Stromberg-Carlson ASR-120 Stereo Tube Amplifier, Pt 2

In Part 1 of this series of posts about a pair of early 1960s Stromberg-Carlson ASR-120 stereo tube power amplifiers being rebuilt / restored / re-imagined, we talked about the basic design of the amp, its circuit, and its origins. While I've been lazy doing other things, like building new literal fences and mending old figurative ones, and merely thinking about getting my tube amp running, friend David from Massachusetts has been busy actually getting his ASR-120 project done.

As you can see in the photo above, the resulting finished and rebuilt amplifier is a real beauty. Here's a couple of pictures showing the circuitry inside of his amp before:

And after:

It's fairly obvious from looking at those two photos, that the amp has been almost completely rebuilt, with new components. I'll let David describe his process:

"I finished working on the ASR-120 late last night and ran some tests this afternoon. All is well, no exploding, no hum and it sounds very nice paired with the Minimus 7's. Some of the voltages are a bit high but that's probably a combination of the 120.5VAC from the wall and/or this specific 6CA4 tube.

The past two weeks have been spent obsessing about this amp and I've probably put 40 hours of thinking/labor into it during that time frame. In the end I decided to rebuild it entirely. All NOS sockets, a complete rewire and I repopulated the tag board with metal film/metal oxide resistors and Sprague 715P and 6PS polypropylene caps. Here's a partial list of some other mods/upgrades:

1) IEC socket and fuse holder on the rear; toggle power switch on the right side. Secondary AC outlet disconnected.
2) Rebuilt the filter section entirely.
3) Wired the heaters like in an old Fender, twisted and floating above the sockets.
4) Two 1N4007 diodes wired in series with the anodes of the 6CA4 rectifier to provide "soft fail" protection.
5) Two sets of RCA inputs - selectable via a mini toggle switch located on the front-right. Inputs were isolated and star grounded.
6) Speaker outputs moved from the top-front corners to the right side (the RCA inputs took their place).
7) 100K Alps stereo pot (excellent tracking and very quiet - it better be for $17!).
8) Bright white LED added off the 6.3V heaters (270 ohm 1W resistor, with a 1N4006 diode across the cathodes. The diode limits the LED's reverse voltage to 1.7v.)
9) Small red glass jewel for the power indicator (front-left). The LED wouldn't fit entirely inside the jewel so I pressed it in as far as I could and then covered the rear with black RTV gasket sealant.
10) I wasn't in love with the chicken head knob that was on the volume control, so I switched back over to my favorite maroon Davies round pointer.
11) Tube shields on the 7199s - why not, I had them in the parts bin.
12) I did take one bit of advice from Tom Bavis and replaced the two .01uF feedback caps with .15uF. Initially I wanted to wire some different value caps to a rotary switch before running it over to the board but in the end I thought it better to keep those wires short.

Here are a couple overhead shots of the chassis, before:

And after:

And finally, a JPEG of the voltage measurements taken at 120.5VAC from the wall, 117VAC from my variac, and also the voltages listed on the schematic (at 117V)":

And how does it sound?

"I've had the ASR-120 in the main system for the past week. I'm actually listening to it as I type. The bass mix using the .15uF feedback caps sounds fine and is actually more "punchy" than I thought it would be (given the 8" full range Stephens Trusonics in the main cabinets). The treble is more present than what usually comes from the Pioneer, but I'm not complaining, drums sound clearer and more well defined. Plus, I've been able to hear some pick strumming where I've never noticed it before. I'm not sure if the difference is due to the amp or the recently rebuilt super tweeters. Probably a mix of both. Anyway, the amp is dead quiet on start up, even with the volume pot maxed out. I'm happy with the results but it's strange to not be able to "hear" whether or not a tube amp is on."

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Thanks to David for the pictures and description, and really nice work!

Next time, in Part 3, we'll take a look at the other Stromberg-Carlson ASR-120 amplifier, my own project, and find out how much I can mess it up, and still manage to get it to operate okay.


Bolderdesh said...

I just pulled a complete 120 including the radio and multiplex from a console yesterday. Would have loved to keep the console but it was weathered beyond reproach. The caps and tubes were all present and in stellar condition. I would like to replace/modify as little components as possible, to keep as close to the amp’s true character. Have not been able to test it yet, because I (A) do not know the ohm load on the speakers. (B) do not know how to easily test the audio input. Any quick tips on this would be helpful. (This is also my first project of this scale).
Looking forward to part 3!!!

James Aoyama said...

Hi Bolderdash -
First, congrats on your find; glad the electronics appear in good shape, that's 1/2 the battle sometimes.

To answer your questions:
A) As for the output impedance of the amp, that's an unknown value. If you still have the console, with speakers in place, you could put a voltmeter on the wire going to one side of the speakers, and take a resistance (ohms) reading. Whatever that value is, the nominal impedance of the speaker(s) will usually be a bit higher - ie, if the resistance is 2 or 3 ohms, the impedance will be 4 ohms; if resistance is 4 to 7 ohms, the impedance is 8 ohms; and if it's 10 - 14 ohms, the impedance is 16 ohms. Keep in mind that impedance is a nominal rating, not an absolute value, since impedance changes with frequency; and since music is dynamic, with constantly changing harmonic content, impedance is always in flux, never static unless the input signal is a steady sine wave, but who listens to an unvarying sine wave tone?

Having said that, somewhere I recall reading that the nominal output impedance of this amp is 6 ohms, so that means either a 4 or 8 ohm speaker load is okay. Also, tube amps really don't care too much about impedance matching, unlike solid state amps. You're good to go with almost any impedance speaker, just don't get too crazy - maybe avoid a 2 ohm load, and stay away from anything higher than 16 ohms.

B) I use a portable CD player for most of my audio testing - a mobile phone headphone output also works well, with a 1/8" stereo plug to 2x RCA plug adapter cable. After hooking up, make sure the headphone output level on the player of phone is at zero, then gradually bring the volume up. Also, make sure you have speakers hooked up to the amplifier.

With this Stromberg amp, first you'll need to install a pair of RCA jacks and hook those up to the circuit in place of the original input jack (that is, bypassing / replacing the input jack that's there, with a pair of regular input jacks). Check out David's nice wiring job in Part 2, or wait for awhile for my own conversion in Part 3, which might take a while!

Lastly, I highly recommend you google "light bulb current limiter", do some reading, and build your own current limiter. It's a great tool in a few ways, invaluable for diagnosing unknown amps (Does it work or not? Are there any shorts in the circuit?), and for forming new capacitors and re-forming caps that have sat unused for years or decades.

For in depth answers, don't hesitate to get in touch through the Contact widget over in the right side bar of any page on the blog.