Akai M7 monoblock valve amp project

[aussievintage]

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.

 

 

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

 

 

Here's some shots of one of the monoblock modules

 

 

 

 

 

 

[aussievintage]

 

 

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.

 

 

 

[aussievintage]

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  

 

Edited March 5, 2021 by aussievintage

[aussievintage]

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?  

[aussievintage]

 

OK, half done.   Before and after pics.

 

Oscillator still in place (before)

 

 

Oscillator removed (after)

 

 

 

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)

 

[aussievintage]

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. 

 

[Interface]

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.

[aussievintage]

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  

 

[btd]

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

[aussievintage]

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.

 

 

 

[aussievintage]

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.

 

[muon*]

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

[aussievintage]

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.

[aussievintage]

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 March 7, 2021 by aussievintage

[aussievintage]

 

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.

 

[muon*]

Can you go up a bit? or down a little?

 

https://www.hificollective.co.uk/catalog/ruby-gold-32uf-32uf-500vdc-electrolytic-capacitor.html

 

https://www.hificollective.co.uk/catalog/ruby-gold-16uf-16uf-500vdc-electrolytic-capacitor.html

[aussievintage]

Just now, muon* said:

Can you go up a bit? or down a little?

 

https://www.hificollective.co.uk/catalog/ruby-gold-32uf-32uf-500vdc-electrolytic-capacitor.html

 

https://www.hificollective.co.uk/catalog/ruby-gold-16uf-16uf-500vdc-electrolytic-capacitor.html

 

 

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.  

[muon*]

Yeah, rectifier dependent

[aussievintage]

 

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.

 

 

[muon*]

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

[aussievintage]

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.

[muon*]

 

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 March 8, 2021 by muon*

[aussievintage]

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.

[xlr8or]

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 March 8, 2021 by xlr8or

[xlr8or]

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.