Pentode "Sag" compression

[tardishead]

Can anyone suggest a simple mic pre out there - preferably SE that I can mod to experiment with Pentode Screen Compression?

 

[gyraf]

redd47?

 

[tardishead]

Anything without feedback??

 

[lassoharp]

The RCA MI-9333/9333A is a 2 stage with a 1620 wired as a pentode on the front end.  It doesn't appear to be using feedback.  There are some involved tone compensation circuits that you can omit, leaving a fairly simple circuit.

I've breadboarded this one up just to play with the bass boosting circuit and it should work fine for experimenting.

I would post a link to the schematic but I can't remember where I found it - it was one of those I just happened to come across.  May take a little searching but it's out there somewhere. 

 

[tardishead]

Yeh I found it - RCA MI-9333/9333A - how does it sound??
http://www.oswaldsmillaudio.com/archive.html

 

[abbey road d enfer]

The 9333 is a very basic two-stage preamp, cluttered with all sorts of EQ that you don't need. It's just one straight-from-the-book common-cathode pentode followed by the same but triode connected, with an output xfmr.
If your purpose is analysing the effects of sag, these will be more pronounced in the second stage. Hence you should just do a two-stage job with a pentode in the second stage, whatever in the first stage, no output xfmr because the iron's character may dominate and mask what you want to analyse.

 

[tardishead]

If your purpose is analysing the effects of sag, these will be more pronounced in the second stage

Why is that? Because an output stage deals with more current - more screen current when it sags and therefore more drain on the power supply?????

I have a great big output transformer 5k : 600 ohms which was designed for 6v6 in pentode mode - would it be better to experiment with this and a 6v6 rather than a small pentode like 6J7 or EF86 in a less powerful preamp??
What I want to do is create a good ammount of sag to see what it does and see if it is musically useful...

Guitarists love this effect - adds sustain to guitar sounds.

 

[abbey road d enfer]

You have to define what effect you're trying to evaluate:
Is it the effect of starving G2 or the B+ sag?
If the latter (your reference to guitarists tends to indicate this is the case), then you have to have a sagging PSU. Then you'd better have a stage that draws significant current, so a power stage is requisite, but more than that, a class B push-pull stage is what you want, because an SE stage(which is inherently class A, unless you make a dirt box) draws an almost constant current.

 

[tardishead]

I am talking of starving G2. like I said earlier in the post.

 

[abbey road d enfer]

OK, then you don't need to do it on a power stage, but if you already have a convenient xfmr, why not?