistor values give you any insight into the circuit, please let me know your thoughts, they are always appreciated.

Do you guys think there would be any harm to swapping a 12AX7 or other 12A** variants in place of the 12AY7 in this circuit?

My plans besides full recap and cleaning are:
-Adding XLR inputs and outputs
-IEC power connector
-Instrument input direct to 12AY7
-Either a switch or pot on the feedback circuit

If you have any thoughts on these modifications, or any other ideas you think are valuable they are more than welcome. I expect to be able to start surgery later this week. I tested the jewel lamp tonight and that is working, and I know it sounds dumb, but man they don't make them like that anymore, just a beautiful glow.

Thanks again.

 

[emrr]

Plate resistors are all very low in value. So low I suspect you misread the decade band.  Maybe 10x.  But maybe not.  And if not, it’s set to minimize gain and maximize inherent 2nd harmonic distortion.  If they are 10x a 12AX7 would be happy. 

The feedback path won’t change input distortion.   

Off the top of my head this thing should be at least 60dB gain. 

The output coupling cap could be a 1mfd.  Little low as is. 

I personally wouldn’t hack this thing much.  Get it working as-is and live with it a bit.  If you decide to resell, hacks kill the Asian collector market where the money is. 

 

[PRR]

Plate resistors are all very low in value. So low I suspect you misread the decade band.  ....

They followed the 12AY7's datasheet suggestion. Page 2:
https://frank.pocnet.net/sheets/127/1/12AY7.pdf

If we believe the transformer impedances we have
iron: 1:20
V1: 12
{V2+V3=12*12=140, but 100k/2.7k makes 37, so say 32}
iron: 5:1
1:1,500 or 63+dB

 

[emrr]

They followed the 12AY7's datasheet suggestion. Page 2:
https://frank.pocnet.net/sheets/127/1/12AY7.pdf

That's sure a silly thing to do.  Not seen in much of anything in the field. 

 

[SirCVH]

I have gone back and forth on what I do and don't want to do to this thing, a full
set of new caps are coming in the mail today, so a couple of questions:

- EmRR said this might have value to a collector. The transformers would sell
as parts (which I'm not going to do) for maybe $400. Other, than that, would an
unmolested unit sell for much more than that?

- I don't plan to alter the chassis at this point. I do want to change the electrolytics
to get it powered on and see if anything else needs fixing. I have new film capacitors
coming in for the rest of the unit, would it be a detriment to the value to change out the
vintage film caps?

- I did recheck the resistors and they do seem correct. Would it still be ok to try out
a 12AT7 or 12AX7 for tube 1, regardless of sound, without damaging anything?

Hoping to have it up and going this weekend!

 

[emrr]

You don't know who will be a buyer, so it's only worth noting possibilities, not count on them. 

The Asian hi-fi guys want it completely untouched.  Don't fix anything.  Don't even blow the dust off. 

Recording people want it working, they are the opposite, they want you to add phantom power, a modern IEC power inlet, XLR's, an LED lit VU meter, etc.  But they're not going to PAY actual added value for any of that.

There's a sweet spot somewhere in between.

Either buyer type is also likely to strip the transformers and throw the rest out, at whatever price.  You never know what crazy thing will follow. 

I'd do filter caps, see if it works fine with no coupling cap leakage. 

No one thinks vintage electrolytics are a thing, unless it's a $30K WE amp, then someone will take the time to restuff the original can so it looks unmolested and no buyer knows it's been done.  But people are goofy about the magic of vintage film caps, even when they have high series resistance and or some leakage.  Only after they live with problems and decide they aren't a great idea do they tend to want things updated.  Until then they could fall on either side of the fence, worth less or worth more because it's been updated. 

Obviously there's a lot of contradictions in all of that.  Bottom line is how much time and money will you put in and get back?  You can devalue your efforts by doing too much. 

You can stick any 12a*7 whatever in there without damage.  Worst you could do is cook plate or cathode resistors if you go crazy with a 12BH7 or something.  Just use powers of observation and measurement along with common sense. If a resistor is smoking, or B+ is sucked way down, it's obviously a bad idea.  Look at the plate resistances of those other tubes, it should be obvious what's a less desirable idea.   

 

[SirCVH]

Thanks again for your input Doug, I really appreciate it!