Hi, I have a pair of AR-410s that I am going to rebuild. I would be grateful for answers to some questions and any suggestions towards best performance. The amp is form 1953. Is that before the riaa curve? Will the 12au7 work well as a phono stage? Can I use just a SUT? Would I have to change the circuit to use 6v6 tubes instead of 6f6? Thank you!
I don't know about the RIAA, possibly you'd need to adapt some part values for that.
But it has a very elaborate loudness control function!
I would keep that, very unique!
Jan
But it has a very elaborate loudness control function!
I would keep that, very unique!
Jan
When looking at an advert for your amplifier (Audio, January 1954), which says it as two setting for phono, being "American" and "Foreign", in combination with what is on Wikipedia about RIAA equalization, including the situation before RIAA was well established, my guess is that it doesn't have RIAA yet. But with the two types of equalization and the tone controls, I would think that you can come close to the RIAA curve.
Advert - Audio, January 1954
RIAA equalization - Wikipedia
Advert - Audio, January 1954
RIAA equalization - Wikipedia
There's a slight discrepancy in the parts value between your schematic and one I found online, especially the feedback resistor in the amp section. In your amp it's R31 = 47K and the below is R36 = 12K. I guess you have to look at the actual part in your amp.
This rig's PHONO AMER is essentially RCA New Orthophonic which is where RIAA got the idea. It sims as -3dB/+1dB from 27Hz to way past 20kHz. The small bump at 250Hz is entirely in keeping with 1950s tastes. The -9dB@10Hz helps with the heavy rumble in most 1950s turntables, and anticipates the later IEC 20Hz amendment. I would not change anything. Just fix what is broke or burnt or rotten.
The SC is a fine machine. The next iteration was my main rig for years; my partner still misses it. I did do a lot of changes and was never convinced it was "better" (except socket-cleaning). It reached a point that I couldn't jiggle the crackle out as fast as it came back, I had OK transistor gear on hand, I took it to the shop for deep cleaning which didn't happen.
Ok. Thanks.
I was wondering what C1(photo)C4(sams) .047 was for? Protection? Because the grid on V1A gets grounded? Thanks.
I was wondering what C1(photo)C4(sams) .047 was for? Protection? Because the grid on V1A gets grounded? Thanks.
V1A is "grid leak bias". The very high (3Meg) grid resistor against the small grid leakage current self-develops a small negative voltage. This trick is so old that most small triodes were adjusted to work this way. Of course if you shunt the 3meg with a 1k DCR needle, or even 47k load, that bias goes away and you can have grid distortion.
Stromberg and Carlson were in business from before tubes were invented until 1955 (and the sell-off stayed in the phone business for decades). Book: Stromberg-Carlson Telephones 1894-1984. They knew their racket. Much of their stuff was telephony, very good quality. They knew home hi-fi was a less demanding market but the stuff is not cheep.
Thank you.
Will the .6v vary depending on the tube used? I guess two quiet au7s that match will be needed. Not exactly easy these days.
Will the .6v vary depending on the tube used? I guess two quiet au7s that match will be needed. Not exactly easy these days.
To answer your questions:
6V6 's can be used here instead of the 6F6's without changing anything. Their characteristics are relatively similar albeit that the 6F6 wants a little higher neg. grid voltage but that's not an issue here.
I would change the fase inverter into a 12ax7. That tube simply sounds better. Again you don't have to change anything you can just put that tube in and listen.
Because of the large amount of coupling caps in the phono stage I would change that into a regular 2 stage 12ax7 amp. many of which can be found on the inernet (look at the Lafayette LA 224a f.e. , I have one myself). You then have 2 coupling c's instead of 4.
Good luck!
6V6 's can be used here instead of the 6F6's without changing anything. Their characteristics are relatively similar albeit that the 6F6 wants a little higher neg. grid voltage but that's not an issue here.
I would change the fase inverter into a 12ax7. That tube simply sounds better. Again you don't have to change anything you can just put that tube in and listen.
Because of the large amount of coupling caps in the phono stage I would change that into a regular 2 stage 12ax7 amp. many of which can be found on the inernet (look at the Lafayette LA 224a f.e. , I have one myself). You then have 2 coupling c's instead of 4.
Good luck!
Thank you KeesB.
FWIW- the sams shows 4-r19 resistors. The 100k (2w) that sits between r16 and r33 is not on original schematic (photo). I think it was in the amp. I have photos that show, I can't put my hand on now.
I'm not going to change the phono stage. Why are there resistors on the heaters of the phono stage?
Thank you for all answers and comments!
FWIW- the sams shows 4-r19 resistors. The 100k (2w) that sits between r16 and r33 is not on original schematic (photo). I think it was in the amp. I have photos that show, I can't put my hand on now.
I'm not going to change the phono stage. Why are there resistors on the heaters of the phono stage?
Thank you for all answers and comments!