Motown Direct Amplifier-inspired Preamp?

[MidnightArrakis]

(very similar-looking (earlier?) unit with a different layout)

There's also a top of chassis pic (I've brightened it since the can cap is difficult to make out in the original) :View attachment 137423

[(very similar-looking (earlier?)] -- Your photo of this chassis more accurately follows the drawing posted earlier than the other photo I had used in my Post #433.

>> Is it possible for you to provide a list of Part Numbers of the other transformers (i.e., HAMMOND 157G, CineMag CM-????, etc.), the large electrolytic capacitors and anything else used inside the chassis? That would prove to be highly useful with my trying to recreate the mechanical design details of this chassis. NOTE: I basically -- assumed -- the chassis height as being 1.50" high. Can you confirm what the actual chassis height is??? THANKS!!!

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[MidnightArrakis]

Nice one midnight. I have been using ChatGPT to find out what tubes are used. It says the first three (from left to right in your chassis) are EF86, 12AX7, 12B4A

It gets confused about the fourth tube (the rectifier). First it says it is a 6X4 (which is a small B7G tube), then it says it used a semiconductor rectifier, and lastly it thinks it is a 5AR4/GZ34 (which seems most likely).

The mains transformer (rear right of the chassis) seems to be a Hammond 309JX if my reading is right. Output transformer is a Cinemag and probably physically identical to their CM-2810 model.

Cheers

Ian

FYI:


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[DaveP]

I don't understand the purpose of R5.
Cheers
Ian

R5 supplies the DC feedback, as in the V72
best
DaveP

 

[abbey road d enfer]

In the V72, the 50k resistor does not provide DC feedback, because the DC loop is open due to the coupling capacitor at the grid of the 2nd pentode.
It does provide some AC POSITIVE feedback, though, sensing the output current, which creates a negative impedance to drive the xfmr, which is a way of reducing LF distortion.
In your circuit, this "fedback" loop does not sense the transformer current, and it senses only DC, because of capacitor C5 across R6.

 

[dai h.]

View attachment 137427

>> Is it possible for you to provide a list of Part Numbers of the other transformers (i.e., HAMMOND 157G, CineMag CM-????, etc.), the large electrolytic capacitors and anything else used inside the chassis? That would prove to be highly useful with my trying to recreate the mechanical design details of this chassis. NOTE: I basically -- assumed -- the chassis height as being 1.50" high. Can you confirm what the actual chassis height is??? THANKS!!!

/

Sorry Midnight, I've got nothing more than the info and the pic in that link.

(err.. too fuzzy to make out clearly, but) possibly CM-2810 (Vacuum tube output; 4:1, 9.6K:600R)?

https://cinemag.biz/output/PDF/CM-2810.pdf

(the colors for the wires in the pdf. (BLK-ORG-RED-YEL) seem to match the ones in the pic (though maybe that's the same for many of Cinemag's XFMRs)

 

[ravachol]

I was led to believe that Electrodyne were the Motown amps?

Electrodyne solid-state and Altec passive EQ —to start– and towels on the drumheads.

 

[DaveP]

• #444

In the V72, the 50k resistor does not provide DC feedback, because the DC loop is open due to the coupling capacitor at the grid of the 2nd pentode.
It does provide some AC POSITIVE feedback, though, sensing the output current, which creates a negative impedance to drive the xfmr, which is a way of reducing LF distortion.
In your circuit, this "fedback" loop does not sense the transformer current, and it senses only DC, because of capacitor C5 across R6.

I take your point, but this is a detail that I was reluctant to discard from the V72 without trying it first on the bench. In fact the actual V72 does not have any ac feedback because it has dc feedback from the second anode to the first tube cathode. This draws an extra 2.5mA which I did not want to do. So I have actually copied the V77, not the V72 and I think as you say, the "dc" feedback could be discarded.
best
DaveP

 

[ruffrecords]

I take your point, but this is a detail that I was reluctant to discard from the V72 without trying it first on the bench. In fact the actual V72 does not have any ac feedback because it has dc feedback from the second anode to the first tube cathode. This draws an extra 2.5mA which I did not want to do. So I have actually copied the V77, not the V72 and I think as you say, the "dc" feedback could be discarded.
best
DaveP

The dc feedback in the V72 and V77 helps to improve LF stability because it ensures there is only a single zero in the loop. Without this there are two zeros in the loop and there will be a resonance at some LF frequency. To minimise its size and frequency requires careful choice of the coupling and feedback capacitors. The dc feedback in many of the V series amps is a major innovation that eliminates this. Another good amp which uses a similar technique is the Pultec MB-1 mic pre:



This design also direct couple the second triode to the cathode follower. There is no reason we could not retain the 6BX7.

Cheers

Ian