AKAI M7 M8 / Roberts 770 Mod updates

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GabrieleP
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Circuit alteration?

Post by GabrieleP » Fri Oct 29, 2010 2:35 am

Hello every AKAI aficionado,
I am about to start the project of redoing a L/R pair of AKAI M-8s into "2 preamps in 1" following Mr. Childers' indications.
I am taking it as a first step into the world of tube electronics , being by no means an expert (I know... PSU voltage & capacitors!!!).
Since I am not intererested in having the power section (although it's a great feature and it sounds lovely), I am asking if I should/could remove also the choke transformer of the power supply section of the circuit.
Please forgive my obvious ignorance! I really don't know what a choke is... :lol:
Thanks to all contributing TapeOperators.
Gabe.

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RodC
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Post by RodC » Fri Oct 29, 2010 5:13 am

Welcome to the board!

Leave the choke in there, it's part of the PS filtering.

Good luck with your mods.

EDIT cause I cant type, lord knows why, I write code 10 hours a day, I guess Im spoiled by intella-crack...
Last edited by RodC on Fri Oct 29, 2010 5:50 am, edited 2 times in total.
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GabrieleP
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Post by GabrieleP » Fri Oct 29, 2010 5:39 am

The man himself!

Thanks a lot Mr. Childers.

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Post by honkyjonk » Fri Dec 24, 2010 12:13 pm

So how do these sound as preamps, now that a few folks have finished them?

I'd love to hear some comparisons
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Post by syzygy » Thu Jan 13, 2011 7:54 pm

I wonder if it would be possible to make a pultec-style passive equalizer out of an akai m8/m7 mono block. i think the eqp-1a uses the same rectifier tube, 1 x 12ax7 and 1 x 12au7 (two halves of each tube in push/pull configuration, i guess with cathode follower). i wonder if a single-ended version could also work? any ideas or suggestions? i have some spare pots, inductors & caps and plan to devise a passive filter network as an experiment.

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M8

Post by wizzo » Mon May 02, 2011 9:32 am

Hi all, new member,i just picked up an M8 on the bay, and I am looking to mod it! Ive got a question though, I found out about the great possibilities w these tube amp sections after i bought the tape machine. The unit i bought is in full working condition as I bought it for real tape saturation for my studio, Id hate to hack a working unit, but i have been tinkering w the idea of hacking the left side preamp and leaving the right side preamp to drive the tape machine, and still be able to use it as analogue tape saturation.... Since I have yet not received the unit i cannot inspect it myself, so i thought I ask a few questions, get a head start on my thought process on how im going to tackle this... Id love to have a MIC pream and DI section as in Robs "The Boris" and Id also like to retain a section to drive the tape machine.... If anyone has any info if and how i could achieve this, id really appreciate it.

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tubetapexfmr
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Post by tubetapexfmr » Tue Aug 16, 2011 7:37 pm

syzygy wrote:I wonder if it would be possible to make a pultec-style passive equalizer out of an akai m8/m7 mono block. i think the eqp-1a uses the same rectifier tube, 1 x 12ax7 and 1 x 12au7 (two halves of each tube in push/pull configuration, i guess with cathode follower). i wonder if a single-ended version could also work? any ideas or suggestions? i have some spare pots, inductors & caps and plan to devise a passive filter network as an experiment.
You could do this, but you need to get the right transformers for it to work right. Sowter has them. I've built one of these in the past using the Sowters and it was pretty awesome. Using other Sowters you can also stuff an Altec 438c circuit, a REDD47 circuit (WITH regulated PSU) and lots of other stuff in thi Akai/Roberts format!

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tubetapexfmr
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Post by tubetapexfmr » Tue Aug 16, 2011 7:41 pm

Id love to have a MIC preamp and DI section as in Robs "The Boris" and Id also like to retain a section to drive the tape machine.
You can't really turn it into anything else if you want to keep tape functionality. It uses a lot of complex switching already to make it do recording and PA duty. It MAY be possible to squeeze some kind of extra functionality out of it if you removed the erase/bias circuit, but certainly is not a project for a novice or beginner.

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Post by buildafriend » Tue Jul 24, 2012 2:04 pm

What is AK-2?

Is it some kind of RC component?

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Post by lassoharp » Tue Jul 24, 2012 2:58 pm

buildafriend wrote:What is AK-2?

Is it some kind of RC component?

Yes. It is a compensation network - resistor and a cap, sealed inside of single unit that looks physically similar to a ceramic disc cap.

buildafriend
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Post by buildafriend » Tue Jul 24, 2012 6:15 pm

oh well look at that.. mine is fully discrete.. no tiny disc lookin component for me.

xfmr
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Post by xfmr » Wed Jul 25, 2012 10:22 am

Here are some time lapse videos of me dismantling, desoldering, and cleaning up one of the Akai Roberts monoblock units.

http://xfmr.blogspot.com/2012/07/time-l ... -tube.html

Also, here are some tracks recorded with a few different modified units:

http://xfmr.blogspot.com/2012/07/preamp ... les-1.html

And then there is my post with schematics and more info:

http://xfmr.blogspot.com/2012/06/brute- ... -hyde.html

Enjoy!

buildafriend
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Post by buildafriend » Wed Jul 25, 2012 11:24 am

1up!

Thanks!

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Post by Matt C. » Thu Oct 04, 2012 12:12 pm

I'm just starting to dig into this project of converting an M8 into a mic preamp, and I have a couple questions:

1. From what I can tell, the EF86 heater is hooked up to the 5.7V secondary from the power transformer. In the EF86 data sheet it says the heater voltage should be 6.3V. I think 5.7V falls just within the acceptable limits, but could I just move the heater over to the 6.3V tap? (I'm removing the power tube, so its heater wont be drawing any current from that tap) Or is that totally unnecessary?

2. has anyone tried adding a second EF86 preamp in place of the power tube? I'm tempted to try it since I don't need the unit to act as both mic preamp and guitar head, but I haven't heard anything about people actually trying this.

thanks!

lassoharp
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Post by lassoharp » Thu Oct 04, 2012 12:46 pm

Matt C. wrote:I'm just starting to dig into this project of converting an M8 into a mic preamp, and I have a couple questions:

1. From what I can tell, the EF86 heater is hooked up to the 5.7V secondary from the power transformer. In the EF86 data sheet it says the heater voltage should be 6.3V. I think 5.7V falls just within the acceptable limits, but could I just move the heater over to the 6.3V tap? (I'm removing the power tube, so its heater wont be drawing any current from that tap) Or is that totally unnecessary?

2. has anyone tried adding a second EF86 preamp in place of the power tube? I'm tempted to try it since I don't need the unit to act as both mic preamp and guitar head, but I haven't heard anything about people actually trying this.

thanks!

They purposefully run the 86 heaters at lower voltage for better noise performance. Assume it best to leave where it is.

2nd ef-86 (ran as pentode) would not be best idea for mic pre use for a host of good reasons. Might be fun to experiment with for guitar amp use though. Actually not great to run one as it is done in the popular mic pre conversion mods but it does work passably and is considered the compromise you have to make in order to squeeze two mic pres out of the unit.

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