Converting a 1959 AKAI Stereo Terecorder Half Side to a Guitar Amp

[TubeLearner1777]

Hello everyone, for an $80 "why not" experiment, I bought the secondary smaller (the "Half Side") unit of a 1959 Akai Stereo Reel to Reel Terecorder. It contains, best I can tell, inputs and outputs as well as a tube amplifier and a small speaker.

Here's the manual in .pdf form: https://museumofmagneticsoundrecording.org/images/R2R/AkaiStereoTerecorder1958.pdf

Note that the schematic is on page 29

Tube chart on page 22

Here's a link to a photo album of pics I took: https://photos.app.goo.gl/s4xNv6dzGryxU9G68

Notes from the manual:

Output is at 8ohm and can be run into a "hifi speaker", or can use 4 or 16 with limited loss

It's adjusted for 50 cycles but can be modded to 60 (is this even an issue?)

Has a switch between 100v to 117v

The output connector has a 1volt power amp

Tubes:

V1: 6AU6T (low noise)

V2: 6AU6 (Intermediate amp)

V3: 6BQ5 (power input stage)

V4: 6AR5 (bias erase oscillator)

V5: 6X4 (rectifier tube)

The person I bought it from demonstrated that it turned on and lit up and produced hum and the pots seemed to impact the hum. I have not yet plugged it in though since buying it.

I posted this on reddit as well and received the following comment with further info if it helps:


"    It's adjusted for 50 cycles but can be modded to 60 (is this even an issue?)

For use as just an audio amp, no issue. I'm guessing you're in North America, which is 120V AC @ 60Hz mains. The mains freqency can affect playback speed of the tape portion on old tape stuff like this though, since it affects electric motor speed. A 50Hz capable power transformer running on 60Hz mains is technically just 20% oversized for the job. The opposite is also true, American amps with 60Hz only PT's running on 50Hz mains with a step-up in Europe/Aus are undersized for the job, and can run hotter or even burn out.

    Has a switch between 100v to 117v

Another quirk of Japanese stuff similar to 50/60Hz, different parts of the country have different electrical infrastructure and voltage frequency standards. I like the map on the last page of the manual. For use in 120V regions like NA, 117V is within the stated +-5% tolerance of modern 120V AC mains.

Looking at the schematic, the audio amp section is pretty straightforward. Here's a copy with the audio signal path traced in blue, and the B+ rail traced in red. The two 6AU6's are pentodes acting as Class A voltage gain stages. Oughta have fairly high gain with cascaded pentodes for the preamp. The 6BQ5 (American designation for an EL84) is the single-ended Class A power output running in cathode bias. Roughly similar to a Champ or AC4. The 6AR5 seems to be a 42kHz oscillator that has somethimg to do with driving the record heads, I'm not familiar with EM tape recording theory though.

Also note that the mic and line inputs first travel through a switch, which must be in the REC position to pass signal to the first tube grid. It also seems the other half of that switch takes the tone control out of circuit while in REC mode. Pretty straightforward to reroute though.

Best practice on old electronics like this in unknown condition is to bring them up slowly on a variac and a dim-bulb current limiter in case of shorted components. Bringing it up slow can also help to reform the insulating aluminum oxide layer of old electrolytic filter caps."

(Attached is a copy of the schematic that the reddit poster generously outlined the signal path on.)



Here's my questions before I plug it in and try it, since all I have done it photograph it thus far;


Does it look safe to plug in and try? I didn't see any obviously bulging or leaky caps or anything, but one of the transformers looked a little... goopy?
Should I look at having it converted to 3-prong?
Can it be used as a janky guitar amplifier either as-is or with modifications? The poster above mentions modifying the signal path as a possible solution.
How heavy do you think those mods might be?
Anything stand out from pictures or that you know about this thing?
Can you give me more specifics on the type of modification that would be required to convert this?


I really only work on guitars and pedals and with amps I know just enough to know what not to touch to stay safe. I can barely read a schematic, and I don't know much about tubes. I'd likely have the work done by a tech if it involves anything more than rerouting a few wires. Safety is my first priority here.

This thing is in great shape and has amazing 1950s/60s Japanese quality at least in terms of hardware that I can see, and I'd love to give it life as a guitar amp to make some noise, and any advice is appreciated.

I thank anyone who takes the time to look at this and help me learn more about this awesome little machine.

[WimWalther]

One thing's for sure, no one can accuse you of trying to take the easy way around. =)

What you have here are potentially the makings of a single-ended (class-a) amp that might make around 3W into 8ohms.

From what I see, the only parts of real interest are TR1, TR2, 6X4, 6BQ5 and possibly the 6AU6 (X2). TR3 might also be useful, as it appears to be a choke rather than an actual transformer.

The rest of it.. basically dead weight, though a few items might be reusable. Can't tell how useful the chassis might be.. it's kind of an odd layout.

As far as "safe" to plug in & try.. it depends what you mean. It won't grenade and take down the house or kill you, but the hum indicates bad filter caps which could leak electrolyte, possibly heat up & go "bang" or, worst case, kill the power transformer.

If you have access to a Variac or dim-bulb tester, now is the time to use it.

[TubeLearner1777]

Thanks for taking the time to type that up. It sounds like it'd be more work than moving a few wires around then. Perhaps I am out of my element here!

Maybe it will go on the project shelf until I have a better understanding of these types of circuits and tube tech. It's not really the kind of thing I want to spend a bunch of money with a technician on.

Again, thanks for the comment!
 

[WimWalther]

If you're looking to convert an existing piece for gitamp use, look for an older PA amp that uses a pair of identical power output tubes (examples are 6BQ5 / EL84, 6V6, 6L6, 6AQ5, 6CA7/EL34).

Avoid any pieces of equipment that have tube types prefixed with numbers above "12" - (examples to avoid are 25L6, 50L6, 50C5, 35Z5, 117??).

Dedicated PA amps that use common audio tube types are much simpler to repurpose as gitamps.

[PRR]

Most tape decks, you gimmick the switching so the preamp is in Record, Mike and the power stage is in Play, with the gain pot in between, and recording EQ lost. A couple of jumper clips, not a rip-and-ream rebirth.

How you do that on some arbitrary cheap deck is left for someone who can care. The circuit is convoluted by economics using same parts for multiple functions.