Motown Direct Amplifier-inspired Preamp?

[ruffrecords]

I just found this article about finding optimum bias points for single ended 6V6 output stages (ultra linear versions anyway):

http://www.r-type.org/pdfs/6v6-se-ul.pdf

Looks like a typical operating point around 250V on the plate would be good. Of particular interest is his conclusion that a cathode bias resistance of around 390 is required which just happens to be the exact values used in the BC-2B schematic. Distortion is around 1% or so which with about 20dB of NFB would drop to an acceptable 0.1%. As with all single ended designs, the transformer is the biggest challenge. It needs to be able to take getting on for 50mA dc yet still have a decent low end response. NFB will help with some of that but may at the same time compromise LF stability.

However, if we drop the original requirement to drive +26dBm into a 300 ohm load we could probably halve the current which would significantly help the transformer design.

Edit: Just discovered that the Hammond 157G choke used in the present day design is only good for up to 40mA so 6V6 current must be somewhat less than this unless its plate is pre the choke.

Cheers

Ian

 

[emrr]

Sa-39’s run 36ma per 6v6 with a 390

 

[btown2009]

I have the original schematic for it, I will not share it, however I can tell you it has nothing to do with the altec or ampex mixers at all. Afair john does not have it either. The guitar amp was built around 65, after the 8 track machine. A musician would plug his guitar/bass/fender or wurlitzer, etc into one of the five channels, adjust the volume on the instrument so that the meter on the preamp wasn’t pegging, and turn the knob below it to a suitable monitor volume, all 5 of those channels were summed and fed into an amp then brought to the monitor speaker below the amp. Each of the 5 channels had its own direct output.
All it essentially has is a preamp stage, output stage, transformer, and high impedance summing with an output, the summing has no master gain control. There isn’t anything special about the circuit except that it uses feedback and different values in each stage to limit the amount of distortion and so no frequency stacks up in the circuit, ie each stage operates at different value points. It was not built by mike, but a guy under him, at that point john wendt was gone or close to it from motown, and mike was working hard with keeping disc cutting, remix, and the daf running. Alot was going on at that point, motown had alot of money coming in with all those supremes hits.

 

[ruffrecords]

I have the original schematic for it, I will not share it,

I would like to understand your reasoning for saying this.

Cheers

Ian

 

[Winston OBoogie]

I still am.

I would like to understand your reasoning for saying this.

Possibly because someone is making a commercial product based on it and, to post the schematic on here could/would hurt them financially. I think that's valid enough reason by itself personally.
We're also not privy to what extent (financially or sheer determination) it took to gain that schematic.
I've been in similar situations in the past.

I still am.


And btown2009 did give up information which does help the OP narrow it down.


James Jamerson would have sounded like James Jamerson no matter what decent guitar to line level amplifier box he went through.

The OP might be best to pick or design a good circuit topology for the job and get on with making music.

 

[jacomart]

Sorry if I say this but I am convinced that behind certain things there is a lot of esotericism. Basically it's not written in stone, as also @Winston OBoogie wrote, that this pre / DI was the reason why James Jamerson became "the Hook", the truth is that he played using only one finger and that he slipped a sponge under the strings at the bridge, we just have to find out the size of the sponge and its brand!

Cheers
JM

 

[Winston OBoogie]

One example regarding an old schematic:

I held onto the RS.124 schematic for years. I didn't talk about it, only built a unit for myself from an Altec 436 to see what it was like. Didn't sell units. Nada.

Some of the reasoning behind holding it back was because, to 'go public' would have taken some of the juice out of the'Recording The Beatles' book which was still being co-written by a friend of mine.
That book cost 10 years of research and fostering relationships for the 2 guys involved.

I'd also spent about a decade doing the same, as well donating a nice REDD.47 amplifier of mine to Abbey Road Studios, the first REDD.47 they'd had back in there in 50 years.

Many hundreds of dollars in transatlantic phone calls. Promises to people that I'd keep it to myself.

I eventually broke that promise and posted the schem on here.
By that time, I'd built a few clones (from converted original Altecs) for friends or acquaintances - Recording The Beatles' authors got units, Ken Scott did. Lenny Kravitz... There were about 20 that I did for engineers and producers in L.A. where I was living. A few up in Canada where I'd also worked.

Then a few units got out into the wider world and an incorrect reverse engineered schematic of one of my units appeared on here.
I could have let it go, but decided it was time.
The Beatles book was out.

So I posted it here to alleviate anyone building a unit from the incorrectly reverse engineered schematic.

It all works out in the end. For most folks.

I'm now persona non grata to some for breaking a promise to not post it.

That schematic cost me BIG TIME.

 

[jacomart]

We were basically having a lot of fun trying to figure out how Mike (or anybody else) could make his toy. So the fun is over, too bad!

Cheers
JM

 

[ruffrecords]

I still am.


Possibly because someone is making a commercial product based on it and, to post the schematic on here could/would hurt them financially. I think that's valid enough reason by itself personally.
We're also not privy to what extent (financially or sheer determination) it took to gain that schematic.
I've been in similar situations in the past.

I still am.

I can quite understand that but if I were in such a position I would not reveal the fact that I owned such a schematic so I am also curious as to why btown did just that.

Cheers

Ian

 

[Winston OBoogie]

Last bit in this thread on the current path:

When you build and sell a unit such as the ACME, you only have a certain amount of time to get as many units out the door before someone will reverse engineer it if it seems worthwhile.

A little understanding and some patience my friends. I think Doug and Jaco get it

Peace.

 

[jacomart]

Last bit in this thread on the current path:

When you build and sell a unit such as the ACME, you only have a certain amount of time to get as many units out the door before someone will reverse engineer it if it seems worthwhile.

A little understanding and some patience my friends. Jaco gets it

Peace.

Given my age (unfortunately) I can say that I have become a wise man

Cheers
JM

 

[Winston OBoogie]

I guess I'm still here

I can quite understand that but if I were in such a position I would not reveal the fact that I owned such a schematic so I am also curious as to why btown did just that.

Cheers

Ian

OK got it. That's a reasonable question Ian.

I can only know why I might do that.
I did it in the REDD history thread regarding the EQ's.
I let it be known that I had the circuit diagrams but wouldn't post.
But this was so there might be some acceptance of validity when, for instance, I was talking to you regarding the general principle behind the design.

I was trying to share information with you and others, without breaking any more promises, to others. And myself

The actual minutia of the original circuit details don't really matter. You could take what info I shared and run with it if you wanted.

As I said to you before: "you pretty much already nailed it"

That's me. Maybe Btown. feels things in a similar way?

 

[ruffrecords]

I guess I'm still here



OK got it. That's a reasonable question Ian.

I can only know why I might do that.
I did it in the REDD history thread regarding the EQ's.
I let it be known that I had the circuit diagrams but wouldn't post.
But this was so there might be some acceptance of validity when, for instance, I was talking to you regarding the general principle behind the design.

I was trying to share information with you and others, without breaking any more promises, to others. And myself

The actual minutia of the original circuit details don't really matter. You could take what info I shared and run with it if you wanted.

As I said to you before: "you pretty much already nailed it"

That's me. Maybe Btown. feels things in a similar way?

I can also understand that. If btown was able to at least give us a few clues - even the tube line up would help - then I would be happy with that.

Cheers

Ian

 

[untune]

Thanks @btown2009 for sharing that info. I understand and respect that you don’t want to reveal the schematic, those reasons are yours. As stated at the beginning, I didn’t want to go too deeply into the ‘current’ production unit if I could avoid it—it was just the only place I could look for clues. No intention to tread on any toes.

I won't go into the above, I understand where everyone is coming from.

If btown was able to at least give us a few clues - even the tube line up would help - then I would be happy with that

I was going to follow up with the same—if he wouldn't mind giving a clue (within reason) on the topology/tube lineup, that would be appreciated

 

[Winston OBoogie]

I can also understand that. If btown was able to at least give us a few clues - even the tube line up would help - then I would be happy with that.

He might. He made sure to steer the OP away from the Ampex and 1567 stuff so he's not keeping all the cards.
Ask him, he seems an approachable chap given his prior posts on here.

 

[emrr]

When you build and sell a unit such as the ACME, you only have a certain amount of time to get as many units out the door before someone will reverse engineer it if it seems worthwhile.

my suggestion back on page one.......

 

[ACME/Helios]

Thanks btown for playing your cards tight to your chest.

 

[btown2009]

I’m not affiliated with anyone making any products.

if you want to build your own version of the 5 channel unit, I would take the altec 1566 preamp, build 5 of those and add summing resistors for all 5 units to give me a collective unit, no it’s not the original circuit, but it’s a closer approach than trying to start with a 1567, it has the right thinking behind it, which was 5 separate entire pres, only coupled together by summing, and a highly filtered common power supply. That would be my take on it if I wanted to build one of these to use for recording.

if it bugs anyone that I’ve joined this thread, I’m happy to stop commenting, I’m not here to argue, just wanted to steer the guys with questions towards what I’d call the right path of thinking, instead of chasing their tails trying to solve it.

 

[jacomart]

Of the two, I honestly like the RCA BC-2B preamps (both mic and program) more than the Altec ones. But it is an absolutely subjective matter.

Cheers
JM

 

[ACME/Helios]

Thanks btown. I actually own Acme Audio and make the MTP66 from Mike Mclean's original schematic. I would love to look at your schematic some day to compare it to the one I got from Mike. It's funny, I always thought Group DIY didn't knock off products that were still in production.