Here we go again... Akai M8 Reel to Reel Conversion

[ElusiveMoose22]

Okay, the last conversion to guitar amp project was a success so it's time for another one!    I have on the way to me 1 Akai M8 Reel to Reel Amplifier, right side only (originally designed as mono with right only engaged or stereo when selected engaging a left side amp).  I'm told it's working condition but we shall see how its doing once it arrives.  In the mean time lets get planning for what needs to be done.

What we have for tubes:
 1 x EF86 & 1 x 12AX7 in preamp,
1 x 6X4 rectifier,
1 x EL84 output

Transformer is putting out 250V

Speaker Out is 8 Ohms

Max output to speakers is 6 watts (probably less)

Schematic is attached with signal path traced out as best I can figure. There is a bit of a convoluted thing where the EF86 is driving the Volume pot and the 12AX7 drives the Tone pot/Speed selector knob.  Or at least that's what I think is going on.  I'll know for sure when the model arrives. 

I'm thinking of gutting this one much further than the last amp and doing something like the 60's Vox AC4 design.  Looks to me like we've got more than sufficient iron for it and the tube compliment is a 90% match already.  I've seen other people do conversions differently on these, keeping the circuit more intact as it was originally designed, but I'm pretty sure a Vox AC-4 mod is the way I'd like to go.  Anyone see anything different or have any tips/twists on the AC4 circuit that I should be looking at?

Before anyone says it, yes I'm aware that EF-86 tubes have issues going microphonic in guitar amps.  I will be using the same adapter that I used on my last conversion allowing the tube socket to accept a 6AU6 pentode.  It works great and is close enough to an EF-86 that the amp functions the same. 

As always any input is welcome and appreciated  !  I may not get to answering any questions or comments for a few days until I have the unit in hand, but rest assured I will respond soon as I'm able. 

[ElusiveMoose22]

specs...

[sluckey]

Last year I built a nice Toast-R-Oven using an old Roberts R2R. 

[shooter]

Looks like she fly's!
you'll have to draw your own schematic this time, I'm busy 

[afinitemind]

Great project...I'll be looking forward to seeing what you do with it!


I've had a pair of these stowed under my bench for a few years, waiting for inspiration. Was considering pulling the iron and building something new entirely in a different chassis, but the AC4 idea is pretty cool!

[ElusiveMoose22]

RECTIFIER TUBE QUESTION:

I'm still working out a schematic for my take on the Vox AC4 circuit for this build. I'm getting a little hung up on a rectifier tube question. Can someone out there with a better understanding of reading these old tube charts double check my conclusions below please? Tube data charts attached.

The 60's Vox AC4 schematic shows a 6V4 rectifier tube, according to the data sheet when supplied with 250V that should yield a max DC output of 90 mA & a little over 260V.
The data sheet for the 6X4 tube in my Akai unit shows a max DC output of 70mA & also a little over 260V when supplied with the same 250V input.

1.) Am I reading this correctly or should I be looking for something different besides the max output current values on these data sheets?

2.) Is this a case of close enough output from the 6X4 rectifier tube to use it the same way the 6V4 is used in the Vox Schematic, or will I need to change any of the B+ caps/resistors supplying the preamp anodes?

Many thanks in advance for any help!

[sluckey]

That's an odd looking tube data sheet. Looks more like a purchase order to me. 

I would not get bogged down thinking about which of those rectifiers to use. The 6X4 in your recorder works well with that PT and OT. I would not hesitate to use it in an AC-4. On the other hand, if you want to use a 6V4 just to keep the Vox circuit original, then use it. I don't see a problem, well other than one is a 7-pin and the other is a 9-pin.

[ElusiveMoose22]

Ooops!    Sorry about that!  See that's why it's not a good idea to work on these things on my down time at the day job lol.  I fixed it 

I thought everything would be close enough but like I said, reading and understanding those old data sheets is something I need to do some more work in.  Thank you sluckey! 

[ElusiveMoose22]

The amp arrived vai Fed-Ex yesterday and all appears as advertised in the photos above  .  Surprisingly clean inside!  No variac at home where I'll be working on this project so I'll need to go to the hardware store and get supplies to rig a dim bulb set up to power it on.  It was advertised as being a working unit, but I'd rather be safe than sorry.  Once I verify it's in working order I will drain all the caps and begin labeling and removing the components to be gutted.  I have my new parts on order already but they won't arrive until next week. 

In the meantime I finally finished my schematic (attached below).  I didn't deviate too much from the original Vox design save for the addition of a bright switch allowing lots of treble to bypass the tone knob and insert after the volume for a hopefully very bright top boost channel kind of feel.  I may adjust it to insert before the volume or change the cap value if it's too much. 

I may also adjust the value of C8 if things are too dark with the bright switch off. Seems like a 0.01uF might be more appropriate there to cut some low end, but I'd like to at least try the original value first. 

The only other place I'm planning to make any changes will be at the tremolo circuit.  I'm still undecided if I'll leave it with a set depth and adjustable speed per the Vox design or if I will try to get fancy and wire in some sort of depth control.  I'm still researching this but a coaxially stacked potentiometer to control speed and depth or a 3 way switch to select varying degrees of set trem depth are both contenders at the moment. 

As always all corrections, suggestions, comments or other input are welcome    Otherwise I will update again once I have some more work done.

[kagliostro]

Very nice score

I like very much that piece of iron 

Franco

[silverfox]

I didn't want to reload the images. If you need to, go back and take a look and tell me, if you would, how much would it cost to build those chassis  today?  And who would the line techs be that tested and repaired the production. Days gone by.


silverfox.

[ElusiveMoose22]

I like very much that piece of iron 

I didn't want to reload the images. If you need to, go back and take a look and tell me, if you would, how much would it cost to build those chassis  today?  And who would the line techs be that tested and repaired the production. Days gone by.

They just don't make many things like this anymore, that's for sure.  That's one of the reasons I've enjoyed doing these conversions so much.  The only downside is that they weight as much as boat anchors lol.  Even this little one is surprisingly heavy and I'm going to have to build some kind of enclosure for it still before I'm done!

[ElusiveMoose22]

Okay, I spent some more time with the unit and realized some things:

1.) I definitely have room for speed and depth knobs to control the tremolo effect so I've added a depth pot to the schematic. I believe this is the correct way to do so. It's essentially the same tremolo circuit as the old Fender 5E8A Tremolux design. If someone more knowledgeable see's something different please let me know.

2.) The "SOUND ON SOUND" button in the upper right is an on/off switch so I'll wire that to control Tremolo ON/OFF

3.) The "STEREO/MONO" switch is basically the power on/off for this unit so I'll just re-label it to reflect this.

4.) The tone knob in place also has a speed selector that works as an on/off switch in the circuit. This is a handy place to wire in my bright switch so I'll take advantage of that 

5.)  The 2 inputs were already there so I went ahead and set one as a high input and the other as a low in the schematic.

Updated schematic attached and labeled photo of the face for reference to the switches and knobs mentioned above 

[ElusiveMoose22]

One more update to the schematic.  I restructured the tremolo network, as the layout was either confusing or incorrect.  Either way it looks better to me now and makes sense from a "looks like it will work as it should" standpoint

[ElusiveMoose22]

Finally got started on gutting the old circuit components the other night.  Pics from right before I started removing things.  When I'm done I'll have just the tubes/sockets, output and power transformers + the choke filter, and the input/output jacks I decide to keep.  The 40 terminals on the fiber board should be more than enough for the simple capacitor and resistor network I'm putting in and there is ample room to work with my soldering iron  .
I'm thinking I'll keep all the B+ connections on top and the rest on the bottom terminals just for organizations sake.  Also there is a large switching mechanism circled in blue) that is separating the EF86 & 12AX7 preamp channels as well as doing some power switching that is supposed to work in conjunction with the rest of the reel-to-reel units in the original design.  That will be coming out for sure too.  Right now I'm about 75% done removing old components, I'll post again once the gutting is complete and I start putting my new circuit in. 

[ElusiveMoose22]

FINALLY finished removing all the old resistor and capacitor components last night.  The leads were all wrapped at least 2 and occasionally 3 times around every post.  I understand the importance of mechanical connection, but through the eyelet AND 3 times around the post is overkill lol 

Now I'm ready to start putting in the new caps and resistors and tiding up a few other connections and wire runs that could be done better.  I hope to be done and have an update by the end of the week *fingers crossed*

[ALBATROS1234]

6 mnths ago i converted a m8 into an early ac4 just like you are thinking, it turned out sahweet. go for it.

[kagliostro]

Seems you really did a very good dismant job   


Franco

[ElusiveMoose22]

6 mnths ago i converted a m8 into an early ac4 just like you are thinking, it turned out sahweet. go for it.

ALBATROS1234, that's good to hear!  I figured I couldn't be the only person with this idea, but I wasn't able to find posts anywhere else about doing a AC4 with this iron and chassis.  Did you make any changes to the AC4 circuit or follow the old design exactly?

[ElusiveMoose22]

More progress and the finish line is finally in sight 
I was able to populate the whole network of new caps and resistors over the weekend and wire in almost everything else.  Only 3 items left to finish out:

1.  The standard size pots for my tremolo speed and depth don't quite fit in the existing holes I'd like to use for them, so I have some mini pots ordered and on the way

2.  I need to wire in a switch to engage 8 ohm load resistors for when the headphone jack is in use.  That was one of the many jobs done by the huge switching mechanism I chose to remove (it's many other functions where no longer necessary).

3.  Final step will be wiring in the new grounded power cord and tidying up loose wires and what not.  Then the amp will be ready to fire up and take for a test run 

Pictures of the work in progress attached.  The first one shows off my fancy cupcake work light  lol.  My good work light is on the fritz so I dug out the ridiculous cupcake reading light received as a gag gift last year and tossed in the back of a closet until now.  It actually works very well  lol

Eagle eyed viewers will also notice something else lurking in the background/periphery of both pics...

[ElusiveMoose22]

    I'm second guessing myself and need some help please to make sure I'm doing this right...

I hand drew a quick schematic for the connections needed to utilize the headphone out last night, but when I looked at my speaker out jack and realized I might be working too hard.  Scan of the drawing and a pic of the speaker out jack attached:

The transformer needs an 8 Ohm load from either a speaker or a resistor if the speaker jack is not in use.  The white braided lead is an 8 Ohm resistor still in place from the original circuit along with the yellow wire that runs up to the output transformer area.  It looks like when there is no tip inserted into the speaker out that the tip and sleeves are connected and the the output transformer would see the 8 Ohms of resistance across both lines out just like a dummy load.  The headphone out jack is just a standard stereo jack without the short when no tip is inserted.

My questions are:
1.  Do I even need a switch at the transformer then or can the yellow line with 8 Ohm resistor and the normal line direct from transformer to the speaker out tip both be connected at all times?  The signal would seek the path of least resistance and pass through the white line unimpeded without the yellow line causing too much extra resistance no?  And when the speaker jack doesn't have a plug in, the jack shorts the tip and sleeve and the 8 Ohms of resistance is seen between them.

2.  Is an additional 8 Ohms of resistance needed across the tip/ring and sleeve at the headphone out jack as well or am I okay just as the schematic is drawn here with an signal coming off the speaker out tip attenuated by a 100 Ohm resistor?

[shooter]

buyer beware 
I think it passes the stink test, maybe not.
with NO speaker you have 8ohm to ground, saving the amp, also forming a 100:8 voltage divider for the phone
with speaker IN, amps still happy, you now have a 100: variable R based on freq(?) for the phone

[ElusiveMoose22]

buyer beware 
I think it passes the stink test, maybe not.
with NO speaker you have 8ohm to ground, saving the amp, also forming a 100:8 voltage divider for the phone
with speaker IN, amps still happy, you now have a 100: variable R based on freq(?) for the phone

Thanks shooter!  I believe this is just what I was trying to figure out, or at least it gets me started in the right direction! 

[shooter]

in the right direction

I tend to wander aimless with good intentions 

[sll]

Don't know if anyone's monitoring this thread but these projects have become popular again and the prices for M8s and Roberts 770x have really started climbing.

https://drive.google.com/file/d/1uA1xuqv3xIFWfHUxbETpxptJHo3dy9rR/view?usp=drivesdk

The Sammies from Charlotte, NC performing a cover of Gang Of Four's "To Hell With Poverty" in the studio for the virtual benefit show "Love Thy Neighbor: A Tribute to Benefit Roof Above". The guitarist on the right, with the darker Strat, is playing through one of these amps in this video. 

[ElusiveMoose22]

Oddly enough I just started working on this project again. I’d put it to the side right after the last post here due to time constraints and other more pressing things needing my attention. I’d always intended to come back to it and finish this one, fine tune the circuit and, and then replicate it with 3 more units that I’d picked up around the same time so I could sell them locally and make some money to support my hobby. A lack of free time, a move to a new home, a pandemic, and about 1000 other things had kept me from doing that until now. I just dug this out last week and ordered the parts to finish it   
I’m going to be making a few slight changes to the circuit design but hopefully I’ll have this first amp up and running in the next week or so. I’ll post an updated schem and hopefully a video demo once it’s done.

[sll]

Hi, I'm sitting with about 11 of these amps to convert. I have completely redesigned the amp circuitry to specifically serve as a guitar amp rather than as hi-fi equipment. Where practical or beneficial I have kept or reused the original vintage components, including the tubes. Of course all filter and bypass capacitors have been replaced and a 3 prong power cord added.

There are 2 instruments inputs which share the common volume and tone controls. The 2 inputs however employ separate, and different, input stage tubes and therefore sound very different. The first input uses an EF86 (6267) tube. These tubes were used in 1960s vintage VOX amps, and are still used in some Dr. Z, Matchless, and other boutique amplifiers. The 2nd input uses a 12AX7 (12AD7) tube. 12AX7s are the most common preamp tubes used in guitar amplifiers such as Fender, Mesa Boogie, and Marshall. This amp is very quiet (background noise and hum) and breaks up nicely at reasonable volume levels.

For the tone circuit I used a modified passive Baxandall/James circuit (Bass and Treble controls) similar to the type of tone stacks used in the original Ampeg, Orange,  and some Dr. Z amps. Additionally, I added a boost switch which increases the gain and frequency response of the amp providing additional tonal flexibility.

The output stage is a single-ended class A power amp. This amp uses an EL84 (6BQ5). The amp uses a 6X4 tube rectifier.

The cabinet was custom designed and constructed by a friend of mine for this amp in return for an amp I built for him.

https://drive.google.com/file/d/1uFpVB3U9Ztwxi8PAmdEuX0nuzv6DtceI/view?usp=drivesdk

https://drive.google.com/file/d/1uAOqTwpguINgy5IyjEBTunLRHcajgqaP/view?usp=drivesdk

https://www.thegearpage.net/board/index.php?threads/5-watt-conversion-amps.2297242/

[jmccanna]

Hello,

I know this a pretty old post but thought I would jump in. I have several of these units that I have acquired over the years.  ElusiveMoose, did you get yours done?

James

[ElusiveMoose22]

James,
Sorry for the late reply. I did get one unit done after reflowing a few suspicious looking solder joints, it worked for about 3 glorious seconds and then failed again lol. I haven’t had time to properly trouble shoot the failure but both of the transformers are still working and the amp stays powered on just fine but there is a short somewhere in the circuit and I have no signal at the speaker output.   
Probably another poorly flowed solder joint somewhere in the signal path. If not for my shoddy workmanship the circuit would work though lol. If you’re looking to do something similar the schematic should be good. I would only suggest testing for circuit integrity at every step along the way so you don’t end up having to go back and re flow a million joints looking for an elusive problem as I have hahah.

[Allbaldo]

Hi! I have a couple of these amps sitting around too, and was considering a similar mod. Did you ever get yours working again?

Best!

Stephen

James,
Sorry for the late reply. I did get one unit done after reflowing a few suspicious looking solder joints, it worked for about 3 glorious seconds and then failed again lol. I haven’t had time to properly trouble shoot the failure but both of the transformers are still working and the amp stays powered on just fine but there is a short somewhere in the circuit and I have no signal at the speaker output.   
Probably another poorly flowed solder joint somewhere in the signal path. If not for my shoddy workmanship the circuit would work though lol. If you’re looking to do something similar the schematic should be good. I would only suggest testing for circuit integrity at every step along the way so you don’t end up having to go back and re flow a million joints looking for an elusive problem as I have hahah.

[jeff]

Reel to reel...
Hmmmm...
Reconfigure play/record heads... built in slapback effect?