AKAI M7 M8 / Roberts 770 Mod updates

[atavacron]

And without further ado, here are the photos of my stripped-and-ready-for-modding M7.

Here's everything I'm keeping (sans lid, out of frame) from the original unit:




I gutted it fo real:




Left channel - dig that crack on the right hand side! Somebody must have slammed the case down on its back long ago. I had to bend back the transformer/can cap mounting plates of both units as the weight of the transformers had totally warped them during the shock. All better. The faceplate is so f'n thick that it's not going to crack any worse.




Left channel top. Note that we leave in place the 6AR5 socket because it's a handy tie point for heater wires and ground. That, and the movement of the 6X4 to the center position, is the only difference between Left and Right channels once you strip everything.




Left channel bottom. See 6AR5/6X4 notes above. Otherwise your Right channel should look exactly like this.




Left channel rear. Oooooo, I forgot to pull that one green wire on the transformer primary.




There you have it. More documentation to come...

[buffalosound]

Hello, I'm doing the 2 pres in one mod to my pair of Akai preamps. I have a question. I have a pair of UTC A-20s. will this work as the input transformer for the EF86? If so what ohms should I connect to for the secondary? it has taps at 500,333,200,125,50. Thanks!

[atavacron]

Hello, I'm doing the 2 pres in one mod to my pair of Akai preamps. I have a question. I have a pair of UTC A-20s. will this work as the input transformer for the EF86? If so what ohms should I connect to for the secondary? it has taps at 500,333,200,125,50. Thanks!

What's the secondary rating? Wow, that's a great input transformer! What was it used in?

Rod will know whether it's appropriate. Rod knows all.....

[lanterns]

I've started stripping all of the guts and have nearly finished. I'm about to start the 12ax7 build. I wish I knew enough to post some pics. This project is really fun. I don't have the slightest idea what kind of transformer to buy. I'd like it to be a compromise between price and performance. If anyone has suggestions, please come forth. I'm willing to find a thrift store piece to gut the transformer, but I don't know what kind of equipment has what I'm looking for.

[/img]

[atavacron]

I have a bunch of Yamaha PM700 input transformers - I have no idea what their impedance and turn ratios are like though. But they're about the same size as the Beyers used in the Shure M67.

[RodC]

Hello, I'm doing the 2 pres in one mod to my pair of Akai preamps. I have a question. I have a pair of UTC A-20s. will this work as the input transformer for the EF86? If so what ohms should I connect to for the secondary? it has taps at 500,333,200,125,50. Thanks!

What's the secondary rating? Wow, that's a great input transformer! What was it used in?

Rod will know whether it's appropriate. Rod knows all.....

Sorry missed this, I think I got a PM on this.

The A20 is a line to line. Not really what you want here.

I usually look for a 1:7 or up to a 1:20 for the input of these. There are a couple of cinmags that are around $50 each I think.

If you go salvaging, most tube equipment with input transformers for a Mic will have this type of ratio. Some of the older transistor has it as well.

[RWood]

Left channel - dig that crack on the right hand side! Somebody must have slammed the case down on its back long ago.

Atavacron,

I have a spare faceplate - it's yours if you want it.

RRRwood at aol dot com

[atavacron]

emailing you in two point five seconds!

PS - For anyone watching this topic, sorry I haven't finished my unit. I'm really looking forward to posting more progress photos.

[RodC]

Update ? Output transformers for EF86 stage

About a year ago I purchased some output transformers for the EF86 stages of these pres. (At the time it was a special order from Edcor) I have been putting off trying them? I?m kicking myself for not doing it sooner. Putting these on the output really gets the impedance to match much better on the EF86 stage with a modern interface (10K to 20K) (Which is why I bought them in the first place!) I always had some distortion/rumble around 100Hz that I thought was mostly my room, well with these on the output it?s much better.
With less distortion and now a balanced output, it?s well worth the effort.

They are no longer a special order and cost around $10:

WSM15K/600:http://www.edcorusa.com/Products/ShowPr ... spx?ID=289

[tubetapexfmr]

I find for an output stage for the EF86 a white cathode follower using a 12AU7 tube works fantastically and no transformer needed. I set up the output going to a TRS 1/4" jack as an impedance balanced output so that it works great with balanced and unbalanced 1/4" cables. Here is a schematic that I adapted for this purpose, just lose the input switching and balance control. Also, chenage the volume pot from 50k to the 30k that comes with the Akai unit for tone control. Bonus if you do this you can put a small cap (.01uf) on the EF86 output and use the switch on the pot to switch in a large cap (.22uf), this effectively is a low cut switch with the large cap switched out! Of course I always tear my mods down to the bare chassis and don't reuse any of the wiring or electrolytics so this may be too advanced for most hobbyists.

http://www.hagtech.com/images/clarinetschem.gif

[RodC]

I have a couple with 12au7 cathode followers as well, they do sound sweet. Just thought I would share the Edcore info.

[tubetapexfmr]

Right on, Rod. I wasn't saying your method wasn't good, I just wanted to present an alternative. My favorite application of the 12AU7 white cathode follower is WITH a transformer and a switch to make the output transformer/transformerless depending on application. I find a 3k:600 ohm to be the best type in this case. Personally I just never liked a pentode alone as a preamp, the loading on the trim control seems to do weird things that the post trim control stage mitigates.

[RodC]

Personally I just never liked a pentode alone as a preamp, the loading on the trim control seems to do weird things that the post trim control stage mitigates.

I hear ya, Im shooting for the sound of a lone Pentode. A lot of what I record with these models is pretty loud stuff, if I dont have the trim control before the input the 1st stage just gets too over driven.

[tubetapexfmr]

if I dont have the trim control before the input the 1st stage just gets too over driven

Oh, so you have the trim before the EF86? Is there one after as well? I usually put a switchable pad before the EF86 with a 680 ohm resistor across the + and - signal and a matched pair of 1.5k ohm resistors in series with the + and - signals before the input transformer. The 12AU7 white cathode follower is just a line stage/buffer to smoothe out loading on the output, but only adds nominal gain.

[RodC]

if I dont have the trim control before the input the 1st stage just gets too over driven

Oh, so you have the trim before the EF86? Is there one after as well? I usually put a switchable pad before the EF86 with a 680 ohm resistor across the + and - signal and a matched pair of 1.5k ohm resistors in series with the + and - signals before the input transformer. The 12AU7 white cathode follower is just a line stage/buffer to smoothe out loading on the output, but only adds nominal gain.

Yes, and there is the risk of noise, but all of them seem pretty darn quiet so far. I basicly size the pot to act as the dampening resistor for the xformer.

If I do a 2nd stage then I will normaly have a level control somewhere else as well so I can control how much each tube is driven.