In the "Reference Material" section below is a set of correspondence between Ed Stiles and Bill Johnson (founder and President of Audio Research) regarding Bill's suggested modifications and upgrades to the ST-70-C3 from early 2004. Thanks Ed, great material!
This amplifier was designed by ARC in the mid 1970s as an entry level product, allowing a low cost path to owning ARC equipment. The amplifier started with a Dynaco Stereo 70, but used only the chassis and transformers from the donor amp. All other components, input/output connectors, tube sockets, power supply capacitors, etc. were new. It created an essentially new amplifier from a new or used Stereo 70. The circuit was significantly more complex than the ST-70, and only a bit simpler than the ARC D-76A amplifiers of its time (only one pair of constant current source tubes was omitted). It used a solid state rectifier (unlike the ST-70), and used 7 small signal tubes in the front end (versus 2 in the Dyna ST-70). Due to the current demands of the heaters for all those input tubes, the output tubes were changed from EL34/6CA7s in the Dyna ST-70 to 6L6GCs in the ST-70-C3, because the latter require significantly less heater current.
ARC originally planned to sell kits for this amp, as well as modification kits for the Dynaco MKIII power amp and PAS preamplifier, as announced in their newsletter below, dated 6-1-74.
Above: ARC Newsletter of 6-1-74 announcing DIY Dynaco mod kits. Click to enlarge.
The plan to sell kits was abandoned shortly thereafter, as indicated in the following newsletter dated 7-29-74, suggesting that a survey of dealers indicated the dealers didn't feel it was a good idea. The new plan was to offer only factory modifications, starting with the Stereo 70.
Above: ARC Newsletter of 7-29-74 announcing factory mods only. Click to enlarge.
The ST-70-C3 did show up on an ARC pricelist dated 11/20/75, under the "Modifications" section, and was listed as "ST-70 to ST-70-C3", for a price of $385, and a 3 week delivery time. Note that that same pricelist also included the "MK-III to MK-III-C3" and "PAS-2(3)X to PAS-()-C3" modifications, each at a price of $295, and delivery of "To be announced". I believe some of these modifications were done at the ARC factory, but cannot say that conclusively. The ST-70-C3 kit idea sat dormant for three years, until in 1977, a deal was struck with Old Colony Sound Labs (a divsion of Audio Amateur Publications, now AudioXPress) to sell and support the kit version, which included a pre-assembled circuit board. A construction article by Ed Dell was published in Audio Amateur in the 4/77 issue, and the kits were sold thru Old Colony for $248 starting shortly thereafter, the first ad appearing on the "Old Colony Kits" page in the 1/78 issue of Audio Amateur, shown below.
Above: Audio Amateur ad for ST-70-C3 kit, 1/78 issue.
Above: Photo showing Old Colony kit contents as packed.
Above: Another view of most of the kit contents spread out.
I bought this ST-70-C3 on ebay a couple years ago. My modifications were primarily limited to replacement of parts for reliability reasons (including all electrolytic caps and some chassis mods for ventilation), plus replacement of audio coupling caps with newer higher quality parts. No circuit changes have been made. I also made a number of parts substitutions suggested by Bill Johnson of ARC, who designed the amp (details below).
Above: Under-chassis view of stock ST-70-C3 before mods. Click to enlarge.
This amplifier had a moderate hum in both channels when I bought it, due to aging and deteriorating filter caps. Since the power supply capacitors are no longer available in the same capacity/voltage ratings, I decide to leave the existing (defective) chassis mounted electrolytic caps in place to maintain a stock look, and install replacements below the chassis. I designed and built a small printed circuit board (PCB) to hold the new electrolytic power supply caps. In the case of the main 230ufd/475 volt power supply cap - since the turn-on surge voltages are high - I decided to put two 470ufd/350 volt Panasonic TSHC caps in series to get a 700 volt rated cap. The other stock 475 volt cap was originally a dual 50ufd unit, which I also left in place for a stock look, but replaced with two 68ufd/500volt Panasonic TSUP units on my new PCB. All of the Panasonic caps are from Digikey.com.
Above: New B+ power supply PCB, replacing both chassis mounted electrolytic caps. The larger black caps are 470ufd/350volt caps in series (effectively creating a 235ufd/700volt cap), and the two smaller black caps are 68ufd/500v units replacing the stock 50ufd/450v caps. These are all Panasonic TS series caps, and only 30mm tall, so they fit below the chassis. New wiring setup for the main B+ supply is above to the left (under the stock 230ufd cap) using a terminal strip.
Above: Bottom view of new power supply PCB for under-chassis mounting of new low-profile Panasonic power supply caps.
Above: New bias caps (top) and new DC supply for V1 tube (below). All of these new caps are Nichicon TVX, from Mouser.com. The original bias supply caps were 50ufd/75volt units, and were replaced with 100ufd/100volt caps; the V1 heater supply caps were originally 500ufd/12volt, and were replaced with 470ufd/25volt units.
On the PCB, I replaced all of the old mylar and non-polar electrolytic coupling caps with Sonicaps (from Soniccraft.com), and replaced all audio ciruit carbon film resistors with metal film 1% types, all per Bill Johnson's 2004 suggestions (see link below in Reference section). Below are before and after photos.
Above: ST-70-C3 PCB stock, before parts replacement. Click to enlarge.
I made a number of hardware changes:
- Replaced the stock Switchcraft RCA jacks with Vampire CM2F gold/teflon units from Percyaudio,
- Replaced the speaker terminals (which were damaged) with new replacements from ARC,
- Replaced the on/off switch with a stock-looking DPDT 6A unit from Radio Shack, part number 275-403A (with the contacts wired in parallel), and
- Installed a new AC cord (16 gauge HPN appliance cord) and strain relief.
- I also added a pair of 1/4" insulated normally closed jacks between each pair of output tubes to simplify checking and setting bias, per note C on ARC's schematic (see below).
After most of the above photos were taken, I replaced all diodes in the amp (which were all 1N4005s) with Telefunken SF4007 ultra-fast soft recovery diodes.
The small signal tubes that came with this amp were Siemens 12AX7s and GE 6FQ7s, which all tested strong and I have kept in the amp. The 6L6GC output tubes that came with the unit were bad, and were replaced with a matched quad of Svetlana Winged C's.
Since this amp runs so warm (okay, HOT!!!), I added ventilation openings to the top and bottom covers, and a fan unit to sit atop the cover. For the chassis bottom plate, I cut out sections below the main PCB and below each set of output tubes. On the top cover, I cut sections out on the sides near the output tubes. All of these cutouts had perforated metal stock epoxied in place over the holes. Although the PCB and associated high voltages in this amp are protected from prying fingers by a perforated metal cover over the PCB (unlike the stock Dynaco Stereo 70), I prefer the added safety of the cover, but with fan cooling (more on the fans below). In addition, I replaced the original feet with new 5/8" tall feet (part number 51620F8G from Mouser.com) to raise the unit a bit higher to aid airflow.
Above: Bottom cover showing new ventilation openings.
Above: Bottom cover, showing view from the bottom.
Above: Top cover showing new ventilation on sides. Painted Rustoleum Satin Black. Click to enlarge.
I planned to add a trio of 80mm computer fans atop the cage to move air out of the unit. After some research, I selected the Panasonic Panaflo FBA08A12L, based on recommendations to use them in silent computers. Unfortunately, after building a nice little mounting bracket for the 3 fans and wiring them up, I was unable to get the fans to operate quietly, even after reducing the supply voltage from the nominal 12volts to 6volts. From my parts bin, I dug out an old pair of 4.75" 120volt Papst 4800N fans I has used years ago, and wired them in series so each sees only 60volts. They still move quite a bit of air and are almost dead quiet in this configuration, so I simply bolted them to the top cover, isolated with rubber grommets. I could have used #4 or #6 hardware to mount the fans, but that would require drilling larger holes in the cover. Instead, I used #3 hardware to fit in the existing ventilation holes of the cover. The amp now runs dramatically cooler than with no cover. It barely gets warm to the touch.
Above: Top cover with two Papst 4800N cooling fans.
Above: Refurbished ST-70-C3, no tubes. Click photo to enlarge.
Above: Rear view. Click photo to enlarge.
Above: With cover. Click photo to enlarge.
Above: With cover, front view. Click photo to enlarge.
2004 Upgrades to the ST-70-C3 suggested by Bill Johnson of ARC:
Thanks to Ed Stiles, the following PDF file is a set of correspondence between Ed and Bill Johnson regarding Bill's suggested modifications and upgrades to the ST-70-C3 from early 2004. It includes Bill's suggested parts upgrades and a circuit revision to remove the bipolar electrolytic caps from the signal path.
Click HERE to see the PDF file containing Bill Johnson's recommended updates.
Above: ST-70-C3 schematic. Click to enlarge.
Above: ST-70-C3 parts list. Click to enlarge.
Click here for the ST-70-C3 Modification manual, 15 pages in PDF format, with step by step instructions, schematics and parts list.
ARCDB, for more info about the ST-70-C3, and all things ARC. Highly recommended!
The following article is available in back issues from AudioXpress:
Audio Research Remakes Dynaco's Stereo 70, article by Ed Dell, Audio Amateur 4/77.
Percyaudio.com - Cardas RCAs, blackgate caps
Mouser.com - Nichicon TVX Caps, feet
Digikey.com - Panasonic TS series low profile caps
Soniccraft.com - Sonicap coupling caps