Searchin' for info on a Ampex console with Tube Monoblocs

I just picked up the same model of Amperex console with the 6973 monoblocks. Would love a set of the schematics if someone would be so kind.
Thanks in advanced
 
Here are a couple different schematics of the preamp. The first is the slightly older 402, and the second is the 403. The image quality is not the best and they are very similar, so you may have to look at both to figure out the component values.
image004.jpg
Ampex403Schematic.jpg

Here is the amplifier.

Ampex-6973-PP-mono-EF86-12AU7-EZ81.-ed.png
 
Not sure what your plans are, but if you are doing a rebuild there are a couple of places in the console where the engineers have added bass-boost features. This would make sense if the console was fitted with the single 8 inch full range driver, but if yours has the 12 inch woofer and tweeter, my opinion is that they are better bypassed.

The first is in the NFB of amplifiers:

nfbChange.jpg

Here is a measurement of the effect of this capacitor, the darker blue is with the capacitor included.

ampsfeedbackmod (Large).jpg
 
Another area is the preamp. It doesn't have a loudness control switch, but was built instead with the loudness always on. This also causes the bass to be over emphasized.

Loudness.jpg

There are two PEC in the unit, this one has only two wires attached, one to the loudness tap of the volume control, the other to ground.

PEC.jpg

I didn't save the measurements on this, but to me it didn't sound right and I was unable to adjust the bass and treble of the tone controls to get a flat response with it always-on.
 
Wow thanks this is great info. I grabbed this off craigslist for $325. Not exactly a steal but I was the first to call. A guy called right after me and offered $500. I couldnt believe my luck. The cabinet is in mint, almost showroom condition! Hard to fathom. The back has never been removed and all the original bugle boy and telefunken tubes are present. Haven't tested them yet but fingers crossed. It has the 403 preamp and the 6973 monos. It has the 12" drivers and bullet tweets. I have recapped quite a few vintage units but have always relied on the knowledge of others. Is it safe/recommended to double the power supply electrolytic capacitors in the amps? What about the pre?
 
Thanks for the 403 schematic. Thats a rare one seeing as there doesnt appear to be a sams fact for this unit. Looks like it's identical to the the 402 aside from the controls. Hard to tell since your scans are lower res. Would you agree its safe to use the lower half of the 402 for the 403?
 
I am having a hard time with the reliability of this schematic for the 6973 PP amplifier. It lists the signal capacitors between the 12AU7 as 47n or .047uF. I am used to coupling capacitor in the .1uF-.47uF range. It also shows the 12AU7 grid to ground capacitor as 4n or .004uF. In my amplifiers these are the same paper capacitors leading me to believe there has to be a typo here. Any thoughts? See attached image.
 

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Yes, I think this is a typo in the original schematic that was carried forward to the one that was cleaned up and redrawn.

The capacitors in my amp were the same as yours, 0.047u to ground, rather than 4n.

I used the 3 .047u caps, plus the .020u on the EF86.
 
Sounds like you got a good deal on your console.

I think you are correct, I haven't seen any differences between the 402 and 403 other than the switches.
 
Actually they appear to be quite different in my case. Some of the coupling capacitors are differnt values as well as the power supply. Found a cleaner copy of the 403 schematic finally.
 
FYI, I'm pretty sure that the OP's console was constructed by Acousti-Craft. Some of the style components and the finish are a dead ringer for my set from 1962.
 
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