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Akai M7 monoblock valve amp project


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This is something I bought many years ago.  I even had a dabble at fixing it up.  The mechanics were shot - the pot metal cams and things had all fallen apart.  So what we have left here is the wooden rack/case and amplifiers.  EL84 based single ended monoblocks with a few extra sockets from the oscillator and preamp valves.  I am starting this thread to goad myself into completing the task this time :) 

 

The Akai M7 is VERY military looking and runs off a low voltage internally (100 volts I think.  Anyway, there's a transformer/choke that steps it down for the monoblocks which have their own power transformers for their own power supplies.  Brushed off the shed growths, and this (pics below) is what I am starting with. 

 

I intend to try and take it slow and turn this into something nice, distractions permitting.

 

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Looking forward to seeing those VU meters lit :)

 

Here's the schematic of one of the channels.  Left and right aren't identical, but it's mostly switching differences and one shared bias/erase oscillator

 

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Here's some shots of one of the monoblock modules

 

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This is something I bought many years ago.  I even had a dabble at fixing it up.  The mechanics were shot - the pot metal cams and things had all fallen apart.  So what we have left here is the wooden

I found one of the meters' glass had been pushed in a bit.  Quite an operation to remove the meter, but it is out.  I will renew the glue and reassemble it.  I did check, those little bulbs light up b

and there it is with new LED lighted VU meters.  I picked yellow for the LED colour as I wanted to keep that nice warm incandescent look.  I think that worked well.  Also, the more directional LED lig

 

 

So, started tracing the circuit.  I am starting from the speaker and working back. 

 

First decision to make.  How many of the original components should I keep.  Here's a closeup of the tag board.  These are fine looking components and many will be working well and not really needing replacing.  I like those japanese tubular resistors with the values all written on them.  Even the Akai electros look fine, and I bet many test fine as well.

 

Also, look at that coil around an electro !!  Will have to figure out why they did that.

 

 

 

2021-03-05_10-07-50.jpg.daf2ca3ffec3bb0bb93c9b889c93600a.jpg

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Posted (edited)

I found one of the meters' glass had been pushed in a bit.  Quite an operation to remove the meter, but it is out.  I will renew the glue and reassemble it.  I did check, those little bulbs light up beautifully :) 

 

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Edited by aussievintage
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Meter fixed.

 

Next step will be to remove the bias oscillator circuitry.  This is the main difference in the two chassis, and strangely they moved the rectifier two sockets to the left in order to fit this oscillator and keep it as far from the high gain valves as possible (both rectifier 6X4 and oscillator 6AR5) have 7 pin sockets.  I will swap the rectifier back to this far right position after I clear the oscillator components and the chassis mounted coil.  I want the two modules to be identical - is that being OCD? :) 

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OK, half done. :)  Before and after pics.

 

Oscillator still in place (before)

2021-03-05_12-54-31.jpg.5826d701e7c89e1b744aa1044e564b99.jpg

 

 

Oscillator removed (after)

 

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In order to swap the rectifier back, I discovered I have to rewire the heaters as the rectifier is supplied from a separate heater winding to the rest of the amplifier valves.  So the blue and blue/green heater and meter light supply wires have to be removed from the socket and joined, then the yellow rectifier heater wires have to be moved into place (along with the HT wires of course)

 

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Also, this afternoon I removed the big record/playback switches.  Some things like the meter signal, and amplifier feedback, and input wiring, all go through this switch, so when it is removed they have to be hardwired into the playback position.

 

Also in this picture you see the power supply electro can and choke.  Yes, it's a nice valve rectifier followed by CLCRC filtering.  Hope the electro is OK.  I hate having to source can electros. 

 

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Even though I don't know the difference between an oscillator and a capacitor I'm looking forward to seeing how this all turns out.

 

Good luck with it all.

 

Cheers.

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58 minutes ago, Interface said:

Even though I don't know the difference between an oscillator and a capacitor I'm looking forward to seeing how this all turns out.

 

Good luck with it all.

 

Cheers.

 

 

Me too :)     It's not as if these old reel 2 reel amps haven't been modified before.  They are such a nice platform for projects.  I have seen them turned into small guitar amps very successfully.  Bit of a waste though, as the specs are much better than that.  The amp itself had to be better than the tape head at top speed - so the 7 1/2 IPS spec from below applies.  As it also says lower down,  about 6 watts as a SE pentode.  I can fit an option for a few watts of triode wired delight as well :) 

 

image.png.a7c44e568795ea788dc115fab6f4e791.png

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I am watching with interest. Might give me the incentive to finish a similar project I started a couple of years ago.

 

An M8 that suffered the pot metal failures common in the 7 & 8`s.

Mounted in a temporary frame I replaced the volume pots and a couple of capacitors. 

It does sound very nice.DSC04025.thumb.jpg.c2bb8d211bce9acddde126dd8e7b10d3.jpg  

Tony

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6 hours ago, btd said:

I am watching with interest. Might give me the incentive to finish a similar project I started a couple of years ago.

 

An M8 that suffered the pot metal failures common in the 7 & 8`s.

Mounted in a temporary frame I replaced the volume pots and a couple of capacitors. 

It does sound very nice.  

Tony

 

 

Your temporary frame brings up another decision I have to make.    The final housing.  I had contemplated using the original box which has a 'rack' system built into it, but I haven't been able to figure out how to make it look good.

 

 

 

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Family and other commitments means I haven't done a lot, but I did measure the power supply can caps.  Each can has 2 x 20 uF caps in it.  All four sections measured very similar to each other (and all about double - i.e. each 40uF).  But it's a cheap cap tester.

 

I think I will try them out and see how they go.  They look good, i.e. no bulges or leaks.

 

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But if known good caps test accurately with that device? then these would be way out of spec, in layman's....dem is kaput!

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37 minutes ago, muon* said:

But if known good caps test accurately with that device? then these would be way out of spec, in layman's....dem is kaput!

 

Funny that the go kaput by increasing capacitance though.  I would have expected capacitance to go down and ESR to go up in a stuffed cap.

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Posted (edited)

I have been doing some searching around, and it seems it is not unknown for old caps to increase in value.  A few plausible theories, none of them good news, sort of explains it.

 

Oh well,  will have to write out a list of all the caps in these amps and order some new ones.

Edited by aussievintage
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The only thing hard to buy these days is the can caps.  This one seems to be skinnier than the first ones I found - only 25mm vs 35mm in the replacement.  The search begins.  Of course, I can always replace it with a couple of normal caps, or maybe even squeeze the bigger replacement cap in by re-drilling the chassis.

 

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Just now, muon* said:

 

 

I wouldn't go up, because it's already more than usually recommended by the manufacturer data sheet for 6X4 rectifiers.  Akai have allowed for internal impedance of their transformer etc I guess, but I can't calculate it to check if higher is OK.

 

The 16+16 uF would probably be fine, but they are the 35mm size I was already looking at (well 34 in this case).

 

Cheapest and most accurate would be to strap two 20uF pigtail electros together and put them in the existing bracket together.  

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Well I am resigned to replacing every cap in these amps.  Just discovered all these coupling caps are stuffed in the same way. (confirmed by two different meters)

 

They look like small electrolytics, and are labelled as "oil", and they read high in value, and fairly lossy.

2021-03-08_09-26-51.jpg.36a8aec82fba159441376cb14fd72f9a.jpg

 

 

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Ah, old paper n oil maybe, those did no age well if the same as the ones I'm thinking of.

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3 hours ago, muon* said:

Ah, old paper n oil maybe, those did no age well if the same as the ones I'm thinking of.

 

Yeah, like those wax paper ones too.    It's ok, I have plenty of 0.01, and 0.02 caps in the drawer, just have to work through them and replace them all. :)     

 

I thought they were all a bit small at first, as coupling caps go, but the design uses 500K input resistors, so that get's us down to about 15 Hz - low enough for audio.  They used a 0.01 uf in one place only, which limits the amp to about 20 Hz.  I guess this is the one they used to control the overall response.   The af bypass electros are way down below that. too.

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18 minutes ago, aussievintage said:

 

Yeah, like those wax paper ones too.    It's ok, I have plenty of 0.01, and 0.02 caps in the drawer, just have to work through them and replace them all. :)     

 

I thought they were all a bit small at first, as coupling caps go, but the design uses 500K input resistors, so that get's us down to about 15 Hz - low enough for audio.  They used a 0.01 uf in one place only, which limits the amp to about 20 Hz.  I guess this is the one they used to control the overall response.   The af bypass electros are way down below that. too.

You can get Russian silver mica in 0.01uf cheaply, very transparent, but apparently sound best with significant DC across them, according to Thorsten Loesch in regards to silver micas.

Need to pick the caps well though as some are said to have a soft bass, others don't have this issue.

Edited by muon*
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1 hour ago, muon* said:

 

You can get Russian silver mica in 0.01uf cheaply, very transparent, but apparently sound best with significant DC across them, according to Thorsten Loesch in regards to silver micas.

Need to pick the caps well though as some are said to have a soft bass, others don't have this issue.

 

 

You don't think it might be overkill for amps like these?   The output transformer is likely to be the bottleneck for SQ in these.

 

I think I will get them up and running at least first.

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Just go down one drawer further and pull out all the old mustard caps instead. 😁

 

The Japanese made power transformers used in these units are really nice quality. I have the exact same one as yours installed in my Eric McChanson's prototype tube pre amp. Plenty of capacity left in reserve on the secondaries.

 

Agreed - try some some new cheap electrolytics in there to get the units up and running then look no further than installing some small-sized fergie or A&R iron for the OPTs in the near future.

 

Nice 'vintage' tube-based project by the way. Have been following it with some interest. 👍👍👍

Edited by xlr8or
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5 hours ago, muon* said:

 

You can get Russian silver mica in 0.01uf cheaply, very transparent, but apparently sound best with significant DC across them, according to Thorsten Loesch in regards to silver micas.

Need to pick the caps well though as some are said to have a soft bass, others don't have this issue.

 

I have 4 of these and they sound excellent in the position of coupling caps. Agree with TL' s verdict on these soundwise. They are however very challenging to find as they come up seldom on eBay. Also, they need to be piggy backed to get higher uF out of them as they only come in a maximum size of 0.1uF. Soldering these is also a challenge due their bent right-angled rigid pin arrangement. I know you luv your caps. 🤗

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8 hours ago, xlr8or said:

The Japanese made power transformers used in these units are really nice quality. I have the exact same one as yours installed in my Eric McChanson's prototype tube pre amp. Plenty of capacity left in reserve on the secondaries.

 

Yes I was impressed with the power trannies when I saw them.  I actually have a third one of these modules,  from one of Akai's mono models. Looks almost identical.

 

 

8 hours ago, xlr8or said:

Agreed - try some some new cheap electrolytics in there to get the units up and running then look no further than installing some small-sized fergie or A&R iron for the OPTs in the near future.

 

They are hard to find.    But yes, plenty of tweaking can be done once they are working.

 

8 hours ago, xlr8or said:

Nice 'vintage' tube-based project by the way. Have been following it with some interest. 👍👍👍

 

Thanks.   I was thinking about that first EF86 socket.  I could put a nice 12AX7 based phono preamp in that.

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I have an M8 that I'm trying to turn into two standalone amps.

 

I got it wired up to run at 100v, just need to put the auto transformer in but don't have access to a multimetre at the moment.  I'm not sure if it'd be the same on the M7, but do you know which cords running out of the auto transformer are which?

 

Thanks

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1 minute ago, lee1000 said:

I have an M8 that I'm trying to turn into two standalone amps.

 

I got it wired up to run at 100v, just need to put the auto transformer in but don't have access to a multimetre at the moment.  I'm not sure if it'd be the same on the M7, but do you know which cords running out of the auto transformer are which?

 

Thanks

 

On mine the taps are all marked.

 

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1 hour ago, aussievintage said:

Thanks.   I was thinking about that first EF86 socket.  I could put a nice 12AX7 based phono preamp in that.

 

Have a look at the design concept used by Abbas Audio around EF86 pentode use in his DAC and phono stage units. @MattyW might be able to share further details. 👍

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I'll refer you to the Abbas Audio website + Google Translate. I know how incredible his gear is sonically, though questions around implementation are a bit beyond me.

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9 minutes ago, MattyW said:

I'll refer you to the Abbas Audio website + Google Translate. I know how incredible his gear is sonically, though questions around implementation are a bit beyond me.

Will have a look, thanks

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First test flight.     I removed all suspect caps, and replaced the essentials - power supply electros and two coupling.

 

Power up.   All voltages rose to desired levels.  Touching EL84 input, suitably loud hum from the speaker.  Wound up the volume, tested from first 12AT7 triode (using 12AT7 for the moment, was designed for 12AD7 or 12AX7.  Blew me away - nice and loud.  Will get louder when I put the AF bypass caps back on the cathode resistors.  Nice and quiet with input shorted.

 

Now, I did not have a 20+20 can cap handy, so I bundled a couple of 10uf into the bracket (see second picture), so it's running a little low on filtering, the main CLC using 10 instead of 20uf caps.  Hum just heard with ear 6" from speaker, so not bad considering.

 

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2021-03-09_08-24-49.jpg.aaa3ada6c6a884dc4b92692cf59a658d.jpg

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39 minutes ago, MattyW said:

I'll refer you to the Abbas Audio website + Google Translate. I know how incredible his gear is sonically, though questions around implementation are a bit beyond me.

 

 

I thought that would be easy to find, but...  Do you have a link?   I did consider an EF86 based phono circuit as well.

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18 minutes ago, aussievintage said:

 

 

I thought that would be easy to find, but...  Do you have a link?   I did consider an EF86 based phono circuit as well.

 

 

I'll PM it through. Not sure I should include the Abbas schematics in the thread. Not mine to share on a public forum.

 

 

Edited by MattyW
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So, for a soak test, I left it running with some nice music playing through it.   Plenty of high end extension, but can't tell about the bass because of the small test speaker.   Very happy, nice clean sound.  The VU meter is set nicely.  It hits 0 db just as the distortion becomes apparent.

 

About the bass, checking all the RC combos (coupling and AF bypass), I found one that limited it to about 30Hz.  I will bump that cap up a bit, aiming for 15 to 20 Hz throughout.

 

Time to get some cap orders organised.

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fantastic restoration project. Great to see these older units get a fresh leash of life! Love those VU meters - something very analogue about them (hard to explain in words).....

 

Good luck with the rest of the restoration.

 

 

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19 minutes ago, anandpkumar said:

 Love those VU meters - something very analogue about them (hard to explain in words).....

 

Yeah me too.  Simple but lovely.

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Posted (edited)

After removing the oscillator earlier, I have now moved the rectifier to the right hand slot nearest the transformer, so it is the same as the other module.    If I am not imagining it, I think it is even quieter now.  Should be quieter,  as the high voltage AC is no longer running near the 12AT7 preamp valve.

 

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Underneath

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Edited by aussievintage
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You have some nice looking Nippon Electric Company and Matsushita bottles in there. Time to break out the Sake. 😁

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1 hour ago, xlr8or said:

You have some nice looking Nippon Electric Company and Matsushita bottles in there. Time to break out the Sake. 😁

 

Yeah, good old Akai quality.  They came with the it of course.   I still have the 12AD7s that were initially in there too.  I was using them in another preamp.

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