Tele-Merc 70

While searching for 60s germanium amplifiers, I came across this cool little thing. It’s an phone amplifier intended to be used as a hearing aid. It’s got a pickup that attaches to a phone and sends the signal to be amplified through a speaker. The best part – I found a new-old-stock unit in its original packaging and bought it very cheap (5 euros).

Looking at the schematic, it looks like a strange arrangement that I haven’t seen in radios or cassette/record players. It’s a class A amplifier, which means it will draw maximum current all the time and drain the battery faster (not too big of a concern for me). Also, it has a weird interstage transformer before the power amplifier. Push-pull amplifiers have a phase-splitter transformer there to split the signal and send each half to one of the power transistors, but there’s no need to do that here.

With help from people who know more than me, I learned that back then transformers were cheaper than transistors and that’s probably the reason they used a transformer here. It works as a buffer, bringing the higher impedance signal of the previous stage down to low impedance that drives the output stage. These days, we’d just put a transistor.

In terms of controls and connectors, it’s got one 2.5mm mono input for the pickup, one power switch and volume pot.

Packaging and user manual are all in German, but based on part brands, it’s safe to assume that these were made in Japan – 2SB54 Toshiba transistors and Sankyo capacitors.

Interestingly enough, the pickup is a coil of thin wire designed to pick up EM signal from the telephone. Does it remind you of anything? A guitar pickup, of course. It’s the same thing. So, just for fun, I installed the suction cup on the pickguard, as close to high E string as possible. Surely enough, you can hear guitar playing on the speaker. Admittedly, it can only reach E and B string, but still it’s cool.


Looking at the schematic, most of it is a straight-forward amplifier, but input stage bugged me. Input jack ground is not connected to circuit ground, but is rather elevated by the 5.1K/4.6K voltage divider. I re-arranged the input to resemble standard deacy-style input with 82K/15K voltage divider and added a 33nF capacitor to block DC from going back to the guitar.

Capacitors looked OK and the device worked, but I replaced all electrolytics just in case. It’s been 50 years, they are probably not the best. I didn’t have all exact values on hand, so I used whatever I had, but tried to use them strategically to tailor the sound. 10uF coupling cap is replaced with a 1uF poly film capacitor. Input stage emitter bypass capacitor is changed from 33uF to 22uF. Filter capacitor value is bumped up from 47uF to 100uF. Second stage emitter bypass capacitor is changed from 33uF to 100uF and output stage emitter bypass capacitor is changed from 33uF to (hefty) 470uF (didn’t have anything smaller 🙂 ). Below is the schematic after the mods I applied.

In terms of construction, there wasn’t a lot of room to play with. I replaced the small 2.5mm input jack with a standard 6.3mm and had to trim the PCB a little to fit it. The wasn’t room for output jack, so I took the puny little speaker out and installed a jack where it used to be.

There wasn’t enough room on the PCB to fit the input capacitor, so I had it installed right at the input jack. I didn’t have confidence in the tiny wire they used, so I replaced all wiring using shielded cable for the input lead.


It’s surprisingly loud for 0.1-0.2W it produces. Plugged into my 1×10″ 8ohm cab it’s noticeably louder than normal TV or conversation level. It’s design to amplify low level signal coming from that suction cup pickup, so it’s very sensitive and easy to overdrive. Guitar pickups easily push it into that distinctive germanium smooth fuzzy territory. It’s hard to get clean sounds from it, but it seems like a cool way to add a fuzzy texture to the sound or as a special effect for a riff or two. And it can be used as a fuzz pedal driving another amp or pedal, as it’s so low powered there’s no risk of frying anything downstream.

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.