Motown Direct Amplifier-inspired Preamp?

[ruffrecords]

Thanks for contributing btown. I must admit in my head I had envisaged the five inputs being split to the mixer at the inputs but your post implies the sum was taken from the individual preamp outputs which actually makes a lot more sense.

Looking at the 1556 and 1557 schematics raises a number of options. Both use a parallel pair of triodes in a single ended output stage with a capacitively coupled output transformer but a 12AX7 is a very poor choice for this job. The 6SN7 is better and with 10mA of quiescent as in the 1557 it should be capable of achieving +20dBu into a 600 ohm load via a suitable transformer. Which in itself raises an interesting question. Why does the Motown DI use a 6V6 for the output stage? Perhaps we are being led astray by thinking it is a true SE stage with a dc current carrying transformer in its plate circuit as in the BC-2B? Maybe it has a simple resistive plate load running at about 10mA with a capacitively coupled output transformer? This certainly much simplifies the output transformer selection process. In triode mode a 6V6 has an ra of about 2K and a mu close to ten so not too shabby The VU meter being fed via a 10\k resistor in the 1557 implies the transformer would have been a 4:1 step down.
wn active DI.

Thank you btown for firing up the inspiration/imagination again.

I don't think any of us are thinking in terms of 5 inputs but just a single channel Motown active DI box.

Cheers

Ian

 

[ruffrecords]

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.

We don't. The Motown Di has not been in production for more than 50 years.

Cheers

Ian

 

[btown2009]

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.

Al, I have the same schematic you do, Mike sent it to me when he discovered it on his HD, Mike stayed with me several times since I’ve known him, I also got to see the sony c37’s before he dropped them off to you. I live in SE Michigan.

 

[Winston OBoogie]

It's funny, I always thought Group DIY didn't knock off products that were still in production.

It's not considered 'fair game' to do that on new designs no.
There is a grey area with old stuff like your Pultec's, LA-2A's, 1176's, Neve's etc., where folks have been building those types of units since this forum's inception in 2004 (and with some of the original members, going even further back in time at a different forum).

The original person who started the thread did state that he wasn't wanting to infringe on anything you were doing, I think his intent was/is more a curiosity as to how Mowtown did things back then.

A lot of us here know that it's hard enough trying to make a living in the Pro Audio market as a manufacturer, and I think you'll find a good number are not interested in hurting your business in any way.

There are folks on here who are more than capable of designing a unit that satisfies whatever whim or need there is for a specific unit like this, and I would think (hope) that this thread will evolve into that, if it even lasts that long. Sometimes a thread creates an initial buzz and there's lots of talk, then it goes into a coma for 3 years.

Thanks for coming along and making yourself known.

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.

Not at all, your input and thoughts helped clear the air around here. Thanks

 

[ACME/Helios]

Hey btown. Are you in Ohio?

 

[abbey road d enfer]

It's funny, I always thought Group DIY didn't knock off products that were still in production.

I don't knock off any products, either current or past, but I like analyzing product schematics, out of sheer curiosity, and sometimes to debunk myths.

 

[jacomart]

... It's funny, I always thought Group DIY didn't knock off products that were still in production.

Personally, I think we're not going to knock off anything that is (still?) in production. I'm curious by nature and investigating certain things is always better than spending the afternoons doing crossword puzzles, isn't it? In any case, I consider the Motown DI a unique piece (more than 50 years old) rather than a series product.

Cheers
JM

 

[btown2009]

Thanks for contributing btown. I must admit in my head I had envisaged the five inputs being split to the mixer at the inputs but your post implies the sum was taken from the individual preamp outputs which actually makes a lot more sense.

Looking at the 1556 and 1557 schematics raises a number of options. Both use a parallel pair of triodes in a single ended output stage with a capacitively coupled output transformer but a 12AX7 is a very poor choice for this job. The 6SN7 is better and with 10mA of quiescent as in the 1557 it should be capable of achieving +20dBu into a 600 ohm load via a suitable transformer. Which in itself raises an interesting question. Why does the Motown DI use a 6V6 for the output stage? Perhaps we are being led astray by thinking it is a true SE stage with a dc current carrying transformer in its plate circuit as in the BC-2B? Maybe it has a simple resistive plate load running at about 10mA with a capacitively coupled output transformer? This certainly much simplifies the output transformer selection process. In triode mode a 6V6 has an ra of about 2K and a mu close to ten so not too shabby The VU meter being fed via a 10\k resistor in the 1557 implies the transformer would have been a 4:1 step down.
wn active DI.

Thank you btown for firing up the inspiration/imagination again.

I don't think any of us are thinking in terms of 5 inputs but just a single channel Motown active DI box.

Cheers

Ian

The output driving tube can be a 6sn7, or a 6v6, the 6v6 was chosen because it’s still in production iirc, the unit is 3 stages,input pre, then another pre /output tube driver, then the last stage is the transformer driving tube, the last stage has no gain but a hefty amount of push to drive the transformer load. A regular 15k to 600 ohm transformer is all you need.

 

[btown2009]

Hey btown. Are you in Ohio?

Michigan Al.

 

[ACME/Helios]

The original 5 channel used a 6BX7GT driving a 5:1 transformer for its output stage. That is a hard tube to find for a production unit. McLean didn't think a 12BH7 would be good enough and opted for the 6v6. That should clear that up for everyone.

 

[btown2009]

The original 5 channel used a 6BX7GT driving a 5:1 transformer for its output stage. That is a hard tube to find for a production unit. McLean didn't think a 12BH7 would be good enough and opted for the 6v6. That should clear that up for everyone.

That’s right, I was holding off the details to protect the original info.

 

[ruffrecords]

Thanks for clearing up the OP tube question. That is very generous. Much appreciated.

The 6BX7 is a double triode designed for a 525 line frame oscillator and amplifier. The Valve Museum notes that "

Although not imagined at the time of original manufacture, the 6BX7GT has been used as an audio amplifier."​

and includes a schematic of a 4.1W audio amplifier using it. It needs a meaty 1.5A heater supply!

6BX7GT @ The Valve Museum

I think it would be worth seeing how far we can go with a 6SN7 or even the B9A equivalent the 6CG7.

Edit: The problem with the 6SN7 is its relatively high ra; even for a parallel pair it will be around 5K compared to the 2K of the 6V6. I can see why the 6V6 was chosen because its mu and ra are not that far away from the 6BX7. Hmmm, needs more thinking.

Cheers

Ian

 

[eskimo]

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.

The original 5 channel used a 6BX7GT driving a 5:1 transformer for its output stage. That is a hard tube to find for a production unit. McLean didn't think a 12BH7 would be good enough and opted for the 6v6. That should clear that up for everyone.

You're right, it is funny.

• btown, who has the original schematic, informs us Acme made alterations to their version (comment now removed).
• Acme tries to spin it into btown not having the the real schematic.
• Acme realizes btown in fact has the real schematic, and now all of a sudden their version has alterations.

 

[ACME/Helios]

That’s right, I was holding off the details to protect the original info.

You're right, it is funny.

• btown, who has the original schematic, informs us Acme made alterations to their version (comment now removed).
• Acme tries to spin it into btown not having the the real schematic.
• Acme realizes btown in fact has the real schematic, and now all of a sudden their version has alterations.

Hello Eskimo

Not spinning anything here. I believe we both have the same information. btown and I were both friends with Mike. Mike was a mentor of mine and I'd like the think I knew him well. He helped me greatly in developing the MTP 66 and also reversed engineered a schematic for the Motown EQ off of my original unit, production coming in 2022. I'm sure btown is not trying to mislead anyone and neither am I.

 

[Winston OBoogie]

Personally speaking, I think ACME and Btown are straight shooters.
Acme didn't need to divulge any info here, yet he did.
Same with Btown who seems to have originally just wanted to steer the ship in the right direction.

If they were both friends with Mike, then that alone would have been reason enough to be protective of their late friend's work.
Yet they've separately come on here and given enough info to satisfy the OP's original curiosity, despite the fact that ACME have a financial investment in this.
And from what he says, it was with the assistance and blessing of Mike that their product is now being made.

Please be respectful.

If you want what the ACME does, then buy the ACME

If not, then build a box that you do want.

Peace guys.

 

[emrr]

It's funny, I always thought Group DIY didn't knock off products that were still in production.

That is/was a general theme, historically some members were run out for making clones of current products or even clones of historical products that looked like fakes......that's the distant past clearly, both here and in the wider world of audio commerce. Look at the number of DIY clone/copy projects here and look at the number of boutique companies cranking out reproductions of historical gear, many of which are in current production from someone commercially.

 

[ruffrecords]

I just ran a quick sim of a triode connected 6V6, its plate load = 12K, Rk = 1K, HT = 300V. Quiescent is about 11mA. The 6V6 dissipates 1.7W and the plate resistor 1.5W. With output capacitively coupled to a 10K load, with +31dBu across the 10K the THD is very close to 3%. With a 4:1 transformer this represents +18dBu into 600 ohms. Stage gain is 14dB which implies ra is close to 5K mainly due to the low quiescent current. Not sure if this is the way to go.

Edit: As with all tubes, the quickest way to reduce distortion is to raise the HT. With 400V HT the THD drops to 2%. The price is the 6V6 dissipation rises to 3W and the 12K plate resistor's foes up to 2.8W. Plate current has increased to 15mA.

Cheers

ian

 

[jacomart]

However, THD may not be that important when dealing with guitars and basses.

Cheers
JM

 

[Winston OBoogie]

And thd etc. might be lower if that 6V6 is a cathode follower (maybe driving a global negative fb loop) which was implied when Btown said the last, transformer driving tube had zero gain

 

[untune]

Just wanted to extend a thankyou to btown and also Acme for the info they have contributed; they did not need to divulge anything and yet they have cleared up some of the mysteries. I'll reiterate that there's no intention to clone Acme's product, I respect that a lot of time and effort has gone into the design, prototyping, manufacture etc and I wouldn't want to infringe on any of that.

Winston is correct; I've had an interest and curiosity about this unit for the last decade at least, it's like a classic car that you never get to see under the bonnet (/hood ) of. I was looking for a project and I've had great help here in the past, and thought it might be interesting to do some detective work and see if we could work out what Mike might have done. Given that there are deeper implications to pursuing that line it might be best at this point to focus instead on branching towards the direction Ian is going in. I had been tinkering with this idea after 1000tinyempires suggested looking at the preamp of a portaflex, and there are plenty of old designs where the octal triodes like 6SN7/6SL7 etc crop up, along with the 6V6, cathode follower etc. Also the RCA BC-2B looked intriguing. I think we can probably make some educated guesses about topology from here?

However, THD may not be that important when dealing with guitars and basses.

And I agree—THD probably isn't as crucial in this application. Engineers talked about the original being clean, maybe that was just based on the sonic character rather than any scientific/measured basis?