Ampex tape preamps

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Yep, it's all tube. There were a couple thick cables with large 12(ish) pin connector type deals, but we've managed to tear all that stuff off at this point.

To be honest, it seems like it almost makes more sense just to build something from scratch using the tubes and iron-- but I'm sort of irrationally attached to this box.

It was "racked" before I got it, but it needs some help.

On another note, I played through a matchless hotbox clone recently that was built into an old big block chevy valve cover! Sounded great too.
 
I don't know what the 601s sound like, but the 351s can be great when overhauled. The job is a total pain in the butt to do though. Not worth the work unless you got the thing for a hundred bucks IMHO. They end up sounding much cleaner and quieter when finished.
 
Hi everyone,

I saw a spec somewhere for the cathode follower output transformer (for the 601) which stated 15k:600. This has confused me because that is the typical Zs for a plate to line trafo like the 15095. One of the typical uses for a cathode follower is to lower the output impedance, so I'm not sure about what is going on there. The reason I dislike the 601 output trafo is because it clips the transients on one phase of the signal. I've seen this mentioned on other sites as "transient distortion of the 601." This effect can be avoided by keeping levels low - but that's no fun. :twisted:

Yes, you will get some cross loading if you don't use a selector switch to choose between the 2 outputs. I saw about a 1 or 2 dB drop when plugging into the second output. I have preferred to eliminate the cathode follower and 4th gain stage entirely in my latest work (dump the 12au7 tube). When I do leave the original circuit these days, I install a switch to select the 15095 which doesn't use the cathode follower or 4th gain stage. From the output of the 12ay7 you disconnect the line going to the record EQ section and tap the second output (plate to line trafo) from there.

If you look at the schematic, you'll see the 4uF 150V blocking cap between the cathode follower and the transformer. This was originally an electrolytic and is a great candidate for replacement with a good polypropylene. I use a 4uF 200V Auricap for that spot. Anyway, you don't need a trafo that will handle DC like some of the old RCA designs. I wouldn't try to replace that cathode follower trafo - just leave it original and install a plate to line trafo as I described as a second output. That gives you the vintage circuit as well as a much airier and cleaner circuit.

hope this helps ...
 
alright, lets squash that piece of misinformation before it spreads like wildfire all over the web. the transformer does NOT clip one side of the signal, the cathode follower does. single ended drive circuits always clip one side sooner than the other
 
Then how do you explain the fact that the waveform inverts and clips the opposite phase when you reverse the trafo leads? If it was the cathode follower this would not happen. The clipping effect inverts when you invert the trafo.
 
It's been over a year since I ran those tests. I will readdress this issue and post if I am wrong. I think I checked it on the scope too, but I can't remember for sure. My first suspicion was the CF and was dismayed when it looked to be a defective trafo design. I will admit to some possible confusion.
 
Sorry if i misled anyone. As far as I can tell from a quick check, the transformer is reasonably symmetrical. I pulled one out of its unit, ran some test tones through it and it looks fine on the scope. I didn't push it very hard, but from these tests, it looks fine. Except for special requests, I've been avoiding the whole cathode follower circuit, but it's nice to have corrected my thinking on this. Thanks, S_C.
(I could have sworn I inverted both sides of the trafo - oh well.)
 
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