Motown Direct Amplifier-inspired Preamp?

[s wave]

I just figured I would interject here. I had the pleasure to talk to Bob O. (hey there) a few times on other site. If no one previously mentioned it; James Jamerson's bass was plugged into an DI/amp that used a UTC A-10. (someone had a probable Motown one with a UTC-11) The old gray ones. https://reverb.com/item/55686219-utc-a-10-transformer. James would listen back to a direct signal through the studio monitor and the ["amp" consisted of a mixer, a McIntosh amp and an Altec monitor speaker.] Interestingly Babbit used a UTC-11 in Philly? Sigma commented on how 2 great bassists/studios ended up using a similar tranny. Prolly cuz the low end bliss. The ol wolfie box schematics is trackable I think. https://en.m.wikipedia.org/wiki/Wolfbox Big Bob Olhsson fan here https://en.m.wikipedia.org/wiki/Wolfbox

 

[abbey road d enfer]

I just figured I would interject here. I had the pleasure to talk to Bob O. a few times on other site. If no one previously mentioned it; James Jamerson's bass was plugged into an DI/amp that used a UTC A-10.

As anyone that's followed the thread from head to toe, this in full contradiction with other testimonies, e.g. Jim Joe Messina who affirms James Jamerson's bass was plugged in the same box the three guitarists were. Now, i'm not saying Bob O. or Jim Messina is wrong but maybe someone is mistaken, OR it could be that different set-ups were used in different sessions, due to a legitimate desire to experiment, so they would both be right.

 

[CJ]

Joe Messina,


Jim was in the band Loggers and Machinist's.

Joe was also the guitarist on the Soupy Sales show.

The UTC A 10 should be the same core and sec turns as the A 11 , it just has an extra tap on the PRI for 125 and 333 ohms so both should sound about the same with those 8,000 turns for the mega Henrys for bass.

On most Motown songs you can barely hear the guitars, they are way down in the mix, so the amp might not he a big as factor as say the instruments and the players. Most of it sounds like a click track tied to the snare to either push the beat forward, keep it the same or drag it back a bit.
It us really hard to hear the guitars on the tracks with a lot of horns, strings, vibes, etc. The best way to hear the subtle effect of the guitars us to listen to the track with and without the guitars.

But I still want to know what was in that box!

 

[abbey road d enfer]

Joe Messina,

You're absolutely correct. I know very well the difference, but fingers did not follow brains.

 

[CJ]

Not you fingers, AI spell check,

 

[olhsson]

Last year I spent a week at the studio with John Windt who had actually assembled the box. It turned out I was wrong about the reverse UTC A-10. It had a high impedance tube input. It may have had a UTC output transformer for each input because it was balanced in the patch bay. Joe's memories match mine. Mike's information in the first post is correct. He designed it and John assembled it.

 

[CJ]

Just some thoughts on this, since the UTC is being used in reverse, you will have a 10:1 gain loss which will need to be made up with a low noise amplifier stage maybe a 7025 instead of 12ax7a ,

DC heaters would be nice,

The 6BX7 is a dual triode, 6V6GT is a single pentode,

The 6BK7 has a plate resistance of 1300 ohms, the 6V6 around 58,000 ohms,


So a 6BX7 with the sections in parallel would have an easier time driving an output transformer than the 6V6, or put another way, the primary inductance on the output transformer could be lower than what us needed for a 6V6. But there was some talk about the 6V6 feeding the OPT from the cathode. You would not need to do thus with the 6BX7 with that low plate resistance, you could drive it from the plate, especially with 1300 ohms in parallel.

So I see this as a 7025 into a 7025 into a 6BX7 into a plate loaded OPT with a gain control in there. Would thus supply enough db to get line level out? The 6BX7 has an amp factor if 10. 7025 yields a u if about 50 in real life, so 50 x 50 x 10 = 25,000, minus the input xfmr so 2500, minus the 5 to 1 opt = 500. And don't forget the gain control and NFB, so call it 200.

200 x 50 MV guitar = 10 volts out, that might be enough,

 

[CJ]

Just some thoughts on this, since the UTC is being used in reverse, you will gave a 10:1 gain loss which will need to be made up with a low noise amplifier stage maybe a 7025 instead of 12ax7a ,

DC heaters would be nice,

The 6BX7 is a dual triode, 6V6GT is a single pentode,

The 6BK7 has a plate resistance of 1300 ohms, the 6V6 around 58,000 ohms,


So a 6BX7 with the sections in parallel would gave an easier time driving an output transformer than the 6V6, or put another way, the primary inductance on the output transformer could be lower than what us needed for a 6V6. But there was some talk about the 6V6 feeding the OPT from the cathode. You would not need to do thus with the 6BX7 with that low plate resistance, you could drive it from the plate, especially with 1300 ohms in parallel.

So I see this as a 7025 into a 7025 into a 6BX7 into a plate loaded OPT with a gain control in there. Would thus supply enough db to get line level out? The 6BX7 has an amp factor if 10. 7925 yields a u if about 50 in real life, so 50 x 50 x 10 = 25,000, minus the input xfmr so 2500, minus the 5 to 1 opt = 500. And don't forget the gain control and NFB, so call it 200. No tone controls to make up for, the engineers doctored the sound at their end.

Eddie "Chank" Willis used those heavy picks and he would take a wind up stroke before slamming the strings with a quick stroke as to not have any delay between strings so I know he was putting out some voltage, at least 200 millivolts so figure 0.2 x 200 amp gain = 40 volts, I bet he turned his volume down,



But what I really want to know us where did they get that reverb?

 

[CJ]

OK there is no way they would parallel the 6BX7 tube sections as that would mean about 120 ma plate current, too much dc imbalance, you would need a huge output transformer so i bet they ran a single section which means they could share one section for the next channel.

This would cut down on heat, cost, and floor space. (3 tubes instead of 5) I doubt they ran the tube in push pull. There is no need for that much voltage swing.

They might have used the extra triode as the summing amp for the Mac monitor amp.

Even with one section of the 6BX7, 60 ma (minus 20 bias) is a lot of DC imbalance. A Triad HS-50 will not handle that. So I bet it is one section of a 6BX7 with a cap feeding a transformer like the MEQ-5 circuit.