Akai M7, M8, Roberts 770 project options

[mishagolin]

So, the skinny is that I have 2 complete machines that are currently not working. I got them pretty cheap and the monoblocks are worth more to me than the tape drive sections. So, i have 4 monoblocks awaiting decomposition/rebirth.

First things first, I don't want to throw away anything that would be useful to anyone here. Is there anyone interested or in need of spare parts for any of the tape drive mechanisms? You pay shipping and i'll send whatever part you need. Just send me a PM.

*DISCLAIMER: I have some experience with high voltage tube electronics. I've built half a dozen tube guitar amps in the past year. I am aware of the dangers and I take proper precautions.

Here is what i have to work with:

- 250v secondary power transformer
- 6x4 rectifier
- EF86 preamp tube
- 12a_7 preamp tube
- EL84 (6bq5) power tube
- spare 7 pin socket from the oscillator no longer needed.
- 5 watt 8ohm output transformer
- Nice pretty 3u-ish chassis
- VU meter

other parts i have on hand
- Cinemag input and output transformers recommended for LA2A
- Shure M68 input transformers
- Edcor output transformers (on order)


So, a list of possible projects and what might be required.

1). RodC mods from the TapeOp forum. I am aware of the mods and have reviewed them. i like the idea of minimal changes but i think i'd like a more purposed project. I'm considering starting with his mods but cascading the EF86 into the next stage permanently and replacing the second stage with a 12au7 cathode follower output. I'm also considering removing the entire el84 section and replacing it with an additional EF86/12au7 pair to make a two channel preamp.

2). LA2a using Cayocosta's turretboard layout. This would require almost a complete gutting of the unit and putting in an octal socket for the T4b. Not a problem. I already have some Cinemags to pull this off but it might be just as useful to just start with a new chassis and salvage the parts i need.

3). REDD47 style preamp. I know the RodC is supposed to have some of the 47 flavor due to the EF86 preamp but i could build one a lot closer using an ECC88 tube as well. I could also do this in two channels in one chassis by sharing the PT (its definitely got the current).

4). Stick with what i'm good at and just build a Vox AC4 head. EF86 preamp with a 12ax7 tremolo driver and an el84 power section. This is the easiest path for me but probably the least interesting as i've already built an AC10 (i stole one of the EF86's from one of the monoblocks already).


So, are there any other suggestions for cool things i can turn these into. If i come away with a pair of LA2a's and a two channel tube mic pre i'll be happy but that leaves one monoblock unused.
Stereo Pultec? Vari-Mu? what else might work?

 

[mishagolin]

With a few slight mods to the main power supply section i think i can get a Urei 175b into this thing. It already has most of the correct tube sockets.  It would be just a bit underpowered with the current PT but i could switch up the first dropping resistor after the choke to keep it near the correct voltage. Also, figuring that i'll be pushing 120v instead of the 110-115 they designed my PT for i think that'll get me at right about the correct voltage. I'd either keep the 6x4 rec in there instead of switching to the GZ34 or switch to a solid state. I could use the existing choke in the PS and replace the OT with the required OT and then try to squeeze in the input and interstage transformers.

Only one question, and its probably a stupid one. What is V6 in the 175b schematic in the PS? Is that the neon, similar to the DS1/NE2 pairing in the LA2a schem?

 

[mishagolin]

Urei schematic

thanks damnyankee. i'd be doing it without the 6386 mod

 

[pH]

I've worked on a ton of these, I used to be able to find them on CL for peanuts.
I made (and sold) several as LA-2As, as the tube sockets and power are perfect for that. Also made a bunch of preamps, it's a nice platform for easy wiring when experimenting. You could also just recap one and put a nice speaker in the case for a cool portable mono hi-fi 

Yes, power runs a bit high, not a problem for B+ but the heaters sit around 6.7vac, kinda high but still acceptable.

I like your idea of a UA175, I had thought of that as well. V6 is a neon regulator tube, easily replaced by a zener diode (which I also did in the LA2A in place of the neon).  And I'd go with a SS rectifier, especially since you need squeeze as much B+ as you can. Keep us posted on your build!

Paul

 

[mishagolin]

Thanks pH.

The solid state rec makes sense to up the voltage for a 175b and the la2a.

I think I'm going to go with a two channel REDD47 style pre, a pair of LA2As and a 175b.  That's s lot of stuff to work through.

 

[akble]

Looking for Play/Record lever for Akai M8 /Roberts 770 if you have one to get rid of -  afraid dog got one of them off my machine

thank you

 

[mjrippe]

Looking for Play/Record lever for Akai M8 /Roberts 770 if you have one to get rid of -  afraid dog got one of them off my machine

thank you

The long ones like in the attached photo?  I think I have some, send me a PM.

 

[fazer]

I have 2 channels of  the 770 and I’m interested these build options .  I too like the idea of building  a mic pre with an ECC88 output for a mic preamp with EF86 front end  in the mono block.  I haven’t thought about an LA 2A.  That’s interesting with tube sockets matching. Unfortunately both meters on mine are damaged.
The guitar amp would be a studio rack amp sort of a copy of the old lexicon tube guitar SE amp .  I have a schematic of that some where.