These two classic amps from Eico are fine examples of the great stereo integrated tube amps that were coming out in the early sixties, shortly after the dawn of stereo.
Amplifiers like the Eico ST-70 and ST-40, the Dynaco ST-70 and the Heathkit AA-32, were real performers and came in kit form or factory wired. These amps were very popular with audio enthusiasts at the time.
This wonderful pair of ST-40s came to us from Patrick in Belleville, Ontario. He requested a full restoration, not only to recap, but also remedy some issues with the amps. One was dubiously assembled as a kit, and the other was factory wired and already partially restored by a competent amplifier technician.
Big Sound, Little Watts
I must say, we were pleasantly surprised with the final result. This is a very, very nice sounding amp, with a big sound for only 20W. The detail is astounding, a testament to the clarity of the very excellent 7247 preamp tubes.
All tubes were tested on my venerable Hickock tube tester. One amp had been upgraded to a complete set of new JJ Electronics tubes. The other amp had a mish-mash of odd tubes at varying strengths. I brought both amps up slowly on my Variac, as some dead electrolytic capacitors can explode if they haven’t been used for years.
Gas and Glass
On the kit amp, the 5AR4 rectifier had something vibrating inside the bottle, something loose. Sure enough, of the two rectifiers in the tube, one half was dead. I replaced it with a NOS Shuguang tube.
The dramatic improvement in dynamic range and bass response can be largely attributed to beefing up the power supply section. The fast film caps result in tighter bass and improved responsiveness, often lacking in stock vintage tube amps.
Plastic Power Filters?
For the power supply filters, I opted for the large-sized WIMA film capacitors in place of the old, dried-out electrolytics. I polished the old can caps and left them in place for cosmetics, and installed the new polys underneath.
For a long time, it wasn’t practical to use film caps at such high voltages, as they were just too big. But recent technology advancements have shrunk the size of capacitors significantly. Fortunately, there was plenty of space under the chassis to accommodate them.
Eico’s labeling of the jacks and front panel selector switch is bizarre. One has to remember, this was the early days of stereo, so they weren’t quite sure what to do with it yet.
What the heck is PHB2 and MX1?
FM stereo required an MPX module to multiplex the two channels. Phono cartridges of the time were magnetic, ceramic and crystal, so it soon gets confusing.
Our customer requested that we relabel these items to make more sense, as well as labeling the tube positions on the chassis.
One interesting feature of this amp: Eico, in trying to future-proof their products, included a set of AM Stereo Tuner RCA inputs, anticipating AM’s eventual migration to stereo, which never really happened.
Eico did a couple of goofy things here. One is the convoluted speaker hookup on the back. They tried to add a centre speaker and ended up making the terminals too complicated. At the client’s request, I rewired the speaker terminals as normal, with simple Left and Right terminals.
It’s a Mute Point
I installed a modification that corrects a well-known flaw with the ST-40 and ST-70 amplifiers’ Loudness switch.
This from the Antique Radio Forum: “Two problems that these amps exhibit: one, all their volume is dialed in within about the first 20% of the volume control’s rotation, and an imbalance favouring the one side at low settings. And two: the stock Loudness switch performs more like a bad Mute switch which cuts the volume but doesn’t add any real bass.”
Is That a TV?
The underside of the chassis reminds me of working on old tube TV sets. They were always massive chassis, packed underneath with hundreds of analog components clustered around the tube sockets.
In this case, I replaced all the electrolytics, ceramics and some old mylars from a previous upgrade that were the wrong values.
I left most of the carbon film resistors in place, but replaced the main power resistor on the rectifier tube with a high-quality Dale part, as it takes quite a beating. A Weller Desoldering Station makes quick work of removing this many components and minimizes damage to the tube sockets.
A lot of attention went into restoring the rotary selector switch. It was common practice with all amplifiers of the time, to attach a fair amount of circuitry around the switch terminals.
I found a pair of resistors mistakenly swapped with another pair. That didn’t help. I replaced the original ceramic and mylar caps (part of the RIAA equalization circuit) with WIMA polys. The selector switch was relabeled to simplify the selections.
Cosmetically Speaking
I was able to clean the chassis to the bare metal, paint the rusty transformers and tube shields and tidy up some of the wiring. The brushed aluminum front panel cleaned up very nicely.
I also added some tube shields that were completely missing.
All Systems Are Go
After setting the hum balance controls and checking all the outputs with a pair of very accurate Simpson analog meters, the amps were ready for final testing in the system.
